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Title: Principles and practices of agricultural analysis
Author: Harvey Washington Wiley
Release date: March 31, 2024 [eBook #73302]
Language: English
Original publication: Easton: Chemical Publishing Co, 1894
Credits: Richard Tonsing, Peter Becker, and the Online Distributed Proofreading Team at https://www.pgdp.net (This file was produced from images generously made available by The Internet Archive)
*** START OF THE PROJECT GUTENBERG EBOOK PRINCIPLES AND PRACTICES OF AGRICULTURAL ANALYSIS ***
PRINCIPLES AND PRACTICE
—OF—
AGRICULTURAL ANALYSIS.
A MANUAL FOR THE ESTIMATION OF SOILS, FERTILIZERS, AND AGRICULTURAL
PRODUCTS.
FOR THE USE OF ANALYSTS, TEACHERS, AND STUDENTS OF AGRICULTURAL
CHEMISTRY.
VOLUME I.
=SOILS.=
BY HARVEY W. WILEY,
CHEMIST OF THE U. S. DEPARTMENT OF AGRICULTURE.
EASTON, PA.,
CHEMICAL PUBLISHING CO.,
1894.
COPYRIGHT, 1895,
BY HARVEY W. WILEY.
PREFACE TO VOLUME FIRST.
In this volume I have endeavored to place in the hands of teachers and
students of Agricultural Analysis, and of analysts generally, the
principles which underlie the science and art of the analysis of soils
and the best approved methods of conducting it.
In the prosecution of the work I have drawn freely on the results of
experience in all countries, but especially in the matter of the
physical examinations of soils, of this country. Science is not
delimited by geographic lines, but an author is not to be blamed in
first considering favorably the work of the country in which he lives.
It is only when he can see nothing of good outside of its own boundaries
that he should be judged culpable. It has been my wish to give full
credit to those from whose work the subject-matter of this volume has
been largely taken. If, in any case, there has been neglect in this
matter, it has not been due to any desire on my part to bear the honors
which rightfully belong to another. With no wish to discriminate, where
so many favors have been extended, especial acknowledgments should be
made to Messrs. Hilgard, Osborne, Whitney, and Merrill, for assistance
in reading the manuscript of chapters relating to the origin of soils,
their physical properties, and mechanical analysis. With the wish that
this volume may prove of benefit to the workers for whom it was written
I offer it for their consideration.
H. W. WILEY.
WASHINGTON, D. C.,
Beginning of January, 1895.
TABLE OF CONTENTS OF VOLUME FIRST.
PART FIRST.
_Introduction_, pp. 1–27.—Definitions; Origin of soil; Chemical elements
in the soil; Atomic masses; Properties of the elements; Relative
abundance of the elements; Minerals occurring in rocks; Classification
of minerals.
_Rocks and Rock Decay_, pp. 28–43.—Types of rocks; Microscopical
Structure of Rocks; Composition of rocks; Color of rocks; Kinds of
rocks; Eruptive rocks.
_Origin of Soils_, pp. 43–63.—Decay of rocks; Effect of latitude on
decay; Action of water; Action of vegetable life; Action of worms and
bacteria; Action of air; Classification of soils; Qualities and kinds of
soils; Humus; Soil and subsoil; Authorities cited in part first.
PART SECOND.
_Taking Samples for Analysis_, pp. 65–86.—General principles; General
directions for sampling; Method of Hilgard; Official French method;
Caldwell’s, Wahnschaffe’s, Peligot’s, and Whitney’s methods; Samples for
moisture; Samples for permeability; Samples for staple crops; Method of
the Royal Agricultural Society; Method of Grandeau; Method of Official
Agricultural Chemists; Method of Lawes; Instruments for taking samples;
Principles of success in sampling.
_Treatment of Sample in the Laboratory_, pp. 87–93.—Preliminary
examination; Treatment of loose soils; Treatment of compact soils;
Miscellaneous methods; Authorities cited in part second.
PART THIRD.
_Physical Properties of Soils_, pp. 95–101.—The soil as a mass; Color of
soils; Odoriferous matters in soils; Specific gravity; Apparent specific
gravity.
_Relation of Soil to Heat_, pp. 102–103.—Sources of soil heat; Specific
heat; Absorption of solar heat.
_Determination of Specific Heat_, pp. 104–110.—General principles;
Method of Pfaundler; Variation of specific heat.
_Soil Thermometry_, pp. 111–115.—General principles; Frear’s method of
stating results; Method of Whitney and Marvin.
_Applications of Soil Thermometry_, pp. 115–116.—Absorption of heat;
Conductivity of soils for heat.
_Cohesion and Adhesion of Soils_, pp. 116–117.—Behavior of soil after
wetting; Methods of determining cohesion and adhesion; Adhesion of soil
to wood and iron.
_Absorption by Soils_, pp. 117–130.—General principles; Summary of data;
Cause of absorption; Deductions of Warington, Way, and Armsby; Selective
absorption of potash; Influence of surface area; Effect of removal of
organic matters; Importance of soil absorption; Methods of determining
absorption; Statement of results; Preparation of salts for absorption.
_Relations of Porosity to Soil Moisture_, pp. 131–150.—Definition of
porosity; Influence of drainage; Capacity of soil for moisture;
Determination of porosity; Whitney’s method; Relation of fine soil to
moisture; Wolff’s and Wahnschaffe’s method; Petermann’s method; Mayer’s
method; Volumetric determination; Wollny’s method; Heinrich’s method;
Effect of pressure on water capacity; Coefficient of evaporation;
Determination of capillary attraction; Inverse capillarity;
Determination of coefficient of evaporation; Wolff’s method; Water given
off in a water-free atmosphere; Porosity of soil for gases;
Determination of permeability in the field.
_Movement of Water Through Soils: Lysimetry_, pp. 151–170.—Porosity in
relation to water movement; Methods of water movement; Capillary
movement of water; Causes of water movement; Surface tension of
fertilizers; Methods of estimating surface tension; Preparation of soil
extracts; Lysimetry; Relative rate of flow of water through soils;
Measurement of rate of percolation; Authorities cited in part third.
PART FOURTH.
MECHANICAL ANALYSIS.
_The Flocculation of Soil Particles_, pp. 171–185.—Relation of
flocculation to mechanical analysis; Effect of potential of surface
particles; Destruction of floccules; Suspension of clay in water; effect
of chemical action; Theory of Barus; Physical explanation of subsidence;
Separation of soil into particles of standard size; Mechanical
separation; Sifting with water.
_Separation of Soil Particles by a Liquid_, pp. 185–207.—Classification
of methods of silt analysis; Methods depending on subsidence of soil
particles; Methods of Kühn, Knop, Wolff, Moore, Bennigsen, and Gasparin;
Method of Osborne; Schloesing’s method.
_Separation of Soil Particles by a Liquid in Motion_, pp.
207–247.—General principles; Nöbel’s Apparatus; Method of Dietrich;
Method of Masure; Method of Schöne; Mayer’s method; Osborne-Schöne
method; Statement of results; Berlin-Schöne method; Hilgard’s method;
Colloidal clay; Properties of pure clay; Separation of fine sediments;
Weighing sediments; Classification of results; Comparison of methods.
_Miscellaneous Determinations_, pp. 247–281.—Mechanical determination of
clay; Effect of boiling on clay; General conclusions; Distribution of
soil ingredients; Percentage of silt by classes; Interpretation of silt
analysis; Number of soil particles; Surface area of soil particles;
Logarithmic constants; Mineralogical examination of silt; Microscopical
examination; Petrographic microscope; Forms and dimensions of particles;
Silt classes; Crystal angles; Refractive index; Polarized light;
Staining silt particles; Cleavage of soil particles; Microchemical
examination of silt particles; Petrographic examination of silt
particles; Separation of silt particles by specific gravity; Separation
with a magnet; Color and transparency; Value of silt analyses;
Authorities cited in part fourth.
PART FIFTH.
_Estimation of Gases in Soils_, pp. 282–300.—Carbon dioxid; Aqueous
vapor; Maximum hygroscopic coefficient; Absorption of aqueous vapors;
Oxygen and air; General method of determining absorption; Special
methods; Diffusion of carbon dioxid; General conclusions; Authorities
cited in part fifth.
PART SIXTH.
_Chemical Analysis of Soils_, pp. 301–342.—Preliminary considerations;
Order of examination; Determination of water in soils; General
conclusions; Estimation of organic matter in soils; Estimation of humus;
Estimation of carbonates in arable soils.
_Digestion of Soils with Solvents_, pp. 342–352.—Treatment with water;
With water saturated with carbon dioxid; With water containing ammonium
chlorid; With water containing acetic acid; Treatment with citric acid;
With hydrochloric acid; With nitric acid; With hydrofluoric and
sulphuric acids.
_Determination of the Dissolved Matter_, pp. 352–367.—Methods of the
Official Agricultural Chemists; Hilgard’s methods; Belgian methods; Bulk
analysis.
_Special Methods of Soil Analysis_, pp. 367–428.—Determination of
potash; Potash soluble in concentrated acids; Soluble in dilute acids;
Estimation as platinochlorid; German Station methods; Raulin’s method;
Russian method; Italian method; Smith’s method; International method;
Dyer’s method; Estimation of total alkalies and alkaline earths; French
method for lime; Estimation of actual calcium carbonate; Estimation of
active calcareous matter; Russian method for lime; Assimilable lime;
German lime method; Estimation of magnesia; Estimation of manganese;
Estimation of iron; Estimation of phosphoric acid; Estimation of
sulfuric acid; Estimation of chlorin; Estimation of silica; Simultaneous
estimation of different elements; Estimation of kaolin in soils.
_Estimation of Nitrogen in Soils_, pp. 428–458.—Nature of nitrogenous
principles; Method of Official Agricultural Chemists; Hilgard’s method;
Moist combustion method of Müller; Soda-lime method; Treatment of soil
containing nitrates; Volumetric method with copper oxid; Estimation of
ammonia; Amid nitrogen; Volatile nitrogenous compounds; Late methods of
the Official Agricultural Chemists; Authorities cited in part sixth.
PART SEVENTH.
_Oxidized Nitrogen in Soils_, pp. 459–496.—Organic nitrogen; Nitric and
nitrous acids; Conditions of nitrification; Production of nitric and
nitrous acids; Production of ammonia; Order of oxidation; Occurrence of
nitrifying organisms; Nitrifying power of soils; Culture of nitrifying
organisms; Isolation of nitrous and nitric ferments; Classification of
nitrifying organisms; Sterilization; Thermostats for cultures;
Conclusions.
_Determination of Nitric and Nitrous Acids in Soils_, pp.
496–531.—Classification of methods; Extraction of nitric acid; The
nitric oxid process; Schloesing’s method; Warington’s method; Spiegel’s
method; Schulze-Tiemann method; DeKoninck’s method; Schmitt’s process;
Merits of the ferrous salt method; Mercury and sulfuric acid method;
Lunge’s nitrometer; Utility of the method; The indigo method.
_Determination of Nitric Nitrogen by Reduction to Ammonia_, pp.
531–542.—Classification of methods; Method of the Official Agricultural
Chemists; German method; Devarda’s method; Stoklassa’s process;
Sievert’s variation; Variation of the sodium-amalgam process; Schmitt’s
method; Process of Ulsch; Reduction by the electric current; Copper-zinc
and aluminum-mercury couples.
_Iodometric Estimation of Nitric Acid_, pp. 543–548.—Method of DeKoninck
and Nihoul; Method of Gooch and Gruener.
_Estimation of Nitric and Nitrous Acids by Colorimetric Comparison_, pp.
548–570.—Delicacy of the process; Hooker’s carbazol method;
Phenylsulfuric acid method; Estimation of nitric in presence of nitrous
acid; Metaphenylenediamin method for nitrous acid. Sulfanilic acid test;
Naphthylamin process; Use of starch as indicator; Method of Chabrier;
Ferrous salt method; Potassium ferrocyanid method; Collecting samples of
rain water.
_Determination of Free and Albuminoid Ammonia_, pp. 570–575.—Nessler
process; Ilosvay’s reagent; Authorities cited in part seventh.
PART EIGHTH.
_Special Examination of Waters_, pp. 576–583.—Total solid matter;
Estimation of chlorin; Estimation of carbon dioxid; Boric acid.
_Special Treatment of Muck Soils_, pp. 583–591.—Sampling; Water content;
Organic carbon and hydrogen; Total volatile matter; Estimation of
sulfur; Estimation of phosphoric acid; Estimation of humus; Special
study of soluble matters in muck.
_Unusual Constituents of Soil_, pp. 580–593.—Estimation of copper;
Estimation of lead; Estimation of zinc; Estimation of boron; Authorities
cited in part eighth. Index, pp. 594–607.
* * * * *
=CORRECTIONS.=—Page 112, second line from bottom, read “Fig. 14” instead
of “13.”
Page 158, insert “and determining soluble matters therein” after “flow”
in paragraph =172=, third line.
Page 468, paragraph =423=, read “calcium carbonate about 200
milligrams,” instead of “calcium carbonate, or gypsum fifty milligrams.”
Page 557, read “red-yellow” instead of “blue” in seventh line from
bottom.
ILLUSTRATIONS TO VOLUME FIRST.
Page.
Plate, figures 1–6. To face 29
Figure 7. Microscopic structure of sandstone 36
„ 8. Microstructure of crystalline limestone 39
„ 9. Microstructure of Gneiss 40
Plate, figure 10. View on the broad branch of Rock Creek, 48
Washington, D. C., to face
Figure 11. 82
„ 12. 84
„ 13. Regnault’s apparatus for determining the specific heat 105
of soils
„ 14. Soil thermometer 113
„ 15. Zalomanoff’s apparatus for determining absorption of 126
salts by soils
„ 16. Müller’s apparatus to show absorption of salts by 127
soils
Plate, figure 17. Capacity of the fine soil for holding moisture. 136
Method of Wolff modified by Wahnschaffe, to face
Figure 18. Fuelling’s apparatus 140
„ 19. Apparatus to show capillary attraction of soils for 145
water
Plate, figure 20. Apparatus for determining coefficient of 148
evaporation, to face
Figure 21. Method of Heinrich 150
„ 22. Method of Welitschowsky 162
„ 23. Ground plan and vertical section of lysimeters and 166
vaults showing position of the apparatus
Plate, figure 24. Deherain’s apparatus for collecting drainage 168
water, to face
Figure 25. Knop’s silt cylinder 190
„ 26. Siphon cylinder for silt analysis 191
„ 27. Bennigsen’s silt flasks 195
„ 28. Nöbel’s elutriator 208
„ 29. Dietrich’s elutriator 209
„ 30. Masure’s silt apparatus 211
„ 31. Schöne’s elutriator 212
„ 32. Schöne’s elutriator outflow tube 213
„ 33. Schöne’s elutriator, arrangement of apparatus 214
„ 34. Schöne’s apparatus for silt analysis, modified by 221
Mayer
„ 35. Hilgard’s churn elutriator 226
„ 36. Improved Schöne’s apparatus with relay 228
„ 37. 257
Plate, figure 38. To
face
264
„ figures 39–44. „
„ figures 45–50. „
„ figures 51–56. „
Figure 57. Machine for making mineral sections 267
„ 58. Thoulet’s separating apparatus 272
„ 59. Harada’s apparatus 275
„ 60. Brögger’s apparatus 276
„ 61. Apparatus of Wülfing 277
„ 62. Schloesing’s soil-tube for collecting gases 291
„ 63. Schloesing’s apparatus for collecting gases from soil 292
„ 64. Schloesing’s apparatus for determination of carbon 293
dioxid
„ 65. Knorr’s apparatus for the determination of carbon 338
dioxid
„ 66. Bernard’s calcimeter 339
„ 67. Smith’s muffle for decomposition of silicates 381
„ 68. Apparatus by Sachsse and Becker 401
Plate, figures 69 and 70. To face 480
Figure 71. Sterilizing oven 491
„ 72. Autoclave sterilizer 492
„ 73. Arnold’s sterilizer 493
„ 74. Lautenschläger’s thermostat 494
„ 75. Schloesing’s apparatus for nitric acid 501
„ 76. Warington’s apparatus for nitric acid 505
„ 77. Spiegel’s apparatus for nitric acid 509
„ 78. Schulze-Tiemann’s nitric acid apparatus 511
„ 79. DeKoninck’s apparatus 514
„ 80. End of delivery-tube 514
„ 81. Schmidt’s apparatus 516
„ 82. Lunge’s nitrometer 519
„ 83. Lunge’s improved apparatus 521
„ 84. Lunge’s analytic apparatus 523
„ 85. Stoklassa’s nitric acid apparatus 535
„ 86. Variation of the sodium amalgam process 537
„ 87. McGowan’s apparatus for the iodometric estimation of 544
nitric acid
„ 88. Apparatus of Gooch and Gruener 547
„ 89. Method of Chabrier 566
„ 90. Schaeffer’s nitrous acid method 568
„ 91. Retort for distilling ammonia 572
„ 92. Gooch’s apparatus for boric acid 581
„ 93. Apparatus for determining sulfur 587
PART FIRST.
INTRODUCTION.
=1. Definitions.=—The term soil, in its broadest sense, is used to
designate that portion of the surface of the earth which has resulted
from the disintegration of rocks and the decay of plants and animals,
and which is suited, under proper conditions of moisture and
temperature, to the growth of plants. It consists, therefore, chiefly of
mineral substances, together with some products of organic life, and of
certain living organisms whose activity may influence vegetable growth
either favorably or otherwise. The soil also holds varying quantities of
gaseous matter and of water, which are important factors in its
functions.
=2. Origin Of Soil.=—Agriculturally considered, the soil proper is the
older and more thoroughly disintegrated superficial layer of the earth,
which has been longest exposed to weathering and the influences of
organic life. It is usually from six to twelve inches, but occasionally
several feet in depth. The subsoil, which lies directly under this, is
not as a rule so thoroughly disintegrated, since it is protected in a
measure by the overlying soil. It usually contains less organic matter
than the soil. There is a freer circulation of air in the soil than in
the subsoil, and the metallic elements usually exist therein as higher
oxids. There is usually a notable difference in color between the soil
and subsoil, and frequently a very sharp color line separating the two.
Geologically considered, the soil is that portion of the earth’s crust
which has been more or less thoroughly disintegrated by weathering and
other forces from the original rock formations, or from the sedimentary
rocks, or from the unconsolidated sedimentary material. The soil has,
therefore, the same essential constitution as the general mass of the
earth, except that this débris has been subjected to the solvent action
of water and the influence of vegetable growth.
Preliminary to the proper understanding of the methods of the analysis
of soils, there should be some definite knowledge concerning the
composition of the earth’s crust, so that the analyst may understand
more thoroughly the origin and nature of the material he has to deal
with, and thereby be better equipped for his work.
=3. The Chemical Elements Present in the Soil.=—The chemical elements
present in the soil are naturally some or all of those which were
present in the original rocks. For analytical purposes relating to
agriculture, it is not necessary to take into account the rare elements
which may occur in the soil, but only those need be considered which are
present in some quantity and which enter as an important factor into
plant growth. Of the whole number of chemical elements less than twenty
are of any importance in soil analysis. These elements may be grouped
into two classes, the non-metals, and the metals as follows:
Non-metals. Metals.
Oxygen, Aluminum,
Silicon, Calcium,
Carbon, Magnesium,
Sulfur, Potassium,
Hydrogen, Sodium,
Chlorin, Iron,
Phosphorus, Manganese,
Nitrogen, Barium.
Fluorin,
Boron.
=4. Atomic Masses.=—For the purpose of facilitating the calculation of
results the latest revised table of atomic masses is given below. All
the known elements are included in this table for the convenience of
analysts who may have to study some of the rarer elements in the course
of their work.
This table represents the latest and most trustworthy results reduced
to a uniform basis of comparison with oxygen = 16 as starting point of
the system. No decimal places representing large uncertainties are
used. When values vary, with equal probability on both sides, so far
as our present knowledge goes, as in the case of cadmium (111.8 and
112.2), the mean value is given in the table.
TABLE OF ATOMIC MASSES OF
THE ELEMENTS.
Revised by F. W. Clarke,
Chief Chemist of the United
States Geological Survey, to
January 1st, 1894.
────────────┬───────┬───────
Name. │Symbol.│Atomic
│ │ mass.
────────────┼───────┼───────
Aluminum │Al │ 27
Antimony │Sb │ 120
Arsenic │As │ 75
Barium │Ba │ 137.43
Bismuth │Bi │ 208.9
Boron │B │ 11
Bromin │Br │ 79.95
Cadmium │Cd │ 112
Cesium │Cs │ 132.9
Calcium │Ca │ 40
Carbon │C │ 12
Cerium │Ce │ 140.2
Chlorin │Cl │ 35.45
Chromium │Cr │ 52.1
Cobalt │Co │ 59
Columbium[A]│Cb[Nb] │ 94
Copper │Cu │ 63.6
Erbium │Er │ 166.3
Fluorin │F │ 19
Gadolinium │Gd │ 156.1
Gallium │Ga │ 69
Germanium │Ge │ 72.3
Glucinum[B] │Gl[Be] │ 9
Gold │Au │ 197.3
Hydrogen │H │ 1.008
Indium │In │ 113.7
Iodin │I │ 126.85
Iridium │Ir │ 193.1
Iron │Fe │ 56
Lanthanum │La │ 138.2
Lead │Pb │ 206.95
Lithium │Li │ 7.02
Magnesium │Mg │ 24.3
Manganese │Mn │ 55
Mercury │Hg │ 200
Molybdenum │Mo │ 96
Neodymium │Nd │ 140.5
Nickel │Ni │ 58.7
Nitrogen │N │ 14.03
Osmium │Os │ 190.8
Oxygen[C] │O │ 16
Palladium │Pd │ 106.6
Phosphorus │P │ 31
Platinum │Pt │ 195
Potassium │K │ 39.11
Praseodymium│Pr │ 143.5
Rhodium │Rh │ 103
Rubidium │Rb │ 85.5
Ruthenium │Ru │ 101.6
Samarium │Sm │ 150
Scandium │Sc │ 44
Selenium │Se │ 79
Silicon │Si │ 28.4
Silver │Ag │ 107.92
Sodium │Na │ 23.05
Strontium │Sr │ 87.6
Sulfur │S │ 32.06
Tantalum │Ta │ 182.6
Tellurium │Te │ 125
Terbium │Tb │ 160.0
Thallium │Tl │ 204.18
Thorium │Th │ 232.6
Thulium │Tu │ 170.7
Tin │Sn │ 119
Titanium │Ti │ 48
Tungsten │W │ 184
Uranium │U │ 239.6
Vanadium │V │ 51.4
Ytterbium │Yb │ 173
Yttrium │Yt │ 89.1
Zinc │Zn │ 65.3
Zirconium │Zr │ 90.6
────────────┴───────┴───────
Footnote A:
Has priority over niobium.
Footnote B:
Has priority over beryllium.
Footnote C:
Standard or basis of the system.
PROPERTIES OF THE ELEMENTS.
Following is a brief description of the most important elements
occurring in the earth’s crust in respect of their relations to
agriculture.
=5. Oxygen= exists in the free gaseous state in the atmosphere of which
it constitutes about one-fifth by bulk, whilst in combination with other
elements it forms nearly half the weight of the solid earth, and
eight-ninths by weight of water. It enters into combination with all the
other elements, except fluorin, forming what are known as oxids, and
with many of the elements it unites in several proportions, forming
oxids of different composition. Combined with silicon, carbon, sulfur,
and phosphorus, it forms an essential part of the silicates, carbonates,
sulfates, and phosphates, most of which are very abundant and all of
which are very widely distributed in the earth’s crust. In this form it
is exceedingly stable and is rarely set free. With the exception of the
oxids of silicon these oxids seldom occur uncombined with the metals as
constituents of rocks or soils. The oxids of iron very commonly occur as
such in rocks and soils, and play a very important part in organic life.
The several oxids of iron very frequently determine the color of soils;
as the iron in a soil is more or less oxidized, or as it is exposed more
or less to access of air, the color of the soil changes. These oxids of
iron also play an important part in the absorption capacities of soils
for moisture and other physical conditions of soils, and also in the
oxidation of organic matters in the soil. Many organic substances, and
even the roots of growing plants when deprived of free access of air,
can readily secure oxygen from the iron oxid, thus reducing the iron to
a lower form of oxidation, the oxygen being used for the oxidation of
the organic matter or for the needs of the growing plant; while the
lower oxid of iron can more readily take up oxygen of the air and again
be converted into a higher oxid, ready again to give up a part of its
oxygen and thus serve as a carrier.
=6. Silicon= never occurs in the free state, but combined with oxygen it
forms silica, which constitutes more than one-half of the earth’s crust.
The oxid of silicon occurs in the very common form of quartz, and
likewise, as silicate of alumina, lime or magnesia. Silicon forms an
essential part of many minerals, such as the feldspars, amphiboles,
pyroxenes, and the micas, besides being an essential ingredient of many
other minerals. Silica is relatively very slightly affected by the
ordinary forces concerned in the decay of rocks, and even after the
crystals of feldspars, micas, and other common minerals occurring in
rocks have been disintegrated the silica remains as hard grains of sand,
forming the bulk of most soils. By far the larger part of silicon in
soils is in the form of grains of quartz or silica. This form, however,
is probably chemically inert in regard to plant growth, but it plays a
very important part in the physical structure of soils and in the
physical relation of soils to plant growth.
=7. Carbon= as an elementary substance occurs as diamond and graphite
and in an impure form as anthracite and bituminous coals. In peats and
mucks carbon is the chief constituent. This substance is also contained
in the organic matters of the soil known as humus, and the relation of
the carbon to nitrogen often throws important light upon the amount and
character of the nitrogenous matters. In composition with oxygen it
forms the chief food of growing plants, the carbon of the carbon dioxid
of the air being elaborated into the tissue of the plants and the oxygen
returned to the atmosphere. The content of carbon dioxid in the air is
from three to five parts per thousand by volume. As carbonates this
element helps to form some of the most important ingredients of the
earth’s crust, namely, limestones, marbles, dolomites, etc., and in an
organic form it is found in the shells of the crustaceans. The
calcareous matter of the soil, that is, the carbonates of the earths
therein found, are of the highest importance from an agricultural point
of view. The carbonates in the soil not only favor the process of
converting nitrogenous bodies into forms suitable for plant food, but
also exert a most potent influence on the physical state of the soil and
its capacity for holding water and permitting its flow to and from the
rootlets of the plant.
=8. Sulfur= occurs in nature in both the free and combined state. In the
free state it is found in volcanic regions such as Sicily, Iceland, and
the western United States. Its usual form of occurrence is in
combination with the metals to form sulfids, or with oxygen and a metal
to form sulfates. Sulfur and iron combine to form iron pyrites or iron
disulfid (FeS₂), while sulfur, oxygen, and calcium are found in gypsum,
an important fertilizing compound.
Sulfur plays an important part in the nourishment of plants, being found
in them both as sulfuric acid and in organic compounds. Methods for
estimating the sulfur in both forms will be found in another part of
this manual.
=9. Hydrogen= is a colorless, invisible gas, without taste or smell. It
occurs free in small proportions in certain volcanic gases, and in
natural gas, but its most common form is in combination with oxygen as
water (H₂O), of which it forms 11.13 per cent by weight. It also occurs
in combination with carbon to form the hydrocarbons, such as the mineral
oils (petroleum, etc.) and gases. Hydrogen is of no importance to
agriculture in a free state, but water is the most important of all
plant foods.
=10. Chlorin= occurs free in nature only in limited amounts and in
volcanic vents. Its most common form is in combination with hydrogen,
forming hydrochloric acid, or with the metals to form chlorids. It
combines with sodium to form sodium chlorid or common salt (NaCl), which
is the most abundant mineral ingredient in sea water and which can
usually be detected in rain and ordinary terrestrial waters. In this
form, also, it exists as extensive beds of rock salt, which is mined for
commercial purposes.
Chlorin is found uniformly in plants and must be regarded as an
essential constituent thereof. Common salt applied to a soil modifies
its power of attracting and holding water.
=11. Phosphorus= never occurs in nature in a free state but exists in
combination in greater or less quantities in all soils. Its combinations
are also found in large deposits of minerals known as phosphorite and
apatite and as so-called pebble deposit and phosphate rock. Phosphorus
in some sort of combination is one of the most essential elements in
animal and plant food. In animals its compounds form almost all of the
mineral matter of the bones, and in plants they are the chief
constituents of the ash of seeds.
The mineral deposits of phosphorus, as well as bones, are chiefly
tri-calcium phosphate, while the slag compound resulting from the basic
treatment of iron ores rich in phosphorus is a tetra-calcium salt.
The pebble deposits and some rock phosphates are supposed to be of
organic origin, derived from the remains of marine, terrestrial, and
aerial animals.
Cereal crops remove about twenty pounds of phosphoric acid per acre from
the soil annually and grass crops about twelve pounds. The total
phosphoric acid removed annually by the cereal and grass crops in the
United States is nearly four billion pounds.
Gautier[1] calls attention to the fact that the oldest phosphates are
met with in the igneous rocks such as basalt, trachyte, etc., and even
in granite and gneiss. It is from these inorganic sources, therefore,
that all phosphatic plant food must have been drawn. In the second order
in age Gautier places the phosphates of hydro-mineral origin. This class
not only embraces the crystalline apatites but also those phosphates of
later formation formed from hot mineral waters in the jurassic,
cretaceous, and tertiary deposits. These deposits are not directly
suited to nourish plants.
The third group of phosphates in order of age and assimilability
embraces the true phosphorites containing generally some organic matter.
They are all of organic origin. In caves where animal remains are
deposited there is an accumulation of nitrates and phosphates.
Not only do the bones of animals furnish phosphates but they are also
formed in considerable quantities by the decomposition of substituted
glycerids such as lecithin. The ammonia produced by the nitrification of
the albuminoid bodies combines with the free phosphoric acid thus
produced, forming ammonium or diammonium phosphates. The presence of
ammonium phosphates in guanos was first noticed by Chevreul more than
half a century ago.
If such deposits overlay a pervious stratum of calcium carbonate, such
as chalk, and are subject to leaching a double decomposition takes place
as the lye percolates through the chalk. Acid calcium phosphate and
ammonium carbonate are produced. By further nitrification the latter
becomes finally converted into calcium nitrate. In like manner aluminum
phosphates are formed by the action of decomposing organic matter on
clay.
Davidson,[2] explains the origin of the Florida phosphates by suggesting
that they arose chiefly through the influx of animals driven southward
during the glacial period. According to his supposition the waters of
the ocean, during the cenozoic period contained more phosphorus than at
the present time. The waters of the ocean over Florida were shallow and
the shell fish existing therein may have secreted phosphate as well as
carbonate of lime. This supposition is supported by an analysis of a
shell of _lingula ovalis_, quoted by Dana, in which there were 85.79 per
cent of lime phosphate. In these waters were also many fishes of all
kinds and their débris served to increase the amount of phosphatic
material. As the land emerged from the sea came the great glacial epoch
driving all terrestrial animals southward. There was, therefore, a great
mammal horde in the swamps and estuaries of Florida. The bones of these
animals contributed largely to the phosphatic deposits. In addition to
this, the shallow sea contained innumerable sharks, manatees, whales,
and other inhabitants of tropical waters, and the remains of these
animals added to the phosphatic store.
While these changes were taking place in the quaternary period, the
Florida Peninsula was gradually rising, and as soon as it reached a
considerable height the process of denudation by the action of water
commenced. Then there was a subsidence and the peninsula again passed
under the sea and was covered with successive layers of sand. The
limestones during this process had been leached by rain water containing
an excess of carbon dioxid. In this way the limestones were gradually
dissolved while the insoluble phosphate of lime was left in suspension.
During this time the bones of the animals before mentioned by their
decomposition added to the phosphate of lime present in the underlying
strata, while some were transformed into fossils of phosphate of lime
just as they are found to-day in vast quantities.
Wyatt,[3] explains the phosphate deposits somewhat differently.
According to him, during the miocene submergence there was deposited
upon the upper eocene limestones, more especially in the cracks and
fissures resulting from their drying up, a soft, finely disintegrated
calcareous sediment or mud. The estuaries formed during this period were
swarming with animal and vegetable life, and from this organic life the
phosphates were formed by decomposition and metamorphism due to the
gases and acids with which the waters were charged.
After the disappearance of the miocene sea there were great disturbances
of the strata. Then followed the pliocene and tertiary periods and
quaternary seas with their deposits and drifts of shells, sands, clays,
marls, bowlders, and other transported materials supervening in an era
when there were great fluctuations of cold and heat.
By reason of these disturbances the masses of the phosphate deposits
which had not been infiltrated in the limestones became broken up and
mingled with the other débris and were thus deposited in various mounds
or depressions. The general result of the forces which have been briefly
outlined, was the formation of bowlders, phosphatic débris, etc. Wyatt
therefore classifies the deposits as follows:
1. Original pockets or cavities in the limestone filled with hard and
soft rock phosphates and débris.
2. Mounds or beaches, rolled up on the elevated points, and chiefly
consisting of huge bowlders of phosphate rock.
3. Drift or disintegrated rock, covering immense areas, chiefly in Polk
and Hillsboro counties, and underlying Peace River and its tributaries.
Darton,[4] ascribes the phosphate beds of Florida to the transformation
of guano. According to this author two processes of decomposition have
taken place. One of these is the more or less complete replacement of
the carbonate by the phosphate of lime. The other is a general
stalactitic coating of phosphatic material. Darton further calls
attention to the relation of the distribution of the phosphate deposits
as affecting the theory of their origin, but does not find any peculiar
significance in the restriction of these deposits to the western ridge
of the Florida peninsula.
As this region evidently constituted a long narrow peninsula during
early miocene time it is a reasonably tentative hypothesis that during
this period guanos were deposited from which was derived the material
for the phosphatization of the limestone either at the same time or soon
after.
Darton closes his paper by saying that the phosphate deposits in Florida
will require careful, detailed geologic exploration before their
relations and history will be fully understood.
According to Dr. N. A. Pratt the rock or bowlder phosphate had its
immediate origin in animal life and to his view the phosphate bowlder is
a true fossil. He supposes the existence of some species in former times
in which the shell excreted was chiefly phosphate of lime. The fossil
bowlder, therefore, becomes the remains of a huge foraminifer which had
identical composition in its skeleton with true bone deposits or of
organic matter.
Perhaps the most complete exposition of the theory of the recovery of
waste phosphates, with especial reference to their deposit in Florida,
has been given by Eldridge.[5] He calls attention to the universal
presence of phosphates in sea water and to the probability that in
earlier times, as during the miocene and eocene geologic periods, the
waters of the ocean contained a great deal more phosphate in solution
than at the present time. He cites the observations of Bischof, which
show the solubility of different phosphates in waters saturated with
carbon dioxid. According to these observations apatite is the most
insoluble form of lime phosphate, while artificial basic phosphate is
the most soluble. Among the very soluble phosphates, however, are the
bones of animals, both fresh and old. Burnt bones, however, are more
soluble than bones still containing organic matter. Not only are the
organic phosphates extremely soluble in water saturated with carbon
dioxid, but also in water which contains common salt or chlorid of
ammonium. The presence of large quantities of common salt in sea water
would, therefore, tend to increase its power of absorbing lime
phosphates of organic origin. It is not at all incredible, therefore, to
suppose that at some remote period the waters of the ocean, as indicated
by these theories, were much more highly charged with phosphates in
solution than at the present time.
According to Eldridge, the formation of the hard-rock and soft
phosphates may be ascribed to three periods: First, that in which the
primary rock was formed; second, that of secondary deposition in the
cavities of the primary rock; third, that in which the deposits thus
formed were broken up and the resulting fragments and comminuted
material were redeposited as they now occur.
“The first of these stages began probably not later than the close of
the older miocene, and within the eocene area it may have begun much
earlier. Whether the primary phosphate resulted from a superficial and
heavy deposit of soluble guanos, covering the limestones, or from the
concentration of phosphate of lime already widely and uniformly
distributed throughout the mass of the original rock, or from both, is a
difficult question. In any event, the evidence indicates the effect of
the percolation of surface waters, highly charged with carbonic and
earth acids, and thus enabled to carry down into the mass of the
limestone dissolved phosphate of lime, to be redeposited under
conditions favorable to its separation. Such conditions might have been
brought about by the simple interchange of bases between the phosphate
and carbonate of lime thus brought together, or by the lowering of the
solvent power of the waters through loss of carbonic acid. The latter
would happen whenever the acid was required for the solution of
additional carbonate of lime, or when, through aeration, it should
escape from the water. The zone of phosphate deposition was evidently
one of double concentration, resulting from the removal of the soluble
carbonate thus raising the percentage of the less soluble phosphate, and
from the acquirement of additional phosphate of lime from the overlying
portions of the deposits.”
“The thickness of the zone of phosphatization in the eocene area is
unknown, but it is doubtful if it was over twenty feet. In the miocene
area the depth has been proved from the phosphates _in situ_ to have
been between six and twelve feet.”
The deposits of secondary origin, according to Eldridge, are due chiefly
to sedimentation, although some of them may have been due to
precipitation from water. This secondary deposition was kept up for a
long period, until stopped by some climatic or geologic change. The
deposits of phosphates thus formed in the Florida peninsula are
remarkably free from iron and aluminum, in comparison with many of the
phosphates of the West Indies.
The third period in the genesis of the hard rock deposits embraces the
time of formation of the original deposits and their transportation and
storage as they are found at the present time. The geologic time at
which this occurred is somewhat uncertain but it was probably during the
last submergence of the peninsula.
In all cases the peculiar formation of the Florida limestone must be
considered. This limestone is extremely porous and therefore easily
penetrated by the waters of percolation. A good illustration of this is
seen on the southwestern and southern edges of Lake Okeechobee. In
following down the drainage canal which has been cut into the southwest
shore of the lake the edge of the basin, which is composed of this
porous material may be seen. The appearance of the limestone would
indicate that large portions of it have already given way to the process
of solution. The remaining portions are extremely friable, easily
crushed, and much of it can be removed by the ordinary dredging
machines. Such a limestone as this is peculiarly suited to the
accumulation of phosphatic materials, due to the percolation of the
water containing them. The solution of the limestone and consequent
deposit of the phosphate of lime is easily understood when the character
of this limestone is considered.
Shaler, as quoted by Eldridge in the work already referred to, refers to
this characteristic of the limestone and says that the best conditions
for the accumulation of valuable deposits of lime phosphate in residual
débris appear to occur where the phosphatic lime marls are of a rather
soft character; the separate beds having no such solidity as will resist
the percolation of water through innumerable incipient joints such as
commonly pervade stratified materials, even when they are of a very soft
nature.
Eldridge is also of the opinion that the remains of birds are not
sufficient to account for the whole of the phosphatic deposits in
Florida. He ascribes them to the joint action of the remains of birds,
of land and marine animals and to the deposition of the phosphatic
materials in the waters in the successive subsidences of the surface
below the water line.
=12. Nitrogen= as a mineral constituent of soils, is found chiefly in
the form of nitrates, but, owing to their solubility, they can not
accumulate in soils exposed to heavy rain-falls. The gaseous nitrogen in
the soil is also of some importance, since it is in this material that
the anaerobic organisms which accumulate on the rootlets of some plants
probably act in the process of the fixation of atmospheric nitrogen in a
form accessible to plants. Nitrogen in the free state, it is believed,
is not directly absorbed into the tissues of plants. It is necessary
that it be oxidized in some way to nitric acid before it can be
assimilated. The importance of nitrogen as a plant food can not be too
highly estimated. It is as necessary to plant growth and development as
water, phosphoric acid, lime, and potash, and far more costly. While a
large quantity of nitrogen exists in the air in an uncombined state, it
is, nevertheless, one of the least abundant of the elements of high
importance in plant nutrition.
The conservation and increase of the stores of available nitrogen in the
soil is one of the chief problems occupying the attention of
agricultural chemistry. Nitrogen, which is not immediately available for
the growth of plants, is conserved and restored by natural processes in
various ways.
The waste nitrogen finds its way sooner or later to the sea, and is
restored therefrom in many forms. Sea-weeds of all kinds are rich in
recovered nitrogen. Many years ago Forchhammer[6] pointed out the
agricultural value of certain fucoids. Many other chemists have
contributed important data in regard to the composition of these bodies.
Jenkins[7] has shown from the analyses of several varieties of sea-weeds
that in the green state they are quite equal in fertilizing value to
stall manure, and are sold at the rate of five cents per bushel. These
data are fully corroborated by Goessmann.[8]
Wheeler and Hartwell[9] give the fullest and most systematic discussion
which has been published of the agricultural value of sea-weeds.
Sea-weed was used as a fertilizer as early as the fourth century, and
its importance for this purpose has been recognized more and more in
modern days, especially since chemical investigations have shown the
great value of the food materials contained therein.
To show the commercial importance of sea-weed, it is only necessary to
call attention to the fact that in 1885 its value as a fertilizer in the
State of Rhode Island was $65,044, while the value of all other
commercial fertilizers was $164,133. While sea-weed, in a sense, can
only be successfully applied to littoral agriculture, yet the extent of
agricultural lands bordering on the sea is so great as to render its
commercial importance of the highest degree of interest.
A large amount of nitrogen is also recovered from the sea in fishes. It
is shown by Atwater[10] that the edible part of fishes has an unusually
high percentage of protein. In round numbers, about seventy-five per
cent of the water free edible parts of fish are composed of albuminoids.
Some kinds of fish are taken chiefly for their oil and fertilizing
value, as the menhaden. Squanto,[11] an American Indian, first taught
the early New England settlers the manurial value of fish.
Immense quantities of waste nitrogen are further secured, both from sea
and land, by the various genera of birds. The well-known habit of birds
in congregating in rookeries during the night and at certain seasons of
the year tends to bring into a common receptacle the nitrogenous matters
which they have gathered and which are deposited in their excrement and
in the decay of their bodies. The feathers of birds are particularly
rich in nitrogen, and the nitrogenous content of the flesh of fowls is
also high. The decay of remains of birds, especially if it take place
largely excluded from the leaching of water, tends to accumulate vast
deposits of nitrogenous matter. If the conditions in such deposits be
favorable to the processes of nitrification, the whole of the nitrogen,
or at least the larger part of it, which has been collected in this
débris, becomes finally converted into nitric acid and is found combined
with appropriate bases as deposits of nitrates. The nitrates of the
guano deposits and of the deposits in caves arise in this way. If these
deposits be subject to moderate leaching the nitrate may become
infiltered into the surrounding soil, making it very rich in this form
of nitrogen. The bottoms and surrounding soils of caves are often found
highly impregnated with nitrates.
While for our purpose, deposits of nitrates only are to be considered
which are of sufficient value to bear transportation, yet much interest
attaches to the formation of nitrates in the soil even when they are not
of commercial importance.
In many of the soils of tropical regions not subject to heavy
rain-falls, the accumulation of these nitrates is very great. Müntz and
Marcano[12] have investigated many of these soils to which attention was
called first by Humboldt and Boussingault. They state that these soils
are incomparably more rich in nitrates than the most fertile soils of
Europe. The samples which they examined were collected from different
parts of Venezuela and from the valleys of the Orinoco as well as on the
shore of the Sea of Antilles. The nitrated soils are very abundant in
this region of South America where they cover large surfaces. Their
composition is variable, but in all of them carbonate and phosphate of
lime are met with and organic nitrogenous material. The nitric acid is
found always combined with lime. In some of the soils as high as thirty
per cent of nitrate of lime have been found. Nitrification of organic
material takes place very rapidly the year round in this tropical
region. These nitrated soils are everywhere abundant around caves, as
described by Humboldt, caves which serve as the refuge of birds and
bats. The nitrogenous matters, which come from the decay of the remains
of these animals, form true deposits of guano which is gradually spread
around, and which, in contact with the limestone and with access of air,
suffers complete nitrification with the fixation of the nitric acid by
the lime.
Large quantities of this guano are also due to the débris of insects,
fragments of elytra, scales of the wings of butterflies, etc., which are
brought together in those places by the millions of cubic meters. The
nitrification, which takes place in these deposits, has been found to
extend its products to a distance of several kilometers through the
soil. In some places the quantity of the nitrate of lime is so great in
the soils that they are converted into a plastic paste by this
deliquescent salt.
The theory of Müntz and Marcano in regard to the nitrates of soils,
especially in the neighborhood of caves, is probably a correct one, but
there are many objections to accepting it to explain the great deposits
of nitrate of soda which occur in many parts of Chile. Another point,
which must be considered also, is this: That the processes of
nitrification can not now be considered as going on with the same vigor
as formerly. Some moisture is necessary to nitrification, inasmuch as
the nitrifying ferment does not act in perfectly dry soil, and in many
localities in Chile where the nitrates are found it is too dry to
suppose that any active nitrification could now take place.
The existence of these nitrate deposits has long been known.[13] The old
Indian laws originally prohibited the collection of the salt, but
nevertheless it was secretly collected and sold. Up to the year 1821,
soda saltpeter was not known in Europe except as a laboratory product.
About this time the naturalist, Mariano de Rivero, found on the Pacific
coast, in the Province of Tarapacá, immense new deposits of the salt.
Later the salt was found in equal abundance in the Territory of
Antofogasta and further to the south in the desert of Atacama, which
forms the Department of Taltal.
At the present time the collection and export of saltpeter from Chile is
a business of great importance. The largest export which has ever taken
place in one year was in 1890, when the amount exported was 927,290,430
kilograms; of this quantity 642,506,985 kilograms were sent to England
and 86,124,870 kilograms to the United States. Since that time the
imports of this salt into the United States have largely increased.
According to Pissis[14] these deposits are of very ancient origin. This
geologist is of the opinion that the nitrate deposits are the result of
the decomposition of feldspathic rocks; the bases thus produced
gradually becoming united with the nitric acid provided from the air.
According to the theory of Nöllner[15] the deposits are of more modern
origin and due to the decomposition of marine vegetation. Continuous
solution of soils, gives rise to the formation of great lakes of
saturated water, in which occurs the development of much marine
vegetation. On the evaporation of this water, due to geologic isolation,
the decomposition of nitrogenous organic matter causes generation of
nitric acid, which, coming in contact with the calcareous rocks, attacks
them, forming nitrate of calcium, which, in presence of sulfate of
sodium, gives rise to a double decomposition into nitrate of sodium and
sulfate of calcium.
The fact that iodin is found in greater or less quantity in Chile
saltpeter is one of the chief supports of this hypothesis of marine
origin, inasmuch as iodin is always found in sea and not in terrestrial
plants. Further than this, it must be taken into consideration that
these deposits of nitrate of soda contain neither shells nor fossils,
nor do they contain any phosphate of lime. The theory, therefore, that
they were due to animal origin is scarcely tenable.
=13. Boron= occurs chiefly in volcanic regions, but is much more widely
distributed in the soil than formerly believed. It is a regular
constituent of the ash of many plants,[16] and is, therefore, thought to
be a true plant food. It is one of the least abundant of the elements,
not occurring in sufficient quantity to find a place in the table
showing their relative abundance, which is to follow. Boracic acid is
used to some extent as a preservative.
=14. Fluorin= does not occur free in nature, but it exists chiefly in
combination with calcium, forming fluorspar, and traces of it are found
in sea water. It occurs in bone, teeth, blood, and the milk of mammals.
It is the only element that does not combine with oxygen, and it can be
isolated only with the greatest difficulty. Only very small traces of it
are found ordinarily and it is usually not considered in the chemical
analysis of soils. Fluorin is found, however, in considerable quantities
in certain phosphate deposits.
=15. Aluminum= is, probably, next to oxygen and silicon, the most
abundant element of the earth’s crust, of which it is estimated to form
about one-twelfth. It has never been found, in nature, in the free
state, but commonly occurs in combination with silicon and oxygen, in
which form it is an abundant constituent of feldspar, mica, kaolin,
clay, slate, and many other rocks and minerals.
By the weathering of feldspar, mica, and other minerals containing
aluminum, kaolin or true clay is formed, which is of the greatest
importance in the constitution of the soil. The compounds of aluminum
are not so important as plant food as they are as the constituents of
the soil, forming a large part of its bulk, and modifying in the most
profound degree its physical properties. It is the custom of some
authors to use the word clay to designate the fine particles of soil
which have in general the same relations to moisture and tilth as the
particles of weathered feldspar, etc. In a strict chemical sense,
however, the term clay is applied only to the hydrated silicate of
alumina formed as indicated above. The fertility of a soil is largely
dependent on the quantity of clay which it contains, its relations to
moisture and amenability to culture being chiefly conditioned by its
clay content. The determination of the percentage of clay in soils is an
operation of the highest utility in forming an opinion of the value of a
soil on analytical data alone.
=16. Calcium= is one of the commonest and most important elements of the
earth’s crust, of which it has been estimated to compose about
one-sixteenth. It does not occur free in nature, but its most common
form is in combination with carbon dioxid, forming the mineral calcite,
marble, and the very abundant limestone rocks. In this form it is
slightly soluble in water containing carbon dioxid, and hence lime has
become a universal component of all soils and is very generally found in
natural waters, in which it furnishes the chief ingredient necessary for
the formation of the shells and skeletons of the various tribes of
mollusca and corals. In combination with sulfuric acid calcium forms the
rock gypsum. Lime is not only a necessary plant food, but influences in
a marked degree the physical condition of the soil and the progress of
nitrification. Many stiff clay soils are rendered porous and pulverulent
by an application of lime, and thus made far more productive. On account
of its great abundance and low price, it has not commanded the degree of
attention from farmers and agricultural chemists which its merits
deserve. It forms an essential ingredient of plants and animals, in the
latter being collected chiefly in the bones, while in plants it is
rather uniformly distributed throughout all the tissues.
=17. Magnesium= occurs chiefly in combination with carbon dioxid or with
lime and carbon dioxid in the mineral dolomite. It is intimately
associated with calcium and a trace of it is nearly always found where
lime occurs in any considerable quantity. The bitter taste of sea water
and some mineral waters is due to the presence of salts of magnesia. In
combination with silica it forms an essential part of such rocks as
serpentine, soapstone, and talc. Magnesia is not of much importance as a
plant food nor as a fertilizing material.
=18. Potassium= combined with silica is an important element in many
mineral silicates as, for instance, orthoclase. Granitic rocks usually
contain considerable quantities of potassium, and on their decomposition
this becomes available for plant food. In the form of chlorid, potassium
is found in small quantities in sea water, and as a nitrate it forms the
valuable salt known as niter or saltpeter. Potassium, as is the case
with phosphorus, is universally distributed in soils, and forms one of
the great essential elements of plant food. Under the form of kainite
and other minerals large quantities of potassium are used for
fertilizing and for the manufacture of pure salts for commercial and
pharmaceutical purposes. The ordinary potassium salts are very soluble
and for this reason they can not accumulate in large quantities in soils
exposed to heavy rain-fall. In the form of carbonate, potassium forms
one of the chief ingredients of hard wood ashes, and in this form of
combination is especially valuable for fertilizing purposes. Potash
salts, being extremely soluble, are likely to be held longest in
solution. Some of them, are recovered in animal and vegetable life, but
the great mass of potash carried into the sea still remains unaccounted
for. The recovery of the waste of potash is chiefly secured by the
isolation of sea waters containing large quantities of this salt and
their subsequent evaporation. Such isolation of sea waters takes place
by means of geologic changes in the level of the land and sea. In the
raising of an area above the water level there is almost certain to be
an enclosure, of greater or less extent, of the sea water in the form of
a lake. This enclosure may be complete or only partial, the enclosed
water area being still in communication with the main body of the sea by
means of small estuaries. If this body of water be exposed to rapid
evaporation, as was doubtless the case in past geologic ages, there will
be a continual influx of additional sea water through these estuaries to
take the place of that evaporated. The waters may thus become more and
more charged with saline constituents. Finally a point is reached in the
evaporation when the less soluble of the saline constituents begin to be
deposited. In this way the various formations of mineral matter,
produced by the drying up of enclosed waters, take place.
The most extensive potash deposits known are those in the neighborhood
of Stassfurt, in Germany. The following description probably represents
the method of formation of these deposits:[17]
“The Stassfurt salt and potash deposits had their origin, thousands of
years ago, in a sea or ocean, the waters of which gradually receded,
leaving near the coast, lakes which still retained communication with
the great ocean by means of small channels. In that part of Europe the
climate was then tropical, and the waters of these lakes rapidly
evaporated but were constantly replenished through these small channels
connecting them with the main body. Decade after decade this continued,
until by evaporation and crystallization, the various salts present in
the sea water were deposited in solid form. The less soluble material,
such as sulfate of lime or ‘anhydrite,’ solidified first and formed the
lowest stratum. Then came common rock salt with a slowly thickening
layer which ultimately reached 3000 feet, and is estimated to have been
13,000 years in formation. This rock salt deposit is interspersed with
lamellar deposits of ‘anhydrite,’ which gradually diminish towards the
top and are finally replaced by the mineral ‘polyhalite,’ which is
composed of sulfate of lime, sulfate of potash, and sulfate of magnesia.
The situation in which this polyhalite predominates is called the
‘polyhalite region’ and after it comes the ‘kieserite region,’ in which,
between the rock salt strata, kieserite (sulfate of magnesia) is
imbedded. Above the kieserite lies the ‘potash region,’ consisting
mainly of deposits of carnallite, a mineral compound of muriate of
potash and chlorid of magnesia. The carnallite deposit is from 50 to 130
feet thick and yields the most important of the crude potash salts and
that from which are manufactured most of the concentrated articles,
including muriate of potash.”
“Overlying this region is a layer of impervious clay which acts as a
water-tight roof to protect and preserve the very soluble potash and
magnesia salts, which, had it not been for the very protection of this
overlying stratum, would have been long ages ago washed away and lost by
the action of the water percolating from above. Above this clay roof is
a stratum, of varying thickness of anhydrite, and still above this a
second salt deposit, probably formed under more recent climatic and
atmospheric influences or possibly by chemical changes in dissolving and
subsequent precipitation. This salt deposit contains ninety-eight per
cent (often more) of pure salt, a degree of purity rarely elsewhere
found. Finally, above this are strata of gypsum, tenacious clay, sand,
and limestone, which crop out at the surface.”
“The perpendicular distance from the lowest to the upper surface of the
Stassfurt salt deposits is about 5000 feet (a little less than a mile),
while the horizontal extent of the bed is from the Harz Mountains to the
Elbe River in one direction, and from the city of Madgeburg to the town
of Bernburg in the other.”
According to Fuchs and DeLauny[18] the saline formation near Stassfurt
is situated at the bottom of a vast triassic deposit surrounding
Madgeburg. The quantity of sea water which was evaporated to produce
saline deposits of more than 500 meters in thickness must have been
enormous and the rate of evaporation great. It appears that a
temperature of 100° would have been quite necessary, acting for a long
time, to produce this result.
These authors therefore admit that all the theories so far advanced to
explain the magnitude of these deposits are attended with certain
difficulties. What, for instance, could have caused a temperature of
100°? The most reasonable source of this high temperature must be sought
for in the violent chemical action produced by the double decompositions
of such vast quantities of salts of different kinds. There may also have
been at the bottom of this basin some subterranean heat such as is found
in certain localities where boric acid is deposited.
Whatever be the explanation of the source of the heat it will be
admitted that at the end of the permian period there was thrown up to
the northeast of the present saline deposits a ridge extending from
Helgoland to Westphalia. This dam established throughout the whole of
North Germany saline lagoons in which evaporation was at once
established, and these lagoons were constantly fed from the sea.
There was then deposited by evaporation, first of all a layer of gypsum
and afterwards rock salt, covering with few exceptions the whole of the
area of North Germany.
But around Stassfurt there occurred at this time geologic displacements,
the saline basin was permanently closed and then by continued
evaporation the more deliquescent salts, such as polyhalite, kieserite,
and carnallite, were deposited.
These theories account with sufficient ease for the deposition of the
saline masses, but do not explain why in those days the sea water was so
rich in potash and why potash is not found in other localities where
vast quantities of gypsum and common salt have been deposited. It may be
that the rocks composing the shores of these lagoons were exceptionally
rich in potash and that this salt was, therefore, in a certain degree, a
local contribution to the products of concentration.
=19. Sodium= is never found free in nature, but its most common form is
in combination with chlorin as common salt, an important ingredient of
sea water. Combined with silica sodium is an important element in many
silicates. Sodium, although closely related to potassium chemically,
cannot in any case be substituted therefor in plant nutrition. In
combination with nitrogen it forms soda or Chile saltpeter which is a
valuable fertilizer on account of its content of nitric acid.
=20. Iron= is the most abundant of the heavy metals, and occurs in
nature both free and combined with other elements. In the free state it
is found only to a limited extent in basaltic rocks and meteorites, but
in combination with oxygen it is one of the most widely diffused of
metals, and forms the coloring matter of a large number of rocks and
minerals. In this form, too, it exists as the valuable ores of iron
known as magnetite and hematite. In combination with sulfur it forms the
mineral pyrite, FeS₂. The yellow and red colors of soils are due chiefly
to iron oxids. It is an important plant food, although not taken up in
any great quantity by the tissues of plants.
=21. Manganese=, next to iron, is the most abundant of the heavy metals.
It occurs in nature only in combination with oxygen, in which form it is
associated in minute quantities with iron in igneous rocks or in the
forms known mineralogically as pyrolusite, psilomelane and wad. As the
peroxid of manganese it occurs in concretionary forms scattered
abundantly over the bottom of the deep sea. It is found in the ash of
some plants but is not believed to be an essential to plant growth.
=22. Barium= occurs in nature combined with sulfuric acid, forming the
mineral barite, or heavy spar, or with carbon dioxid forming the mineral
witherite. It is of small importance from an agricultural standpoint.
=23. Relative Abundance of the More Important Chemical Elements.=—It
will be of interest to the agricultural analyst to know as nearly as
possible the relative abundance of the more important chemical elements.
This subject has been carefully studied by Prof. F. W. Clarke in a paper
read before the Philosophical Society of Washington.[19] The materials
considered in these calculations are the atmosphere, the water, and the
solid crust of the earth to the depth of ten miles below the sea level.
Of these materials the relative quantities of the three constituents
named are as follows:
Per cent.
Atmosphere 0.03
Water 7.08
Solid crust of the earth to the depth of ten miles 92.89
According to these calculations the relative abundance of the important
elements composing the atmosphere, the water of the ocean and the solid
crust of the earth to the depth given is as follows:
Solid crust, Ocean, seven per Mean, including
ninety-three per cent. air.
cent.
Oxygen 47.29 per cent. 85.79 per cent. 49.98 per cent.
Silicon 27.21 „ „ „ „ 25.30 „ „
Aluminum 7.81 „ „ „ „ 7.26 „ „
Iron 5.46 „ „ „ „ 5.08 „ „
Calcium 3.77 „ „ 0.05 „ „ 3.51 „ „
Magnesium 2.68 „ „ 0.14 „ „ 2.50 „ „
Sodium 2.36 „ „ 1.14 „ „ 2.28 „ „
Potassium 2.40 „ „ 0.04 „ „ 2.23 „ „
Hydrogen 0.21 „ „ 10.67 „ „ 0.94 „ „
Titanium 0.33 „ „ „ „ 0.30 „ „
Carbon 0.22 „ „ 0.002 „ „ 0.21 „ „
Chlorin 0.01 „ „ 2.07 } „ „ 0.15 „ „
Bromin „ „ 0.008} „ „ „ „
Phosphorus 0.10 „ „ „ „ 0.09 „ „
Manganese 0.08 „ „ „ „ 0.07 „ „
Sulfur 0.03+ „ „ 0.09 „ „ 0.04+ „ „
Barium 0.03 „ „ „ „ 0.03 „ „
Nitrogen „ „ „ „ 0.02 „ „
Chromium 0.01 „ „ „ „ 0.01 „ „
—————— ——————— ——————
100.00 „ „ 100.000 „ „ 100.00 „ „
=24. Fluorin= is not mentioned in this table but it is stated that its
probable percentage is 0.02 to 0.03 making it thus slightly more
abundant than nitrogen.
One of the chief points of interest in connection with this table is
that the nitrogen which is regarded by most persons as one of the most
abundant of the elements is almost the least abundant of those
mentioned.
THE MINERALS OCCURRING IN ROCKS.
=25. The Soil=, as before stated, being comprised almost exclusively of
decayed rocks, its characteristics would naturally be determined by the
character of the minerals contained in the rocks.
A rock may be composed of a single mineral or an aggregation of several
minerals.
According to the authority of the National Museum[20] it may occur,
either in the form of stratified beds, eruptive masses, sheets or dikes,
or as veins and other chemical deposits of comparatively little
importance as regards size and extent. The mineral composition of rocks
is greatly simplified by the wide range of conditions under which the
commonest minerals can be formed. Thus quartz, feldspar, mica, the
minerals of the hornblende, or pyroxene group, can be formed from a mass
cooling from a state of fusion; they may be crystallized from solution,
or be formed from volatilized products. They are therefore the commonest
of minerals and are rarely excluded from rocks of any class, since there
is no process of rock formation which determines their absence.
Most of the common minerals, like the feldspars, micas, hornblendes,
pyroxenes, and the alkaline carbonates possess the capacity of adapting
themselves to a very considerable range of compositions. In the
feldspars, for example, lime, soda, or potash may replace one another
almost indefinitely, and it is now commonly assumed that true species do
not exist, but all are but isomorphous admixtures passing into one
another by all gradations, and the names albite, oligoclase, anorthite,
etc., are to be used only as indicating convenient stopping and starting
points in the series. Hornblende or pyroxene, further, may be pure
silicate of lime and magnesia, or iron and manganese may partially
replace these substances. Lime carbonate may be pure, or magnesia may
replace the lime in any proportion.
These illustrations are sufficient to show the reason for the great
simplicity of rock masses as regards their chief mineral constituents.
Whatever may be the conditions of the origin of a rock mass, the
probabilities are that it will be formed essentially of one or more of a
half a dozen minerals in some of their varieties.
But however great the adaptability of these few minerals may be they
are, nevertheless, subject to very definite laws of chemical
equivalence. There are elements which they cannot take into their
composition, and there are circumstances which retard their formation
while other minerals may be crystallizing. In a mass of rock of more or
less accidental composition formed under these widely varying conditions
it may, therefore, be expected that other minerals will form, in
considerable numbers, but minute quantities. It is customary to speak of
those minerals which form the chief ingredients of any rock, and which
may be regarded as characteristic of any particular variety, as the
essential constituents, while those which occur in but small quantities,
and whose presence or absence does not fundamentally affect its
character, are called accessory constituents. The accessory mineral
which predominates, and which is, as a rule, present in such quantities
as to be recognizable by the unaided eye, is the characterizing
accessory. Thus a biotite granite is a stone composed of the essential
minerals quartz and potash feldspar, but in which the accessory mineral
biotite occurs in such quantities as to give a definite character to the
rock.
=26. Classification Of Minerals.=—The minerals of rocks may also be
conveniently divided into two groups, according as they are products of
the first consolidation of the mass or of subsequent changes. This is
the system here adopted. We thus have:
(1) The original or primary constituents, those which formed upon its
first consolidation. All the essential constituents are original, but on
the other hand all the original constituents are not essential. Thus, in
granite, quartz and orthoclase are both original and essential, while
beryl and zircon or apatite, though original, are not essential.
(2) The secondary constituents are those which result from changes in a
rock subsequent to its first consolidation, changes which are due in
great part to the chemical action of percolating water. Such are the
calcite, chalcedony, quartz, and zeolite deposits which form in the
druses and amygdaloidal cavities, of traps and other rocks.
Below is given a list of the more common, original and secondary
minerals occurring in rocks. It will be observed that the same mineral
may, in certain cases, occur in both original and secondary forms. The
tables following were prepared by Dr. George P. Merrill.
ORIGINAL MINERALS.
1. Quartz, SiO₂.
2. The Feldspars:
2a. Orthoclase. Anhydrous silicate of alumina with varying amounts of lime, potash, or soda and rarely barium.
2b. Microcline. „
2c. Albite. „
2d. Oligoclase. „
2e. Andesite. „
2f. Labradorite. „
2g. Bytownite. „
2h. Anorthite. „
3. The Amphiboles:
3a. Hornblende. Anhydrous silicates of lime and magnesia with iron and alumina in the dark varieties.
3b. Tremolite. „
3c. Actinolite. „
3d. Arfvedsonite. „
3e. Glaucophane. „
3f. Smaragdite. „
4. The Monoclinic Pyroxenes:
4a. Malacolite. Anhydrous silicates of magnesia and lime with alumina and iron in the dark varieties.
4b. Diallage. „
4c. Augite. „
4d. Acmite. „
4c. Aegerite. „
5. The Rhombic Pyroxenes:
5a. Enstatite (bronzite). Silicates of magnesia and iron.
5b. Hypersthene. „
6. The Micas:
6a. Muscovite. Anhydrous silicates of alumina with potash, soda, and iron.
6b. Biotite. „
6c. Phlogopite. „
7. Calcite,
8. Dolomite.
9. Gypsum.
10. Olivine.
11. Beryl.
12. Tourmaline.
13. Garnet, variable common form.
14. Vesuvianite.
15. Epidote.
16. Zoisite.
17. Allanite.
18. Andalusite.
19. Staurolite.
20. Fibrolite.
21. Cyanite.
22. Scapolite.
23. Apatite.
24. Elaeolite and Nepheline.
25. Leucite.
26. Cancrinite.
27. The Sodalite Group:
27a. Sodalite.
27b. Haüyn (noseau).
28. Zircon.
29. Chondrodite.
30. Cordierite.
31. Topaz.
32. Corundum.
33. Titanite (sphene).
34. Rutile.
35. Menaccanite.
36. Magnetite.
37. Hematite.
38. Chromite.
39. The Spinels:
39a. Pleonast.
39b. Picotite.
40. Pyrolusite.
41. Halite (common salt).
42. Fluorite.
43. The Elements:
43a. Graphite.
43b. Carbon.
43c. Iron.
43d. Copper.
44. The Metallic Sulfids:
44a. Galena.
44b. Sphalerite.
44c. Pyrrhotite.
44d. Marcasite.
44e. Pyrite.
44f. Chalcopyrite.
44g. Arsenopyrite.
SECONDARY MINERALS.
1. Quartz:
1a. Chalcedony.
1b. Opal.
1c. Tridymite.
2. Albite.
3. The Amphibole Group:
3a. Hornblende.
3b. Tremolite.
3c. Actinolite.
3d. Uralite.
4. Muscovite (sericite).
5. The Chlorites:
5a. Jefferisite.
5b. Ripidolite.
5c. Penninite.
5d. Prochlorite.
6. Calcite (and aragonite).
7. Wollastonite.
8. Scapolite.
9. Garnet.
10. Epidote.
11. Zoisite.
12. Serpentine.
13. Talc.
14. Kaolin.
15. The Zeolites:
15a. Pectolite.
15b. Laumontite.
15c. Prehnite.
15d. Thomsonite.
15e. Natrolite.
15f. Analcite.
15g. Datolite.
15h. Chabazite.
15i. Stilbite,
15k. Heulandite.
15l. Harmotome.
16. Magnetite.
17. Hematite.
18. Limonite.
19. Siderite.
20. Pyrite.
21. Pyrrhotite.
ROCKS AND ROCK DECAY.
=27. Types Of Rocks.=—Rocks may be divided in reference to their
structure into four types: First, crystalline; second, vitreous; third,
colloidal; fourth, fragmental.
Of these classes there may be selected, as types of the first order,
granite and crystalline limestone.
The second class is typically represented by obsidian. Rocks of this
kind are confined to a volcanic origin.
The third class of rocks is completely amorphous in its structure and is
less common than the others. It is found only in rocks of chemical
origin. Types of this class are the siliceous sinters, opals, flint
nodules, and many serpentines.
Of the fourth class of rocks, sandstone is typical, being comprised
wholly of fragments of rocks pre-existing. The particles may be held
together either by cohesion or by a cement composed of silica, iron
oxids, carbonate of lime or clayey matter.
=28. The Microscopical Structure of Rocks.=—A great deal more light is
thrown upon the nature of rock materials by microscopical study than by
their study in bulk. The requisites for a microscopical study of rock
are that the material should be cut into extremely thin laminae with
parallel sides and polished so as to transmit the light freely. The
study of the crystalline structure of the material is then conducted by
means of a microscope furnished with polarizing and analyzing
appliances. The light before passing through the mineral film is
polarized by a Nicol prism. After passing through the film it is
analyzed by a second Nicol prism. In this way the crystalline structure
of the rock as affecting polarized light is distinctly brought out. The
thickness of the films examined should be from ¹⁄₅₀₀ to ¹⁄₆₀₀ of an
inch.
[Illustration:
FIG. 1. Microstructure of granite.
FIG. 2. Microstructure of micropegmatite.
FIG. 3. Microstructure of quartz porphyry.
FIG. 4. Microstructure of porphyritic obsidian.
FIG. 5. Microstructure of trachyte.
FIG. 6. Microstructure of serpentine.
]
The method of rock study by thin microscopic sections is one of
comparatively recent origin. It is scarcely more than a dozen years
since the process was fairly adopted by mineralogists. The value of the
method is based upon the fact that every crystalline mineral has certain
definite optical properties. Therefore, when a crystalline mineral is
distorted or misshapen so as to be incapable of identification by the
ordinary method, it can be at once identified by its optical examination
in the manner just described. In this way not only can one mineral be
distinguished from another, but the crystalline system to which it
belongs can be accurately pointed out. The value of the method is well
summed up by Merrill,[21] who says that it is not merely an aid in
determining the mineralogical composition of a rock, but also, which is
often much more important, its structure and the various changes which
have taken place in it since its first consolidation. Rocks are not the
definite and unchangeable mineral compounds they were once considered,
but are rather ever varying aggregates of minerals which even in
themselves undergo structural and chemical changes almost without
number.
Another valuable result of such a study is illustrated by the discovery
that the structural features of a rock are not dependent upon its
chemical composition or geologic age, but upon the conditions under
which it cooled from the molten magma. Portions of the same rock may
vary all the way from a wholly crystalline to a pure vitreous form.
Some typical microstructures of crystalline rocks are shown in the
accompanying figures 1–6.[22]
Although this method of study has thus far been confined mainly to
crystalline rocks, its efficiency is by no means limited to them. The
fragmental rocks and their decomposed débris to which the name soil is
given are equally worthy of study by this method. Indeed, the full value
of a chemical analysis of any rock or soil can not be ascertained unless
such an analysis is accompanied by a microscopic examination. It is
desirable to know not merely what there is in any soil, but in what form
these compounds exist. To this latter question the chemical analysis as
ordinarily made will give no clew. In Germany a beginning has been made
in this line of work, and American scientists are beginning to realize
its importance. An outline of this method of analysis will be given in
the proper place.
=29. Specific Gravity.=—Much information in regard to the properties of
a rock, or mineral constituent thereof, may be derived from its specific
gravity.
The internal structure of a rock may have much to do with its apparent
specific gravity. As an instance of this, it may be stated that an
obsidian pumice will float upon water, buoyed up by the air contained in
its vesicles, while a compact obsidian of the same composition will sink
immediately. A careful discrimination must, therefore, be made between
apparent and true specific gravity. In general it may be said that
crystalline rocks have a higher specific gravity than those of a
vitreous nature. The specific gravity is, therefore, largely dependent
upon chemical and crystallographic properties; for instance, among
siliceous rocks those which contain the largest amount of silica are the
lightest, while those with a comparatively small amount, but rich in
iron, lime, and magnesia, are heaviest.
=30. Chemical Composition of Rocks.=—Rocks are often classified with
respect to the chief mineral constituent which they contain. Rocks which
are composed largely of lime are termed calcareous; of silica,
siliceous; of iron, ferruginous; and of clay, argillaceous. In respect
of eruptive rocks, it is customary to speak of those which show above
sixty per cent of silica as acidic, while those containing less than
fifty per cent of silica and a correspondingly larger amount of iron,
lime, and magnesia, are spoken of as basic. Illustrations of the
classification of rocks on the above principles are given below.[23]
STRATIFIED ROCKS.
──────────────────────────┬──────────────┬─────────────────────────────
Kind. │ Specific │ Composition.
│ Gravity. │
──────────────────────────┼──────────────┼─────────────────────────────
Calcareous: │ │
Compact limestone │ 2.6 to 2.8 │ Carbonate of lime.
Crystalline limestone │ „ │ „
│ │
Compact dolomite │ 2.8 to 2.95 │ Carbonate of lime and
│ │ magnesia.
Crystalline dolomite │ „ │ „
│ │
Siliceous: │ │
Gneiss │ 2.6 to 2.7 │ Same as granite.
Siliceous sandstone │ 2.6 │ Mainly silica.
Schist │ 2.6 to 2.8 │ 60 to 80 per cent silica.
│ │
Argillaceous: │ │
Clay slate (argillite) │ 2.5 │ Mainly silicate of alumina.
──────────────────────────┼──────────────┼─────────────────────────────
│ │
ERUPTIVE ROCKS.
──────────────────────────┬──────────────┬─────────────────────────────
│ Specific │ Per cent silica.
│ Gravity. │
──────────────────────────┼──────────────┼─────────────────────────────
Acidic Group: │ │
Granite │ 2.58 to 2.73 │ 77.65 to 62.90
Liparite │ 2.53 to 2.70 │ 76.06 to 67.61
Obsidian │ 2.26 to 2.41 │ 82.80 to 71.19
Obsidian pumice │ Floats on │ 82.80 to 71.19
│ water. │
│ │
Intermediate Group: │ │
Syenite │ 2.73 to 2.86 │ 72.20 to 54.65
Trachyte │ 2.70 to 2.80 │ 64.00 to 60.00
Hyalotrachyte │ 2.4 to 2.5 │ 64.00 to 60.00
Andesite │ 2.54 to 2.79 │ 66.75 to 54.73
│ │
Basic Group: │ │
Diabase │ 2.66 to 2.88 │ 50.00 to 48.00
Basalt │ 2.90 to 3.10 │ 50.59 to 40.74
Peridotite │ 3.22 to 3.29 │ 42.65 to 33.73
Peridotite (iron rich) │ 3.86 │ 23.00
Peridotite (meteorite) │ 3.51 │ 37.70
──────────────────────────┴──────────────┴─────────────────────────────
=31. Color Of Rocks.=—The color of rocks is determined chiefly by the
oxids of metals which they contain and the degree of oxidation of the
mineral in each particular case. There are, however, many colors of
rocks which seem to depend not upon any particular mineral ingredient
which they contain, but upon some particular crystalline structure or
physical condition.
The chief coloring matters in minerals are those which form colored
bases such as iron, manganese, chromium, etc. The yellow, brown, and red
colors, common to fragmental rocks, are due almost wholly to free oxids
of iron. The gray, green, dull brown, and even black colors of
crystalline rocks are due to the presence of free iron oxids or to the
prevalence of silicate mineral rich in iron, as augite, hornblende, or
black mica. Rarely copper and other metallic oxids than those of iron
are present in sufficient abundance to impart their characteristic hues.
As a rule, a white or light-gray color denotes an absence of an
appreciable amount of iron in any of its forms. The bluish and black
colors of many rocks, particularly the limestones and slates, are due to
the presence of carbonaceous matter.
In still other cases, and particularly the feldspar-bearing rocks, the
color may be due in part to the physical condition of the feldspar.
Inasmuch as the color of rocks is due so largely to metallic oxids, it
is easy to see that they may undergo changes when exposed to weathering,
or the degree of oxidation may change, and either, together with changes
in the physical structure of the rock, may cause a distinct change in
color. Luster is often considered in connection with color, and is due
almost exclusively to physical conditions.
=32. Kinds of Rocks.=—The rocks which form any essential part of the
earth’s crust are grouped under four main heads, the distinction being
based upon their origin and structure.[24] Each of the main divisions
may be subdivided into groups or families, the distinction being based
mainly upon chemical composition, structure, and mode of occurrence. The
four chief families are:
First, aqueous rocks, formed mainly through the agency of water as
chemical precipitates or as sedimentary beds.
Second, aeolian rocks formed from wind-drifted materials.
Third, metamorphic rocks, changed from their original condition through
dynamic or chemical agencies, and which may have been partly of aqueous
and partly of igneous origin.
Fourth, igneous or eruptive rocks, which have been brought up from below
in a molten condition, and which owe their present structural
peculiarities to variations in composition and conditions of
solidification.
=33. Aqueous Rocks.=—Aqueous rocks may be divided into the following
general classes:
First, rocks formed as chemical precipitates.
Second, rocks formed as sedimentary deposits and fragmental in
structure. The second class may again be subdivided into rocks formed by
mechanical agencies and mainly of inorganic materials; and second, rocks
composed mainly of the débris of plant and animal life.
In regard to the first form of aqueous rocks, namely, those formed as
chemical precipitates, it may be said that while their quantity is not
large they are yet of considerable importance from an agricultural point
of view. They embrace those substances which, having once been in a
condition of vapor or aqueous solution, have been deposited or
precipitated, either by cooling or by the evaporation of the liquor
holding them in solution, or by coming in contact with chemical
substances capable of precipitating them. The influence of water as a
solvent is perhaps not fully appreciated. Its solvent influence will be
noted particularly under the head of weathering or decay of rocks. Its
importance, however, in producing stratified rocks has been very great.
Water, especially when under great pressure and at a high temperature,
has the power of dissolving many minerals. This power is often greatly
increased by the mineral matter previously in solution in the water or
by the gases which it may contain. As an illustration of the latter
property, the solvent action of water charged with carbon dioxid on
limestone may be cited.
When mineral matters have been dissolved by the water in the ways
mentioned and carried with the water beyond the condition where the
solution has taken place, new conditions are found favorable to the
precipitation of the dissolved matters. The water, which before may have
been very hot, may reach a place where it cools, and being a
supersaturated solution, the excess of the material is thrown down as
the water cools.
On the other hand, if the solution be due to the presence of carbon
dioxid and the water reach a place where it is exposed to the air or
where the pressure under which the abundance of the gas has been due is
diminished, the carbon dioxid will escape and the mineral matters which
have been dissolved thereby will be precipitated.
The incrustations which often appear round the mouth of springs and the
occurrence of stalagmites and stalactites in caves are illustrations of
this action.
In respect of the formation of rocks as precipitates from a state of
vapor we have scarcely any illustrations excepting in volcanic regions.
Rocky materials with which we are generally acquainted are practically
non-volatile at the highest temperature which can be secured on the
earth’s surface, but it is possible that in the interior of the earth
the temperature may be so high as to maintain many substances in a state
of vapor.
They may, in this case, become disassociated so that the compounds or
elements exist distinctly in a vaporous condition. Such a vapor
transported to regions of diminished temperature would first of all on
cooling permit a union of the chemical elements forming new compounds
less volatile, which, of course, would be at once precipitated.
The rocks and minerals formed in this way which are of some agricultural
importance may be classified as follows:
Oxids, carbonates, silicates, sulfur, sulfids, sulfates, phosphates,
chlorids, and hydrocarbon compounds, the most important from an
agricultural point of view being the phosphates.
The second group of rocks, namely those formed as sedimentary deposits,
differ from those just described in that they are comprised mainly of
fragmental materials derived from the breaking down of pre-existing
rocks. The formation of fragmental rocks includes, therefore, the same
processes as are active in the formation of arable soil. They are
deposited from water, and are as a rule distinctly stratified.
Through the action of pressure and the heat thereby generated, or simply
through the chemical action of percolating solutions, such rocks pass
over into the crystalline sedimentary forms known as metamorphic. All
metamorphic rocks, however, are not of a sedimentary origin. For
instance, by pressure, heat, and the chemical changes thereby induced,
granite may be changed into gneiss and the latter would then be a
metamorphic rock.
This group of sedimentary rocks and of sedimentary material, either
unchanged or metamorphosed, is of vast extent and includes materials of
widely varying chemical and mineralogical nature. They form by far the
greater portion of the present surface of the earth, even the mountain
ranges being composed mainly of this sedimentary material. Indeed, in
the whole of this country there is only a comparatively very small
extent of igneous or irruptive rocks. They are of great importance from
a purely scientific, as well as agricultural standpoint, since they
contain the fossil records of past geologic ages. From them it is
possible to study the variations in climate, the meteorological
conditions in circumstances and periods far remote, and thus form some
idea of the process by which the crust of the earth has been modified by
natural forces from its original form to the present time.
The sedimentary rocks may be divided, with sufficient accuracy for our
purposes, into two great classes: First, rocks formed by mechanical
agencies and mainly of inorganic materials. These are subdivided again
as follows:
(a) The arenaceous group.
(b) The argillaceous group.
(c) The volcanic group.
The second class of sedimentary rocks is formed largely, or in part at
least, by mechanical agencies, but is comprised chiefly of the débris of
plant and animal life. It may be subdivided as follows:
(a) The siliceous group, such as infusorial earth.
(b) The calcareous group, fossiliferous formations, limestone, etc.
(c) The carbonaceous group, such as peat, lignite, coals, etc. The
different classes of rock described above are distinguished by special
qualities represented largely by the name. The first division, the
arenaceous group, is composed mainly of the siliceous or coarsely
granular materials derived from the disintegration of older crystalline
rocks, which have been rearranged in beds of varying thickness through
the mechanical agency of water. They are, in short, consolidated or
unconsolidated beds of sand and gravel. In composition and texture they
vary almost indefinitely. Many of them having suffered little during the
process of disintegration and transportation are composed essentially of
the same materials as the rocks from which they were derived.
The sandstones, which are the type of these rocks, vary greatly in
structure as well as in composition, in some the grains being rounded
while in others they are sharply angular.
The microscopic structure of sandstone is shown in figure 7.[24]
The material by which the individual grains of a sandstone are bound
together is usually the material of some of the other classes. The
calcareous, ferruginous, and siliceous cements being the chief ones.
This cementing substance is deposited among the granules forming the
sandstone by percolating water.
The colors of sandstone are dependent usually upon iron oxids.
Especially is this true of the red, brown, and yellow colors. In some of
the light grey varieties, the color is that of the minerals comprising
the stone. Some of the darker colored sandstones contain organic matter.
[Illustration:
FIG. 7.
Microscopic Structure of Sandstone.
]
The rocks of the argillaceous group are composed essentially of a
hydrous silicate of alumina, which is the basis of common clay, and
varying amounts of free silica, oxids of iron and manganese, carbonates
of lime and magnesia, and small quantities of organic matter. They may
have originated _in situ_ from the decomposition of feldspars or as
deposits of fine mud or silt at the bottom of large bodies of water. The
older formations of these rocks are known as shales, argillites, and
slates and the fissile structure which enables this to be split into
thin sheets is probably due to the conditions under which they have been
formed and not to any properties of the clays themselves.
One of the purest forms of this rock is kaolin, which is almost a pure
hydrous silicate of alumina formed from the decomposition of feldspathic
rocks from which the alkalies, iron oxids and other soluble constituents
have been removed by water.
Under the volcanic group are included the materials ejected from
volcanic vents in a more or less finely comminuted condition and which
through the drifting power of atmospheric currents may be scattered over
many miles of territory. Various names are applied to such products,
names dependent in large part upon their state of subdivision. Volcanic
dust and sand, or ashes, includes the finer dust-like or sand-like
materials, and lapilli, or rapilli the coarser. The general name tuff
includes the more or less compacted and stratified beds of this
material, while trass, peperino, and pozzuolano are local varietal names
given to similar materials occurring in European volcanic regions.
The second division, namely sedimentary rocks composed of the débris of
plant and animal life includes many forms of great agricultural
importance.
The first subdivision of this group is the infusorial or diatomaceous
earth. It forms a fine white or yellowish pulverulent rock composed
mainly of minute shells, or tests of diatoms, and is often so soft and
pliable as to crumble readily between the thumb and fingers. According
to Whitney the beds are of comparatively limited extent and for this
reason are of little agricultural value, although the weathering of this
diatomaceous material gives rise to a light yellow clay forming very
fertile agricultural lands.
The second subdivision of this group includes the rocks of a calcareous
nature derived from animal life; that is to say, what are properly
called limestones. They vary in color, structure, and texture almost
indefinitely, and include all possible grades of materials from those
which can be used only as a flux, or for lime burning, through ordinary
building materials to the finest marbles. These rocks are world-wide in
their distribution and limited to no one particular geologic horizon,
but are found in stratified beds among rocks of all ages from the most
ancient to the most recent.
Owing to the fact that their chief constituent, carbonate of lime, is
soluble in ordinary meteoric waters, the rocks have undergone extensive
decomposition, their lime being removed, while their less soluble
constituents or impurities remain to form soil. A single ton of residual
soil represents not infrequently a loss of 100 tons of original rock
matter. As this mass of lime carbonate is removed by solution the
residual soil settles, and as the limestone rocks are more soluble than
the adjacent rock formations limestone formations usually form valley
lands with ridges on either side. Caves are frequently found in such
formations. Furthermore, as the lime is almost all in the form of the
easily soluble lime carbonate it can be very completely removed and the
fertile “limestone soils” are often very deficient in lime and respond
readily to an application of burnt lime, which, not infrequently, is
quarried from the same field. From an agricultural standpoint this group
is of very great interest and importance.
The third subdivision of this group, namely, that of vegetable origin,
includes peat, lignite, coals, etc. Rocks of this group are made up of
more or less fragmental remains of plants. In many of them, as the peats
and lignites, the traces of plant structure are still apparent. In
others, as the anthracite coals, these structures have been wholly
obliterated by metamorphisms.
Plants when decomposing on the surface of the ground give off their
carbon to the atmosphere in the shape of carbon dioxid gas leaving only
the strictly inorganic or mineral matter behind. When, however, they are
protected from the oxidizing influence of the air, by water or by other
plant growth, decomposition is greatly retarded, and a large portion of
the carbonaceous and volatile matters is retained, and by this means
together with pressure from the overlying mass, the material becomes
slowly converted into coal. When this process goes on near the surface
of the earth, and without much pressure, peat or muck is the product.
The fourth subdivision of this group, the phosphatic, forms a class of
rocks limited in extent but of the greatest economic importance. Guano,
coprolites, and phosphatic rocks (the phosphorites) come under this
head.
=34. Aeolian Rocks.=—This class of rocks is of less importance than the
others, either geologically or agriculturally. It is formed from
materials drifted by the winds and this material has various degrees of
compactness. Usually the components of these drifts form rocks or
deposits of a friable texture and of a fragmental nature. The very
extensive deposits of loess in China, forming their most fertile lands,
are admitted now to have been formed in this way, but it is now
generally admitted that similar deposits in this country are of
subaqueous origin.
Chief among these rocks, are the volcanic ashes which are often carried
to a long distance by the wind before they are deposited and
consolidated into rock masses. Many loose soils may be carried to great
distances by the wind, the deposits forming new aggregates. This is
particularly the case in arid regions.
=35. Metamorphic Rocks.=—This class of rocks includes all sedimentary or
eruptive rocks, which, after their deposition and agglomeration, have
been changed in their nature through the action of heat, pressure, or by
chemical means. Sometimes these changes are so complete that no
indication of the character of the original rocks remains. At other
times the changes may be found in all the stages of progress, so that
they can be traced from the original fragmental or irruptive to the
completely metamorphosed deposit. This is especially true of rocks
containing large quantities of lime. In those containing silica, the
changes are less readily traced.
[Illustration:
FIG. 8.
MICROSTRUCTURE OF CRYSTALLINE LIMESTONE.
(West Rutland, Vermont.)
]
The metamorphic rocks may be divided into two subclasses, namely,
stratified or bedded, and foliated or schistose.
The rocks of the first class are represented by the crystalline
limestones and dolomites. The microstructure of a crystalline limestone
is shown in Fig. 8.[25] When lime and magnesia occur together in
combination with carbon dioxid, the substance is known as dolomite. The
chemical nature of these rocks and their soil-forming properties are the
same as those of the ordinary, non-metamorphosed limestones and
dolomites to which reference has already been made. The subject need
not, therefore, be further dwelt upon here.
[Illustration:
FIG. 9.
MICROSTRUCTURE OF GNEISS.
(West Andover, Massachusetts.)
At _a a_ are shown plagioclase crystals broken and rounded by the
sheering force producing the foliation.
]
The second variety of metamorphic rocks is represented by the gneisses
and crystalline schists. Gneiss has essentially the same composition as
granite and can frequently hardly be distinguished from it, except by a
microscopic study of its sections, and even thus it is sometimes
difficult to determine. Frequently a number of new minerals is formed in
the metamorphic changes. The microstructure of a gneiss is shown in Fig.
9.[26] The schists include an extremely variable class of rocks, of
which quartz is the prevailing constituent, and which, as a rule, are
deficient in potash and other important ingredients.
=36. Rocks Formed Through Igneous Agencies, or Eruptive Rocks.=—This
group includes all those rocks, which, having been at some time in a
state of igneous fusion, have been solidified into their present form by
a process of cooling. It may be stated, as a general principle, that the
greater the pressure under which a rock solidifies and the slower and
more gradual the cooling the more perfect will be found the crystalline
structure. Hence, it follows that the older and more deep-seated rocks
which are forced up in the form of dikes, bosses, or intrusive sheets,
into the overlying masses, and which have become exposed only through
erosion and removal of the overlying rocks, are the more highly
crystalline.
The eruptive rocks are divided into two main groups, _viz._:
(a) Intrusive or plutonic rocks, and
(b) Effusive or volcanic rocks.
Among the more important of the first division of the plutonic form,
from an agricultural point of view, are the granites. The essential
constituents of granite are quartz, potash feldspar, and plagioclase.
One or more minerals of the mica, amphibole or pyroxene groups are also
commonly present, and in microscopic proportions apatite and particles
of magnetic iron. The more valuable constituents, from an agricultural
standpoint, are the minerals potash feldspar, and apatite, which furnish
by their decomposition the essential potash and phosphoric acid.
In addition to the granites, which have already been mentioned, the
group includes the syenites, the nepheline syenites, the diorites, the
gabbros, the diabases, the theralites, the peridotites, and the
pyroxenites.
The second group, the effusive or volcanic rocks, includes those igneous
rocks, which, like the first group, have been forced up through the
overlying rocks, but which were brought to the surface, flowing out as
lavas. These, therefore, represent, in many cases, only the upper or
surface portions of the first group, differing from them structurally,
because they have cooled under little pressure more rapidly, and hence
are not so distinctly crystalline. These comprise the following groups:
(a) Quartz porphyries. (b) Liparites. (c) Quartz-free porphyries.
(d) Trachytes. (e) Phonolites. (f) Porphyrites. (g) Andesites.
(h) Melaphyrs and augite porphyrites. (i) Basalts. (j) Tephrites
and Basanites. (k) Picrite porphyrites. (l) Limburgites and
augitites. (m) Leucite rocks. (n) Nepheline rocks. (o) Melilite
rocks.
It is, in most cases, impossible to state which of the above classes is
of most importance from an agricultural standpoint, since, in the
process of soil formation, both chemical and physical processes are
involved, whereby the character of the resultant soil is so modified as
to but remotely resemble its parent rock. In most soils, the prevailing
constituent is but the least soluble one of the rock mass from which it
was derived. Thus a limestone soil may contain upwards of ninety per
cent of silica and alumina, while the original limestone itself may not
have carried more than one or two per cent of these substances. Of
course, if a rock mass contains none of the constituents essential to
plant growth, its resultant soil must by itself alone be quite barren.
It does not absolutely follow, however, that those rocks containing the
highest percentages of valuable constituents will yield the most fertile
soils, since much depends on the manner in which they have been formed,
the amount of leaching, etc., they may have undergone. Nevertheless, the
study of the composition of the rocks in their relation to soils, is an
extremely interesting and by no means unimportant one.
A comparative table of rock compositions is here given. It will be
observed that there is a considerable range of variation even among
rocks of the same class, a fact due to the varying abundance of their
mineral constituents. The figures given are not those of actual analyses
on any one particular rock, but are selected from a number of
comparatively typical cases; and, it is thought, fairly well represent
the composition of the class of rocks indicated.
COMPOSITION OF ROCKS.—THE FIGURES INDICATE PARTS PER HUNDRED.
──────────────────────┬────────┬────────────┬────────────┬────────
│ SiO₂. │ Al₂O₃. │ Fe₂O₃. │ MgO.
│ │ │ FeO. │
──────────────────────┼────────┼────────────┼────────────┼────────
Granite │ │ │ │
Quartz poryhyries} │ 63–78 │ 10–15 │ 2–3 │0.3–0.5
Liparite │ │ │ │
│ │ │ │
Syenite │ │ │ │
Orthoclase porphyries}│ 55–73 │ 12–16 │ 5–7 │ 2–6
Trachyte │ │ │ │
│ │ │ │
Nepheline syenites} │ 54–56 │ 16–22 │ 4–6 │0.4–0.88
Phonolites │ │ │ │
│ │ │ │
Diorites │ │ │ │
Porphyrites} │ 52–65 │ 16–20 │ 7–10 │ 5–7
Andesites │ │ │ │
│ │ │ │
Gabbros │ │ │ │
Norites} │ 48–55 │ 12–20 │ 8–15 │ 2–7
Melaphyrs │ │ │ │
│ │ │ │
Theralites │ │ │ │
Tephrites} │ 43–47 │ 15–23 │ 9–18 │ 1–6
Basanites │ │ │ │
│ │ │ │
Peridotites │ │ │ │
Picrite porphyrites} │ 23–43 │ 1–10 │ 10–15 │ 15–45
Limburgites │ │ │ │
│ │ │ │
Pyroxenites} │ 50–55 │ 0.5–4 │ 4–10 │ 20–25
Augitites │ │ │ │
│ │ │ │
Leucite rocks │ 48–50 │ 15–20 │ 7–10 │ 1–2
│ │ │ │
Nepheline rocks │ 40–45 │ 8–20 │ 10–20 │ 1–13
──────────────────────┴────────┴────────────┴────────────┴────────
──────────────────────┬─────┬────────┬───────┬───────────
│CaO. │ Na₂O. │ K₂O. │P₂O₅.
│ │ │ │
──────────────────────┼─────┼────────┼───────┼───────────
Granite │ │ │ │
Quartz poryhyries} │ 1–2 │ 2–3 │ 4–5 │ 0.05–0.15
Liparite │ │ │ │
│ │ │ │
Syenite │ │ │ │
Orthoclase porphyries}│ 3–5 │ 2–6 │ 4–7 │ trace.
Trachyte │ │ │ │
│ │ │ │
Nepheline syenites} │ 2–4 │ 3–7 │ 4–6 │ 0.15
Phonolites │ │ │ │
│ │ │ │
Diorites │ │ │ │
Porphyrites} │ 5–7 │ 2–4 │ 1–2 │ 0.1–0.3
Andesites │ │ │ │
│ │ │ │
Gabbros │ │ │ │
Norites} │6–10 │ 2–4 │ 0.5–2 │ 0.1–0.33
Melaphyrs │ │ │ │
│ │ │ │
Theralites │ │ │ │
Tephrites} │6–10 │ 5–7 │ 2–4 │ trace.
Basanites │ │ │ │
│ │ │ │
Peridotites │ │ │ │
Picrite porphyrites} │ 1–4 │ 0–4 │trace. │ 0.0
Limburgites │ │ │ │
│ │ │ │
Pyroxenites} │8–15 │ │ │
Augitites │ │ │ │
│ │ │ │
Leucite rocks │5–10 │ 3–5 │ 5–7 │ 0.5–2
│ │ │ │
Nepheline rocks │4–10 │ 4–8 │ 1–3 │ 0.2
──────────────────────┴─────┴────────┴───────┴───────────
=37. Origin of Soils.=—The soils in which crops grow and which form the
subject of the analytical processes to be hereinafter described have
been formed under the combined influences of rock decay and plant and
organic growth. The mineral matters of soils have had their origin in
the decay of rocks, while the humic and other organic constituents have
been derived from living bodies. It is not the object of this treatise
to discuss in detail the processes of soil formation, but only to give
such general outlines as may bear particularly on the proper conceptions
of the principles of soil investigation.
=38. The Decay of Rocks.=—The origin and composition of rocks are fully
set forth in works on geology and mineralogy. Only a brief summary of
those points of interest to agriculture has been given in the preceding
pages. The soil analyst should be acquainted with these principles, but
for practical purposes he has only to understand the chief factors
active in securing the decay of rocks and in preparing the débris for
plant growth.
The following outline is based on the generally accepted theories
respecting the formation of soils.[7]
The forces ordinarily concerned in the decay of rocks are:
1. Changes of temperature, including the ordinary daily and
monthly changes, and the conditions of freezing and thawing.
2. Moving water or ice.
3. Chemical action of water and air.
4. Influence of vegetable and animal life:
(a) Shades the rock or soil surface.
(b) Penetrates the rock or soil material with its roots, thus
admitting air.
(c) Solvent action of roots.
(d) Chemical action of decaying organic matter.
5. Earth worms.
6. Bacteria.
=39. The Action of Freezing and Thawing.=—In those parts of a rock
stratum exposed near the surface of the earth the processes of freezing
and thawing have perhaps had considerable influence in rock decay. The
expansive force of freezing water is well known. Ice occupies a larger
volume than the water from which it was formed. The force with which
this expansion takes place is almost irresistible. The phenomenon of
bursted water pipes which have been exposed to a freezing temperature is
not an uncommon one. While the increase in volume is not large, yet it
is entirely sufficient to produce serious results. The way in which
freezing affects exposed rock is easily understood. The effect is
unnoticeable if the rock be dry. If, on the other hand, it be saturated
with water, the disintegrating effect of a freeze must be of
considerable magnitude. This effect becomes more pronounced if the
intervals of freezing and thawing be of short duration. The whole
affected portion of the rock may thus become thoroughly decayed. But
even in the most favorable conditions this form of disintegration must
be confined to a thin superficial area. Even in very cold climates the
frost only penetrates a few feet below the surface, and therefore the
action of ice cannot in any way be connected with those changes at great
depths, to which attention has already been called. Nevertheless,
certain building stones seem very sensitive to this sort of weathering,
and the crumbling of the stone in the Houses of Parliament is due
chiefly to this cause.
On the whole it appears that the action of ice in producing rock decay
has been somewhat overrated, although its power must not by any means be
denied. But on the other hand a freeze extending over a long time tends
to preserve the rocks, and it therefore appears that the entire absence
of frost would promote the process of rock decay.
At best it must be admitted that frost has affected the earth’s crust
only to an insignificant depth, but its influence in modifying the
arable part of the soil is of the utmost importance to agriculture.
=40. The Action of Glaciers.=—The action of ice in soil formation is not
confined alone to the processes just described. At a period not very
remote geologically, a great part of our Northern States was covered
with a vast field of moving ice. These seas of ice crept down upon us
from more northern latitudes and swept before them every vestige of
animal and vegetable life. In their movement they leveled and destroyed
the crests of hills and filled the valleys with the débris. They crushed
and comminuted the strata of rocks which opposed their flow and carried
huge boulders of granite hundreds of miles from their homes. On melting
they left vast moraines of rocks and pebbles which will mark for all
time the termini of these empires of ice. When the ice of these vast
glaciers finally melted the surface which they had leveled presented the
appearance of an extended plain. No estimate can be made of the enormous
quantities of rock material which were ground to finest powder by these
glaciers. This rock powder forms to-day no inconsiderable part of those
fertile soils which are composed of glacial drift. The rich materials of
these soils probably bear a more intimate relation to the rocks from
which they were formed than of any other kinds of soil in the world. The
rocks were literally ground into a fine powder, and this powder was
intimately mixed with the soils which had already been formed _in situ_.
The melting ice also left exposed to disintegrating forces large
surfaces of unprotected rocks in which decay would go on much more
rapidly than when covered with the débris which protected them before
the advent of the ice. The area of glacial action extended over nearly
all of New England and over the whole area of the northern tier of
States. It extended southward almost to the Ohio river, and in some
places crossed it. The effect of the ice age in producing and modifying
our soil must never be forgotten in a study of soil genesis. It is not a
part of our purpose here to study the causes which produced the age of
ice. Even a brief reference to some of the more probable ones might be
entirely out of place. Before the glacial period it is certain that a
tropical climate extended almost, if not quite, to the North Pole. The
fossil remains of tropical plants and animals which have been found in
high northern latitudes are abundant proofs of this fact.
In the opinion of Sterry Hunt,[27] rock decay has taken place largely in
preglacial and pretertiary times. The decay of crystalline rocks is a
process of great antiquity. It is also a universal phenomenon. The fact
that the rocks of the southern part of this country seem to be covered
with a deeper débris than those further north is probably due to the
mechanical translation of the eroded particles towards the south. The
decay and softening of the material were processes necessarily preceding
the erosion by aqueous and glacial action.
It is possible that a climate may have existed in the earlier geologic
ages more favorable to the decay of rocks than that of the present time.
=41. Progress of Decay as Affected by Latitude.=—Extensive
investigations carried on along the Atlantic side of the country show
wide differences in the rate of decay in the same kind of rocks in
different latitudes. In general, the progress of decay is more marked
toward the south. The same fact is observed in the great interior
valleys of the country; at least, everywhere except in the arid and
semi-arid regions. Wherever there is a deficiency of water the processes
of decay have been arrested. Where the rock strata have been displaced
from a horizontal position the progress of decay has been more rapid.
This is easily understood. The percolation of water is more easy as the
displacement approaches a vertical position.
A most remarkable example of this is seen in the rocks of North
Carolina.[28] A kind of rock known as trap is found in layers called
dikes in the Newark system of rocks in that State. These dikes have been
so completely displaced from the horizontal position they at first
occupied as to have an almost vertical dip. The edges thus exposed vary
from a few feet to nearly 100 feet in thickness. The trap rock in those
localities is composed almost exclusively of the mineral dolerite, which
is so hard and elastic in a fresh state as to ring like a piece of metal
when struck with a hammer. In building a railroad through this region
these dikes were in some places uncovered to a depth of forty feet and
more. At this depth they were found completely decomposed and with no
indications of having reached the lower limit of disintegration. The
original hard bluish dolerite has been transformed into a yellowish
clay-like mass that can be molded in the fingers and cut like putty.
Similar geologic formations in New Jersey and further North do not
exhibit anything like so great a degree of decomposition, thus
illustrating in a marked degree the fact that freezing weather for a
part of the year is a protection against rock decay. The ice of winter
at least protects the rocks from surface infiltration, although it can
not stop the subterranean solution which must go on continuously.
Other things being equal, therefore, it appears that as the region of
winter frost is passed the decay of the rocks has been more rapid than
in the North, because the chief disintegrating forces act more
constantly.
=42. The Solvent Action of Water.=—The water of springs and wells is not
pure. It contains in solution mineral matters and often a trace of
organic matter. The organic matter comes from contact with vegetable
matter and other organic materials near the surface of the earth. The
mineral matter is derived from the solvent action of the water and its
contents on the soil and rocks.
The expressions “hard” and “soft” applied to water indicate that it has
much or little mineral matter in suspension, as the case may be. When
surface and spring waters are collected into streams and rivers they
still contain in solution the greater part of the mineral matters which
they at first carried.
When well or spring waters have more than forty grains of mineral matter
per gallon they are not suitable for drinking waters. Mineral waters, so
called, are those which carry large quantities of mineral matter, or
which contain certain comparatively rare mineral substances which are
valued for their medicinal effects.
The analysis of spring, well, or river waters will always give some
indication of the character of the rocks over which they have
passed.[29] The vast quantities of mineral matters carried into oceans
and seas are gradually deposited as the water is evaporated. If,
however, these matters be very soluble, such as common salt, sulfate of
magnesia, etc., they become concentrated, as is seen with common salt in
sea waters. In small bodies of waters, such as inland seas, which have
no outlet, this concentration may proceed to a much greater extent than
in the ocean. As an instance of this, it may be noted that the waters of
the Dead Sea and Great Salt Lake are impregnated to a far greater degree
with soluble salts than the water of the ocean. The solvent action of
water on rocks is greatly increased by the traces of organic or carbonic
acids which it may contain. When surface water comes in contact with
vegetable matter it may become partially charged with the organic acids
which the growing vegetable may contain or decaying vegetable matter
produce. Such acids coming in contact with limestone under pressure will
set free carbon dioxid. Water charged with carbon dioxid acts vigorously
on limestone and other mineral aggregates. If such solutions penetrate
deeply below the surface of the earth their activity as solvents may be
greatly increased by the higher temperature to which they are subjected.
Hence, all these forces combine to disintegrate the rocks, and through
such agencies vast deposits of original and secondary rocks have been
completely decomposed.
The gradual passing of the firm rock into an arable soil is beautifully
shown in Fig. 10, a print from a negative taken by Mr. Geo. P. Merrill,
near Washington, D. C.
[Illustration:
FIGURE 10.
View on the Broad Branch of Rock Creek, Washington, D. C.
The fresh but badly decomposed granitic rock is shown passing upward
into material more and more decomposed until it becomes sufficiently
pulverulent and soluble to support plant life. The roots showing in
the upper part of the picture formerly penetrated the decomposed
rock, but have been exposed through quarrying operations. Photograph
by George P. Merrill, 1891.
]
=43. Action Of Vegetable Life.=—The preliminary condition to vegetation
is the formation of soil, but once started, vegetation aids greatly in
the decomposition of rocks. Some forms of vegetation, as the lichens,
have apparently the faculty of growing on the bare surface of rocks, but
the higher order requires at least a little soil. The vegetation acts by
shading the surface and thus rendering the action of water more
effective, by mechanically separating the rock particles by means of its
penetrating roots and by the positive solvent action of the root juices.
The rootlets of plants in contact with limestone or marble dissolve
large portions of these substances, and while their action on more
refractory rocks is slower, it must be of considerable importance. The
organic matter introduced into the soil by vegetation also promotes
decay still further both directly and by the formation of acids of the
humic series. This matter also furnishes a considerable portion of
carbon dioxid which is carried by the water to assist in its solvent
action.
=44. Action of Earth-Worms.=—Of animal organisms those most active in
the formation of soil are earth-worms. The work of earth-worms has been
exhaustively studied by Darwin.[30] The worms not only modify the soil
by bringing to the surface portions of the subsoil, but also influence
its physical state by making it more porous and pulverulent. According
to Darwin the intestinal content of worms has an acid reaction, and this
has an effect on those portions of the soil passing through their
alimentary canal. The acids, which are formed in the upper part of the
digestive canal are neutralized by the carbonate of lime secreted by the
calciferous glands of the worms thus neutralizing the free acid and
changing the reaction to alkaline in the lower part of the canal. There
is a fair presumption that the acids of the worm are of a humic nature.
The worms further modify the composition of the soil by drawing leaves
and other organic matter into their holes, and leaving therein a portion
of such matter which is gradually converted into humus. Stockbridge[31]
gives a striking illustration of this process due to an experiment by
von Hensen. Darwin estimates that about eleven tons of organic matter
per acre are annually added to the soil in regions where worms abound. A
considerable portion of the ammonia in the soil at any given time may
also be due to the action of worms, as much as 0.18 per cent of this
substance having been found in their excrement. It is probable that
nearly the whole of the vegetable matter in the soil passes sooner or
later through the alimentary canals of these ceaseless soil builders,
and is converted into the form of humus.
=45. Action of Bacteria.=—The intimate relations which have been found
to subsist between certain minute organisms and the chemical reactions
which take place in the soil is a sufficient excuse for noting the
effect of other similar organisms in the formation of soils.
In addition to the usual forces active in decomposing rocks Müntz[32]
has described the effects of a nitrifying bacillus contributing to the
same purpose.
According to him the bare rock usually furnishes a purely mineral
environment where organisms cannot be developed unless they are able to
draw their nourishment directly from the air. Some nitrifying organisms
belong to this class. It has been shown that these bodies can be
developed by absorbing from the ambient atmosphere carbonate of ammonia
and vapors of alcohol, the presence of which has been determined in the
air. According to the observations of Winogradsky, they assimilate even
the carbon of the carbon dioxid just as vegetables do which contain
chlorophyl. Thus even in the denuded rocks of high mountains the
conditions for the development of all these inferior organisms exist. In
examining the particles produced by attrition, it is easily established
that they are uniformly covered by a layer of organic matter evidently
formed by microscopic vegetations. Thus we see, in the very first
products of attrition, appearing upon the rocky particles the
characteristic element of vegetable soil, viz., humus, the proportion of
which increases rapidly with the products of disaggregation collected at
the foot of declivities until finally they become covered with
chlorophyliferous plants.
In a similar manner the presence of nitrifying organisms has been noted
upon rocky particles received in sterilized tubes, and cultivated in an
appropriate environment where they soon produce nitrification. The naked
rocks of the Alps, the Pyrenees, the Auvergnes and the Vosges, comprise
mineralogical types of the most varied nature, _viz._, granite,
porphyry, gneiss, micaschist volcanic rocks and limestones and all these
have shown themselves to be covered with the nitrifying ferment. It is
known that below a certain temperature these organisms are not active;
their action upon the rock is, therefore, limited to the summer period.
During the cold season their life is suspended but they do not perish,
inasmuch as they have been found living and ready to resume all their
activity after an indefinite sleep on the ice of the glaciers where the
temperature is never elevated above zero.
The nitrifying ferment is exercised on a much larger scale in the normal
conditions of the lower levels where the rock is covered with earth.
This activity is not limited to the mass of rock but is continued upon
the fragments of the most diverse size scattered through the soil and it
gradually reduces them to a state of fine particles. It is therefore a
phenomenon of the widest extension.
The action of these micro-organisms according to Müntz is not confined
to the surface but extends to the most interior particles of the rocky
mass. Where, however, there is nothing of a nitrogenous nature, to
nitrify such an organism must live in a state of suspended animation.
When the extreme minuteness of these phenomena is considered there may
be a tendency to despise their importance, but their continuity and
their generality in the opinion of Müntz place them among the geologic
causes to which the crust of the earth owes a part of its actual
physiognomy and which particularly have contributed to the formation of
the deposits of the comminuted elements constituting arable soil.
The general action of nitrifying organisms in the soil, the nature of
these bodies, and the method of isolating and identifying them will be
fully discussed in another part of this work.
=46. Action of the Air.=—The air itself takes an active part in rock
decay. Wherever rocks are exposed to decay, there air is found or, at
least, the active principle of air, _viz._, oxygen. The air not only
penetrates to a great depth in the earth, but is also carried to much
greater depths by water which always holds a greater or less quantity of
air in solution. The oxygen of the air is thus brought into intimate
contact with the disintegrating materials and in a condition to assist
wherever possible in the decomposing processes.
The oxygen acts vigorously on the lower oxids of iron, converting them
into peroxids, and thus tends to produce decay.
There are other constituents of rocks which oxygen affects injuriously
and thus helps to their final breaking up. It is true that, as a rule,
the constituents of rocks are already oxidized to nearly as high a
degree as possible, and on these constituents of course the air would
have no effect. But on others, especially when helped by water with the
other substances it carries in solution, the air may greatly help in the
work of destruction.
In a general view, the action of the air in soil formation may be
regarded as of secondary importance, and to depend chiefly on the
oxidation of the lower to the higher basic forms. These processes, while
they seem of little value, have, nevertheless, been of considerable
importance in the production of that residue of rock disintegration
which constitutes the soil.
=47. Classification of Soils According to Deposition.=—In regard to
their deposition soils are divided into five classes:
1. Those which are formed from the decomposition of crystalline or
sedimentary rocks or of unconsolidated sedimentary material _in situ_.
2. Those which have been moved by water from the place of their original
formation and deposited by subsidence (bottom lands, alluvial soils,
lacustrine deposits, etc.).
3. Those which have been deposited as débris from moving masses of ice
(glacial drift).
4. Soils formed from volcanic ashes or from materials moved by the wind
and deposited in low places or in hills or ridges.
5. Those formed chiefly from the decay of vegetable matter, (tule, peat,
muck).
=48. Qualities of Soils.=—In respect of quality, soils have been
arbitrarily divided into many kinds. Some of the more important of these
divisions are as follows:
1. _Sand._ Soils consisting almost exclusively of sand.
2. _Sandy Loams._ Soils containing some humus and clay but an excess of
sand.
3. _Loams._ Soils inclining neither to sand nor clay and containing some
considerable portions of vegetable mold, being very pulverulent and
easily broken up into loose and porous masses.
4. _Clays._ Stiff soils in which the silicate of alumina and other fine
mineral particles are present in large quantity.
5. _Marls._ Deposits containing an unusual proportion of carbonate of
lime, with often some potash or phosphoric acid resulting from the
remains of sea-animals and plants.
6. _Alkaline._ Soils containing carbonate and sulfate of soda, or an
excess of these alkaline and other soluble mineral substances.
7. _Adobe._ A fine grained porous earth of peculiar properties
hereinafter described.
8. _Vegetable._ Soils containing much vegetable débris in an advanced
state of decomposition. When such matter predominates or exists in large
proportion in a soil the term tule, peat or muck is applied to it.
With the exception of numbers six, seven and eight these types of soil
are so well-known as to require no further description for analytical
purposes. The alkaline, adobe and vegetable soils on the contrary demand
further study.
=49. Alkaline Soils.=—The importance of a more extended notice of this
class of soils for analytical purposes is emphasized by their large
extent in the United States.
Chiefly through the researches of Hilgard attention has been called to
the true character of these soils which are found throughout a large
part of the Western United States and which are known by the common name
of alkali. The following description of the origin of these soils is
compiled chiefly from Hilgard’s papers on this subject. Wherever the
rain-fall is scanty, and especially where the rains do not come at any
one time with sufficient force to thoroughly saturate the soil and carry
down through the subsoil and off through the drainage waters the alkali
contained therein, favorable conditions exist for the production of the
alkaline soil mentioned above. The peculiar characteristic of this soil
is the efflorescence which occurs upon its surface and which is due to
the raising of soluble salts in the soil by the water rising through
capillary attraction and evaporating from the surface, leaving the salts
as an efflorescence.
Soils which contain a large amount of alkali are usually very rich in
mineral plant food, and if the excess of soluble salt could be removed,
these lands under favorable conditions of moisture would produce large
crops.
The formation of the alkali may be briefly described as follows: By the
decomposition of the native rocks, certain salts soluble in water are
formed. These salts in the present matter are chiefly sodium and
potassium sulfates, chlorids and carbonates. The salts of potash
together with those of lime are more tenaciously held by the soil than
the soluble salts of soda, and the result of this natural affinity of
the soil for soluble potash, lime and magnesian salts is seen in the
formation at the surface of the earth, by the process of evaporation
above described, of a crust of alkaline material which is chiefly
composed of the soluble salts of soda. In countries which have a
sufficient amount of rain-fall, these soluble salts are carried away
either by the surface drainage or by the percolation of water through
the soil, and the sodium chlorid is accumulated in this way in the
waters of the ocean. But where a sufficient amount of rain-fall does not
occur, these soluble salts carried down by each shower only to a certain
depth rise again on the evaporation of the water, reinforced by any
additional soluble material which may be found in the soil itself. The
three most important ingredients of the alkali of the lands referred to
are sodium chlorid, sulfate, and carbonate. When the latter salt,
namely, sodium carbonate, is present in predominant quantity, it gives
rise to what is popularly known as black alkali. This black color is due
to the dark colored solution which sodium carbonate makes with the
organic matters or humus of the soil. The black alkali is far more
injurious to growing vegetation than the white alkali composed chiefly
of sodium sulfate and chlorid.
This black alkali has been very successfully treated by Hilgard[33] by
the application of gypsum which reacting with the sodium carbonate
produces calcium carbonate and sodium sulfate, thus converting the black
into the white alkali and adding an ingredient in the shape of lime
carbonate to stiff soils which tends to make them more pulverulent and
easy of tillage.
This method of treatment, however, as can be easily seen, is only
palliative, the whole amount of the alkaline substances being still left
in the soil, only in a less injurious form.
The only perfect remedy for alkaline soils, as has been pointed out by
Hilgard, is in the introduction of underdrainage in connection with
irrigation. The partial irrigation of alkaline soils, affording enough
moisture to carry the alkali down to and perhaps partially through the
subsoil, can produce only a temporary alleviation of the difficulties
produced by the alkali. Subsequent evaporation may thus increase the
amount of surface incrustation. For this reason in many cases the
practice of irrigation without underdrainage may completely ruin an
otherwise fertile soil by slowly increasing the amount of alkali in the
soil by the total amount of the alkaline material added in the waters of
irrigation.
As Hilgard has pointed out, if a soil can be practically freed from
alkali by underdrainage connected with a thorough saturation by
irrigation, it may be centuries before the alkali will accumulate in
that soil again when ordinary irrigation only is practiced. It may thus
become possible to reclaim large extents of alkaline soil little by
little by treating them with an excess of irrigation water in connection
with thorough underdrainage. The composition of the alkali on the
surface of the soil due to the causes above set forth is thoroughly
illustrated by the analyses of Hilgard and Weber, which follow:
TABLE SHOWING COMPOSITION OF ALKALI SALTS IN SAN JOAQUIN VALLEY.
═════════════╤════════════════════════════════════════════
│ FRESNO COUNTY.
─────────────┼────────────────────────────────────────────
│ Sections 13 and 24 T. 14 S. R. 19 E., 4
│ miles S. W. from Fresno.
│
─────────────┼────────┬───────────────────────────────────
│ Alkali │ Alkali Spot, 1889.
│ soil, │
│ 1888. │
─────────────┼────────┼────────┬────────┬────────┬────────
│ „ │ 1 inch │ 18 │ 26 │ 42
│ │surface.│ inches │ inches │ inches
│ │ │ bel. │ bel. │hardpan.
│ │ │surface.│surface.│
─────────────┼────────┼────────┼────────┼────────┼────────
Soluble salts│ │ 0.76 │ 0.20 │ 0.18 │ 0.16
in 100 │ │ │ │ │
parts soil │ │ │ │ │
Potassium │ │ │ │ │
sulfate │ │ │ │ │
[D]Potassium │ │ │ │ │
nitrate │ │ │ │ │
Potassium │ │ │ │ │
carbonate │ │ │ │ │
(Saleratus)│ │ │ │ │
Sodium │ large │ small │ small │ very │ very
sulfate │ │ │ │ slight │ slight
(Glauber’s │ │ │ │ │
salt) │ │ │ │ │
Sodium │ very │ large │ small │ large │ large
carbonate │ slight │ │ │ │
(Sal-soda) │ │ │ │ │
Sodium │chiefly │moderate│chiefly │moderate│moderate
chlorid │ │ │ │ │
(Common │ │ │ │ │
salt) │ │ │ │ │
[D]Sodium │ │ │ │ │
phosphate │ │ │ │ │
Calcium │moderate│ small │ very │ very │ very
sulfate │ │ │ slight │ slight │ slight
(Gypsum) │ │ │ │ │
Magnesium │ │ │ │ │
sulfate │ │ │ │ │
(Epsom │ │ │ │ │
salt) │ │ │ │ │
Organic │ │ │ │ │
matter │ │ │ │ │
─────────────┴────────┴────────┴────────┴────────┴────────
═════════════╤══════════════════════════════════════════════════
│ FRESNO COUNTY.
─────────────┼────────────────┬────────────────┬───────┬────────
│ Miss Austin’s │ N.W. Cor. N ½ │Easton.│Emigr’nt
│ Ranch, Central │Sec. 20 T. 14 S.│ │ Ditch.
│ Colony. │ R. 21 E. │ │
─────────────┼───────┬────────┼───────┬────────┼───────┼────────
│Surface│Surface │Surface│Surface │ „ │ „
│ soil, │ soil, │ soil. │ soil. │ │
│No. 1. │ No. 2. │ │ │ │
─────────────┼───────┼────────┼───────┼────────┼───────┼────────
│ „ │ „ │ „ │ „ │ „ │ „
│ │ │ │ │ │
│ │ │ │ │ │
│ │ │ │ │ │
─────────────┼───────┼────────┼───────┼────────┼───────┼────────
Soluble salts│ 3.54 │ 1.90 │ 1.20 │ 2.69 │ │
in 100 │ │ │ │ │ │
parts soil │ │ │ │ │ │
Potassium │ small │moderate│ │ │ │
sulfate │ │ │ │ │ │
[D]Potassium │ │ small │ │ │ │
nitrate │ │ │ │ │ │
Potassium │ │ │ │ │ │
carbonate │ │ │ │ │ │
(Saleratus)│ │ │ │ │ │
Sodium │ large │ large │ much │moderate│ large │
sulfate │ │ │ │ │ │
(Glauber’s │ │ │ │ │ │
salt) │ │ │ │ │ │
Sodium │ small │chiefly │ small │ small │ │chiefly
carbonate │ │ │ │ │ │
(Sal-soda) │ │ │ │ │ │
Sodium │chiefly│ large │chiefly│chiefly │ large │ little
chlorid │ │ │ │ │ │
(Common │ │ │ │ │ │
salt) │ │ │ │ │ │
[D]Sodium │ │ │ │ │ │
phosphate │ │ │ │ │ │
Calcium │ small │moderate│ small │ small │ much │
sulfate │ │ │ │ │ │
(Gypsum) │ │ │ │ │ │
Magnesium │ │ small │ │ │ much │
sulfate │ │ │ │ │ │
(Epsom │ │ │ │ │ │
salt) │ │ │ │ │ │
Organic │ │ │ │ │ │
matter │ │ │ │ │ │
─────────────┴───────┴────────┴───────┴────────┴───────┴────────
Footnote D:
Very generally present, but not always in quantities sufficient for
determination.
═════════════════════╤═════════════════════════════════════════════════
│ TULARE COUNTY.
─────────────────────┼───────┬───────┬────────┬───────┬────────┬───────
│Goshen │Peopl’s│ Near │Visalia│Lemoore │Tulare
│ │ Ditch │ Lake │ │ │ Exp’t
│ │ │ Tulare │ │ │Station
─────────────────────┼───────┼───────┼────────┼───────┼────────┼───────
│Surf’ce│Alkali │Surf’ce │Surf’ce│ Alkali │Alkali
│ soil │ crust │ soil │ soil │ crust │ crust
─────────────────────┼───────┼───────┼────────┼───────┼────────┼───────
Soluble salts in 100 │ 1.40│ │ 0.83│ 1.26│ │
parts soil │ │ │ │ │ │
Potassium sulfate │ │ │ │ │ │ small
[E]Potassium nitrate │ │ │ │ │ │ small
Potassium carbonate │ │ │ │ 18.80│ │
(Saleratus) │ │ │ │ │ │
Sodium sulfate │ 44.24│ 1.22│31.30[F]│ 13.4│chiefly │ 32.8
(Glauber’s salt) │ │ │ │ │ │
Sodium carbonate │ 32.98│ 88.09│ 18.2│ 45.3│ │ 36.16
(Sal-soda) │ │ │ │ │ │
Sodium chlorid │ 16.74│ 1.00│ │ 4.4│ little │ 31.16
(Common salt) │ │ │ │ │ │
[E]Sodium phosphate │ 1.97│ │ 0.22│ 10.4│ │
Calcium sulfate │ │ │ │ │ little │
(Gypsum) │ │ │ │ │ │
Magnesium sulfate │ │ │ │ 8.1│moderate│
(Epsom salt) │ │ │ │ │ │
Organic matter │ 1.59│ 9.21│ 7.5│ │ │ 5.37
─────────────────────┴───────┴───────┴────────┴───────┴────────┴───────
═════════════╤════════════════════════════════════════════════════════
│ KERN COUNTY.
─────────────┼────────────────────────────────────────────────────────
│ Alkali crusts from the Smyrna artesian belt. Townships
│ 25 and 26 R. 23 E. W. S. W. from Delano, S. P. R. R.
─────────────┼─────┬────────┬────────┬────────┬─────┬─────┬─────┬─────
│No. 1│ No. 2 │ No. 3 │ No. 4 │No. 5│No. 6│No. 7│No. 8
│ │ │ │ │ │ │ │
─────────────┼─────┼────────┼────────┼────────┼─────┼─────┼─────┼─────
Soluble salts│ │ │ │ │ │ │ │
in 100 │ │ │ │ │ │ │ │
parts soil │ │ │ │ │ │ │ │
Potassium │ │ │ │ │ │ │ │
sulfate │ │ │ │ │ │ │ │
[E]Potassium │ │ │ │ │ │ │ │
nitrate │ │ │ │ │ │ │ │
Potassium │ │ │ │ │ │ │ │
carbonate │ │ │ │ │ │ │ │
(Saleratus)│ │ │ │ │ │ │ │
Sodium │small│moderate│moderate│moderate│large│small│small│small
sulfate │ │ │ │ │ │ │ │
(Glauber’s │ │ │ │ │ │ │ │
salt) │ │ │ │ │ │ │ │
Sodium │ │ │ │ │ │ │ │
carbonate │ │ │ │ │ │ │ │
(Sal-soda) │ │ │ │ │ │ │ │
Sodium │large│moderate│ large │ large │small│large│small│large
chlorid │ │ │ │ │ │ │ │
(Common │ │ │ │ │ │ │ │
salt) │ │ │ │ │ │ │ │
[E]Sodium │ │ │ │ │ │ │ │
phosphate │ │ │ │ │ │ │ │
Calcium │small│ small │ small │ small │small│small│small│small
sulfate │ │ │ │ │ │ │ │
(Gypsum) │ │ │ │ │ │ │ │
Magnesium │small│ small │ small │ small │small│small│small│small
sulfate │ │ │ │ │ │ │ │
(Epsom │ │ │ │ │ │ │ │
salt) │ │ │ │ │ │ │ │
Organic │ │ │ │ │ │ │ │
matter │ │ │ │ │ │ │ │
─────────────┴─────┴────────┴────────┴────────┴─────┴─────┴─────┴─────
═════════════╤══════════════
│ KERN COUNTY.
─────────────┼───────┬──────
│Summer.│ Kern
│ │Island
─────────────┼───────┼──────
│Alkali │Alkali
│ crust │crust
─────────────┼───────┼──────
Soluble salts│ │
in 100 │ │
parts soil │ │
Potassium │ │ 4.72
sulfate │ │
[E]Potassium │ │
nitrate │ │
Potassium │ │
carbonate │ │
(Saleratus)│ │
Sodium │ 19.20│ 70.61
sulfate │ │
(Glauber’s │ │
salt) │ │
Sodium │ │ 14.82
carbonate │ │
(Sal-soda) │ │
Sodium │ 37.14│ 4.13
chlorid │ │
(Common │ │
salt) │ │
[E]Sodium │ │
phosphate │ │
Calcium │ 0.96│ 0.08
sulfate │ │
(Gypsum) │ │
Magnesium │ 18.31│
sulfate │ │
(Epsom │ │
salt) │ │
Organic │ 20.87│
matter │ │
─────────────┴───────┴──────
Footnote F:
Common and Glauber’s salts.
=50. Adobe Soils.=—In many parts of the arid regions of this country
which can be recovered for agricultural purposes by irrigation the soil
has peculiar characteristics.
The name adobe as commonly used applies to both the sundried bricks of
the arid regions of the West and Southwest, and to the materials of
which they are composed. The material is described by Russell[34] as a
fine grained porous earth, varying in color through many shades of gray
and yellow, which crumbles between the fingers, but separates most
readily in a vertical direction. The coherency of the material is so
great that vertical scarps will stand for many years without forming a
noticeable talus slope.
_Distribution._—The area over which adobe forms a large part of the
surface has not been accurately mapped, but enough is known to indicate
that it is essentially co-extensive with the more arid portions of this
country. In a very general way it may be considered as being limited to
the region in which the mean annual rain-fall is less than twenty
inches. It forms the surface over large portions of Colorado, New
Mexico, Western Texas, Arizona, Southern California, Nevada, Utah,
Southern Oregon, Southern Idaho, and Wyoming. Adobe occurs also in
Mexico and may there reach a greater development than in the United
States, but observations concerning it south of the Rio Grande are
wanting.
In the United States it occurs from near the sea-level in Arizona, and
even below the sea-level in Southern California, up to an elevation of
at least six or eight thousand feet, along the eastern border of the
Rocky Mountains, and in the elevated valleys of New Mexico, Colorado,
and Wyoming. It occupies depressions of all sizes up to valleys having
an area of hundreds of square miles. Although occurring throughout the
arid region, it can be studied to best advantage in the drainless and
lakeless basins in Nevada, Utah, and Arizona.
_Composition._—When examined under the microscope, the adobe is seen to
be composed of irregular, unassorted flakes and grains, principally
quartz, but fragments of other minerals are also present. An exhaustive
microscopic study has not been made, but the samples examined from
widely-separated localities were very similar. The principal
characteristics observed were the extreme angularity of the particles
composing the deposit and the undecomposed condition of the various
minerals entering into its composition. It is to be inferred from this
that the material was not exposed even to a very moderate degree of
friction, and had not undergone subaerial decay before being deposited.
Adobe collected, at typical localities is so fine in texture that no
grit can be felt when it is rubbed between the fingers; in other
instances it contains angular rock fragments of appreciable size.
The composition of the material is illustrated by the following
analyses:
ANALYSES OF ADOBE.
BY L. G. EAKINS.
Constituents. No. 1. No. 2. No. 3. No. 4.
Sante Fe, New Fort Wingate, Humboldt, Salt Lake
Mexico. New Mexico. Nevada. City, Utah.
SiO₂ 66.69 26.67 44.64 19.24
Al₂O₃ 14.16 0.91 13.19 3.26
Fe₂O₃ 4.38 0.64 5.12 1.09
MnO 0.09 trace 0.13 trace
CaO 2.49 36.40 13.91 38.94
MgO 1.28 0.51 2.96 2.75
K₂O 1.21 trace 1.71 trace
Na₂O 0.67 trace 0.59 trace
CO₂ 0.77 25.84 8.55 29.57
P₂O₅ 0.29 0.75 0.94 0.23
SO₃ 0.41 0.82 0.64 0.53
Cl 0.34 0.07 0.14 0.11
H₂O 4.94 2.26 3.84 1.67
Organic matter 2.00 5.10 3.43 2.96
————— ————— ————— ——————
99.72 99.97 99.84 100.35
=51. Vegetable Soils.=—The heavy soils whose origin has been described
are essentially of a mineral nature. The quantity of organic matter in
such soils may vary from a mere trace to a few per cent, but they never
lose their mineral predominance. When a soil on the other hand is
composed almost exclusively of vegetable mold it belongs to quite
another type. Such soils are called tule, peat or muck. In this country
there are thousands of acres of peat or muck soils; the largest
contiguous deposits being found in Southern Florida. The origin of these
soils is easily understood. Whenever rank vegetation grows in such a
location as to secure for the organic matter formed a slow decay there
is a tendency to the accumulation of vegetable mold in shallow water or
on marshy ground and where conditions are favorable to such
accumulations. In Florida the muck soils have been accumulated about the
margins of lakes. During the rainy season the marshes bordering these
are partly covered with water, but the vegetation is very luxuriant. The
water protects the vegetable matter from being destroyed by fire. It
therefore accumulates from year to year and is gradually compacted into
quite a uniform mass of vegetable mold.
The composition of the muck is illustrated in the following table which
shows the character of the layers at one, two and three feet in
depth:[35]
Carbon. Hydrogen. Nitrogen. Volatile
matter.
1 foot 57.67 per cent 4.48 per cent 2.24 per cent 90.60 per cent
2 feet 47.07 „ 5.15 „ 1.40 „ 72.00 „
3 feet 8.52 „ 0.53 „ 0.31 „ 15.00 „
In this sample, No. 3, the muck was only three feet deep, resting on
pure sand. As the bottom of the deposit is approached the admixture of
sand becomes greater and the percentage of organic matter less.
No reliable estimate of the time which has been required to form these
deposits can be given, but in the Okeechobee region in Florida the
deposit of vegetable mold in some places exceeds ten feet in depth.
The purest muck or peat soils contain only small quantities of potash
and phosphoric acid, and especially is this true of the Florida mucks
which have been formed of vegetable growth containing very little
mineral matter.
It is not at all probable that the flora now growing on any particular
area of virgin peat contains all the plants that have contributed to its
formation. The principal vegetable growths now going to make up the muck
soils of Florida are the following:
Common names. Botanical names.
Saw grass Cladium effusum
Yellow pond lily Nymphea flava
Maiden cane grass Panicum Curtisii
Alligator Wampee Pontederia cordata
Sedge Cyperus species
Fern brake Osmunda „
Mallow Malva „
Broom sedge Andropogon „
Arrow weed Sagittaria „
The above are only the plants growing in the greatest profusion and do
not include all which are now contributing to increase the store of
vegetable débris.
=52. Humus.=—The active principle of vegetable mold is called humus, a
term used to designate in general the products of the decomposition of
vegetable matter as they are found in soils. In peat and muck are found
a mixture of humus with undecomposed or partially decomposed vegetation.
According to Kostytchoff[36] vegetable matter decays under the influence
of molds and bacteria. Molds alone produce the dark colored matters
which give soils rich in vegetable matter, their color. One chief
characteristic of humus is its richness in nitrogen. Black Russian soil
contains from 4 to 6.65 per cent of nitrogen. This soil is estimated to
contain sixty million organisms per gram and much of the nitrogen which
it holds must be in the form of proteids. The first development in
decaying vegetable matter is of bacteria and there is a tendency of the
decaying matter to become acid. This causes a decay of the bacteria and
the ammonia produced by this neutralizes the acid. The various kinds of
mold grow when the reaction becomes neutral. Afterwards the bacteria and
the molds develop together. This statement of Kostytchoff is not a very
satisfactory explanation of even our limited knowledge of the
decomposition of organic matters in the soil. Ammonia and ammonia salts
are formed not by the decay of some forms of bacteria but by the
activities of other forms. Warington found that in nitrification there
were three distinct forms of bacteria concerned in the final products of
ammonia, nitrites, and nitrates. Humus always contains easily
decomposable matter and consequently the rate of decay at any observed
periods is nearly the same. In humus which is produced above the
water-level Kostytchoff states that all trace of the vegetable structure
is destroyed by the leaves being gnawed and passed through the bodies of
caterpillars and wire-worms. Under the water-level the vegetable
structure is preserved and peat results. The decay of humus is most
rapid in drained and open soils. For this reason the presence of clay in
a soil promotes the accumulation of humus. Inferior organisms are the
means of diffusing organic matter through the soil. The mycelia of fungi
grow on a dead root for instance, ramify laterally and thus carry
organic matter outward and succeeding organisms extend this action and
the soil becomes darkened in proportion. Humic acid in black soil is
almost exclusively in combination with lime.
A more common view of the difference between the formation of humus
above and below the water-level is that above the water-level there is a
very free access of air and even the harder parts of the leaf skeleton
can be oxidized through the agency of bacteria, while under the
water-level there is a very limited supply of air and this oxidation
cannot proceed as rapidly. The harder parts of the leaf skeleton are
preserved, and from the freer access of air humus is oxidized more
readily in drained and open soils, and accumulates in clay soils where
there is less circulation of air.
The real composition of humus is a matter which has never been
definitely determined. Composed of many different but closely related
substances it has been difficult to isolate and determine them.
Stockbridge[37] gives the following composition of the bodies which form
the larger part of humus:
Ulmin and Ulmic Acid.
Carbon 67.1 per cent Corresponding to C₄₀H₂₈O₁₂ + H₂O.
Hydrogen 4.2 „ „
Oxygen 8.7 „ „
Humin and Humic Acid.
Carbon 64.4 per cent Corresponding to C₂₁H₂₄_O₁₂ + 3H₂O
Hydrogen 4.3 „ „
Oxygen 31.3 „ „
Crenic Acid.
Carbon 44.0 per cent Corresponding to C₁₂H₁₂O₈?
Hydrogen 5.5 „ „
Nitrogen 3.9 „ „
Oxygen 46.6 „ „
Apocrenic Acid.
Carbon 34.4 per cent Corresponding to C₂₄H₂₄O₁₂?
Hydrogen 3.5 „ „
Nitrogen 3.0 „ „
Oxygen 39.1 „ „
He further states that there are, aside from these humus compounds,
others still less known and the action of which is not yet understood;
among them xylic acid, C₂₄H₃₀O₁₇, saccharic acid, C₁₄H₁₈O₁₁, glucinic
acid, C₁₂H₂₂O₁₂, besides a brown humus acid containing carbon, 65.8 per
cent, and hydrogen, 6.25 per cent, and a black humus acid yielding
carbon, 71.5 per cent, and hydrogen, 5.8 per cent.
According to Mulder humic acid has the following composition, C₆₀H₅₄O₂₇,
while Thenard[38] ascribes to it the formula, C₂₄H₁₀O₁₀.
At the present time we can only regard the various forms of humus bodies
as mixtures of many substances mostly of an acid nature and resulting
from a gradual decomposition of organic matter under conditions which
partially exclude free access of oxygen.
For analytical purposes it is only necessary to separate these bodies by
the best approved processes. A further knowledge of their composition
can then be derived by determining the percentages of carbon dioxid and
water which they yield on combustion.
=53. Soil and Subsoil.=—Many subdivisions have been made of the above
varieties of soil, but they have little value for analytical purposes.
For convenience in description for agricultural purposes, the soil,
however, is further divided into soil and subsoil. In this sense the
soil comprises that portion of the surface of the ground, usually from
four to nine inches deep, containing most of the organic remains of
plants and animals and in which air circulates more or less freely for
the proper humification of the organic matter, which usually gives a
darker color to the soil than to the subsoil. The subsoil proper lies
below this, and has usually more characteristic properties, especially
in respect of color and texture, as it has been less influenced by
artificial conditions of cultivation and the remains of vegetation.
The subsoil extends to an indefinite depth and is limited usually by
deposits of undecomposed or partly decomposed rock matter, or by layers
of clay, sand or gravel.
Inasmuch, however, as the influence of the subsoil on growing crops is
of little importance below the depth of eighteen inches the analysis of
samples from a greater depth has more of a geologic than agricultural
value.
Hilgard regards as subsoil whatever lies beneath the line of change, or
below the minimum depth of six inches. But should the change of color
occur at a greater depth than twelve inches, the soil specimen should
nevertheless be taken to the depth of twelve inches only, which is the
limit of ordinary tillage; then another specimen from that depth down to
the line of change, and then the subsoil specimens beneath that line.
The depth to which the last should be taken will depend upon
circumstances. It is always desirable to know what constitutes the
foundation of a soil to the depth of three feet at least, since the
question of drainage, resistance to drought, etc., will depend
essentially upon the nature of the substratum. But in ordinary cases ten
or twelve inches of subsoil will be sufficient. The sample should be
taken in other respects precisely like that of the surface soil, while
that of the material underlying this subsoil may be taken with less
exactness, perhaps at some ditch or other easily accessible point, and
should not be broken up like the other specimens.
In the method of soil sampling adopted by the Royal Agricultural College
of England, the soil is regarded as that portion of the surface of the
ground which is reached by ordinary tillage operations, generally being
from six to nine inches deep; the subsoil is that portion which is
ordinarily not touched in plowing.
AUTHORITIES CITED IN PART FIRST.
Footnote 1:
Comptes rendus, Tome 110, p. 1271.
Footnote 2:
Wyatt, Phosphates of America, p. 66.
Footnote 3:
Engineering and Mining Journal, August 23, 1890.
Footnote 4:
American Journal of Science, Vol. 41, February, 1891.
Footnote 5:
Preliminary Sketch of Florida Phosphates, Author’s edition, pp. 18, et
seq.
Footnote 6:
Journal für praktische Chemie, 1st series, Band 38, S. 388.
Footnote 7:
Annual Report Connecticut Experiment Station, 1890, p. 72.
Footnote 8:
Annual Report Massachusetts Experiment Station, 1887, p. 233.
Footnote 9:
Bulletin 21, Rhode Island Experiment Station, 1893.
Footnote 10:
Chemical Composition of Food-Fishes. Report of U. S. Commissioner of
Fish and Fisheries, 1888, pp. 679 et seq.
Footnote 11:
G. Brown Goode, American Naturalist, Vol. 14, July, 1890.
Footnote 12:
Comptes rendus, Tome 101, 1885, pp. 65, et seq.
Footnote 13:
Royal Agricultural Society Journal, Vol. 13, 1852, pp. 349 et seq.
Footnote 14:
Gîtes Mineraux, par Fuchs et DeLauny, Tome 1, p. 425.
Footnote 15:
El Salitre de Chile; René F. LeFeuvre y Artūro Dagnino, 1893, p. 12.
Footnote 16:
Crampton, American Chemical Journal, Vol. II, 1890, p. 227.
Footnote 17:
Potash, pamphlet of German Kali Works, pp. 3, 4.
Footnote 18:
Gîtes Mineraux, p. 429.
Footnote 19:
Bulletin of the Philosophical Society of Washington, Vol. II, p. 142.
Footnote 20:
Handbook for the Department of Geology of the U. S. National Museum,
by Geo. P. Merrill.
Footnote 21:
Vid. supra, p. 506.
Footnote 22:
Vid. supra, Plate 120.
Footnote 23:
Merrill, op. cit. p. 521.
Footnote 24:
Merrill, op. cit. p. 536.
Footnote 25:
Merrill, op. cit. p. 545.
Footnote 26:
Merrill, op. cit. p. 547.
Footnote 27:
Mineral Physiology and Physiography, p. 251.
Footnote 28:
Bulletin No. 52, United States Geological Survey, p. 16.
Footnote 29:
bis (p. 48), Vid. supra, p. 38.
Footnote 30:
The Formation of Vegetable Mold through the Action of Worms.
Footnote 31:
Rocks and Soils, pp. 131–2.
Footnote 32:
Comptes rendus, Tome 110, p. 1370.
Footnote 33:
Bulletin No. 83, California Experiment Station.
Footnote 34:
Geological Magazine, Vol. 7, No. 6, pp. 291–92.
Footnote 35:
Wiley, Agricultural Science, 1893, pp. 106 et seq.
Footnote 36:
Travaux de la Société des Naturalistes St. Petersburg, Tome 20, 1889.
Footnote 37:
Rocks and Soils, p. 134.
Footnote 38:
Beilstein’s Handbuch der Organischen Chemie, Band I, Ss. 891–2.
PART SECOND.
TAKING SAMPLES OF SOIL FOR ANALYSIS.
=54. General Principles.=—It would be unwise to attempt to give any
single method of taking soil samples as the only one to be practiced in
all circumstances. In the methods which follow it is believed will be
found directions for every probable case. The particular method to be
followed will in each case have to be determined by circumstances.
The sole object in taking a sample of soil should be to have it
representative of the type of soils to which it belongs. Every
precaution should be observed to have each sample measure up to that
standard.
The physical and chemical analyses of soils are long and tedious
processes and are entirely too costly to be applied to samples which
represent nothing but themselves.
The particular place selected for taking the samples as well as the
method employed are also largely determined by the point of view of the
investigations. The collection of samples to illustrate the geologic or
mineralogical relations of soils is quite a different matter from
gathering portions to represent their agricultural possibilities. In a
given area the sum of plant food in the soil would only be determined by
the analyses of samples from that particular field, while samples
illustrating geologic relations could or should be taken at widely
distant points. Again the chemist is content with a sample of a few
grams in weight while the physicist would require a much larger
quantity. Much popular ignorance exists respecting the importance of the
collection of soil samples. As an illustration of this may be cited a
recent instance in which a sample of soil was received by the author
with a request for a complete analysis and a statement of the kinds of
crops it was suited to grow. No data relating to the locality in which
the sample was taken accompanied this request. The sample itself, which
weighed a little less than 3.6 grams, was not a soil at all in an
agricultural sense but a highly ferruginous sand.
The collector of samples who understands the purpose for which he is
working will find among the approved methods which follow some one or
some combination of methods, by means of which his work can be made
successful. In these cases it is the collector rather than the method on
which reliance must be placed to secure properly representative samples.
=55. General Directions for Sampling.=—The locality having been selected
which presents as nearly as possible the mean composition of the field a
square hole is dug with a sharp spade to the depth of eighteen inches.
The walls of this hole should be smooth and perpendicular. The soil to
the depth of six to nine inches is then removed from the sides of the
hole in a slice about four inches thick; or the sample of soil may be
taken to the depth indicated by a change of color. Any particles which
fall into the bottom of the hole are carefully collected and added to
the parts adhering to the spade. The whole is thrown into a suitable
vessel for removal to the laboratory. The sample of soil having been
thus secured, the subsoil is taken in the same way. To insure uniformity
in the samples, it is well to take several of them from the same field.
Where more than one sample is taken it is advisable to mix all the
sub-samples in the field, remove large sticks, stones, roots, etc., and
take a general sample of from three to five kilograms. The character of
the débris, etc., removed should be carefully noted.
It is sometimes desirable to take samples of the subsoil to a greater
depth than eighteen inches. A post-hole auger or large wood auger will
be found very useful for this purpose. It is rarely necessary to take
samples of subsoil to a greater depth than six feet. In taking samples
the geologic formation and the general topography of the field should be
noted, also the character of the previous crops, kind and amount of
fertilizers employed, character of drainage and any other data of a
nature to give a more accurate idea of the forces which have determined
the physical and chemical properties of the sample.
=56. Method Of Hilgard.=—Hilgard[39] recommends that samples should not
be taken indiscriminately from any locality you may chance to be
interested in, but that you should consider what are the two or three
chief varieties of soil which, with their intermixtures, make up the
cultivable area of your region, and carefully sample these first of all.
As a rule, and whenever possible, samples should be taken only from
spots that have not been cultivated, or are otherwise likely to have
been changed from their original condition of virgin soils and not from
ground frequently trodden over such as roadsides, cattle paths, or small
pastures, squirrel holes, stumps, or even the foot of trees, or spots
that have been washed by rains or streams, so as to have experienced a
noticeable change, and not be a fair representative of their kind. He
further suggests that the normal vegetation, trees, herbs, grass, etc.,
should be carefully observed and recorded, and spots showing unusual
growth be avoided whether in kind or quality, as such are likely to have
received some animal manure or other outside addition.
Specimens should be taken from more than one spot judged to be a fair
representative of the soil intended to be examined as an additional
guarantee of a fair average.
After selecting a proper spot pull up the plants growing on it, and
scrape off the surface lightly with a sharp tool to remove half-decayed
vegetable matter not forming part of the soil. Dig a vertical hole, like
a post-hole, at least 20 inches deep. Scrape the sides clean so as to
see at what depth the change of tint occurs which marks the downward
limit of the surface soil, and record it. Take at least half a bushel of
the earth above this limit, and on a cloth (jute bagging should not be
used for this purpose, as its fibers, etc., become intermixed with the
soil) or paper break it up and mix thoroughly, and put up at least a
pint of it in a sack or package for examination. This specimen will,
ordinarily, constitute the soil. Should the change of color occur at a
less depth than six inches the fact should be noted, but the specimen
taken to that depth nevertheless, since it is the least to which
rational cultures can be supposed to reach.
In case the difference in the character of a shallow surface soil and
its subsoil should be unusually great, as may be the case in tule or
other alluvial lands or in rocky districts, a separate example of that
surface soil should be taken, besides the one to the depth of six
inches.
Specimens of salty or alkali soils should, as a rule, be taken only
toward the end of the dry season, when they will contain the maximum
amount of the injurious ingredients which it may be necessary to
neutralize.
Whatever lies beneath the line of change, or below the minimum depth of
six inches, will constitute the subsoil. Should the change of color
occur at a greater depth than twelve inches the soil specimen should
nevertheless be taken to the depth of twelve inches only, which is the
limit of ordinary tillage; then another specimen from that depth down to
the line of change, and then the subsoil specimen beneath that line.
Hilgard justly calls attention to the fact that all peculiarities of the
soil and subsoil, their behavior in wet and dry seasons, their location,
position and every circumstance in fact, which can throw any light on
their agricultural qualities or peculiarities should be carefully noted
and the notes sent with the samples. Unless accompanied by such
information, samples can not ordinarily be considered as justifying the
amount of labor involved in their examination.
=57. Whitney=[40] suggests that a geologic map of the region to be
sampled should always be at hand and that all samples should be rejected
from spots showing local discrepancies, washings or other disturbances.
The kind of analyses to which a sample is to be subjected also largely
determines the method to be pursued in selecting it: For instance, a
sample to be used for determining the size of the particles therein, may
be taken in quite a different manner from that designed only for the
determination of moisture.
=58. In= the directions collated by Richards[41] and which have been
largely followed by the correspondents of the Department of Agriculture,
it is recommended to select in a field, four or five places, at least,
per acre, taking care that these places have an homogeneous aspect, and
represent as far as possible the general character of the whole ground.
If the field, however, present notable differences, either in regard to
its aspect or its fertility, the samples gathered from the different
parts must be kept separate.
The sampling of arable soil should be made only after the raising of the
crop and before it has received any new manure. In other soils the
sample should be taken only from spots that have not been cultivated.
=59. In= the method of soil sampling adopted by the German Experiment
Stations[42] it is directed that the samples of soil should be taken
according to the extent of the surface to be sampled, in three, five,
nine, twelve or more places at equal distances from each other. They
should be taken in perpendicular sections to the depth turned by the
plow; and for some studies of the subsoil to a depth of sixty to ninety
centimeters. The single samples can be either examined separately or
carefully mixed and an average portion of the mixture taken.
=60. Method of the Official French Commission.=—The official French
commission[43] emphasizes the fact that the sample of soil taken for
analysis, should represent a layer of equal thickness through the total
depth of its arable part. An analysis of the subsoil taken in the same
way, will often be useful to complete the data of the soil study.
First of all, according to this authority, it is necessary to determine
the point of view from which the sample is to be taken. If the object is
a general study, having for its aim the determination of the general
composition of the soils of a definite geologic formation, the sample
should be taken in such a way that the different characteristics of the
soil alone should enter into consideration without paying any attention
to its accidental components, which have been determined by local
causes, such as are produced by continued high cultivation, the
application of abundant fertilizers, or the practice of a particular
line of agriculture. The samples of soil therefore, with such an object
in view, should be taken on parts of the earth which are beyond the
reach of the causes mentioned above and which tend to modify the nature
of the primitive soil. In such a case it is the soil which has not been
modified, or better still, virgin soil, such as is found in the
woodlands and prairies, which should be taken for a sample, choosing
those places in which the geologic formation is most perfectly
characterized. In such a case a soil taken in one spot corresponding to
the conditions before mentioned, would be the best for the purposes in
view. The sample would thus represent a true type to be studied, not one
of a mean composition got by taking samples from different localities
and mixing them into a homogeneous parcel. This last method of
proceeding could introduce into the sample earth modified by culture or
by influences purely accidental. However, it would be wise, in a region
characterized by the same geologic formation, to take a certain number
of samples in different localities, and examine singly each one of them
in order to be assured that there is a uniformity of composition in the
whole of the soils.
If, on the contrary, it is the purpose of the investigation to furnish
information to the cultivator concerning the fields which are worked, it
is necessary to approach the problem from a different point of view. In
this case the earth which is under cultivation should be first of all
considered with all the modifications which nature causes or practical
culture has caused, in it. But it often happens that upon the same farm
the natural soil is variable, caused either by the washings from the
adjacent soils, by the accumulation at certain points of deposits formed
from standing water, or from other reasons. In such a case it would be
necessary to take samples from every part of the field which exhibited
any variation from the general type, in order to get a complete mean
sample of the whole. It is necessary to be on guard against making a
mixture of these different lots which would neither represent the
different soils constituting the farm nor their mean composition. It
would be better to examine each of these samples alone and then from
those parts which appear to have a similar composition, to take a
general sample for the mean analysis.
Most often it is necessary to confine our studies to the really
important part of the farm the composition of which would have a
practical interest. The aspect of the spontaneous vegetation in such a
case, will often serve as a guide to determine the parts of the farms
which are similar in nature. The sample should represent the arable
layer, properly so-called, that is, that part of it which is stirred by
the agricultural implements in use and in which the root system of the
plant takes its greatest development and which is the true reservoir of
the fertilizing materials.
When a trench is dug in the soil it is easy to distinguish the arable
layer from the subsoil. In the first place, its color is different,
generally being modified by vegetable débris which forms the supply of
humus. The depth of the arable layer is variable, but it is most
frequently between 200 and 300 millimeters. In the analysis the depth
and layers should be indicated since the chemical composition of the
earth varies according as the sample is taken to a greater or less
depth. As an example of this it may be said that the quantity of
nitrogen decreases in general in proportion as the depth of the layer is
increased. The sample, therefore, should be limited exactly to the
arable layer of soil.
=61. Caldwell=[44] advises that according to the purpose of the analysis
samples be taken:
_a_, from one or from several spots in the field, in order to subject
each sample to a separate analysis; or
_b_, for an average representation of the soil of the whole field; in
this case, several portions of earth are taken from points distributed
in a regular manner over the field, all of which are most carefully
mixed together, and 4–6 kilograms of the mixture, free from any large
stones, are preserved as the average sample.
An excavation in the soil 30–50 centimeters deep, or through to the
subsoil, and 30–50 centimeters square, with one side as nearly vertical
as possible is made and a slice taken from this side of uniform
thickness throughout, weighing 4–5 kilograms. If the subsoil is to be
examined, a sample of it should be taken out in the same manner as
directed for the upper soil, to the depth of about 60 centimeters.
If the character of the soil varies materially in different parts of the
field, samples from several spots should be analyzed separately.
A small portion of the sample should be put at once in a well-stoppered
bottle; the remainder may be allowed to become air-dried, by exposing it
in a thin layer, in summer, to the common temperature in the shade, or,
in winter, to that of a warm room, or a moderately warm drying-chamber,
heated to 30°–40°; in either case it should be carefully protected from
dust.
At the time of taking the sample of the soil, observations should be
made in regard to the following points:
_a._ The geognostic origin of the soil.
_b._ The nature of the underlying strata, to the depth of 1–2 meters, if
practicable.
_c._ The meteorology of the locality, by consulting meteorological
records, if possible; otherwise, by the general opinion of the
neighborhood; in this connection, the height of the locality above the
level of the sea should be noted also.
_d._ The management and rotation of crops in previous years.
_e._ The character of the customary manuring.
_f._ The amount of the crops removed in the preceding year, and, if
possible, the average amount of each of the more important crops yielded
by the field.
_g._ The practical judgment of neighboring farmers in regard to the
field.
Caldwel’s method is practically identical with that of Wolff[45] which
was one of the earliest of the systematic schemes for taking soil
samples.
=62. Wahnschaffe=[46] insists on rather a fuller preliminary statement
to accompany soil samples but gives essentially the method of Wolff with
some unimportant variations which add little to the value of the
process.
=63. Method Of Peligot.=—According to Peligot[47] the taking of samples
of soil of which the physical and chemical properties are to be
determined is a delicate operation.
These samples should represent as nearly as possible both the good and
bad qualities of a soil.
In the field selected are chosen a certain number of places at least
four or five per hectare. The spots selected should have a homogeneous
appearance—resembling as nearly as possible the general aspect of the
field.
By means of a spade a few kilograms of earth are removed to the depth of
the subsoil being careful to include in the sample no accidental
detritus which the upper part of the soil especially may contain.
The samples should be taken immediately after the crop is harvested and
before any fresh fertilizer is applied. The samples are carefully mixed
and placed in a glass bottle or flask.
The sample of subsoil is obtained in the same manner. If the field
presents notable differences in surface or fertility all the samples
taken should be examined separately.
=64. Method of Whitney.=[48]—An ordinary wood auger, 2½ inches in
diameter is so arranged as to admit of additions to the stem to enable
the operator to take samples at different depths. It may be fitted with
a short piece of gas pipe for a handle and the several pieces of which
it is composed may be taken apart and carried in a knapsack.
In taking a soil sample the boring is continued until a change in color
shows that the subsoil has been reached. The auger cuts a very clean
sample save in excessively sandy soil. After the soil sample is secured
the hole is cleaned out and the sample of subsoil taken by the same
instrument. The soil is conveniently preserved in heavy cloth bags of
which the usual size is 6 by 8½ inches. Where larger samples are
required the size of the bag is correspondingly increased. Each bag is
to be tagged or labeled to correspond with the entry in the note book.
Samples to determine the amount of empty space in a soil are taken as
follows: The sampler is a piece of brass cylinder about nine inches long
and about 1½ inches in diameter. A piece of clock spring is soldered in
one end and sharpened to give a good cutting edge. This arrangement
permits the sample to pass into the cylinder without much friction. The
area enclosed by the clock spring is accurately determined and a mark is
placed in the cylinder six inches from the cutting edge. The cylinder is
driven into the soil to a depth of six inches, a steel cap being used to
prevent the hammer from injuring the cylinder. The earth is next removed
from about the cylinder with a trowel, and the separated cylinder of
earth is cut smoothly off by a sharp knife and removed together with its
brass envelope. The sample is taken to the laboratory in a cloth bag,
dried and weighed.
=65. Taking Samples for Moisture Determination.=—A number of brass tubes
is provided nine inches long and ¾ inch in diameter and with a mark six
inches from the bottom.
The tube is pushed down into the soil to the mark and the sample of soil
removed with the tube. There is but little danger of the sample dropping
out of the tube even in sandy soils. When the tube is withdrawn each end
is capped with a rubber finger tip making a perfectly air tight joint.
The tubes containing the samples can be kept several days with no fear
of losing moisture. This method is especially useful in having samples
taken by observers in different localities who can enclose the tubes in
a cloth sack and send them to the laboratory by mail daily or at stated
intervals. A tube of the size given holds about fifty grams of soil.
=66. Taking Samples to Determine the Permeability of Soil to Water or
Air.=—Whitney[49] determines the permeability of soil or subsoil to
water or air in the following manner:
An excavation two feet square and eighteen inches deep is made in the
soil. On one side of this hole the sample of soil or subsoil is taken by
means of a narrow saw blade and a sharp carving knife. The sample of
soil taken should be two inches square and 3½ to 4 inches long.
It is placed in a brass cylinder three inches long and 3¼ inches in
diameter. The open space in the cylinder is filled with paraffin heated
just to its melting point. As the paraffin cools the upper surface
should be kept stirred to prevent the mass when set from receding from
the square column of soil. Care must be taken to keep the paraffin from
the ends of the soil columns and these should be left, as far as
possible in their natural condition.
The rate of percolation of the water may be determined at the time the
sample is taken. For this purpose an additional section of brass tube
two inches deep is secured to the one holding the sample by a rubber
band. An iron rod is driven into the earth carrying a retort stand ring
supporting a funnel filled with fine gravel. The lower end of the soil
column in the brass cylinder is placed on this gravel. Water is next
carefully poured upon the top of the sample of soil being careful not to
disturb the surface. The surface of the sample may be protected with a
little fine sand. The water should be poured on the paraffin thus
affording an additional protection to the soil surface. When the water
begins to drop from the funnel a graduated glass is set under it and the
time required for a given volume to pass through under an initial
pressure of two inches is noted. The volume required represents one inch
in depth over the four square inches of soil surface, _viz._: four cubic
inches.
=67. Sampling of Soil for Staple Crops.=—Some variations from the usual
methods are recommended by Whitney when the samples are taken from
fields growing staple crops.
The immediate object of the work, for which these samples are desired,
is to make a thorough study of the physical and chemical properties of a
number of typical soils adapted to the different staple crops, such as
grass, wheat, truck, and the different types of tobacco. They should be
taken for a careful study of the texture of the soils, the relative
amount and arrangement of sand and clay, the relation of the soils to
moisture and heat, and the ease with which they can maintain a proper
water supply for the different staple crops under existing climatic and
cultural conditions. The ultimate object of such a study is to see how
these conditions can be changed so as to make the soils more productive,
and make them yield a better quality of crop, or to change the
conditions in other soils, which differ from these, so that the culture
of the different staple crops can be extended over wider areas by
improved methods of cultivation and manuring.
The soil selected for sampling for these investigations should be
typical, should represent fairly well a considerable area of land. It
should represent either the very best type of land for the staple crop
or crops of the locality, or the very poorest lands for these same
crops. Both of these extremes are desired for contrast. For example, if
the staple crop of the locality is wheat or a certain type of tobacco,
select the soil best adapted to this staple crop, and another soil, if
possible, in the same locality, representing considerable area of land
upon which this staple crop cannot be successfully grown on account of
the inferior yield, quality, or the time of ripening of the crop. The
soil sampled should be, or should recently have been, under actual
cultivation in the crop or crops best adapted to it, so that the real
agricultural value of the land can be accurately known.
The samples should be taken inside the field, some distance away from
fences, roads, or trees. If there are plants growing in the field, the
sample should be taken about midway between two plants. The samples
should be taken where they will typify fairly well the average soil of
the field and of the large area of land which they are to represent.
The samples are taken in some one of the ways described herein. Each
sample should be carefully labelled at the time of taking. The following
blank form will be found convenient for this purpose:
│Locality:
LABORATORY No.: │
────────────────────────┼──────────────────────────────────────────────
No. of sack: │Description: (virgin or cultivated).
│ (_a_) Natural herbage:
│ (_b_) Crops best adapted to land (grass,
│ wheat, tobacco, truck, barren).
│ „
------------------------│ „
Date: │ „
│ „
------------------------│ „
Collector: │ „
│ „
------------------------│------------------------
Depth of sample: │
(Soil or Subsoil?) │Geologic formation:
... in. to ... inches. │
=68. Method of the Royal Agricultural Society.=[50]—Have a wooden box
made, six inches long and wide, and from nine to twelve inches deep,
according to the depth of soil and subsoil in the field. At one of the
selected places mark out a space of twelve inches square; dig around it
in a slanting direction a trench, so as to leave undisturbed a block of
soil, with its subsoil, from nine to twelve inches deep; trim this block
to make it fit into the wooden box, invert the open box over it, press
down firmly, then pass a spade under the box and lift it up and gently
turn it over.
In the case of very light, sandy, and porous soils, the wooden box may
be at once inverted over the soil and forced down by pressure, and then
dug out.
Proceed in the same way for collecting the samples from all the selected
places in the field, taking care that the subsoil is not mixed with the
surface soil. The former should be sampled separately.
In preparing the plot for the gathering of the sample, take care to have
it lightly scraped so as to remove any débris which may be accidentally
found there.
The different samples thus procured are emptied on a clean, boarded
surface, and thoroughly mixed, so as to incorporate the different
samples of the same field together. The heap is then divided into four
divisions, and the opposite quarters are put aside, taking care to leave
the two remaining ones undisturbed; these are thoroughly mixed together,
the heap divided into quarters, and the opposite ones taken away as
before. This operation of mixing, dividing into quarters and taking away
the opposite quarter is continued until a sample is left weighing about
ten or twelve pounds.
Thus is obtained the average sample of the soil. Of course where only a
single sample is taken from the field this method of quartering is not
resorted to, but the bottom of the box is nailed directly on and sent to
the laboratory, where the soil is to be analyzed.
=69. Grandeau=[51] suggests that in taking soil samples there are two
cases to be considered; first a homogeneous soil and second, a soil
variable in its appearance and composition. First, if the soil is
homogeneous, being of the same geologic formation it will be sufficient
to take a mean sample in accordance with the following directions:
The field is first divided by diagonals or by transverse lines the
direction of which need not be fixed in advance but as inspection of the
form and configuration of the field may indicate. In the ordinary
conditions, of homogeniety (marly, granite, argillaceous or silicious
soils) it will be sufficient to select about five points per hectare
from which the samples are to be taken. These points having been
determined the surface is cleaned in such a way as to remove from it the
detritus which may accidentally cover it; such as dry leaves, fragments
of wood, foreign bodies, etc. The surface having been prepared, (five to
six square meters) a hole is dug four-tenths of a meter long and as wide
as the spade employed. The sides should be as nearly vertical as
possible. As to depth it varies with the usage of the country in regard
to tillage. The layer of arable earth is what in effect properly
constitutes the soil. It ought not to be mixed with any fragments of the
subsoil. When the hole is properly cleaned the samples are secured with
a spade from the sides of the excavations. About five kilograms are
taken. The soil is placed in a proper receptacle as it is removed from
the hole.
This operation is repeated on as many points as may be necessary to
obtain a mean sample of the soil of the whole field.
All the samples are now collected on a table sufficiently large, and
intimately mixed together. Two samples, each of about five kilograms,
are then taken from the mixed material. One sample is immediately placed
in bottles and carefully stoppered and sealed; the other is dried in the
sun or on the hearth of a furnace. When sufficiently dry the second
sample is also placed in bottles and well stoppered. While mixing the
samples, pebbles, etc., of the size of a nut and larger are removed, the
weight of the rejected matter being determined. The nature of the
pebbles should also be noted; whether silicates, limestone, etc.
The sample of subsoil is taken in exactly the same manner, using the
same holes from which the samples of soil were taken. The nature, the
arrangement and the appearance of the strata will indicate the depth to
which the subsoil should be taken. In general, a depth equal to that of
the sample of soil will be sufficient. The depth to which the roots of
cultivated plants reach is also a good indication in taking a sample of
the subsoil. In forests the sample of subsoil should be taken from four
to five-tenths of a meter below the surface.
If the soil in respect of its geologic formation, its fertility or its
physical aspect presents great differences, special samples should be
taken in each part in accordance with the directions given above.
=70. Method of the Official Agricultural Chemists.=—In the directions
given by the Association of Official Agricultural Chemists[52] it is
stated that the soil selected should be as far as possible in its
natural condition, not modified by recent applications of manure, or
changed by the transporting action of water or wind. Surface
accumulations of decaying leaves, etc., should be removed before taking
the sample.
To eliminate accidental variations in the soil, select specimens from
five or six places in the field which seem to be fair averages of the
soil, remove two or three pounds of the soil, taking it down to the
depth of nine or ten inches[53] so as to include the whole depth. Mix
these soils intimately, remove any stones, shake out all roots and
foreign matter, and dry the soil until it-becomes friable.[54] Break
down any lumps in a mortar with a wooden pestle, but avoid pulverizing
any mineral fragments; pass eight to ten pounds of the soil through a
sieve, having circular perforations one twenty-fifth of an inch in
diameter, rejecting all pebbles and materials too coarse to pass through
the sieve. Once more mix intimately the sifted soil. Expose in thin
layers in a warm room till thoroughly air dry (or dry it in an air-bath
at a temperature of 40°), place six to eight pounds in a clean bottle,
with label of locality and date, and cork the bottle containing the
soil, for analysis.
The soil is rapidly dried to arrest nitrification; it is not heated
above 40° lest there should be dissipation of ammonia compounds, or a
change in solubility. The normal limit to which the soil may be heated
in place by the sun’s rays should not be exceeded in preparing a sample
for an agricultural chemical analysis.
The relative amount of fragments too coarse to pass through the sieve
should be made a matter of record. They are soil material, but not yet
soil, so far as agricultural purposes are concerned.
=71. Method of Lawes.=—In a late method of sampling proposed by Sir J.
B. Lawes[55] a steel frame ten by twelve inches, and nine inches deep
open at top and bottom is driven into the earth until its upper edge is
level with the surface of the soil. All above-ground vegetation is then
cut off as closely as possible with scissors. The soil within the frame
is then removed exactly to the depth of the frame, and immediately
weighed. It is then partially dried, and mechanically separated by a
series of sieves, all visible vegetable matter being at the same time
picked out. The stones and roots and the remaining soil are thus
separated, and the determinations of dry matter, nitrogen, etc., are
made in the separated soil after being finely powdered. The loss of
water at each stage of preparation and on drying the samples as analyzed
is also carefully determined. This method, which requires the soil to be
taken to an arbitrary depth of nine inches, could not be used when
samples of strictly arable soil are to be taken.
=72. In taking= a sample by the French commission[56] method it is
necessary to remove from the surface, the living and dead vegetation
which covers the soil. With a spade a square hole is then dug to the
depth of about 500 millimeters; in other words, to a depth considerably
exceeding that of the arable layer. Afterwards on each of the four sides
of the hole there is removed by the spade, a prismatic layer of the
arable portion of a thickness equal to its depth. The samples thus
obtained are united together and carefully mixed for the purpose of
forming a sample for analysis. If there are large stones they are
removed by hand and their proportion by weight determined.
In all cases it would prove useful to take a sample of the subsoil which
is far from playing a secondary rôle. The rootlets bury themselves
deeply in it and seek there a part of their nourishment. The subsoil,
therefore, furnishes an important addition to the alimentation of the
plants. For taking a sample of the subsoil a ditch is dug of sufficient
depth, say one meter, and the arable soil carefully removed from the top
portion. Afterwards pieces are taken from the four sides of the hole at
variable depths, which should always be indicated, and which should
extend in general, from six to eight-tenths of a meter below the arable
soil since it is demonstrated that the roots of nearly all plants go at
least to this depth. The analysis of the subsoil, however, is less
important than that of the soil, properly so-called, because the
agronomist does not act directly upon it and takes no thought of
modifying it and enriching it as he does the layer of arable soil. But
the composition of the subsoil is a source of information capable of
explaining certain cultural results and capable sometimes, of leading to
the correct way of improving the soil, as in cases where the subsoil can
be advantageously mixed with the superficial layer.
=73. Wolff=[57] suggests that a hole thirty centimeters square be dug
perpendicularly and a section from one of the sides taken for the
sample. To the depth of thirty centimeters the sample shall be taken as
soil and to the additional depth of thirty centimeters as subsoil. The
thickness of the section taken may vary according to the quantity of the
sample desired. For analytical purposes, five kilograms will usually be
sufficient. When culture experiments are also contemplated a larger
quantity will be required.
=74. Method of Wahnschaffe.=—The method of sampling advised by
Wahnschaffe[58] is but little different from that of Wolff already
mentioned.
A square sample hole is dug with a spade having its sides perpendicular
to the horizon. The soil which is removed is thrown on a cloth and
carefully mixed. From the whole mass a convenient amount is next removed
care being taken not to include any roots. In a similar manner it is
directed to proceed for the sample of subsoil. At first the subsoil
should be removed to a depth of two to three decimeters. The number and
depth of subsequent samples will depend chiefly upon the character of
the soil. Where samples are taken to the depth of two meters the use of
a post-hole auger is recommended.
The samples taken should not be too small. In general from two to three
kilograms should remain after all preliminary sampling is finished.
=75. Method of König.=—The directions given by König[59] for taking soil
samples are almost identical with those prescribed by Wahnschaffe and do
not require any further illustration.
=76. Special Instruments Employed in Taking Samples.=—In general a sharp
spade or post-hole auger is quite sufficient for all ordinary sampling
but for certain special purposes other apparatus may be used.
The instrument which is used by King[60] consists of a thin metal tube
of a size and length suited to the special object in view, provided with
a point which enables it to cut a core of soil smaller than the internal
bore of the tube and at the same time make a hole in the ground larger
than its outside diameter. Its construction is shown in figure 11, in
which A B represent a soil tube intended to take samples down to a depth
of four feet. A′ is a cross-section of the cutting end of the tube,
which is made by soldering a heavy tin collar, about three inches wide,
to the outside of a large tube allowing its lower end to project about
one-half an inch. Into this collar a second one is soldered with one
edge projecting about one-quarter of an inch and the other abutting
directly against the end of the soil tube. Still inside of this collar
is a third about one-half an inch wide which projects beyond the second
and forms the cutting edge of the instrument.
[Illustration:
FIGURE 11.
]
The construction of the head of the tube is shown at B′. It is formed by
turning a flange on the upper end of the tube and then wrapping it
closely with thick wire for a distance of about three inches, the wire
being securely fixed by soldering. The soil tube should be of as light
weight as possible not to buckle when being forced into the ground, and
the cutting edge thin. The brass tubing used by gas fitters in covering
their pipes has been found very satisfactory for ordinary sampling. With
a one inch soil tube four feet long it is possible to get a clear
continuous sample of soil to that depth by simply forcing the tube into
the ground with the hand and withdrawing it, or the sample may be taken
in sections of any intermediate length. Later in the season when the
soil becomes dryer it is necessary to use a heavy wooden mallet to force
the tube, and this should be done with light blows.
The closeness with which it is possible to duplicate the samples in
weight by this method will be seen below, where from each of four
localities three samples were taken from the surface to a depth of four
feet.
SHOWING VARIATIONS IN THE DRY WEIGHT OF TRIPLICATE SAMPLES OF SOIL.
─────────────────────────┬──────────┬──────────┬──────────
│ A. │ B. │ C.
─────────────────────────┼──────────┼──────────┼──────────
I. Surface to four feet│716.6 gms.│715.5 gms.│710.3 gms.
II. Surface to four feet│715.4 gms.│687.1 gms.│731.2 gms.
III. Surface to four feet│654.0 gms.│688.3 gms.│709.0 gms.
IV. Surface to four feet│714.0 gms.│687.8 gms.│719.3 gms.
These four series of samples were taken at the four corners of a square
twelve feet on a side and serve to show how much samples may vary in
that distance. The large difference shown in III, A is due to the fact
that the soil tube penetrated a hole left by the decay of a rather large
root as shown by the bark in the sample.
=77. Auger for Taking Samples.=—It has already been said that the
ordinary auger used for boring fence post-holes may be used to advantage
in taking soil samples. Large wood augers can also be used to advantage
for the same purpose. For special purposes, however, other forms of
augers may be used.
Norwacki and Borchardt[61] have described a new auger for taking samples
of soil for analytical purposes.
[Illustration:
FIGURE 12.
]
In figure 12, A, B and C show the general exterior and interior form of
the instrument. The handle is hollow and made of iron gas pipe covered
with leather. On the inside of this, in the middle, is fixed a wooden
plug a, which leaves two compartments, one in each end for holding the
brass plug bb,’ and the wicker lubricating wad cc.’ The stem of the
auger a, is heavy and made of eight-sided steel and the under end is
strengthened with a heavy casting fitting into the auger guide g g. The
end of the auger I I′ is triangular and hardened. The auger guide g g,
is made out of a single piece of drawn steel tubing. Above it is
strengthened by a ring-shaped piece of iron or copper and its lower end
is furnished with saw teeth as shown in K and is hardened. The fixing
key e, is bent in the form of a hook and can be passed through the two
holes o o, of the auger stem and through the one hole o′ in the
strengthened part of the auger guide. It permits the auger guide to be
fixed upon the auger stem in two different positions, higher and lower.
On one end it is cut squarely across and on the other provided with a
conical hole drilled into it. It fits on the one hand exactly in the
auger guide and on the other loosely plays in the cavity of the handle
at b, designed to hold it when not in use. The cap d′ is made of heavy
sheet brass and is fastened upon the end of the handle at c c′ after the
manner of a bayonet. The wicker cartridge is made of rolled and sewed
wicker-work. At the upper end it is provided with a metallic button and
before use it is saturated with paraffin oil. It fits on the one side
firmly in the auger guide and on the other in the cavity, of the handle
c where it is kept when not in use. The union h is made of a brass tube
which below is closed with a piece of solid brass upon the inside of
which a hole is bored. In this hole rests the end of the auger stem when
the union is placed firmly upon the auger guide.
The auger is placed together as is shown in A B, the union h is taken
off and it is driven with gentle blows, turning it back and forth, to
the proper depth into the soil. After the key is loosened the auger is
lifted high enough so that the second hole appears and then it is fixed
in position by the key. Then the boring is continued, turning the auger
to the right, by which the auger, eating its way with its saw teeth,
presses deeper into the ground and withdraws the material for analysis.
After the auger guide has been filled through any desired length, say
five to ten centimeters with the sample of soil, the whole auger is
drawn out of the soil, the key removed, the auger stem withdrawn from
the auger guide, the apparatus opened by turning the bayonet fastening
of the stopper on the handle, the brass plug placed in the end and then
with the smooth part forward, from above, it is allowed to fall into the
auger guide until it reaches the soil. The auger stem is then put back,
the point of it fitting into the hole of the plug and the sample of soil
shoved out of the auger guide. The auger guide is again fixed on the
auger stem by the key and then the apparatus is ready for a second
operation. When the borings cease the wicker cartridge is drawn out of
the handle and shoved, the soft end forward, from above, into the auger
guide and the brass plug after it and pushed through with the auger
stem. By this process the wicker cartridge gives up a sufficient amount
of paraffin oil to completely grease the inside of the auger guide and
to protect it from rust. After use the instrument should be cleaned on
the outside by means of a cloth, the plug and wicker wad replaced in
their proper positions, the cap fixed on the handle and the union on the
point of the instrument.
The length of the whole apparatus may reach one meter or more; the
internal diameter sixteen millimeters. The apparatus weighs with a
length of one meter, together with all its belongings, about two
kilograms. For the investigation of peat and muck soils as well as sand,
instead of the steel auger guide one of brass or copper can be used. For
this purpose the length of the apparatus may reach three to four meters.
In comparison with other apparatus which are used for taking samples, it
appears without doubt that with the one just described a better and less
mixed portion of the soil can be obtained at great depths. The apparatus
is said to have many advantages over a similar one known as Fraenkel’s,
and is much more easy to clean. The advantages of the apparatus are said
to be the following: The farmer with this piece of apparatus in a short
time can go over his whole farm taking samples to the depth of ninety
centimeters since a single boring does not take more than one minute.
Geologists and others interested in the soil at greater depths can use
an apparatus three to four meters in length and obtain unmixed samples
from these lower depths. These are also interesting from a bacteriologic
point of view. The entire apparatus is especially valuable for the
investigation of the lower parts of peat and muck soils. The apparatus
has been tried in the collection of samples for the laboratory of the
Department of Agriculture and is too complicated to be recommended for
ordinary use. When however samples are to be taken at great depths as in
peat soils it is highly satisfactory.
=78. Soil Sampling= depends for its success more on the judgment and
knowledge of the collector than on the method employed and the apparatus
used. One skilled in the art and having correct knowledge of the purpose
of the work will be able to get a fair sample with a splinter or a
jack-knife while another with the most elaborate outfit might fail
entirely in collecting anything of representative value.
There are some special kinds of soil sampling, however, which cannot be
left to the method of the individual and it is believed that with the
descriptions given above nearly all purposes for which samples are
desired may be served.
For the study of nitrifying organisms, however, special precautions are
required and these will be noted in a more appropriate place.
In taking samples for moisture determinations the method of Whitney is
recommended as the best. For the general physical and chemical
analytical work the standard methods are all essentially the same. The
principles laid down by Hilgard will be found a sufficient guide in most
cases.
TREATMENT OF SAMPLE IN THE LABORATORY.
=79. The Sample=, or mixed sample, taken by one of the methods above
described, is placed on a hard smooth board, broken up by gentle
pressure into as fine particles as possible and all pieces of stone and
gravel carefully removed and weighed; all roots, particles of vegetable
matter, worms, etc., are also to be weighed and thrown out. This can be
done very well by using a sieve of from one to two millimeter mesh. Care
should be taken that the soil be made to pass through, which can be
accomplished by subjecting the lumps to renewed pressure with a
rubber-tipped pestle. In the above operation the soil should be dry
enough to prevent sticking. The relative weights of the pebbles, roots,
etc., and the soil should be determined.
=80. Order of Preliminary Examination.=—Hilgard[62] commences the
examination of a soil sample by washing about ten grams of it into a
beaker with a water current of definite velocity, stirring meanwhile
actively the part carried into the vessel. The residue not carried by
the current is examined macro- and microscopically to determine the
minerals which may be present, and the condition in which the fragments
exist—whether sharp or rounded edges, etc.
This examination will give some general idea of the parent rocks from
which the sample has been derived and of the distance the particles have
been transported. Next follows the hand test, _viz._, rubbing the soil
between the thumb and fingers first in the dry state and afterwards
kneading it with water and observing its plasticity. Following this
should come a test of the relations of the sample to water, _viz._, its
capacity for absorbing and retaining moisture. Finally the separation of
the soil into particles of definite hydraulic value and a chemical
examination of the different classes of soil concludes the analytical
work.
=81. Air Drying.=—The sifted soil should be thoroughly mixed and about
one kilogram spread on paper and left for several days exposed in a room
with free circulation of air and without artificial heat. The part of
the sample to be used for the determination of nitrates should be dried
more quickly as described in another place. The sample is then placed in
a clean, dry glass bottle, corked, sealed, and labeled. The label or
note book should indicate the locality where the sample was taken, the
kind of soil, the number of places sampled, and other information
necessary to proper description and identification.
=82. Caldwell=[63] directs that having taken the sample to the
laboratory, the stones and larger pebbles should be separated from the
finer parts by the hand, or by sifting with a very coarse sieve, and
examined with reference to their mineralogical character, weight and
size, making note, in this last respect, of the number that are as large
as the fist or larger, the number as large as an egg, a walnut,
hazel-nut, and pea, or give the percentage of each by weight.
Pulverize the air-dried soil in a mortar with a wooden pestle, and
separate the fine earth by a sieve with meshes three millimeters wide;
this sieve should have a tightly fitting cover of sheepskin stretched
over a loop, and it should be covered in the same manner underneath, so
that no dust can escape during the process of sifting.
Wash the pebbles and vegetable fibers remaining on the sieve with water,
dry and weigh the residue; the water with which this gravel was washed
should be evaporated to dryness at a temperature not exceeding 50°
towards the close of the evaporation, and the residue mixed with what
passed through the dry sieve.
The sifted fine earth is reserved for all the processes hereinafter
described, and is kept in well-stoppered bottles, marked air-dried fine
earth. The sieve mentioned above is too coarse for the more modern
methods of analysis.
=83. Wolff=[64] directs that the air-dried earth (in summer dried in
thin layers at room temperature, in winter in ovens at 30° to 50°) be
freed from all stones, the latter washed, dried, and weighed. The soil
is next passed through a three millimeter mesh sieve, the residual
pebbles and fiber washed, dried and weighed. The fine earth passing the
sieve is used for all subsequent examinations. It is air-dried at
moderate temperatures and preserved in stoppered glass vessels.
=84. The French=[65] commission calls especial attention to the method
of subsampling, and prescribes that the sample of earth which has been
taken in the manner indicated, and of which the weight should be greater
as the material is less homogeneous should not be analyzed as a whole.
It should be divided into two parts. The first includes the finer
particles constituting the earth, properly so-called, with the elements
which alone enter into play in vegetable nutrition and on which it is
necessary to carry out the analysis. The second embraces the coarser
particles to which only a superficial examination should be given and
which may have a certain importance from a physical point of view but
which cannot take any part from a chemical point of view, in the
nutrition of plants. It is, however, useful to examine its mineralogical
constitution and to look for the useful elements such as lime, potash,
etc., which it may be able to furnish to the earth, and in proportion as
it is decomposed, finer particles which may be useful in plant
nutrition.
How are we to distinguish between the fine and coarse elements? All
grades of fineness are observed in the soil, from the particles of
hydrated silica so small that with the largest magnifying power of the
microscope it is scarcely possible to distinguish them, up to grains of
sand which are of palpable size and visible to the naked eye, and
extending to pebbles of varying sizes. All intermediate stages are found
between these and if it should be asked what is the precise limit at
which it is necessary to stop in distinguishing the fine from the coarse
elements of the soil, the answer is that this can only be determined by
a common understanding among analysts. In general, it may be said, that
the mark of distinction should be the separation which can be secured
with a sieve having ten meshes per centimeter.
=85. Loose Soils.=—Having agreed upon a sieve of the above size, the
process of separation in loose soils is as follows: The earth is exposed
to the air and when the touch shows that it is sufficiently dry the
conglomerated particles should be simply divided without breaking the
rocky material which exists in a state of undivided fragments. There are
some special precautions to be taken. Rubbing in a mortar must be
forbidden since it reduces the earth to particles which are unnatural in
size, by securing the breaking up of the fragments consisting of the
débris of rocks. When it is possible the earth should be rubbed simply
in the hand and after having separated that which passes the sieve, the
large particles which have not passed should be again rubbed with the
hand, until all the particles which can be loosened by this simple
treatment have passed the sieve. The separation should be as complete as
possible in order that a sample of the particles passing the sieve
should represent as nearly as possible, a correct sample of the fine
particles of the soil.
In regard to the pebbles, they should be washed with water upon the
sieve in order to carry through the last of the particles of earth
adhering to them. They are then dried and their weight taken. The fine
part of the earth is also weighed. On an aliquot part, say 100 grams,
the moisture is determined and then by simple calculation the whole
sample of the air-dry soil can be calculated to the dry state. The
sample is then placed in a glass flask.
The pebbles are examined with a view of determining their mineralogical
constitution; as for instance, on being touched with a little
hydrochloric acid it can be determined whether or not they are carbonate
of lime. The nature of the rock from which they have been derived is
often to be determined by a simple inspection.
=86. Compact Soils.=—If the soils are not sufficiently loose to be
treated as before described, it is necessary to have recourse to other
means of division, which should not, however, be sufficiently energetic
to reduce the rocky elements to fine particles. For this purpose the
earth may be broken by means of a wooden mallet, striking it lightly and
separating the fine elements from time to time by sifting. A wooden
roller may also be used with a little pressure, for breaking up the
particles or a roller made out of a large glass bottle. These methods
will permit of a sufficiently fine division of the soil without breaking
up any of the pebbles. Sometimes, however, a soil can not be broken up
by such treatment. It is then necessary to have recourse to the
following process: The soil is thoroughly moistened and afterwards
rubbed up with water. The paste which is thus formed, is poured upon the
sieve and washed with a stream of water until all the fine particles are
removed. The wash water and the fine particles are left standing until
the silt is thoroughly deposited when the supernatant water is poured
off and the deposited moist earth is transferred into a large dish and
dried on a sand or water-bath. In this way a firm paste is formed which
can be worked up with the hand until rendered homogeneous and afterwards
an aliquot portion be taken to determine moisture.
=87. Method of Peligot.=—The method recommended by Peligot[66] for the
preparatory treatment of the sample is essentially that already
described. The sample is at first dried in the air and then in an oven
at 120°. When dry and friable 100 grams are placed in a mortar and
rubbed with a wooden pestle. It is then passed through a sieve of ten
meshes per centimeter. The largest particles which remain in the sieve
should have about the dimensions of a pin’s head. The stones are
separated by hand. They should be shaken with water in order to detach
any pulverulent particles adhering thereto. The turbid water resulting
from this treatment is added to that which is used in separating the
sand from the impalpable part of the soil.
=88. Wahnschaffe= prescribes[67] in the further preparation of the
sample for analysis that the coarse pieces up to the size of a walnut be
separated in the field where the sample is taken and their relative
weight and mineralogical character determined. The soil sample is then
to be placed in linen or strong paper bags and carefully labelled. In
order to avoid any danger of loss of label the description or number of
the sample should be put on the cloth or paper directly.
The sample when brought to the laboratory should be spread out to dry,
in a room free of dust. In the winter the room should be heated to the
usual temperature. The air drying should continue until there is no
sensible loss of weight. The samples then are to be placed in dry,
glass-stoppered glass bottles where they are kept until ready for
examination. This method of keeping the samples avoids contact with
ammonia or acid fumes with which a laboratory is often contaminated.
=89. The Swedish= chemists[68] direct that samples which are to be used
for chemical examination in the manner described below, are most
conveniently brought to such a condition of looseness and humidity that
the soil feels moist when pressed between the fingers without, however,
sticking to the skin. To prepare the sample in this manner, spread it in
a large porcelain dish or on a glass plate in a place where it is not
reached by the laboratory atmosphere; stir it frequently till it assumes
the mentioned humidity (if the sample when sent is too dry, moisten it
with distilled water till its condition is as indicated); then pulverize
carefully between the fingers and finally sift through a sieve with five
millimeter holes. In this way free the sample from stones, undecayed
roots and similar parts of plants, pieces of wood, and other matter
strange to the soil, which remain on the sieve; mix the sample carefully
and put it into a glass bottle provided with a stopper well ground in;
keep it in a cool place. Samples prepared in this way will usually
contain 20–30 per cent moisture; boggy soils 60–80 per cent and peat
soils 50 per cent.
=90. Petermann=[69] follows the method below in preparing samples of
soil for analysis.
The soil is gently broken up by a soft pestle and all débris if of
organic nature, cut fine with scissors. About 2500 grams of this soil
are passed through a one millimeter mesh sieve. The organic débris is
removed by forceps, washed free of adhering earth dried at 120° and
weighed. The nature of the organic débris should be noted as carefully
as possible.
The pebbles and mineral débris not passing the sieve are worked in a
large quantity of water by decantation. They are also dried at 120° and
weighed. This débris is examined mineralogically and thus some idea of
the origin of the soil obtained.
=91. The= various methods for the preliminary treatment as practiced by
the best authorities have been somewhat fully set forth in the foregoing
résumé. The common object of all these procedures is to get the soil
into a proper shape for further physical and chemical examination and to
determine the comparative weights of foreign bodies contained therein.
The essential conditions to be observed are the proper sifting of the
material and avoidance of mechanical communition of the solid particles
too large to pass the meshes of the sieve. If possible the material
should be passed through a sieve of one millimeter mesh. In cases where
this is impracticable a larger mesh may be used, but as small as will
secure the necessary separation. Before final chemical analysis a half
millimeter mesh sieve should be employed if the soil be of a nature
which will permit its use. Over-heating of the sample should be avoided.
Rapid drying is advisable when the samples are to be examined for
nitrates.
The method recommended by the French commissions seems well adapted to
the general treatment of samples, but the analyst must be guided by
circumstances in any particular soil.
AUTHORITIES CITED IN PART SECOND.
Footnote 39:
Bulletin 38, pp. 61–2.
Footnote 40:
Ms. communication to author.
Footnote 41:
Bulletin No. 10.
Footnote 42:
Landwirtschaftliche Versuchs-Stationen, Band 38, Ss. 309 et seq.
Footnote 43:
Annales de la Science Agronomique, Tome 1, Part 2, p. 240.
Footnote 44:
Agricultural Chemical Analysis, p. 166.
Footnote 45:
Zeitschrift für analytische Chemie, Band 3, S. 87.
Footnote 46:
Anleitung zur Wissenschaftlichen Bodenuntersuchung, S. 17.
Footnote 47:
Traité de Chimie Analytique, p. 149.
Footnote 48:
Bulletin 35, p. 108.
Footnote 49:
Op. cit.
Footnote 50:
Bulletin 10, p. 33.
Footnote 51:
Analyse des Matières Agricoles, p. 131.
Footnote 52:
Bulletin 38, p. 200.
Footnote 53:
This in some instances would include a part of the subsoil.
Footnote 54:
All soils do not become friable on drying.
Footnote 55:
Journal of the Royal Agricultural Society, (2), Vol. 25, p. 12.
Footnote 56:
Annales de la Science Agronomique, Tome 1, Part Second, pp. 240 et
seq. The personnel of the commission is as follows: MM. Risler,
Grandeau, Joulie, Schloesing, and Müntz.
Footnote 57:
Vid. 7.
Footnote 58:
Anleitung zur Wissenschaftlichen Bodenuntersuchung, S. 17.
Footnote 59:
Untersuchung Landwirtschaftlich und Gewerblich Wichtiger Stoffe, S. 5.
Footnote 60:
Seventh Annual Report of the Wisconsin Agricultural Experiment
Station, p. 161.
Footnote 61:
Deutsche Landwirtschaftliche Presse, Band 19, No. 35, Ss. 383–4.
Footnote 62:
Journal American Chemical Society, Vol. 16, p. 36.
Footnote 63:
Agricultural Chemical Analysis, p. 168.
Footnote 64:
Vid. 7.
Footnote 65:
Vid. 16.
Footnote 66:
Vid. 9.
Footnote 67:
Vid. 8, p. 19.
Footnote 68:
Methods of Analysis of Soils, Fertilizers, etc., adopted by the
Swedish Agricultural Chemists, translated for the author by F. W.
Woll.
Footnote 69:
L’Analyse du Sol, p. 14.
PART THIRD.
PHYSICAL PROPERTIES OF SOILS.
=92. The Soil as a Mass.=—The soil constituted as indicated in the
preceding pages, is now brought to the analyst for investigation. The
properties with which he first becomes acquainted are those which
impress his senses as mass characteristics. There is a perception of
color, consistence, weight and other features which the soil possesses
as a whole. The several constituents of the soil must first be
considered as molecular and mole aggregates. In other words, the soil in
its natural state is a mechanical mixture of particles which must first
be considered as a whole. The physical properties of the soil,
therefore, should engage the attention of the analyst before he proceeds
to the investigation of the properties of its several constituents as
classified by the relative size or hydraulic value of the particles of
which they are composed, or to a chemical determination of the compounds
or elements therein contained.
DETERMINATION OF PHYSICAL PROPERTIES.
=93. Color.=—The color of a soil depends chiefly upon the proportion of
organic matter and iron compounds which it contains and the state of
subdivision of its particles. When a soil contains a large amount of
organic matter, especially when this organic matter is in an advanced
state of decay, it assumes more or less a black color when moist. This
black color is to be distinguished from the black alkali tint which is
produced by the action of carbonate of soda on organic matter. The
naturally black color of a soil containing a large amount of organic
matter depends, however, either upon the action of mineral matters upon
this organic matter, as in the case of the black alkali mentioned, or
upon the blackish color of carbon resulting from the slow combustion of
the organic matter during the period of decay.
The presence of a large amount of ferric oxid in soil gives the
well-known red color so well-marked in the soils of southwestern
Kentucky and other portions of the United States. The preponderance of
sand in a soil tends to produce a light yellow or whitish tint, while
certain kinds of clay have a bluish tint probably due to the presence of
ferrous salts. The influence of the color of the soil upon the color of
the vegetation is also well-marked, the black soils as a rule producing
a much deeper green tint of foliage than the light colored soils. This
effect should not be attributed to color alone for as a matter of fact
highly colored soils are usually very close and very retentive of
moisture, which is one reason, probably, for their not being more highly
oxidized. Such soils will produce a more vigorous and ranker growth of
vegetation, but it is the texture of the soil and the more moist
condition which it maintains, rather than the color, which produce the
deeper green tint of foliage.
The color of a soil is also used as an index of its fertility, the black
and red soils being usually the most fertile.
It may be well to add here the probable reason as given by Whitney for
this, _viz._, that the deeper color shows that the oxids of iron and the
organic compounds have less oxygen and indicate that the soils are quite
retentive of moisture and rather tend to the exclusion of air, so that
part of the oxygen of the iron compounds and of the organic matters has
been used up in the oxidation processes within the soil. It is known,
for example, that wood oxidizes much more rapidly around a rusty nail
than where it is simply exposed to the air, the iron oxid acting as a
carrier between the oxygen of the air and the organic matter. In a sandy
soil, on the contrary, where there is usually less moisture and much
freer circulation of air, the iron compounds have more oxygen and
usually have a light yellow color. If this sand is heated, however, with
the exclusion of air, and especially in the presence of organic matters,
part of this oxygen will be given off and there will be the same red
color as in the heavier clay soils. It is frequently noticed, also, in
compact clays that where air gains access through cracks or root-holes,
the color is altogether modified.
=94. Determination of Color.=—There is no process which will give
experimentally and accurately the color of a soil sample. The changes
which the color of a soil undergoes in passing from a saturated to an
anhydrous state are well-marked. The analyst will have to be content
with giving as nearly as possible a description of the color of the
sample when taken and the changes which it undergoes in air drying or on
heating in a bath to 100°–110°, or in heating to redness with or without
exclusion of the air. These changes in color will give some indication
of the character of the organic and mineral matters present.
=95. Odoriferous Matters in Soil.=—It is known that the soil emits a
peculiar odor which is not disagreeable except when it has been recently
wet, for instance, after a short rain. Several attempts have been made
to discover the nature of this odor. These researches have established
the fact that the essential principle of this odor resides in an organic
compound of a neutral nature of the aromatic family and which is carried
by the vapor of water after the manner of a body possessing a feeble
tension. The odor is penetrating, almost piquant, and analogous to that
of camphorated and quite distinct from other known substances. In regard
to the quantity of this substance, it is extremely minute and can be
regarded as being only a few millionths of a per cent.
According to Berthelot and André[70] this new principle is neither an
acid nor an alkali nor even a normal aldehyd. It is, in a concentrated
aqueous solution, precipitable by potassium carbonate with the
production of a resinous substance. Heated with potash it develops a
sharp odor similar to the aldehyde resin. It does not reduce the
ammoniacal nitrate of silver. Treated with potash and iodin it gives an
abundant formation of iodoform, which, however, is a property common to
a great number of substances. For the qualitative and quantitative
estimation of the odoriferous matter the following process is employed:
About three kilograms of the soil are mixed with sand containing a small
amount of carbonate of lime and some humic substance; after having freed
it from all organic débris which is visible, it is placed in a glass
alembic. The soil should contain from ten to twelve per cent of water at
least. The alembic is placed in a sand bath and is kept at 60° for
several hours. The water evaporated is condensed until about
seventy-five cubic centimeters are distilled over. This distilled water
is again rectified so as to obtain in all about twenty cubic
centimeters. The odoriferous matter appears to be nearly all contained
in this twenty cubic centimeters. The liquid thus obtained shows an
alkaline reaction; it contains some ammonia and reduces ammoniacal
silver nitrate. This last reaction is due to some pyridic alkali or
analogue thereof, and is cause for it to be distilled anew with a trace
of sulfuric acid which gives a neutral liquor deprived of all reducing
action but which preserves the odor peculiar to the soil. The twenty
cubic centimeters obtained as before are subjected to two additional
distillations and in the final one only one cubic centimeter of liquid
is distilled over.
The peculiar odor is intensified proportionately as the volume of the
liquid is decreased. To this one cubic centimeter, is added some pure
crystallized potassium carbonate. The liquor is immediately troubled and
some hours are required for it to become clear again. Meanwhile there is
formed upon its surface a resinous ring almost invisible, amounting at
most to from ten to twenty milligrams of a matter which has not been
identified with any known principle. The reactions described above,
however, permit of its general character being known. This resinous
matter contains the odoriferous principle, the composition of which is
not yet definitely known.
=96. Specific Gravity.=—The density of a soil depends on its
composition, the fineness of its particles and upon the packing which it
has received. It has in other words an apparent and a real specific
gravity. It is easy to see that a soil in good tilth would weigh less
per cubic foot than one which had been pressed closely together, as in a
road or well-pastured field. Ordinary soils in good tilth have an
apparent specific gravity of about 1.2, and when entirely free from air,
a real specific gravity of about 2.5. If the apparent specific gravity
of a soil sample were 1.2 and the air were removed, leaving a vacuum in
the interstices of the soil, the apparent specific gravity would not be
sensibly increased. The figure 1.2 is the apparent specific gravity of a
mixture of soil material which is about 2½ times heavier than water, and
of an extremely small proportion by weight of air which is about 1000
times lighter than water. The figure 2.5 is about the true specific
gravity of the real soil material, and shows that this material is about
2½ times heavier than an equal volume of water.
The weights of a cubic foot of different kinds of soil as given by
Schübler[71] are as follows;
Pounds.
Sand 110
Sand and clay 96
Common arable soil 80 to 90
Heavy clay 75
Vegetable mold 78
Peat 30 to 50
In general the specific gravity of soil decreases inversely as its
content of humus.
=97. Determination of Specific Gravity.=—The ordinary method of
proceeding to determine the true specific gravity is by means of a
pyknometer. The pyknometer should have a capacity of from twenty-five to
fifty cubic centimeters.
From ten to fifteen grams of earth dried to constant weight at 100° are
taken, boiled for a time with a few cubic centimeters of water to remove
air and poured into the pyknometer. All soil particles are washed out of
the vessel in which the boiling took place into the pyknometer with
freshly boiled distilled water and after cooling to the temperature at
which the calibration took place, the pyknometer is filled with
distilled water at the given temperature and weighed. If the soil
contain materials soluble in water, alcohol of definitely known specific
gravity may be employed and the number thus obtained calculated to a
water basis.
The calculations when water is used are made as follows:
Grams.
Weight of pyknometer 13.4789
„ „ pyknometer full of distilled water at 20° 62.8934
„ „ water in pyknometer 49.4145
„ „ dry soil taken 10.0000
„ „ pyknometer + dry soil + filled with water at 20° 67.9834
„ „ soil and water 54.5045
„ „ water 44.5045
„ „ water displaced by ten grams soil 4.9100
Then specific gravity = 10.000 ÷ 4.9100 = 2.04.
=98. Specific Gravity of Undried Soils.=—It is often desirable to
determine the specific gravity of an undried portion of the soil. For
this purpose a portion of the sample is dried at 100° to determine its
percentage of moisture. The specific gravity is then determined on a ten
gram sample of the undried soil as just given. The actual weight of soil
taken is calculated from the percentage of moisture obtained in the
first instance. In the case given if the percentage of moisture at 100°
be ten then the actual weight of dry soil taken is nine grams. This
number is therefore used in making the calculations. In all statements
of specific gravity taken in the manner described the temperature at
which the pyknometer is calibrated should be stated and all weighings
where water is involved made at that degree.
=99. Volume of Soil.=—If it be desired to calculate the volume occupied
by a soil it is easily done by dividing the weight of water displaced by
the weight of one cubic centimeter of water of the temperature at which
the determination took place.
In the case given one cubic centimeter of water at 20° weighs 0.998259.
Then 4.9100 ÷ 0.998259 = 4.9186 cubic centimeters = volume occupied by
ten grams of dry soil excluding interstitial spaces between particles.
=100. Volumetric Methods.=—The water displaced by a given weight of soil
may also be measured volumetrically by the method of Knop.[72]
Place 200 grams of the soil in a flask of from three to five hundred
cubic centimeters capacity. Add a measured quantity of water, and shake
thoroughly to eliminate air, and fill up to the mark from a burette. The
quantity of water required to complete the volume subtracted from the
number expressing the volume of the flask will give the volume of water
displaced by the earth.
Another method consists in thoroughly shaking about thirty grams of the
soil in a graduated cylinder with fifty cubic centimeters of water
containing a little ammonium chlorid and after twenty-four hours
recording the volume occupied by the whole. The increase in volume over
fifty cubic centimeters shows the quantity of water displaced. This
method may also be used to determine the volume occupied by a soil when
saturated with water. The above methods are only to be used when
approximately correct results are all that are desired.
=101. Apparent Specific Gravity.=—The apparent specific gravity of a
soil is obtained by dividing its volume, interstitial spaces included,
by the weight of an equal volume of water.
The real and apparent specific gravities of six samples of soil are
given below.[73]
Real specific gravity 2.5445, 2.6315, 2.6508, 2.6400, 2.7325, 2.6603
Apparent specific
gravity of air-dried
soil 1.0940, 1.1710, 1.3570, 1.2810, 1.4060, 1.2730
Apparent specific
gravity of soil dried
at 125° 1.0990, 1.1770, 1.3750, 1.2910, 1.4640, 1.2850
It is to be noted that in computing the apparent specific gravity of a
soil dried at 125° the volume occupied by the water is assumed to occupy
the same space as if it existed in a free state. The volume of this
water is therefore to be subtracted from the contents of the flask
before proceeding with the computations.
=102. Determination of Apparent Specific Gravity.=—Place in small
quantity portions of the air-dried sample properly prepared, into an
open glass cylinder, holding one liter, and about 170 millimeters high
(if the height is exactly the mentioned one, the diameter of the
cylinder will be 86.6 millimeters); pack the sample by striking the
bottom of the cylinder hard against the palm of the hand after each new
filling; close the cylinder thus filled by a glass plate and weigh on a
balance sensitive to 0.1 gram; deduct the weight of the cylinder and
glass plate, and the weight of one liter of soil in approximately
similar conditions as it is found on the dry land prepared for
cultivation, is thus ascertained. The weight of one liter of the soil in
grams multiplied by 2000 will give in kilograms the weight of the
surface soil from a hectare (2.47 acres) of the field from which the
sample is taken when the depth of this is calculated at twenty
centimeters.[74]
RELATION OF THE SOIL TO HEAT.
=103. Sources of Soil Heat.=—The heat of the soil comes from three
sources, _viz._: solar heat, as the sun’s rays, heat of chemical and
vital action within the soil, and the original heat of the earth’s
interior. The latter is sensibly a constant quantity, and of great value
to plants. The heat of chemical and vital action is not great in amount
except in a few special cases but is often, as in germination, of the
greatest importance to plant growth. The sun, therefore, remains the
greatest source of heat of practical importance in relation to the
production of crops. Dark-colored soils, absorbing most and radiating
the fewest rays, must attain the highest temperature. Schübler’s
classical researches on soil temperatures, show that there is at times a
difference of over 7° in temperature between white and black soils, all
other conditions being alike. Schübler’s researches, being made on dry
soils in the laboratory, do not, however, apply wholly to conditions in
the field.
=104. Influence of Specific Heat.=—The heat which a soil receives and
retains is largely due to the specific heat of the soil. The specific
heat of a body is expressed by a number which shows the amount of heat
necessary to raise a given weight of the body 1° of temperature, as
compared with the amount necessary to raise the same weight of water 1°.
The specific heat of the soil is usually between 0.20 and 0.25, while
that of water taken as the standard is unity.
=105. Influence of Moisture.=—The moisture of the soil possesses great
influence on the soil temperature, so much so that a dry, light-colored
soil may attain a greater degree of warmth than a moist, dark-colored
one. The action of water in reducing soil temperature is easily
explained. In our latitude, we see the water in all its forms, solid,
liquid, and gaseous, and we know that these forms are the direct result
of temperature. The changing of water from the solid to the liquid or
gaseous form is performed at the expense of heat; the more water
evaporated from the soil the more heat must be used for the evaporation.
Therefore, the more water contained in the soil at any given time the
lower must be its temperature during subsequent exposure to sun heat
because of the greater evaporation. The experiments of Liebenberg,
Pattner, Schübler and Dickenson have practically settled all the
questions of soil temperatures. The radiation of heat from the soil, and
the consequent cooling propensity of the latter, are directly
proportional to the absorptive power of the soil. Two soils of like
absorptive power towards heat possess, as a rule, equal radiating power.
In a general way, it can be said the greater the heating capacity and
conductivity of a soil the more readily and rapidly does it give off its
heat and become cooled.
=106. Absorption of Solar Heat.=—The quantity of heat absorbed from the
sun by the earth is an important factor in the growth of vegetation. As
has been established in the physics of heat, a black surface, other
things being equal, will absorb a larger amount of heat than one of any
other color; so, other things being equal in the physical and chemical
composition of a soil, variations in the amount of organic matter
producing greater or less black coloration will affect the heat
absorption. Thus, black soils, in the conditions above mentioned, will
absorb more heat than lighter colored soils. As a result, the vegetation
in such soils gets an earlier start in the Spring and matures more
rapidly. As an illustration of this it may be noted that the black
prairie soils of Iowa produce uniformly crops of maize which are matured
before the early frosts, while crops grown on lighter soils much farther
South often suffer injury from that source.
DETERMINATION OF SPECIFIC HEAT.
=107. General Principles.=—The quantity of heat stored in any given
weight of soil is capable of being measured and compared with the
quantity stored in an equal weight of water at the same temperature. The
ease, however, with which disturbing influences operate during the
determination makes the manipulation somewhat difficult. The specific
heat of the containing vessels must be carefully determined. Fortunately
this has been done for most materials and the data thus obtained are
recorded in standard works on physics. The material operated on must be
protected from thermal influences from sources not controlled by the
experiment and even the heat of the operator’s body may often disturb
the conduct of the work. The general conditions which should control the
experiment as well as the details thereof are given in the following
method which, however, the ingenious analyst may profitably simplify.
=108. Method of Pfaundler.=—The process of estimating the specific heat
of soils by the method of mixture, is essentially that of Regnault and
is described as follows by Pfaundler[75].
The apparatus used is illustrated in Fig. 13.
A and A′ show the heating apparatus. It consists of a vessel of sheet
iron in which a test tube E is fixed by means of a cork. The test tube
holds the soil whose specific heat is to be determined. The apparatus
contains water, which is brought to the boiling point by means of a
lamp, and the excess of steam is conducted away, as indicated in the
figure, through one of the axes of the apparatus; the opposite axis is,
of course, closed. It requires about thirty-five minutes boiling to
bring the contents of the test tube to the temperature of the aqueous
vapor. The exact temperature at which the water boils is determined by
observing the barometer at the time and consulting a table of the
boiling temperature of water at different barometric pressures.
The calorimeter is shown in the figures B and B′. It consists of a
wooden box closed on one side by a glass plate G and on the other to the
heighth F by a small board on which a calorimeter of ordinary
construction is placed. The cylinder of the calorimeter is seventy
millimeters high and forty-seven millimeters in diameter.
[Illustration:
FIGURE 13.
REGNAULT’S APPARATUS FOR DETERMINING THE SPECIFIC HEAT OF SOILS.
]
This part of the apparatus is supported by triangular pieces of cork. A
delicate thermometer is fastened to the top of the box of the
calorimeter and the value of the degrees is so arranged that about
twelve of them correspond to about one degree C. The scale of the
instrument can be arbitrarily fixed and the temperature of any part of
it determined by comparison with a delicately graduated thermometer.
Near the thermometer in the calorimeter is a stirrer made of a very thin
copper disk with a bent rim. This stirrer is operated by means of a silk
cord moved by appropriate machinery.
The reading of the thermometer is made through a glass plate and this
should be protected from the heat of the body of the observer by a paper
screen.
The test tube E is first filled with the substance, whose specific heat
is to be determined, and weighed. It is then placed in the water bath
until constant weight is reached. After constant weight has been
obtained the apparatus is again dried and the exact weight of the
moisture lost thus determined. The test tube is then placed in the
apparatus A closed with a well-fitted cork, the top covered with cotton
and heated in the aqueous vapor for about one hour. The heating
apparatus should be far removed from the calorimeter so that the
temperature of the latter cannot be influenced thereby. Meanwhile the
calorimeter is filled with water which has stood in the room for a long
time until it has acquired, as nearly as possible, the room temperature.
The quantity of water is such that the water value of the whole of the
calorimeter together with the immersed portions of the thermometer and
stirrer shall amount to exactly 100 grams. A few minutes before bringing
the substance into the calorimeter, the stirring apparatus is put in
motion and the temperature observations are commenced. These should be
at intervals of twenty seconds and should be continued until ten
observations have been made. Meanwhile the height of the barometer is
also read. A few seconds before the tenth interval the apparatus A is
brought quickly to the calorimeter and its contents emptied into it at
the moment of the tenth interval. The apparatus A should be removed as
quickly as possible after its contents are emptied.
After the introduction of the substance and its thorough incorporation
with the water of the calorimeter by the stirring apparatus, the
thermometer is again read, at intervals of twenty seconds, until its
maximum has been reached and as much longer thereafter as may be
necessary to show that an appreciable fall of temperature has taken
place. The test tube, in which the substance was heated is weighed and
the exact quantity of the added substance thus determined.
In order that the sample of soil may be easily removed from the test
tube in which it is heated, it is best to have it molded into
appropriate forms before being placed in the heating tube. This is
easily accomplished by pressing it into molds of convenient shape and of
a size so that six or eight pieces (best of cylindrical shape) will be
necessary to give the quantity sufficient for the experiment. Since some
soils will not retain their shape after molding, the molds may be made
of zinc foil whose water values in the calorimeter are previously
determined and they can be placed with their contents in the calorimeter
thus securing the total immersion of all the particles of soil in the
water. With very dusty materials, it is necessary that these little
cylinders should be closed with pieces of foil at the ends in order to
prevent the particles of dust from escaping and rising to the surface of
the water.
Another source of error consists in the solution of the soluble salts
which the soil may contain. This is avoided by the use of turpentine
instead of water. If the cylinder containing the soil be made
water-tight, this danger from the solubility of the salts in water is
avoided. Another method of correcting these errors is in making a blank
experiment in which a quantity of the earth taken is kept at the
temperature of the water in the calorimeter until both are of the same
temperature. The earth is then mixed with the water and the change of
temperature produced noted. In this way the corrections made necessary
by the solution of the salts in water and other causes are determined.
=109. Method of Calculating Results.=—Let t represent the mean
temperature of the beginning period of the experiment, and v equal the
loss in heat per interval. Let t′ and v′ represent the same values for
the end period. Let θ₁, θ₂, θ₃, etc., represent the temperature at the
end of the first, second and third intervals of the middle period and θ₀
the temperature at the beginning of the middle period and θₙ the end
temperature of of the middle period. Let τ₁, τ₂, τ₃, ... τₙ, represent
the mean temperature of the single intervals; then τ₁ = (Θ₀ + Θ₁)/(2);
τ₂ = (Θ₁ + Θ₂)/(2), and τₙ = (Θ_{n–1} + Θₙ)/(2). The constant C
represents the correction which must be applied in order to determine
the true increase of temperature in the calorimetric system. The
expression θₙ − θ₀ + C represents the true temperature increase of the
calorimetric system which we may represent by Δθ and θₙ + C represents
the true maximum, that is, the end temperature, which by exclusion of
external influences is reached. The correction C, as already indicated,
is to be added to θₙ − θ₀ when it is positive and is to be subtracted
therefrom when it is negative. The numerical value of C is usually very
small, and, in the experiments indicated, varied between zero and one
division of the thermometer employed, that is it seldom exceeded one
degree.
=110. Illustration.=—The method of determining value of specific heat is
best illustrated by an example:
In one determination the water value of the calorimetric system,
including stirrer and thermometer was 2.50 grams, the weight of water
added was 97.50 grams and the total water value of the system 100 grams.
The substance was dried at 100° and weighed in five envelopes:
Total weight 31.423 grams.
The envelopes alone weighed 10.654 „
Weight of the soil taken 20.769 „
The envelopes holding the soil were made of brass with zinc ends, the
specific heat of which is 0.0939 and the water value of the whole of the
envelopes was 1.0004 grams. Since, however, the ends were soldered on
with zinc the true water value was somewhat smaller being equal to
0.8692 gram. The data of the observations were as follows:
Corrected height of barometer 699.6 millimeters.
Intervals between the observations 20 seconds.
No. of Temperature on the
Observations. arbitrary scale of the
thermometer.
First Period { 0 162°.6
„ {10 162°.9 = θ₀ (Moment of
immersion.)
Second Period {11 185°.0
„ {12 200°.0
„ {13 206°.1
„ {14 209°.5
„ {15 210°.7
„ {16 211°.3
„ {17 211°.5 Differences.
„ {18 211°.5 0
„ {19 211°.5 0
„ {20 211°.5 0
„ {21 211°.5 0
„ {22 211°.4 –0°.1
= θₙ
–0°.1
–0°.1
Third Period {23 211°.3
„ {24 211°.2 –0°.1
„ {25 211°.1 –0°.1
„ {26 211°.0 –0°.1
„ {27 210°.9 –0°.1
„ {28 210°.8 –0°.1
„ {29 210°.6 –0°.2
„ {30 210°.5 –0°.1
From the twenty-second interval, the regular fall of temperature begins
and 211°.4 is therefore taken as θₙ. The mean temperature of the
beginning period is therefore (162°.6 + 162°.9)/(2) = 162°.75 = t. The
value of v is (162°.6 − 162°.9)/(10) = –0°.03. For the end period the
value of t′ is (211°.4 + 210°.5)/(2) = 210°.95 and the value of v′ is
(211.4 − 210.5)/(8) = + 0.11. Then the sum of the observations from
eleven to twenty-one inclusive = Σ′_{n–1}θ = 2280.1
(θ₀ + θₙ)/(2) = 187.15
The sum = 2467.25
nt = 1953.00
Difference 514.25
This difference multiplied by v − v′ = 0.14
gives a product equal to 71.995
This product divided by t′ − t = 48.20
gives a quotient equal to 1.49
nv = –0.36
The sum = 1.13 = C
Then Δθ = θₙ − θ₀ + C = 211°.4 − 162°.9 + 1°.13 = 49°.63. The true end
temperature = θₙ + C = 212°.53. The zero point of the thermometer =
24°.70, and the actual rise of temperature = 187°.83. The rise of
temperature due to the proximity of the warming apparatus at the
beginning was found by experiment to be equal to 0°.1 of the division of
the scale. On comparing the thermometer used with a standard centigrade
scale it was found that one division of the calorimetric thermometer was
equal to 0°.0858. Converting these numbers into expressions of the
centigrade scale we have the following summary:
The true rise of temperature, Δθ = 4°.25
The true end temperature, θₙ + C = 16°.10
The temperature of the steam, as determined by the height of the
barometer, was equal to 97°.70
From these data the specific heat is calculated according to the
following formula:
Σ = 1/20.769 × ((100 × 4.25)/(97.70 − 16.10) − 0.8692) = 0.2089.
From this formula the following rule for calculating specific heat is
deduced:
Multiply the water value of the calorimetric system by the true rise in
temperature in degrees Celsius and divide the product by the difference
between the temperature of boiling water under the conditions of the
experiment and the true end temperature. From the quotient subtract the
water value of the envelopes holding the soil sample. Divide the
remainder by the weight of soil taken.
=111. Variations in Specific Heat.=—Different soils deport themselves
very differently in respect of specific heat. In a large number of soils
examined by Pfaundler, the specific heats were found to vary from 0.19
to 0.51. The highest specific heat was observed in the case of a peaty
soil. Next to peaty soils came those soils which were highest in humus,
and in general it was found that the specific heat varied directly with
the humus content.
SOIL THERMOMETRY.
=112. General Principles.=—The measurement of the temperature of the
soil at stated depths is often of use in analytical processes connected
with agricultural chemistry and physics. The general principles on which
the process rests, depend on bringing the bulb of the thermometer into
as intimate contact as possible with the particles of soil at the depth
required, disturbing as little as possible the normal state of the soil
particles.
In the thermometer chiefly used for this purpose in this country, the
stem is strong and carries the degrees figured on the glass. The whole
is inclosed in a wooden case which is cut away to expose the face of the
scale. The scale is about eleven inches long. The part which enters the
soil is of varying lengths, according to the depth at which the
temperature is desired.
=113. Method of Procedure.=—An excellent method of determining soil
temperatures and of recording results is well illustrated by Frear.[76]
The thermometers are set in niches cut in a trench, the earth being
afterwards carefully tamped about the bulbs to secure a good contact,
the trench being filled at the same time. The surface of the soil is
freed from vegetation and kept in good tilth.
The depths at which observations are made are at the surface and one,
three, six, twelve, and twenty-four inches. The soil tested was
moderately dark and loamy to a depth of seven inches and below that a
stiff clay. Solid rock existed at from five to seven feet below the
surface. Readings were made three times a day.
=114. Method of Stating Results.=—The individual readings of the
thermometers should be entered at the time they are made. At the end of
each month the mean of the readings should be determined, together with
the maxima and minima, and a comparison made between the mean readings
of the temperature of the air and maxima and minima. As a sample of the
method of stating these mean results the data are given for the month of
May, 1891, for the atmosphere, surface, and for the depths mentioned
above:
MAY.
T° Fahrenheit.
ATMOSPHERE.
Monthly mean 57.1
Maximum 85.0
Minimum 31.0
Mean daily range 22.5
Greatest daily range 32.0
Least daily range 8.0
SURFACE.
Monthly mean 56.7
_Extremes._
Maximum (10th of month) 77.0
Minimum (5th) 36.0
Mean maximum 65.2
Mean minimum 49.9
_Range._
Monthly 41.0
Mean daily 14.9
Greatest daily (19th) 25.0
Least daily (21st) 4.0
ONE INCH.
Monthly mean 56.8
_Extremes._
Maximum (10th) 74.5
Minimum (5th) 36.5
Mean maximum 62.9
Mean minimum 49.5
_Range._
Monthly 38.0
Mean daily 11.9
Greatest daily (10 and 19) 20.0
Least daily (23rd) 1.0
THREE INCHES.
Monthly mean 56.7
_Extremes._
Maximum (31st) 71.0
Minimum (6th) 40.0
Mean maximum 60.9
Mean minimum 49.7
_Range._
Monthly 31.0
Mean daily 9.3
Greatest daily (19th) 15.5
Least daily (23rd) 1.5
SIX INCHES.
Monthly mean 56.3
_Extremes._
Maximum (31st) 66.0
Minimum (6th and 7th) 43.0
Mean maximum 56.7
Mean minimum 53.2
_Range._
Monthly 23.0
Mean daily 4.65
Greatest daily (8 and 19) 8.5
Least Daily (5th) 1.0
TWELVE INCHES.
Monthly mean 55.6
_Extremes._
Maximum (31st) 64.0
Minimum (6th and 7th) 46.0
Mean maximum 56.6
Mean minimum 54.4
_Range._
Monthly 18.0
Mean daily 2.18
Greatest daily (8th) 4.5
Least daily (3rd and 20th) 0.0
TWENTY-FOUR INCHES.
Monthly mean 53.1
_Extremes._
Maximum (31st) 58.0
Minimum (6th and 8th) 48.0
Mean maximum 53.4
Mean minimum 52.8
_Range._
Monthly 10.0
Mean daily 0.48
Greatest daily (23rd) 2.0
Least daily (on 12 days) 0.0
[Illustration:
FIG. 14. SOIL THERMOMETER—Whitney and Marvin.
]
=115. Method of Whitney and Marvin.=[77]—The thermometer devised by
Whitney and Marvin is shown in Fig. 14. The principle on which this
modification depends is as follows:
A mercurial thermometer of the ordinary construction is liable to give
wrong indications of the temperature because it is difficult to
determine the temperature of the column of mercury from the bulb to the
surface of the ground. To avoid this source of error the thermometer
figured was constructed.
The bulb of the thermometer is made quite small and a slender portion of
the stem extends into its spherical portion. The top portion of the
thermometer stem does not differ in any essential respect from the stem
of an ordinary thermometer.
The bulb is almost wholly filled with alcohol, which acts as the
principal thermometric fluid and has the advantages of a high
coefficient of expansion. The thermometer bulb and the stem of the
thermometer up to a point convenient for graduation, are filled with
mercury. In the drawing the mercury is represented by the heavy black
marking in and just above the small bulb. The peculiar construction at
this point is for the purpose of retaining the mercury about the point
of the slender capillary stem inside the bulb and preventing the
entrance of alcohol into the stem when the thermometer is horizontal.
In order to register the maximum and minimum temperatures a short column
of alcohol is placed in the upper portion of the stem, above the
mercury, and within this are arranged two small steel indexes, so
constructed that they will not slide in the tube of their own weight,
but are easily pushed upward by the mercury column or pulled downward by
the top meniscus of the alcohol column. The indexes are set by means of
a small magnet, the one being drawn down upon the top of the mercurial
column and the other raised up against the meniscus of the alcohol
column.
The rise of the mercury carries its index upward, leaving it to register
the highest point reached, while the alcohol meniscus withdraws the
other index and leaves it at a point representing the minimum
temperature. It remains only to mention that the graduations are fixed
in the usual way, having reference only to the positions of the
mercurial column. Beyond the highest point supposed to be reached by the
mercury, say about 120°, the graduations are extended in an arbitrary
manner. The scale numbers represent temperatures by the mercurial column
and are continued in regular sequence beyond the 120°. On this plan the
readings for minimum temperatures are on a purely arbitrary scale and
are converted into true degrees of temperature by use of a table
prepared for each thermometer, which table embodies as well all the
corrections for instrumental error.
The arrangement of the alcohol columns above the mercurial column and
the indexes are shown enlarged at one side of the illustration. The
readings of the maximum temperature are made from the bottom end of the
index next to the mercurial column. The minimum temperature is the
reading of the top of the uppermost index. Thus in the figure the
maximum temperature indicated is 76.5°, and the minimum 125.7°, which,
by reference to the table of correction for this thermometer, No. 10, is
found to be 53.3°.
The use of mercury in the stem of the thermometer not only admits of the
use of the index for registering the maximum temperature, but possesses
the additional advantage of reducing the error due to uncertain
temperature of the stem to about one-sixth what it would be if alcohol
were used. Moreover, if necessary, as in the case with thermometers for
greater depths than that figured, the ungraduated portion of the stem
can be made of very much finer bore than the graduated portion, the
effect of which is to diminish the objectionable error to a
comparatively unimportant quantity.
The chief objection to thermometers of this construction is the
liability of alcohol getting from the bulb into the stem during the
processes of construction, graduation and subsequent handling, and the
difficulty of safely shipping them.
When once set up, however, there seems to be little or no possibility of
derangement and the error common to mercurial thermometers due to rise
of the freezing point with age does not apply owing to the high
coefficient of expansion of the alcohol used in the bulb.
APPLICATIONS OF SOIL THERMOMETRY.
=116. Estimation of the Absorption of Heat by Soils.=—A cubical zinc
box, six centimeters square, is filled with the sifted air dried soil.
The box, one side of which is left open, is encased snugly in a wooden
cover, exposing only the open end, and placed for a few hours in the
direct rays of the sun. The temperature is then taken at a given depth.
The box may be provided with thermometers at different depths, the bulbs
thereof extending to the center. In this case the box should be covered
with thick felt instead of wood. The temperature of the layers of soils
of different depths can thus be read off directly. The air temperature
directly above the box should be accurately noted while the experiment
continues.
Any other kind of box well protected against all heat save the direct
sunlight on the open surface of the soil will answer as well as the one
described.
To determine the action of moist earth in similar conditions the soil
may be previously moistened; the per cent of moisture being determined
in a separate portion of the soil or the amount of water added to the
air-dried soil being noted.
=117. Estimation of the Conductivity of Soils for Heat.=—The bulb of a
thermometer is placed in the middle of a mass of fine earth which is
then exposed, best in a metallic box painted with lamp black, in a warm
place. The time required for the thermometer to reach a certain degree
is noted. By reversing the experiment and placing the mass of earth
heated to a given degree in a cool place the conductivity can be
determined by the time required for the mercury in the thermometer to
fall to any given point.
The experiment may also be made by packing the soil by gently jolting it
in a glass tube six to eight centimeters in diameter. One end of the
tube is closed with a piece of metal or fine wire gauze painted with
lamp black and is exposed to the source of heat. The bulb of a
thermometer is placed at a given distance from the end of the tube and
the time for the mercury to be affected observed.
COHESION AND ADHESION OF SOILS.
=118. Behavior of Soil After Wetting.=—The deportment of a soil when
thoroughly wet in respect of its physical state on drying out is a
matter of great practical concern to the agronomist. Some soils on
becoming dry fall into a pulverulent state and are easily brought into
proper tilth; others become hard and tenacious, breaking into clods and
resisting ordinary methods of pulverization. The physical laws which
determine these conditions depend largely on the principles of
flocculation soon to be described. The present task is to describe
briefly some of the methods of estimating the force of cohesion and
adhesion.
=119. General Method.=—The fine earth, air-dried, is mixed with enough
water to make a paste and molded into forms suitable for trial in a
machine for testing strength of cement, etc. The forms most used are
cakes three to five centimeters in length and one to two centimeters
thick. These are used for determining crushing power. For longitudinal
adhesion the paste may be molded in prismatic or cylindrical shape.[78]
The prisms should show one to two centimeters in cross section or the
cylinder be one to two centimeters in diameter. Before use they are to
be exposed for several days until thoroughly air-dried. The force
required to separate or crush these prepared pieces will measure the
adhesive or cohesive property of the sample. A great number of trials
should be made and the mean taken.
=120. Method of Heinrich.=[79]—This process consists in mixing the
air-dried earth with water until its aqueous content is fifty per cent
of the highest water capacity determined by experiment. The sample is
next placed between two pieces of sheet iron of ten centimeters square,
each of which in its middle point is provided with a hook. The thickness
of the layer between the two pieces of iron should be about five to ten
centimeters. The exuding particles of soil are cut off with a knife. The
upper piece of sheet iron is next suspended by a cord in such a way that
the iron piece occupies a horizontal position. A small basket is
attached to the lower surface and sand added thereto, little by little,
until the column of earth is separated. The sand basket and iron plate
are weighed, and the total weight gives the power necessary to separate
a column of soil ten centimeters square in cross section. The iron
plates may be roughened so that the adhesion thereto of the soil is
greater than its cohesive force.
=121. Adhesion of Soil to Wood, Iron, Etc.=—The adhesive power of moist
soil for wood, iron, etc., is measured by Heinrich[80] in the following
way: The soil is mixed with water, as above, until it contains just
fifty per cent of its total water-holding content. It is then placed in
a large vessel and the upper surface made as smooth as possible. A plate
of wood, iron, etc., of ten centimeters square is then pressed on the
surface until a complete contact is secured. This plate, by means of a
hook and cord passing over a pulley, is then subjected to stress by
weighting the cord which carries a basket for that purpose. The basket
should be of the same weight as the plate in contact with the soil. The
weight added to the basket necessary to separate the plate from the soil
is taken to represent the cohesive force. The author of the method
appears to take no account of the pressure of the air on the plate
caused by the exclusion of the air from its under surface.
THE ABSORPTIVE POWER OF SOILS FOR SALTS IN SOLUTION.
=122. General Principles.=[81]—It is a fact of every-day observation
that soils have a particular property of absorbing certain materials
with which they come in contact. If it were not for this property all
our wells would soon become unwholesome from the reception of decayed
animal and vegetable matter carried to them in the drainage water from
the surface. It is also a well-known fact that burying dead bodies
prevents the gaseous products of decomposition from reaching and
vitiating the atmosphere.
Besides this well-known power of soils to absorb the decomposition
products of animal and vegetable matter, they also possess a property
which is of far greater importance in plant economy; that is, the power
of withdrawing and retaining certain mineral constituents from their
solutions.
As far back as the sixteenth century mention is made by Lord Bacon of a
process for obtaining pure water on the seashore by simply digging a
hole in the sand and allowing it to fill with filtered sea water, which
by this means is deprived of its salt. Although certain facts were
observed by some of the earlier writers in regard to soil absorption, no
systematic researches were conducted with a view of demonstrating the
extent and cause of this power until within a comparatively few years.
In 1850 Prof. Way published in the _Journal of the Royal Agricultural
Society of England_, the results of a thorough and most excellent
investigation of the subject. Since then many distinguished chemists,
such as Henneberg, Stohmann, Peters, Heiden, Knop, Ullik, Pillitz,
Biedermann, Tuxen, and others have given their attention to this matter.
=123. Summary of Data.=—If a solution of a soluble sulfate, chloride or
nitrate of an alkali or an alkaline-earth metal be placed in contact
with a soil, the result is that the soil takes up a part of the base but
none of the acid. This absorption of base is attended with the
liberation of some other base from the soil which combines with the acid
of the solution. Any alkali or alkaline earth base has the power of
replacing any other such base. However, if soluble phosphates and
silicates of these bases be placed in contact with the soil both the
base and the acid are removed from the solution.
Peters[82] has shown that the amount of absorption depends upon the
concentration of the solution, the relation between the quantity of
solution and the soil and the kind of salt used. He treated 100 grams of
earth with 250 cubic centimeters of solutions of different potash salts
with the following results:
Strength of solution. ⅒ Normal. ¹⁄₂₀ Normal.
Grams Grams
Salt Used K₂O absorbed. K₂O absorbed.
KCl 0.3124 0.1990
K₂SO₄ 0.3362 0.2098
K₂CO₃ 0.5747 0.3154
Biedermann[83] proves that, for phosphoric acid at least, the absorption
increases with the temperature.
It has also been found that the amount of absorption depends upon the
time of contact between the soil and solution. Way found that the
absorption of ammonia was complete in half an hour, while Henneberg and
Stohmann[84] noticed that the phosphoric acid continued to be fixed
after the expiration of twenty-four hours.
It is a very important fact that the absorption of a base is never
complete; no matter how dilute the solution it will still carry a small
portion of the base with it. Peters states that it requires about 28,000
parts of water to remove one part of absorbed potash and Stohmann found
that it required about 10,000 parts of water to remove one part of
absorbed ammonia. With phosphoric acid, the resulting compound seems to
be much more insoluble.
According to Tuxen[85] the presence of salts of soda and potash in
solution decreases the power of a soil to absorb ammonia compounds and
the presence of sodium salts decreases the power of a soil to absorb
potash. On the other hand the presence of potassium compounds
considerably increases the absorption of phosphoric acid. He further
affirms that the compounds of potash, phosphoric acid, etc., formed in
the soil, are decidedly more soluble in sodium salts than in pure water.
=124. Cause of Absorption.=—The withdrawing and fixing of phosphoric
acid from solutions by the soil is not very difficult to understand as
this acid forms insoluble compounds of iron, lime, and magnesium, some
or all of which are present in all soils. As to the absorption of the
alkalies, the explanation is far more difficult as nearly all of their
ordinary compounds are readily soluble in water.
As lime is usually found combined with the acid part of an alkali salt,
from which the base has been absorbed by the soil, it might naturally be
supposed that the absorptive power of the soil would depend upon the
amount of lime present. Way found, however, that the addition of chalk
in no way influenced the absorption of ammonia by a soil which contained
but a small amount of lime. This fact was also confirmed by Knop[86] who
found that chalk exerted no influence on the absorption of ammonia
salts. These facts would seem to point to the conclusion that lime was
present in sufficient quantity in these experiments, or that it is not
essential to the phenomena of absorption. However, as any alkali or
alkaline-earth base can replace any other such base, the presence of
lime in the filtrate is probably more of an accidental occurrence, owing
to the comparatively large amount of that substance in most soils, than
a necessary condition, as any other base would doubtless answer in the
absence of lime.
=125. Warington=[87] has shown that hydrated oxides of iron and
aluminum, and especially the former, are capable of absorbing potash and
ammonia, and as more or less of these hydrates exist in nearly all
soils, a part, at least, of absorptive phenomena is to be ascribed to
them.
=126. Way= tried to determine which of the constituents of a soil
exercised chiefly the absorptive power. He passed a solution of ammonia
through tubes containing pure sand and found that it came through
apparently unaltered from the first, while a soil treated in the same
way removed the ammonia for a considerable time. He concluded from this
that the absorptive power does not exist in the sand. He next oxidized
the organic matter in a soil with nitric acid and then treated it with
ammonia in the same way. The first portions of the filtrate showed no
ammonia in any form, hence he concluded that organic matter is not
essential to the act of absorption. He further showed that clay alone is
capable of causing absorption phenomena, by treating powdered clay
tobacco pipes with ammonia.
Having shown that clay was the main constituent in a soil which caused
the absorption of alkalies, he tried next to trace out the particular
compound which caused the absorption. Having tried various natural
silicates he at last succeeded in producing a hydrated silicate of
aluminum and soda which exhibited displacement and absorptive properties
very similar to those shown by the soil.
As Way had succeeded in producing an artificial hydrated silicate
possessing absorptive properties, Eichorn[88] thought of trying natural
hydrated silicates or zeolites and found that they exhibited the same
power as Way’s artificial preparation. It has also been shown by
Biedermann,[89] Rautlenberg,[90] and Heiden[91] that the absorptive
power bears a close relation to the amount of soluble silicates present.
In view of these facts it is now generally accepted that the absorption
of salts of the alkalies, accompanied by the change of base, is due
chiefly to the presence of decomposed zeolite minerals in the soil.
Besides the purely chemical absorption of salts by the soil, we have a
physical absorption of various substances similar to the action of
charcoal when used as a filter.
=127. Conclusions of Armsby.=—The data connected with the absorption of
bases by a soil have also been reviewed by Armsby.[92] He shows that the
absorption is accompanied by a chemical reaction between the salt whose
base is absorbed and some constituent of the soil, and this change seems
to be due particularly to certain zeolitic silicates, although Liebig
and others were disposed to credit this absorption largely to physical
causes.
Knop advances the idea that the soil has the power of disintegrating
salts in the presence of some substances like calcium carbonate which
can unite with the acid. In experiments made with hydrous silicates it
was shown that the absorption resembled in all cases like phenomena in
the soil; hence the supposition already advanced in regard to the
influence of such silicates is doubtless true.
In respect of absorption in general, the following conclusions were
reached:
1. The absorption of combined bases by the soil consists in an exchange
of bases between the salt and the hydrous silicates of the soil.
2. This exchange, which is primarily chemical, is only partial, its
extent varying
(a) with the concentration of the solution, and
(b) with the ratio between the volume of the solution and the quality of
soil used.
3. The cause of these variations is probably the action of mass or the
tendency of resulting compounds to re-form the original bodies, the
absorption actually found in any case marking the point where the two
forces are in equilibrium.
=128. Selective Absorption of Potash.=—As a rule more potash is absorbed
from the sulfate than from the chlorid. This fact would seem to point to
the advisability of using sulfate as a fertilizer in preference to
chlorid. However, as with the exception of nitrates, the absorptive
power of a soil, for the salts used as fertilizers, is many times
greater than it is ever called upon to exert in fixing applied
fertilizers, we need not trouble ourselves in regard to the absorption
of phosphoric acid, potash or ammonia, in so far as the practical side
of the matter is concerned. For example, an acre of soil to the depth of
nine inches weighs about 900 tons. Now it has been found by Huston,[93]
that 100 parts of a soil experimented upon absorbed over 0.25 part of
P₂O₅, hence 900 parts would absorb over 2.25 parts of P₂O₅; or an acre
of this soil to the depth of nine inches would absorb over two and
one-fourth tons of phosphoric acid. 500 pounds per acre is a large
dressing of a phosphatic fertilizer for field crops and 500 pounds of a
high grade fertilizer would contain about 100 pounds of P₂O₅; hence the
power of such a soil to absorb phosphoric acid is more than forty-five
times as great as it is ever likely to be called upon to exert in fixing
the phosphoric acid added to it as a fertilizer.
Huston has further shown that an acre of soil nine inches deep will
absorb more than 2.7 tons of potash (K₂O) from potassium chlorid from
which salt less potash is absorbed than from the sulfate. Now one-tenth
ton of potassium chlorid per acre would be a large dressing of potash,
hence this soil possesses the power of absorbing more than twenty-seven
times as much potash as is ever likely to be applied as a fertilizer.
In like manner it may be shown that the power of an acre of soil nine
inches deep to absorb ammonia from ammonium sulfate is more than
thirty-two times as great as it would be called upon to exert in fixing
the ammonia from a dressing of one-quarter ton of ammonium sulfate per
acre.
With sodium nitrate, however, there is no absorption; hence great care
is necessary in the application of nitrogen as a nitrate, for, if it be
put on in large quantities, at a season when the plant is not prepared
to assimilate it, or during a period of heavy rains, there must
unavoidably result loss from drainage. The best time to apply a nitrate
is evidently during the active growing season.
=129. Whitney=[94] places great emphasis on the surface area of soil
particles in respect to their power to absorb solutions of salts. The
approximate surface area of a cubic foot of each of the different
typical soils of Maryland is as follows:
Pine barrens 23,940 square feet.
Truck lands 74,130 „ „
Tobacco lands 84,850 „ „
Wheat lands 94,540 „ „
River terrace 106,260 „ „
Limestone subsoil 202,600 „ „
It will be seen that there are about 24,000 square feet of surface area
in a cubic foot of the subsoil of the pine barrens, no less then 100,000
square feet or two and three-tenths acres of surface area in a cubic
foot of the subsoil of the river terrace, and 200,000 square feet of
surface area in a cubic foot of the limestone subsoil.
These figures seem vast, but they are probably below rather than above
the true values, on account of the wide range of the diameters of the
clay group. This great extent of surface and of surface attraction,
which has been described as potential, gives the soil great power to
absorb moisture from the air, and to absorb and hold back mineral
matters from solution. A smooth surface of glass will attract and hold,
by this surface attraction, an appreciable amount of moisture from the
surrounding air. A cubic foot of soil, having 100,000 square feet of
surface, should be able to attract and hold a considerably larger amount
of moisture.
It might have been added that if the potential of the surface,
separating the solution from the soil, be greater than the potential in
the interior of the liquid mass, there will be a tendency to concentrate
the liquid on this surface of separation. It has been shown that certain
fluids have greater density on a surface separating the fluid from a
solid. On the other hand, if the potential were low there might be no
tendency for this concentration, and even the reverse conditions would
prevail and the soluble substance could be readily washed out of the
soil.
=130. Removal of Organic Matters.=—It is probably largely due to this
straining power that organic matters are removed from solutions in
percolating through the soil. Whitney[95] has observed that the organic
matter may be coagulated and precipitated from solution by the soil
constituents, and held in the soil in loose flocculent masses, while the
liquid passes through nearly free of organic matter.
=131. Importance of Soil Absorption.=—The importance of the absorptive
power of the soil can hardly be overestimated. By means of this power
those mineral ingredients of plant food, of which most soils contain but
little, are held too closely to allow of rapid loss by drainage, and
still sufficiently available to answer the needs of vegetation, provided
the store is large enough. The only important plant food liable to be
deficient in the soil which does not come under the influence of
absorption is nitrogen in the form of salts of nitric acid, and nature
has made a wide provision for this element by binding it in the form of
organic bodies which nitrify but slowly, and by supplying each year a
small quantity from the atmosphere.
By means of the absorptive power of soils the farmer, if he puts on an
excess of potash or phosphoric acid as a fertilizer, does not lose it
but is able to reap some benefits from it in the next and even in
succeeding crops. If it were not for this power the best method for
applying fertilizers would be a much more complicated problem than it is
at present; and it would be necessary to apply them at just the proper
season and in nicely regulated amounts to insure against loss.
=132. Method of Determining Absorption of Chemical Salts.=—The soil
which is to be used for this experiment should be treated as has been
indicated and passed through a sieve the meshes of which do not exceed
half a millimeter in size. From twenty-five to fifty grams of the fine
earth may be used for each experiment.
The fine earth should be placed in a flask with 100 to 200 cubic
centimeters of the one-tenth to one-hundredth normal solution of the
substance to be absorbed. The flask should be well shaken and allowed to
stand with frequent shaking twenty-four to forty-eight hours at ordinary
temperatures. The whole is then to be thrown upon a folded filter and an
aliquot part of the filtrate taken for the estimation. The methods of
determining the quantities of the substances used will be found in other
parts of this manual. It is recommended to conduct a blank experiment
with water under the same conditions in order to determine the amount of
the material under consideration abstracted from the soil by the water
alone. The difference in the strength of the solution as filtered from
the soil, corrected by the amount indicated by the blank experiment, and
the original solution will give the absorptive power of the soil for the
particular substance under consideration.
If it should be desired to determine the absorptive power of the soil
for all the ordinary chemical fertilizing materials at the same time, a
larger quantity of the sample should be taken corresponding to the
increased amount of the standard solutions used. About 500 cubic
centimeters of the mixed salt solution should be shaken with 125 grams
of the earth and the process carried on in general as indicated above.
The absorption coefficient of an earth for any given salt according to
Fesca,[96] is the quantity of the absorbed material expressed in
milligrams calculated to a unit of 100 grams of the soil.
=133. Method of Pillitz and Zalomanoff.=—It is recommended by Pillitz
and Zalomanoff[25] to reject the old method, _viz._, shaking the soil
with the solution in a flask, and substitute the filtration method both
because it gives a more natural process and because the results are more
constant. The apparatus is shown in Fig. 15.
[Illustration:
FIGURE 15.
ZALOMANOFF’S APPARATUS FOR DETERMINING ABSORPTION OF SALTS BY SOILS.
]
Two cylinders are placed vertically, one over the other. The lower
cylinder is graduated in cubic centimeters, the upper cylinder is closed
at each end by perforated rubber stoppers A and B through the openings
of which the glass tubes _c_ and _d_ pass. Within the cylinder A the
opening of the small tube _d_ is closed with a disk of Swedish filter
paper. The lower part of the small tube is _d_ connected by means of a
rubber tube carrying a pinch-cock C, with another small tube _e_ which
passes through the stopper _f_. In carrying out the process the weighed
quantity of soil is placed in the upper cylinder and afterwards the
measured quantity of the solution, the whole thoroughly mixed and the
cylinder closed. The valve C is then opened, a given quantity of the
solution, but not all, is made to drop into the lower cylinder and the
valve C is then closed. The liquid which has passed into the lower
cylinder as well as that which remains in the upper cylinder, is
thoroughly stirred and the quantity of the material remaining in both
liquids determined and the absorbing power of the soil estimated from
their difference. It does not appear that this method of estimation of
the absorption power possesses any special advantages over the old and
far simpler method of shaking in a flask.
[Illustration:
FIGURE 16.
MÜLLER’S APPARATUS TO SHOW ABSORPTION OF SALTS BY SOILS.
]
=134. Method of Müller.=—The method of Müller[97] for illustrating
absorption is carried out by means of the apparatus shown in Fig. 16. A
glass cylinder A about 750 centimeters long and four to five centimeters
wide is closed at each end with rubber stoppers with a single
perforation. The cylinder A is for the reception of the soil with which
the experiment is to be made. Before using, the lower part of it is
filled with glass pearls or broken glass and above this a layer of glass
wool is placed about one centimeter thick. The object of this is to
prevent the soil from passing into the small tube below. As soon as the
soil has all been placed in the cylinder A the upper part of the tube is
also filled with glass wool. The cylinder A is connected with the
pressure bottle B by means of a rubber tube and the small glass bulb
tube shown in the figure. The bottle B should have a content of about
two liters. It is filled with the standard solution of the material of
which the absorption coefficient is to be determined. At _c_ the rubber
tube is connected with a glass T one arm of which is provided with a
piece of rubber tubing which can be closed by means of a pinch-cock. At
_c_ a screw pinch-cock is placed which can be used to regulate the flow
of the solution from B to A. By opening the pinch-cock at _e_ on the
short arm of the T piece, a sample of the original liquid can be taken
and this can be compared with the part which runs to _b_. If it is
desired for instance, to show that potassium carbonate has been absorbed
by the soil the two bulbs shown on the small glass tubes connecting with
A can be filled with red litmus paper. This paper will at once be turned
blue in the lower bulb while in the upper one it will retain its
original color because the liquid in passing through the soil will have
lost its alkaline reaction. The solutions used should be very dilute.
The apparatus is designed for lecture experiments and not for
quantitative determinations.
=135. Method of Knop.=—For rapid determination of the absorption
coefficient of the soil Knop’s method may be used.[98]
The fine earth which is employed is that which passes a sieve with
meshes of half a millimeter. From 50 to 100 grams of this soil are mixed
with from five to ten grams of powdered chalk and with about twice the
weight of ammonium chlorid solution of known strength, _viz._, from 100
to 200 cubic centimeters. The ammonia solution should be of such a
concentration that the ammonia by its decomposition for each cubic
centimeter of the liquid evolves exactly one cubic centimeter of
nitrogen. This solution is prepared by dissolving in 208 cubic
centimeters of water one gram of ammonium chlorid. With frequent shaking
the solution is allowed to stand in contact with the soil for
forty-eight hours. The whole is now allowed to settle and the
supernatant clear liquid is poured through a dry filter. From the
filtrate twenty to forty cubic centimeters are removed by a pipette, and
evaporated to dryness in a small porcelain dish, with the addition of a
drop of pure hydrochloric acid. The ammonium chlorid remaining in the
porcelain dish is washed with ten cubic centimeters of water into one of
the compartments of the evolution flask of the Knop-Wagner azotometer.
It is decomposed with fifty cubic centimeters of bromin lye and the
nitrogen estimated volumetrically. The difference between the amount of
nitrogen in this material and that of the original material will give
the amount of absorption exercised by the fine earth. This number,
without any further calculation, can be taken as the coefficient of
absorption.
=136. Method of Huston.=—The salt solutions recommended by Huston[99]
are sodium phosphate (Na₂HPO₄), potassium chlorid, potassium sulfate,
ammonium sulfate and sodium nitrate.
The solutions should be approximately tenth normal, the actual strength
in each case being determined by analysis. The phosphorus is determined
as magnesium pyrophosphate in the usual way, the potash as potassium
platinochlorid, the ammonia by collecting the distillate from soda in
half normal hydrochloric acid and titrating with standard alkali, and
the nitrate by Warington’s modification of Schlösing’s method for gas
analysis. The details of these methods of determination will be given
later. One hundred grams of the sifted, air-dried soil are placed in a
rubber stopped bottle and treated with 250 cubic centimeters of the
solution to be tested. The digestion is continued for forty-eight hours
in each case, the bottles being thoroughly shaken at the end of
twenty-four hours. At the end of the treatment the solutions are
filtered and the salts determined in aliquot portions. The details of
this method are essentially those already described.
=137. Statement of Results.=—Duplicate analyses should be made and the
tabulation of the data is illustrated in the following analyses by
Huston:
Na₂HPO₄ cubic Weight of Weight of P₂O₅ Salt
centimeters Mg₂P₂O₇ in Mg₂P₂O₇ in absorbed removed
filtrate taken. twenty-five filtrate. by 100 per
cubic grams cent.
centimeters of soil.
the solution.
(a) 25 0.1368 gram 0.0962 gram
(b) 25 0.0963 „ 0.2589 29.6
gram
——————
Mean 0.0963 „
KCl cubic Weight of Weight of K₂O Salt
centimeters K₂PtCl₆ in K₂PtCl₆. absorbed removed
filtrate taken. twenty-five by 100 per
cubic grams cent.
centimeters of soil.
solution.
(a) 25 0.6154 gram 0.4505 gram
(b) 25 0.4540 „ 0.3161 26.5
gram
——————
Mean 0.4523 „
K₂SO₄ cubic Weight of Weight of K₂O Salt
centimeters K₂PtCl₆ in K₂PtCl₆. absorbed removed
filtrate taken. twenty-five by 100 per
cubic grams cent.
centimeters of soil.
solution.
(a) 25 0.6113 gram 0.4426 gram
(b) 25 0.4371 „ 0.3324 28.0
gram
——————
Mean 0.4399 „
(NH₄)₂SO₄ cubic Number cubic Half normal acid N absorbed Salt
centimeters centimeters neutralized. by 100 absorbed
filtrate taken. one-half normal grams per
acid neutralized soil. cent.
by fifty cubic
centimeters of
solution.
(a) 50 10.00 7.25 grams
(b) 50 7.25 „ 0.0964 27.5
gram
————
Mean 7.25 „
NaNO₃ cubic Number cubic Cubic centimeter N absorbed Salt
centimeters centimeters N₂O₂ N₂O₂ at 0° and by 100 absorbed
filtrate taken. afforded by ten 1000 grams per
cubic millimeters. soil. cent.
centimeters of
solution at 0°
and 1000
millimeters
pressure.
(a) 10 16.63 16.77 grams
(b) 10 16.70 „ none 00.00
—————
Mean 16.73 „
Upon comparing the figures it will be found that the absorption, passing
from the greatest to the least, is as follows: phosphoric acid (P₂O₆),
potassium sulfate, ammonium sulfate, potassium chlorid and sodium
nitrate.
It will be seen that there was no absorption in the case of the nitrate,
while with each of the other salts there was quite a marked absorption.
It will also be noticed that the percentages of absorption are not very
different, and especially is this true of the potassium and ammonium
salts, the P₂O₅ being somewhat higher. Whether this fact is merely an
accidental occurrence or is due to the law of combination by equivalents
could hardly be predicted from the single soil experimented upon; but
taking into consideration the possibility of difference in solubility of
the resulting compounds in the saline solutions used, and of other
varying conditions, the percentages are evidently not far enough apart
to exclude the possibility of the bases uniting in equivalent
proportions.
=138. Preparation of Salts for Absorption.=—The salts employed in the
foregoing determinations are conveniently prepared, in fractional normal
strength.
In grams per liter the following quantities in grams are recommended,
_viz._, 5.35 g NH₄Cl; 10.11 g KNO₃; 16.40 g Ca(NO₃)₂; 24.60 g MgSO₄ +
7H₂O; 23.4 g CaH₄(PO₄)₂, etc.
The ammonium chlorid, potassium nitrate and magnesium sulfate can be
weighed as chemically pure salts and the standard solution be directly
made up. Calcium nitrate is so hygroscopic that a stronger solution must
be made up, the calcium determined and the proper volume taken and
diluted to one liter.
Monocalcium phosphate is prepared as follows:
A solution of sodium phosphate is treated with glacial acetic acid and
precipitated with a solution of calcium chlorid. It is then washed with
water until all chlorin is removed. The fresh precipitate is saturated
with pure, cold phosphoric acid of known strength. After filtering the
solution is placed in a warm room and left for two or three weeks until
crystallization takes place.
The crystals are pressed between blotting papers and finally dried over
sulfuric acid and washed with water-free ether, and again dried. Since
this salt is decomposed in strong solutions it should be used only in
one hundredth normal strength, viz., 2.34 grams per liter.
POROSITY AND ITS RELATIONS TO MOISTURE.
=139. Porosity.=—The porosity of a soil depends upon the state of
divisibility and arrangement of its particles, and upon the amount of
interstitial space within the soil. If a soil be cemented together into
a homogeneous mass, its porosity sinks to a minimum; if it be composed,
however, of numerous fine particles, each preserving its own physical
condition, the porosity of the soil will rise to a maximum. The porosity
of a soil may be judged very closely by the percentage of fine particles
it yields by the process of silt analysis to be described further on. In
general, the more finely divided the particles of a soil, the greater
its fertility. This arises from various causes; in the first place, such
a soil has a high capacity for absorbing moisture and holding it; thus
the dangers of excessive rain-falls are diminished, and the evil effects
of prolonged drought mitigated. In the second place, a porous soil
permits a freer circulation of the gases found in the soil. The
influence of lime in securing the proper degree of porosity of a soil is
very great, especially in alluvial deposits and other stiff soils. It
prevents the impaction which will necessarily follow in a soil which is
too finely divided. In general, the porosity of the soil may be said to
depend on three factors, _viz._: 1. Upon the state of divisibility or
the number of particles per unit volume; 2. Upon the nature and
arrangement of these particles; 3. Upon how much interstitial space
there is in the soil.
=140. Influence of Drainage.=—Good underdrainage increases the porosity
of a soil by removing the excess of water during wet seasons and
rendering the soil more suitable to capillary attraction which will
supply moisture during dry seasons. The influence of tile drainage on
the production of floods has been carefully studied by Kedzie,[100] who
shows that surface ditching in conjunction with deforesting may increase
floods and contribute to droughts, and that tile-draining may increase
flood at the break-up in spring, when the water accumulated in the
surface soil by the joint action of frost and soil capillarity during
the winter, and the surface accumulations in the form of snow are
suddenly set free by a rapid thaw.
He also points out that during the warm months tile-draining tends to
prevent flood by enabling the soil to take up the excessive rain-fall
and hold it in capillary form, keeping back the sudden flow that would
pass over the surface of the soil if not absorbed by it, and it
mitigates summer drought by increased capacity of the soil to hold water
in capillary form and to draw upon the subsoil water supply.
=141. Soil Moisture.=—The capacity of a soil to absorb moisture and
retain it depends on its porosity and is an important characteristic in
relation to its agricultural value.
The following general principles relating to soil moisture are adapted
from Stockbridge:[101]
During dry weather plants require a soil which is absorptive and
retentive of atmospheric moisture. The amount of this retention is
generally in direct ratio to two factors, _viz._, the amount of organic
matter and its state of division. The capillary water of the soil is
very closely related to its percolating power, since all waters in the
soil are governed in their movements by what is known as capillary
force. Liebenberg has shown that this movement may be either upwards or
downwards, according as the atmosphere is dry or supplies
soil-saturating rain. The water absorbed by the roots passes into the
plant circulation, and the greater part is evaporated from the leaves.
Where the supply of water is insufficient, the plant wilts, and if the
evaporation long continue in excess of the supply obtained from the
soil, the plant must die. The experiments of Hellriegel have shown that
any soil can supply plants with all the water they need, and as fast as
they need it, so long as the moisture within the soil is not reduced
below one-third of the whole amount that it can hold. The quantity of
water required and evaporated by different agricultural plants during
the period of growth has been found to be as follows:
One acre of wheat exhales 409,832 pounds of water.
„ „ „ clover „ 1,096,234 „ „ „
„ „ „ sunflowers „ 12,585,994 „ „ „
„ „ „ cabbage „ 5,049,194 „ „ „
„ „ „ grape-vines „ 730,733 „ „ „
„ „ „ hops „ 4,445,021 „ „ „
Dietrich estimates the amount of water exhaled by the foliage of plants
to be from 250 to 400 times the weight of dry organic matter formed
during the same time. Cultivation conserves soil moisture. It must be
remembered that this water contains soil ingredients in solution.
Hoffmann has estimated that the quantity of matter dissolved from the
soil by water varies from 0.242 to 0.0205 per cent of the dried earth.
The experiments of Humphrey and Abbott have shown that about one-sixth
of the total sediment of the Mississippi river is soluble in water.
=142. Determination of the Porosity of the Soil.=—The porosity of the
soil is fixed by the relative volume of the solid particles as compared
with the interstitial space. It is most easily determined by dividing
the apparent by the real specific gravity.
Let the real specific gravity of a soil be 2.5445 and the apparent
specific gravity of the same soil be 1.0990.
The porosity is then calculated according to the following ratios,
_viz._:
2.5445 : 1.099 = 100 : X
Whence X = 43.2 = per cent volume occupied by the solid particles of the
soil.
The per cent volume occupied by the interstitial space is therefore
56.8.
=143. Method of Whitney.=—The total volume of interstitial space within
the soil, in which water and air can enter, is best determined by
calculation from the specific gravity and the weight of a known volume
of soil. To determine this in the soil in its natural position in the
field, a sample is taken in the following way: A brass tube, about two
inches in diameter and nine inches long, has a clock spring securely
soldered into one end, and this end turned off in a lathe to give a good
cutting edge of steel. The area enclosed by this steel edge is
accurately determined, and a mark is placed on the side of the tube
exactly six inches from the cutting edge. A steel cap fits on top of the
brass cylinder to receive the blows of a heavy hammer or wooden mallet.
The cylinder is driven into the ground until the six-inch mark is just
level with the surface. The whole is then dug out, care being taken to
slip a broad piece of steel under the cylinder before it is removed, so
as to prevent the soil which it contains from falling out. The cylinder
is then carefully laid over on its side, and the soil is cut off flush
with the cutting edge of steel. The soil is then removed from the
cylinder, carried to the laboratory and properly dried and weighed. The
object of the steel inserted in one end of the cylinder is to reduce the
friction on the inside of the tube to a minimum, and thus prevent the
soil inside the cylinder being forced down below the level of the
surrounding earth. The volume of the soil removed with this sampler can
readily be determined by calculation, as the area of the end of the tube
is known and the sample is six inches deep. In a sampler, such as
described here, this volume is about 300 cubic centimeters. From the
weight of soil and the volume of the sample, the volume of interstitial
space may be found by the following formula:
S = ([V − W/ω] × 100)/V
S is the per cent by volume of interstitial space, V is the volume of
the tube in cubic centimeters, W is the weight of soil in grams, and ω
is the specific gravity of the soil. The specific gravity can be
determined for each soil, or the factor 2.65 can be used, which is
sufficiently accurate for most work.
The per cent by volume of interstitial space in the undisturbed subsoil
is found to range from about thirty-five for sandy land, to sixty-five
or seventy for stiff clay lands.
For the determination of the amount of water an air-dried soil will
hold, if all the space within it is completely filled with water, an
eight-inch straight argand lamp chimney, with a diameter of about two
inches, can be conveniently used. A mark is placed on the side of the
tube, six inches from one end, and the volume of the tube up to this
mark is found by covering the end with a piece of thin rubber cloth, or
by pressing the chimney down firmly on a glass plate, and making a
water-tight joint with paraffin or wax. Water is then poured into the
tube up to the six-inch mark, and the weight or volume of water
determined. The tube can then be dried, a piece of muslin tied tightly
over the top and the whole then weighed. Soil is carefully poured in and
the tube gently tapped on a soft support until the soil is six inches
deep in the tube, and has the desired degree of compactness. The weight
and volume of the soil can thus be determined, and the volume of the
interstitial space from the formula already given. This can also be
determined directly by introducing water from above, or by immersing the
cylinder of soil up to the six-inch mark in water, and allowing the
water to enter the soil from below. With such a short depth of soil,
very little water will flow out when the cylinder is suspended in the
air. The amount which will flow out when the cylinder is thus suspended,
will depend both upon the texture and the depth of soil. It is
impossible, however, by this method, to completely remove the air or to
completely fill the space within the soil with water; for as the water
enters the soil, a considerable amount of air becomes entangled in the
capillary spaces, and this could not be removed except by boiling and
vigorous stirring, which would altogether change the texture of the
soil. The amount of water held by the soil, or the amount of space
within the soil into which water and air can enter, will evidently
depend upon the compactness of the soil, and this is best expressed in
per cent by volume of space.
=144. Capacity of the Fine Soil for Holding Moisture.=—The soil, as it
is taken from the field, may have quite a different water coefficient
from the same soil after it has been passed through a fine sieve or been
dried at air temperatures or at 100° or 110°. The method of
determination which depends upon adding excess of water to a given
weight of fine earth, and afterwards eliminating the excess by
percolation or filtration, is apt to give misleading results. If,
however, the results are obtained by working on the same weight of soil,
and in the same conditions, they may have value in a comparative way.
The comparison between soils must be made with equal weights, in like
apparatus and with the same manipulation, to have any value. These
determinations, however, cannot have the same practical value as those
made in the samples in a natural condition as has just been described.
=145. Method of Wolff Modified by Wahnschaffe.=[102]—A cylindrical zinc
tube (Fig. 17), sixteen centimeters long and four centimeters internal
diameter, is used, the cubical capacity of which is 200 cubic
centimeters.
The cylinder is graduated by placing the moist linen disk on the gauze
and tying a piece of rubber cloth over the bottom. Water is now poured
in until the level is even with the gauze bottom. Add then exactly 200
cubic centimeters of water, mark its surface on the zinc, throw out the
water, and file the zinc down to the mark.
The bottom of the tube is closed with a fine nickel-wire gauze. Below
this a piece of zinc tubing, of the size of the main tube, is soldered;
pierced laterally with a number of holes.
Before using, the gauze bottom of the cylinder is covered with a moist,
close fitting linen disk, and the whole apparatus weighed. It is then
filled with the fine earth, little by little, jolting the cylinder on a
soft substance after each addition of soil to secure an even filling.
When filled even full the whole is weighed, the increase in weight
giving the weight of soil taken.
[Illustration:
FIGURE 17.
CAPACITY OF THE FINE SOIL FOR HOLDING MOISTURE. METHOD OF WOLFF
MODIFIED BY WAHNSCHAFFE.
]
A large number of cylinders can be filled at once and placed in a large
glass crystallizing dish containing water and covered with a bell jar
(Fig. 17). The water should cover the gauze bottoms of the cylinders to
the depth of five to ten millimeters. More water should be added from
time to time as absorption takes place. The cylinders should be left in
the water until when weighed at intervals of an hour no appreciable
increase in weight takes place. The temperature and barometer reading
should be noted in connection with each determination. With increasing
temperature the water coefficient is diminished.
The method of Wolff, as practiced in the laboratory of the Chemical
Division of the U. S. Department of Agriculture, has given very
concordant results. Five determinations were made on a sample of
vegetable soil with the Wolff cylinders, which were weighed at intervals
of ten, twenty, and thirty days, with the following results:
No. 1. Water absorbed after ten days 106.25 per cent
„ 2. „ „ „ „ „ 105.68 „ „
„ 3. „ „ „ „ „ 105.86 „ „
„ 4. „ „ „ „ „ 106.11 „ „
„ 5. „ „ „ „ „ 105.83 „ „
——————
Mean 105.95 „ „
No. 1. Water absorbed after twenty days 106.44 per cent
„ 2. „ „ „ „ „ 105.98 „ „
„ 3. „ „ „ „ „ 106.56 „ „
„ 4. „ „ „ „ „ 106.52 „ „
„ 5. „ „ „ „ „ 106.38 „ „
——————
Mean 106.38 „ „
No. 1. Water absorbed after thirty days 108.35 per cent
„ 2. „ „ „ „ „ 107.60 „ „
„ 3. „ „ „ „ „ 108.32 „ „
„ 4. „ „ „ „ „ 107.86 „ „
„ 5. „ „ „ „ „ 107.87 „ „
——————
Mean 108.00 „ „
The data obtained show that there was a very slight increase in the
amount of moisture absorbed after the tenth day.
As will be seen, however, from the following data, the soil within the
cylinder does not contain in all parts the same percentage of moisture,
the lower portions of the cylinder containing notably larger proportions
than the upper parts. The cylindrical soil column was divided into four
equal parts and the moisture determined in each part. Beginning with the
top quarter the percentages of moisture were as follows:
First quarter 97.52 per cent
Second „ 105.91 „ „
Third „ 112.83 „ „
Fourth „ 116.48 „ „
=146. Method of Petermann.=[103]—The method of Wolff as practiced by the
Belgian Experiment Station, at Gembloux, is essentially the same as
described above.
Petermann recommends the use of tared cylinders twenty to twenty-five
centimeters long and six to eight centimeters in diameter. The cylinder
is to be filled with the fine earth, little by little, with gentle
tapping after each addition. The bottom of the cylinder is closed with a
perforated rubber stopper on which is spread a moistened disk of linen.
The cylinder, thus prepared and filled, is weighed and afterwards placed
in a vessel containing distilled water, to such a depth as to secure a
water level about two centimeters above the lower surface of the soil in
the cylinder. The level of the water is kept constant as the contents of
the cylinder are moistened by capillarity. When the earth appears to be
thoroughly moistened, as can be told by the appearance of the upper
surface, maintain the contact with water for about five or six hours.
The cylinder is then removed, the upper surface covered to avoid
evaporation, allowed to drain for a few hours, wiped and weighed. The
cylinder is again placed in water to see if any increase in weight takes
place. The weight of the fine earth and of the absorbed water being
known, the percentage of absorption is easily calculated.
=147. Method of A. Mayer.=[104]—A glass tube, one and seven-tenths
centimeters in diameter, composed of two pieces, seventy-five
centimeters and twenty-five centimeters in length, is united by a piece
of rubber tubing. The lower free end of the seventy-five centimeter
piece is closed with a piece of linen. The tube is filled, with gentle
jolting, to the depth of one meter with fine earth, the earth column
thus extending twenty-five centimeters above the point of union of the
two pieces. Thus prepared, a quantity of water is poured into the upper
tube sufficient to temporarily saturate the whole of the soil.
During the sinking of the water in the tube there is thus effected a
moistening of the material before it is wholly filled with water. After
waiting until the water poured on top has disappeared the tube is
separated at the rubber tube connection and a sample of the moist soil
taken at that point. This is at once weighed and then dried at 100°. The
loss in weight gives the water absorbed.
The number thus obtained is calculated to the standard by volume, by use
of the number representing the apparent specific gravity of the fine
earth.
For sand of different degrees of fineness the following numbers were
found:
Degree of fineness 2 3 4
Per cent water absorbed 7.0 13.7 44.6
The numbers thus obtained are taken to represent the absolute water
capacity of a mineral substance in powder.
The full water capacity, _i. e._, the power of holding water when the
powder is immersed in water, the excess of which is then allowed to flow
away is much greater than the absolute number.
This difference is shown in the following data:
Quartz, size three. Clay, size three.
Full water capacity 49.0 per cent 46.8 per cent
Absolute water capacity 13.7 „ „ 24.5 „ „
In general the absolute is markedly inferior to the full water capacity.
Only in the finest dust do the two numbers approach each other.
=148. Volumetric Determination.=—A convenient apparatus for this
determination has been devised by Mr. J. L. Fuelling, of the Chemical
Division, Department of Agriculture. It is shown in Fig. 18.
It consists of an ordinary percolator the diameter of which decreases
slightly towards the lower end, a thick-wall rubber tube and an ordinary
burette, divided in tenths. A rubber stopper is fitted to the mouth of
the percolator and perforated twice—in the middle and at the side, the
former for a small tube provided with pinch-cock and the latter for the
neck of a small funnel. The whole is supported on a convenient stand,
the clamp holding the percolator being placed above that supporting the
burette, both clamps arranged to slide on the stand-rod.
[Illustration:
FIGURE 18.
FUELLING’S APPARATUS.
]
The method is as follows:
A mark is placed upon the projecting tube at the lower end of the
percolator, and the tube at this point may be drawn out sufficiently to
decrease the width of meniscus to one-eighth inch. Into the percolator
is first introduced a small disk of wire gauze or perforated porcelain,
with heavy wire pendant in the tube. Through the rubber stopper a small
glass tube is passed and its lower end pressed firmly upon the wire or
porcelain disk, its upper end being curved and supplied with a
pinch-cock. Into the percolator is now poured one inch of fine shot (No.
20) and then one inch of fine sand which has been previously digested
with hydrochloric acid and well cleaned of dust by washing.
_The zero._—After the shot and sand have been shaken even, the burette
is filled with water and raised above the level of the sand, wetting the
percolator for four inches of its length. The burette is lowered and the
shot and sand bed allowed to drain by opening the pinch-cock of the
inner tube. The burette is raised and the shot-sand flooded repeatedly
until, by lowering the burette until the zero mark of the percolator
tube is reached, a uniform reading on the burette is secured. Thus the
shot-sand bed is completely charged with water. The water level is now
made zero on the percolator stem, the burette filled to its zero mark
and the apparatus is prepared for introduction of the soil.
_The Determination._—From 100 to 200 grams of soil, previously dried
free of moisture, are weighed, the burette raised until the water level
is three inches above the sand, and the soil gently dropped through a
funnel into the water. When the soil has been introduced and wetted
completely the water level is raised above the soil and allowed to
remain thus two hours. The burette is then lowered and the water allowed
to drain from the wetted soil. Four to six hours are usually given the
draining, the reading taken on the burette after establishing the zero
on the percolator stem, the volume of absorbed water thus ascertained
and divided by the weight of soil multiplied by 100; the result
expresses the water absorbed per hundred of soil.
Example:
Water required to saturate disk, etc. 0.50 cubic centimeter.
Weight of air-dried soil taken 20.00 grams.
Moisture at 105° therein 14.25 per cent.
Weight water in soil 2.85 grams.
Reading of burette after saturation 10.75 cubic centimeters.
Less water required for disk, etc 9.25 „ „
Temperature 20°.00
Weight of 9.25 cubic centimeters H₂O at 20° 9.22 grams.
Total weight of water retained by soil 12.07 „
Per cent water retained by soil 60.35 per cent.
For general analytical work the correction for variations in the weight
of water for different temperatures is of no practical importance.
=149. Accuracy of Results.=—A sample of soil from the beet sugar
station, in Nebraska, gave the following duplicate results:
First trial 45.75 per cent water.
Second trial 44.85 „ „ „
Muck soils from Florida, containing varying proportions of sand, gave
the following numbers:
Soil number one, 144.85 per cent, and 145.43 per cent; soil number
two, 109.13 per cent, and 107.93 per cent; soil number three (very
sandy), 46.86 per cent, and 46.51 per cent.
=150. Method of Wollny.=[105]—A zinc tube, ninety centimeters long and
four centimeters internal diameter, carries at each end, at right angles
to the axis, a flattened rim 1.5 centimeters broad. The lower end of the
tube is closed with a strong piece of coarse linen. The soil to be
examined is then filled in little by little, with gentle tamping.
On the upper end two glass tubes are placed, each ten centimeters long
and four centimeters internal diameter. These tubes are furnished at
each end with cemented brass cylinders which are expanded to a circular,
evenly ground rim, 1.5 centimeters wide, also at right angles to the
axis of the main tube. These rims are greased and placed together, one
on the other, and held together by wooden clamps.
The glass tube in immediate connection with the zinc tube is also firmly
filled with the soil sample, while the second tube is only partly
filled, so that any settling which may take place in the soil on the
addition of water may still find the first glass tube full of the
sample.
The empty part of the upper glass tube is now filled with water and
additional quantities of water are added from time to time until the
soil is saturated. In order to be able to observe when this takes place
there is a slit at the lower end of the zinc tube which is closed with a
piece of glass. This slit should be about two centimeters broad and ten
centimeters long. The lower end of the zinc tube is set on a glass plate
to prevent evaporation.
As soon as the water shows itself at the lower end of the zinc tube, the
excess of water in the upper glass tube is at once removed by a pipette
and a stopper inserted through which a glass tube passes drawn out into
a fine point above. The object of this is to avoid evaporation on the
upper surface. The apparatus is then left at rest for thirty-six hours.
At the end of this time the clamps are removed and the column of moist
earth cut with a piece of platinum foil, and the two ends of the glass
tube, next to the zinc tube, covered with glass plates. It is then
weighed and the weight of moist earth determined by deducting the weight
of the tube and its glass covers. The moist earth is carefully removed
to a large porcelain dish and dried at 100°. Before weighing it is
allowed to stand twenty-four hours in the air. The data obtained are
used to calculate the water content to volume per cent.
The volume of the glass tubes should be determined by careful
calibration.
=151. Method of Heinrich.=[106]—The soil to the depth turned by the plow
is dug out and in the hole a lead vessel without bottom, twenty
centimeters in diameter and forty centimeters high, is placed. The soil
is then thrown back around and outside the lead vessel until the latter
appears buried in the fragments.
The rest of the soil is passed into the lead vessel, through a sieve
having four meshes to the centimeter, using for this purpose enough
water to thoroughly moisten it. Care should be taken not to use enough
water to cause any separation of the fine from the coarse particles.
By this process all coarse stones, sticks, etc., are separated. In sandy
soils the flask is left for a few hours while in clay soils a much
longer time is necessary. When the excess of water has disappeared the
lead cylinder is removed, and a piece cut out of the center of it placed
in a weighed drying flask and dried at 100°.
=152. Effect of Pressure on Water Capacity.=[107]—The increasing
capacity of soil to hold water developed by shaking or pressure, is
determined by Henrici in the following way:
Into a glass cylinder of twenty millimeters internal diameter are poured
twenty cubic centimeters of water. A given quantity of soil is next
added, and after standing until thoroughly saturated, the residual water
is measured by pouring off, or better, by graduations on the side of the
tube. The increase in the volume of the clear water is also measured,
after shaking, in the same way. The data of a determination made as
above described follow:
Water in cylinder 30 cubic centimeters.
Water and saturated soil 40 „ „
Volume of unsaturated soil = e = 10 „ „
Volume of saturated soil = e + w = 20.5 „ „
Water contained therein = w = 10.5 „ „
By repeated shaking the volume of e + w, the content of w therein, and
the relative values of e/w were found to be as follows:
Cubic Cubic Cubic Cubic
centimeters. centimeters. centimeters. centimeters.
e + w 20.5 16.0 15.7 15.0
w 10.5 6.0 5.7 5.0
─────────────────────────────────────────────────────────
w/e 1.05 0.60 0.57 0.50
If e′ represent the volume of the saturated soil then e′ = e + w, and
this gives the relation to the volume of dry earth represented by the
equation e′/e = 1 + w/e. This indicates that the relative volume of the
saturated soil is equal to unity increased by the relative content of
water.
=153. Coefficient of Evaporation.=—At an ordinary room temperature in
the shade, samples of soil, if they are subjected to experiment in
tolerably thin layers have nearly an equal coefficient of evaporation.
That is, the absolute quantity of water evaporated in a given time is
almost entirely conditioned upon the magnitude of the surface exposed
and the temperature of the surrounding air. Only when exposed in
conditions as nearly as possible natural in thin layers to the action of
the sunlight and shade do the soils show their peculiarities in respect
of the evaporation of moisture. In order to see these peculiarities,
samples of soil which have been previously examined must be subjected to
examination at the same time with the soil whose properties are to be
determined.
The zinc box, before described, should be protected with a well fitting
cover of thick paper, and the different samples of soil which are to be
tested placed therein. This should now be placed in a wooden box, the
top of which is exactly even with the top of the zinc vessel. This box
containing the vessel should be exposed to the sunlight. After
twenty-four hours the zinc boxes can be taken away from position and
their loss in moisture determined, and these weighings, according to the
condition of the atmosphere, can be continued from fourteen days to
three weeks, the temperature of the air of course being carefully
determined at each time. At first, all the different soils being
saturated with moisture, it will be observed that the loss of moisture
is proportionately the same for all. Soon, however, the rapidity of the
evaporation in the samples of soil rich in humus and clay will be
decreased as compared with the sandy soils, and in general, those which
possess a high capillary power capable of bringing the moisture rapidly
from the deeper layers to the surface. There soon comes a point when the
difference in evaporation is at its greatest; and then there will be a
gradual diminution until the samples lose no further moisture. This
point, for the different soils, can be determined by frequent weighings
of the vessel.
=154. Determination of Capillary Attraction.=—Long glass tubes graduated
in centimeters may be used for this determination, or plain tubes so
arranged as to admit of easy measurements with a rule. The tubes may be
from one to two centimeters internal diameter and about one meter long.
The fine earth should be evenly filled in little by little, with gentle
jolting. The lower end of each tube, before filling, is closed with a
piece of linen.
The tubes, after filling, are supported in an upright position by a
frame AE, Fig. 19, in a vessel B containing water in which the linen
covered ends D dip to the depth of two centimeters. The height of the
water in the several tubes should be read or measured at stated
intervals. The water contained in the supply vessel should be kept at a
constant height by a Mariotte bottle.
[Illustration:
FIGURE 19.
APPARATUS TO SHOW CAPILLARY ATTRACTION OF SOILS FOR WATER.
]
The observations may be discontinued after one hundred and twenty hours,
but even then the water will not have reached its maximum height.
It is recommended by some experimenters to cut the tubes, after the
above determination is completed, into pieces ten centimeters in length,
and to determine the per cent of water in each portion.
=155. Statement of Results.=—The following table illustrates a
convenient method of tabulating the observed data as given by
König.[108]
Number of sample 1 2 3 4 5 6
───────────────────────────────────────────────────────────────────────
Height of 24 hours. 27.3 38.0 16.7 36.4 8.0 28.8 centimeters.
moisture column
after:
„ 48 „ 35.9 50.8 24.5 49.2 11.9 40.5 „
„ 72 „ 41.5 59.5 30.0 57.9 15.2 49.1 „
„ 96 „ 44.4 66.2 33.5 63.8 17.5 55.2 „
„ 120 „ 46.7 70.0 36.3 68.5 19.2 60.5 „
=156. Inverse Capillarity.=—In tubes filled with fine earth, as
described in paragraph =154=, water is quickly poured, the same quantity
into each tube of the same diameter, or such quantities in tubes of
different diameters as would form a water column of the same depth over
the surface of the sample. The rate at which the water column descends
in each tube, the time of the disappearance of the water at the surface
and the final depth to which it reaches, are the data to be entered.
=157. Statement of Results.=—The points to be observed in the
determination of inverse capillarity are the number of hours required
for the total absorption of a column of water of a given height, the
depth of the moisture column at that moment, and the total depth to
which the moisture column finally reaches. The data of observations with
six samples with a water column four centimeters high are given by
König[109] as follows:
Number of sample 1 2 3 4 5 6
─────────────────────────────────────────────────────────────────────────
Number of hours required for
water to disappear 4.3 1.8 10.3 3.0 21.0 4.3
Depth of moisture at time of
disappearance of water 11.0 12.0 11.4 13.3 11.7 12.0 centimeters.
Total depth of moisture 13.0 18.1 13.0 19.0 12.0 16.5 „
=158. Determination of the Coefficient of Evaporation.=—The coefficient
of evaporation is the number of milligrams of water evaporated from a
square centimeter of soil surface in a given unit of time. It is evident
that this number will vary with the physical state of the soil, the
velocity of the wind, the saturation of the air with aqueous vapor and
the temperature. In all statements of analyses these factors should
appear.
The process may be carried on first (a) with soil samples kept
continually saturated with water and (b) with samples in which the water
is allowed to gradually dry out.
_Method a._—The determination may be made in the shade or sunlight.
_In the Shade._—A zinc cylinder (Z Fig. 20), fifteen centimeters in
diameter and 7.5 centimeters high, with a rim one centimeter wide and
one centimeter from top, is covered at one end with linen or cotton
cloth and filled with fine earth, with gentle jolting, until even with
the top. It is then placed in a zinc holder H, into the circular opening
of which it snugly fits as in A. This holder is twenty centimeters in
diameter and 7.5 centimeters deep. It has an opening at O through which
water can be added until it is filled so as to wet the bottom of Z when
in place. As the water is absorbed by the soil more is added and, the
top being covered, the apparatus is allowed to stand for twenty-four
hours. At the end of this time the soil in the zinc cylinder is
saturated with water to the fullest capillary extent.
The whole apparatus, after putting a stopper in O, is now weighed on a
large analytical balance and placed in an open room, with free-air
circulation, for twenty-four hours. At the end of this time it is again
weighed and the loss of weight calculated to milligrams per square
centimeter. Where large and delicate balances can not be had, the
apparatus can be constructed on a smaller scale suitable for use with a
balance of the ordinary size.
_In the Sunlight._—The apparatus described above is enclosed in a wooden
box having a circular opening the size of the soil-zinc cylinder. In the
determination of the rate of evaporation, the apparatus, charged and
weighed as above described, is exposed to the sun for a given period of
time, say one hour. On the second weighing the loss represents the water
evaporated. The time of year, time of day, velocity of wind and
temperature, and degree of saturation of the air with aqueous vapor,
should be noted. The data obtained can then be calculated to milligrams
of water per square centimeter of surface for the unit of time.
_Method b._—As in method a the determination may be made in the shade or
in the sunlight. The rate of evaporation is, in this method, a
diminishing one and depends largely on the reserve store of water in the
sample at any given moment.
The same piece of apparatus may be used as in the determinations just
described. After charging the sample with moisture all excess of water
in the outer zinc vessel is removed and the rate of evaporation
determined by exposure in an open room or in the sunlight, as is done in
the operations already described.
_Alternate Method._—The zinc cylinders used in determining saturation
coefficient, paragraph =145=, may also be employed in determining the
rate of evaporation. Each cylinder should be wrapped with heavy paper or
placed in a thick cardboard receptacle, and all placed in a wooden box,
the cover of which is provided with circular perforations, just
admitting the tops of the cylinders, which should be flush with the
upper surface of the cover.
Arranged in this way the cylinders previously weighed are exposed in the
shade or to direct sunlight and reweighed after a stated interval. On
account of the small surface here exposed in comparison with the total
quantity of soil and moisture it is recommended to weigh the cylinders
once only in twenty-four hours. The weighings may be continued for a
fortnight or even a month.
In soils fully saturated with water the rate of evaporation is at first
nearly the same on account of the surface being practically that of
water alone. As the evaporation continues, however, the rate changes
markedly with the character of the soil.
=159. Rapid Method of Wolff.=—In order to expose a larger surface to
evaporation and to secure the results in a shorter period of time,
Wolff[110] fills square boxes, having wire-gauze bottoms, with fine
earth, and after saturating with moisture weighs and suspends them in
the open air. The wire-gauze bottoms are previously covered with filter
paper to prevent loss of soil.
[Illustration:
FIGURE 20.
APPARATUS FOR DETERMINING COEFFICIENT OF EVAPORATION.
]
=160. Estimation of Water Given up in a Water-Free Atmosphere.=—The
air-dried sample, in quantities of from five to ten grams in a thin
layer on glass, is placed over a vessel containing strong sulfuric acid.
It is then placed on a ground glass plate and covered with a bell jar.
The sample is weighed at intervals of five days until the weight is
practically constant.
This method is valuable in giving the actual hygroscopic power of a soil
depending on its structure alone.
=161. Estimation of the Porosity of the Soil for the Passage of
Gases.=—Some further notion of the physical state of the soil known as
porosity, may also be derived by a study of the rate at which it will
admit of the transmission of gases. A method for estimating this has
been devised by Ammon.[111]
Air is compressed in two gas holders by means of a column of water of
proper height to give the pressure required.
The tubes through which the air passes out of the gas holders are each
furnished with a stop-cock and united with a glass tube having a side
tube set in at right angles for carrying off the air.
The use of two holders makes it possible to carry on the experiment as
long as may be desired, one holder being filled with air while the other
is emptying.
The common conducting tube is joined with a meter which is capable of
measuring, to 0.01, the volume of air passing through it. The pressure
is regulated by means of the stop-cocks. The air passing from the meter
is received in a drying tube filled with calcium chlorid.
From the drying tube the air enters a drying flask filled below with
concentrated sulfuric acid and above with pumice stone saturated
therewith. Next the dried air passes through a worm, eight meters long,
surrounded with water at a given temperature. The dried air of known
temperature next enters the experimental tube. This tube is made of
sheet zinc 125 centimeters in length and five centimeters in internal
diameter. It is placed in an upright position, and about six centimeters
from its upper end carries a small tube at right angles to the main one
for connection with a water-filled manometer.
The upper and lower ends of the tube are closed with perforated rubber
stoppers carrying tubes for the entrance and exit of the air.
In the inside of the zinc tube are found two close-fitting but movable
disks, of the finest brass wire gauze, between which the material to be
experimented upon is held.
The layer of fine soil is held between these disks and may be of such a
depth as is required for the proper progress of the experiment. With
soils of firm texture opposing a great resistance to the passage of the
air the column of earth tested should be shorter than with light and
very permeable soils. The experimental tube is surrounded with a water
jacket, which may also be made of sheet zinc, carrying small tubes
directed upwards for holding thermometers. The water jacket should be
kept at the same temperature as the air which is used in the experiment.
The process of filling the tube, the amount of pressure to be used and
the air and soil temperature, will naturally vary in different
determinations.
The volume of air at a given pressure and temperature which passes a
column of soil of a given length in a unit of time will give the
coefficient of permeability.
=162. Determination of Permeability in the Open Field.=—A method for
determining the rate of transmission of a gas through the soil in the
field has been devised by Heinrich.[112]
A box C (Fig. 21) is made of strong sheet iron and has an opening below,
ten centimeters square, and a height of about twenty centimeters. At
exactly ten centimeters from the bottom, the box has a rim at right
angles to its length so that it can be placed only ten centimeters deep
in the soil. The box holds a volume of earth equal to 1,000 cubic
centimeters.
[Illustration:
FIGURE 21.
METHOD OF HEINRICH.
]
The part of the box above ground is connected with the bottle B by a
glass tube as indicated in the figure. The bottle B should have a
capacity of about ten liters. The air in B is forced out through C by
water running in from the supply A and the pressure in B is recorded by
the manometer D. The experiment should be tried on a soil thoroughly
moist.
In measuring the pressure in B the water pressure should be cut off by
the pinch-cock between A and B, and the pressure on the manometer
observed after the lapse of one to two minutes.
MOVEMENT OF WATER THROUGH SOILS: LYSIMETRY.
=163. Porosity in Relation to Water Movement.=—The intimate relation
which water movement in a soil bears to fertility makes highly important
the analytical study of this feature of porosity. A soil deficient in
plant food, in so far as chemical analysis is concerned, will produce
far better crops when the flow of moisture is favorable than a highly
fertile soil in which the water may be in deficiency or excess.
Aside from the actual rain-fall the texture of the soil, in other words
its porosity, is the most important factor in determining the proper
supply of moisture to the rootlets of plants. Even where the rain-fall
is little, a properly porous soil in contact with a moist subsoil will
furnish the moisture necessary to plant growth. This fact is well
illustrated by the beet fields in Chino Valley, California. In this
locality most excellent crops of sugar beets are produced without
irrigation and almost without rain.
=164. Methods of Water Movement=—The translocation of soil water is
occasioned in at least two ways; namely,
1. By changing the porosity of a given stratum of soil.
2. By changing the amount of water a given stratum contains.
The following experiment by King[113] illustrates a convenient method of
studying this movement of water:
On a rich fallow ground of light clay soil, underlaid at a depth of
eighteen inches by a medium-grained sand, water, to the amount of two
pounds per square foot on an area of eight by eight feet, was slowly
added with a sprinkler, samples of soil having been previously taken in
six-inch sections down to a depth of three feet. The samples were taken
along a diagonal of the square under experiment and one foot apart. The
middle sample of the line being from the center of the area. The
sampling and wetting occurred between one and three P. M., on July 22,
and on the evening of the 23 a corresponding series of samples was taken
along a line parallel to the first but eight inches distant. The changes
in the percentages of water in the soil are given in the following
table, showing the translocation of water in soil due to wetting the
surface:
PER CENT OF WATER. DIFFERENCE.
Inches. Before After In per cent. In pounds per
wetting. wetting. cu. ft.
0–6 14.00 22.23 +8.23 +2.873
6–12 15.14 15.71 +0.57 +0.199
12–18 16.23 15.75 –0.48 –0.213
18–24 17.70 16.92 –0.78 –0.347
24–30 16.76 14.41 –2.35 –1.032
30–36 15.51 15.21 –0.30 –0.132
The figures given in the last column of the table are computed from the
absolute dry weights of the upper three feet of soil as determined in a
locality some rods from the place of experiment, and are therefore only
approximations, but the error due to this cause is certainly small. It
will be seen that while only two pounds of water to the square foot were
added to the surface, the upper six inches contained 2.87 pounds per
square foot more than before the water was added, and the second six
inches contained 0.199 pound more, and this too in the face of the fact
that the evaporation per square foot from a tray sitting on a pair of
scales close by, was 0.428 pound during the interval under
consideration. Similar experiments were made by taking the samples of
soil at 5.30 P. M. in one-foot sections down to four feet, at four
equally distant places along the diagonal of a square, six by six feet,
and having the ground sprinkled. At the same time four similar sets of
samples were taken on lines vertical to each of the sides of the square
but four feet distant from them. The amount of water the soil contained
was then determined, and at 11.30 A. M., nineteen hours later, another
series of samples was taken at points about four inches distant from the
last and the amount of water determined with the result given below.
TRANSLATION OF WATER OCCASIONED BY WETTING THE SURFACE.
─────────────┬───────────────────────────────────────────────────────
Depth of │
samples. │ WET AREA.
─────────────┼───────────────────────────╥───────────────────────────
„ │ Before wetting. ║ After wetting.
─────────────┼─────────────┬─────────────╫─────────────┬─────────────
„ │ │ Pounds of ║ │ Pounds of
│ Per cent of │ water per ║ Per cent of │ water per
│ water. │ cubic foot. ║ water. │ cubic foot.
─────────────┼─────────────┼─────────────╫─────────────┼─────────────
0–12 inches │ 16.86│ 11.78║ 20.15│ 14.06
12–24 „ │ 17.76│ 15.79║ 19.71│ 17.52
24–36 „ │ 16.76│ 14.73║ 17.72│ 15.58
36–48 „ │ 15.01│ 14.03║ 16.47│ 15.40
─────────────┼─────────────┼─────────────╫─────────────┼─────────────
Averages │ 16.59│ 14.08║ 18.51│ 15.64
│ │ ║ │
Total amount │ │ ║ │
of water │ │ 56.33║ │ 62.56
Amount of │ │ ║ │
change │ │ ║ │ +6.23
─────────────┴─────────────┴─────────────╨─────────────┴─────────────
─────────────┬───────────────────────────────────────────────────────
Depth of │
samples. │ AREA NOT WET.
─────────────┼───────────────────────────╥───────────────────────────
„ │ First samples. ║ Second samples.
─────────────┼─────────────┬─────────────╫─────────────┬─────────────
„ │ │ Pounds of ║ │ Pounds of
│ Per cent of │ water per ║ Per cent of │ water per
│ water. │ cubic foot. ║ water. │ cubic foot.
─────────────┼─────────────┼─────────────╫─────────────┼─────────────
0–12 inches │ 17.72│ 12.38║ 18.27│ 12.75
12–24 „ │ 19.18│ 17.05║ 19.94│ 17.72
24–36 „ │ 16.97│ 14.92║ 17.52│ 15.40
36–48 „ │ 15.49│ 14.48║ 15.16│ 14.17
─────────────┼─────────────┼─────────────╫─────────────┼─────────────
Averages │ 17.34│ 14.71║ 17.71│ 15.01
│ │ ║ │
Total amount │ │ ║ │
of water │ │ 58.83║ │ 60.04
Amount of │ │ ║ │
change │ │ ║ │ +1.21
─────────────┴─────────────┴─────────────╨─────────────┴─────────────
The above data show sufficiently well the method of investigation to be
pursued in studies of this kind.
=165. Capillary Movement of Water.=—The method of investigation proposed
by King[114] consists in taking samples of soil at intervals of one,
two, three, or four feet in depth, and determining the amount of
moisture in each in connection with the amount of rain-fall during the
period. The quantity of water contained in a given soil, at various
depths and on different dates, is shown in the following table:
Depth in Date. Per cent Pounds per Increase or decrease.
feet. water. cubic foot. Pounds per cubic foot.
1 March 8th 24.33 16.98
1 April 18th 22.37 15.61 –1.37
2 March 8th 15.80 14.05
2 April 18th 21.64 19.24 +5.19
3 March 8th 11.16 9.81
3 April 18th 16.24 14.27 +4.46
4 March 8th 7.87 7.36
4 April 18th 11.19 10.46 +3.10
The rain-fall during the interval was 4.18 inches, equal to 21.77 pounds
per square foot.
=166. Lateral Capillary Flow.=—To determine the lateral capillary flow
of water in a soil the following method, used by King[115] may be
employed:
A zinc lined tray, six by six feet in area and eight inches deep, is
filled with a soil well packed. In one corner of this tray a section of
five inches of unglazed drainage tile, having its lower end broken and
jagged, is set and the dirt well filled in round it. By means of a
Mariotte bottle water is constantly maintained in the bottom of this
tile, three-quarters of an inch deep, so that it will flow laterally by
capillary action into the adjacent soil, the object being to determine
the extent and rate of capillary flow laterally.
The water content of the soil is determined at the time of starting the
experiment, on the circumferences of circles described with the tile as
a center, the distance between the circles being one foot. At stated
periods, usually at intervals of one day, the content of moisture is
again determined at the same points. The investigations show that the
lateral movement of water in the soil is not rapid enough to extend much
beyond three feet in thirty-one days, for beyond that distance the soil
was found to be drier than at the beginning of the experiment. A record
is to be kept of the amount of water delivered to the soil by weighing
the supply bottle at intervals, and the rates given at which the soil
takes up the water in grams per hour and pounds per day. Also the amount
of flow per square foot of soil section together with the mean daily
evaporation should be noted. The mean flow per foot of soil section is
computed on the assumption that the outer face of the zone of completely
saturated soil is the delivering surface. In King’s work this point, as
nearly as could be determined, was twelve inches from the corner of the
tray and hence the figures at best can only be regarded as
approximations. The method of stating results is shown in the following
table:
SHOWING THE RATE OF LATERAL CAPILLARY FLOW OF WATER IN CLAY LOAM.
Date. No. Total mean, Total mean, Mean daily flow Mean daily
of hourly flow, daily flow, per square evaporation,
days. grams. pounds. foot, pounds. pounds.
Jan. 28 to 5 70.70 3.73 2.38
Feb. 2
Feb. 2–7 5 85.98 4.54 2.91
Feb. 7–12 5 79.33 4.19 2.64
Feb. 12–17 5 79.41 4.19 2.64 0.598
Feb. 17–22 5 70.79 3.74 2.38 0.534
Feb. 22–28 6 59.89 3.16 2.01 0.451
Feb. 28 to 6 60.74 3.21 2.04 0.458
March 6
Mar. 6–13 7 60.37 3.14 2.00 0.448
───────────────────────────────────────────────────────────────────────
Means 2.38 0.498
From this table it will be seen that the flow of water in the soil
varied in rate, being slower during the first five days than in the
succeeding fifteen days. After twenty days the flow dropped again to the
beginning rate and then fell below, but remained quite constant during
the following nineteen days. For the sake of uniformity in units of
measure the daily quantity of flow should be given in kilograms when the
hourly flow is given in grams.
=167. Causes of Water Movement in the Soil.=—The movement of water in a
soil as explained by Whitney[116] is due to two forces, _viz._,
gravitation and surface tension.
The force of gravitation in a given locality is always uniform, both in
direction and magnitude per unit volume of water.
Surface tension is the tendency of any exposed water surface to pull
itself together. It may act in any direction, according to
circumstances, and may thus sometimes help and sometimes antagonize the
force of gravitation.
According to the law of surface tension any particle of moisture tends
to assume the smallest possible area. This tendency is a constant
definite force per unit of surface at a given temperature. In the soil
this constant strain on the free surface of water particles serves, in a
high degree, to move them from place to place, in harmony with the
requirements of the different portions of the field.
When a soil is only slightly moist the water clings to its grains in the
form of a thin film. When these soil particles are brought together the
films of water surrounding them unite, one surface being in contact with
the soil particles and the other exposed to the air. If more water enter
the soil the film thickens until finally, when the point of saturation
is reached, all the space between the soil particles becomes filled with
water, and surface tension within the soil is thus reduced to zero.
Gravity then alone acts on the water and with a maximum force.
In a cubic foot of ordinary soil the total surface of the soil particles
will be at least 50,000 square feet. It follows that when the soil is
only slightly moist the exposed water surface of the films surrounding
the soil particles approximates that of the particles themselves. If
such a mass of slightly moist soil be brought in contact with a like
mass saturated with water, the films of water at the point of contact
will begin to thicken in the nearly dry soil at the expense of the water
content of the saturated mass. The water will thus be moved in any
direction.
During evaporation the surface tension near the surface of the soil is
increased, and water is thus drawn from below. In like manner, when rain
falls on a somewhat dry soil, the surface tension is diminished and the
greater surface tension below pulls the moisture down even when
gravitation would not be sufficient for that purpose.
Certain fertilizers have the faculty of modifying surface tension and
thus change the power of the soil in its attraction for moisture. In
this way such fertilizers act favorably on plant growth, both by
providing plant food and by supplying needed moisture.
=168. Surface Tension of Fertilizers.=—Whitney gives the following data
in respect of the surface tension of aqueous solutions of some of the
more common fertilizing materials. It is expressed in gram meters per
square meter, _i. e._, on a square meter of liquid surface there is
sufficient energy to lift the given number of grams to the height of one
meter.
SURFACE TENSION OF VARIOUS FERTILIZING SOLUTIONS.
Solution of— Specific gravity. Gram meters per square meter.
Salt 1.070 7.975
Kainite 1.053 7.900
Lime 1.002 7.696
Water 1.000 7.668
Acid phosphate 1.005 7.656
Plaster 1.000 7.638
Ammonia 0.960 6.869
Urine 1.026 6.615
Magnesium chlorid 1.1000 7.964
Basic slag 1.0012 7.890
Marl 1.0013 7.855
Potassium chlorid 1.1000 7.853
Ammonium sulfate 1.1000 7.834
Dried blood 1.0001 7.764
Ground bone 1.0007 7.749
Sodium nitrate 1.1000 7.730
Sodium sulfate 1.1000 7.730
Wood ashes 1.0038 7.674
Potassium nitrate 1.1000 7.661
Potassium sulfate 1.0830 7.658
Ammonium nitrate 1.1000 7.656
Dried fish 1.0026 7.594
Stable manure 1.0013 7.464
Cotton-seed meal 1.0054 6.534
Tankage 1.0169 4.844
Cotton seed 1.0070 4.788
SURFACE TENSION OF SOIL EXTRACTS.
Kind of Soil. Specific gravity. Surface tension.
Kentucky blue grass 1.000 7.244
Triassic red sandstone 1.000 7.244
Wheat soil 1.000 7.098
Garden soil 1.000 7.089
=169. Method of Estimating Surface Tension.=—The determination of
surface tension is made by measuring the rise of the liquid in a
capillary tube. A short piece of thermometer tubing is used, the
diameter of the bore being determined by careful microscopic
measurements with a micrometer eyepiece. The diameter of the tube should
be about 0.5578 millimeter. The tube is very thoroughly cleaned after
each observation, or set of observations, with a strong caustic potash
solution, and, after washing, is allowed to stand for some time in a
saturated solution of potassium bichromate in strong sulfuric acid. The
height of the rise in the capillary tube is measured with a
cathetometer.
The following formula is used for the calculation of the results:
T = (_h d_ ω)/(4 cos. _a_)
Where T is the surface tension, _d_ is the diameter of the tube in
centimeters; _h_ the height to which the liquid rises in the capillary
tube in centimeters; ω is the specific gravity of the solution; and 4
cos. _a_ refers to the angle of the liquid with the sides of the glass
tube. For a tube of the size given above, 5° 24′ is the value of this
edge angle. In regard to saline solutions, Quincke[117] says, that the
edge angle appears to increase a little with augmenting concentration of
the saline solution, but otherwise to differ only inconsiderably from
the edge angle of pure water.
=170. Effect of the Solutions on Surface Tension.=—The mineral
fertilizers, as a rule, increase the surface tension of water, while
organic matters in solution decrease it. But it must not be forgotten in
this connection that but little of the organic matter in the fertilizers
employed for the experiment passes into solution. Moreover, with these
substances, the accuracy of the work is impaired somewhat by the
increased viscosity. In general, the results of the experiment are in
harmony with the well-known effect of magnesium, sodium, and potassium
chlorids, and sodium nitrate, to make the soil more moist in dry
weather, and the opposite effect produced by the application of organic
matter.
=171. Method of Preparing Soil Extracts.=—The soil extracts used in
determining the surface tension, as given in the above table, are
prepared as follows:
Ten grams of the soil are rubbed up with fifteen to twenty cubic
centimeters of distilled water and allowed to stand for twenty-four
hours with frequent stirring. Any fine particles not removable by a
filter are neglected, although they may give a turbid appearance to the
solution.
=172. Lysimetry.=—The process of measuring the capacity of a soil to
permit the passage of water and of collecting and determining the amount
of flow and determining soluble matters therein is known as lysimetry.
In general, the rate at which water will pass through a soil depends on
the fineness and approximation of its particles. Water will pass through
coarse sand almost as rapidly as through a tube, while a fine clay may
be almost impervious. The study of the phenomena of filtration through
soil, and the methods of quantitatively estimating them, are therefore
closely related to porosity.
Two cases are to be considered, _viz._: First, percolation through
samples of soil prepared for analysis, and second, the passage of the
water through soil _in situ_, whether it be virgin or cultivated.
The determination of the rate of flow through a soil in laboratory
samples, gives valuable information in respect of its physical
properties, while the same determination made on the soil _in situ_, has
practical relations to the supply of moisture, to growing plants, and
the waste of valuable plant food in the drainage waters. The
determination of the rate of flow of water through a small sample,
disturbed as little as possible in its natural condition, is classed
with the first divisions of the work, inasmuch as the removal of a
sample of soil from a field, and its transfer to the laboratory,
subjects it to artificial conditions, even if its texture be but little
disturbed by the removal.
=173. Calculation of the Relative Rate of Flow of Water Through
Soils.=—There will evidently be one space, or opening, into the soil for
every surface grain, as pointed out by Whitney,[118] and the approximate
number of grains, or of openings, on a unit area of surface may be found
by the following formula:
N = (√((M × W)/(V))^⅔
where N is the number of grains, or openings, on one square centimeter
of surface, M is the approximate number of grains in one gram of soil, W
is the weight of soil, V is the total volume of the soil grains and the
empty space.
If the grains are assumed to be symmetrically arranged and the spaces
between them cylindrical in form, the radii of the spaces can be found
by the following formula:
_r_ = √(V₁)/(πNL)
where _r_ is the radius of a single space, V is the total volume of the
empty space, N is the number of grains or spaces on one square
centimeter of surface, and L is the depth of the soil.
If the space within the soil is completely filled with water the
relative rate of flow of water through the soil will be according to the
fourth power of the radius of a single space multiplied by the number of
spaces on the unit area of surface, as shown by the following formula:
T₁ = (N(_r_)⁴T)/(N₁(_r_₁)⁴)
where N-N₁ are the numbers of spaces, and _r_-_r_₁ are the radii of
single spaces in the respective soils, and T-T₁ the times required for a
unit volume of water to flow through the soils under the same head or
pressure.
The space within the soil is rarely filled with water in agricultural
lands, and the most favorable amount of water for the soil to hold, as
Hellriegel and others have shown, is from thirty to fifty per cent of
the total amount of water the soil can hold if all the space within it
were filled.
If the space within the soil be only partly filled with water, as in
most arable lands, the water will move in a thin film surrounding the
soil grains and according to the fourth power of the thickness of the
film. The mean thickness of the film surrounding the soil grains may be
theoretically determined by the following formula, which is based on the
conception that the film is cylindrical and of uniform size throughout:
_t_ = _r_(1 − √(_s_)/(_s_ + _p_))
where _s_ is the per cent by weight of water which the soil will hold
when the empty space is filled with water, _p_ the per cent of water
actually contained in the soil, _r_ the radius of a single space, and
_t_ the mean thickness of the film surrounding the soil grains.
The relative rate of flow of water through the soils will then be
according to the following formula:
T₁ = (N(_t_)⁴T)/(N₁(_t_)₁⁴))
It must be remembered that these formulæ give only approximate and
comparative values for comparing one soil with another. The structure of
the soil is altogether too intricate to expect ever to obtain absolute
values.
If the observed rate of flow varies widely from the relative rate
calculated from the mechanical analysis, it will indicate a difference
in the arrangement of the soil grains, or in the amount or condition of
the organic matter in the soils. In the older agricultural regions of
the United States, south of the influence of the glacial action, the
great soil areas appear to have sensibly similar arrangements of the
soil grains, and sensibly uniform conditions of organic matter, save
where these have been modified by local conditions.
=174. Measurement of Rate of Percolation in a Soil Sample.=—In order to
measure the power of the soil for permitting the passage of water, a
box, about twenty-five centimeters high and having a cross section of
about three centimeters square, is used. Below, this box has a
funnel-shaped end with a narrow outlet tube, which at its lower end is
closed with cotton, in such a way that a portion of the cotton extends
through the stem of the funnel. A little coarse quartz sand is scattered
over the cotton and afterwards the funnel part of the apparatus filled
with it. The sand and the cotton are saturated with water and the
apparatus weighed. The box is then filled with the fine sample of earth,
with light tapping, until the depth of earth has reached about sixteen
centimeters. The apparatus, after the addition of the air-dried earth,
is again weighed to determine the amount of earth added, and the soil is
then saturated by the careful addition of water. After the excess of
water has run down the funnel, the total quantity of absorbed water is
determined by reweighing the apparatus and the total water-holding power
of the soil is determined. There is carefully added, without stirring up
the surface of the soil, a column of water eight centimeters high,
making in all from sixty to seventy grams. The time is observed until
the water ceases to drip from the funnel. The dripping begins
immediately after the water is poured on and ceases as soon as the
liquid on the surface of the soil has completely disappeared. On the
repetition of this operation a longer time for the passage of the water
is almost always required than at the first time. The experiment,
therefore, must be tried three or four times and the mean taken.
=175. Method of Welitschowsky.=[119]—The soil is placed in the vessel
_a_, Fig. 22, which is cylindrical in shape and five centimeters in
diameter. The lower end of the cylinder is closed with a fine wire-gauze
disk and the upper end is provided with an enlargement for the reception
of the tube _b_, which is connected to _a_ with a wide rubber band. The
lower end of the tube _b_ is also closed with a wire-gauze disk. These
tubes may be conveniently made of sheet zinc. The tube _b_ carries on
the side, at distances of ten centimeters, small tubes of fifteen
millimeters diameter. On the opposite side it is provided with a glass
tube set into a side tube near the bottom for the purpose of showing the
height of the water. The side tube carrying the water meter is provided
with a stop-cock as shown in the figure.
[Illustration:
FIGURE 22.
METHOD OF WELITSCHOWSKY.
]
In conducting the experiment, after the apparatus has been arranged as
described, the small lateral tubes are, with one exception, closed with
stoppers. On the open one, _d_, a rubber tube is fixed for the purpose
of removing the water. The required water pressure is secured by taking
the lateral opening corresponding to the pressure required. Water is
introduced into the apparatus slowly through the glass tube _f_.
The water rises to _d_ and then any excess flows off through _e_. By a
proper regulation of the water supply the pressure is kept constant at
_d_. The water flowing off through _a_ is collected by the funnel and
delivered to graduated flasks where its quantity can be measured for any
given unit of time. Since the rate of flow at first shows variations,
the measurement should not be commenced until after the flow becomes
constant.
In general, the experiments should last ten hours, and, beginning with a
water pressure of 100 centimeters, be repeated successively with
pressures of eighty, sixty, forty, and twenty, centimeters, etc. In
coarse soils, or with sand, one hour is long enough for the experiment.
=176. Statement Of Results.=—In the following tables the results for
ninety centimeters, seventy centimeters, etc., are calculated from the
analytical data obtained for 100 centimeters, eighty centimeters, etc.
MATERIAL—QUARTZ SAND.
│
│ LITERS OF WATER PASSING IN TEN
│ HOURS.
No. Diameter of│
of sand │
Exp. particles │ Water
in │pressure in Thickness of Soil Layer.
mm. │ cm. 10 cm. 20 cm. 30 cm.
1. 0.01–0.71 │ 10 0.244 0.187 0.151
„ „ │ 20 0.282 0.198 0.154
„ „ │ 30 0.320 0.209 0.158
„ „ │ 40 0.358 0.220 0.161
„ „ │ 50 0.396 0.231 0.165
„ „ │ 60 0.434 0.242 0.168
„ „ │ 70 0.472 0.253 0.172
„ „ │ 80 0.510 0.264 0.175
„ „ │ 90 0.548 0.275 0.179
„ „ │ 100 0.586 0.286 0.182
2. 0.071–0.114│ 10 2.194 1.724 1.425
„ „ │ 20 2.898 2.012 1.578
„ „ │ 30 3.602 2.300 1.731
„ „ │ 40 4.306 2.588 1.884
„ „ │ 50 5.010 2.876 2.037
„ „ │ 60 5.714 3.164 2.190
„ „ │ 70 6.418 3.452 2.343
„ „ │ 80 7.122 3.740 2.496
„ „ │ 90 7.826 4.028 2.649
„ „ │ 100 8.530 4.316 2.802
Similar sets of data have been collected with powdered limestone, clay
and humus.
The general conclusions from the experiments are as follows:
1. Clay (kaolin) and humus (peat) are almost impermeable for water, and
fine quartz and limestone dust are also very impermeable.
2. The permeability of a soil for water increases as the particles of
the soil increase in size, and when particles of different sizes are
mixed together the permeability approaches that of the finer particles.
3. The quantity of water passing through a given thickness of soil
increases with the water pressure but is not proportional thereto,
increasing less rapidly than the pressure.
4. The quantity of water passing under a given pressure is inversely
proportional to the thickness of the soil layer when the particles are
very fine and the pressure high.
=177. Method of Whitney.=—To determine the permeability of the soil or
subsoil to water or air, in its natural position in the field, the
following method, due to Whitney, can be recommended:
A hole should be dug, and the soil and subsoil on one side removed to
the depth at which the observation is to be made. A column of the soil
or subsoil, two inches or more square, and four or five inches deep, is
then to be carved out with a broad bladed knife, or a small saw can be
conveniently used for cutting this out. A glass or metal frame, a little
larger than the sample and three or four inches deep, is slipped over
the column of soil, and melted paraffin is run in slowly to fill up the
space between the soil and the frame. The soil is then struck off even
with the top and bottom of the frame, preferably with a saw, or at any
rate taking care not to smooth it over with a knife, which would disturb
the surface and affect the rate of flow. The frame is then placed upon
some coarse sand or gravel, contained in a funnel, to prevent the soil
from falling out and to provide good drainage for the water to pass
through. Another similar frame can then be placed on top and secured by
a wide rubber band. A little coarse sand, which has been thoroughly
washed and dried, is then placed on the soil, and water carefully poured
on until it is level with the top of the frame. When the water begins to
drop from the funnel more water must be added to the top, so as to have
the initial depth of water over the soil the same in all the
experiments. A graduated glass is then pushed under the funnel, and the
time noted which is required for a quantity of water to pass through the
soil. The quantity usually taken for measurement is equivalent to one
inch in depth over the soil surface. In taking the sample, root and
worm-holes are to be avoided, and these are particularly troublesome in
clay lands.
=178. Measurement of Percolation through the Soil in Situ.=—If lateral
translocation could be prevented, the measurement of the quantity of
water descending in the soil through a given area would be a matter of
simplicity. But to secure accurate results all lateral communication of
a given body of soil with adjacent portions must be cut off. Various
devices have been adopted to secure this result. An elaborate system of
lysimetric measurements is illustrated by the apparatus erected by the
Agricultural Experiment Station, of Indiana.
The plan and section of the apparatus are shown in Fig. 23.
Each lysimeter box, when finished, resembles somewhat a hogshead with
one head out. The sides, however, are perfectly straight inside, having
a slight thickening in the center, on the outside, for making them
stronger. The sides and bottom of the apparatus are constructed of oak
and lined with sheet copper carefully soldered so as to be water-tight.
Six inches above, and parallel to the bottom of each of the boxes, is a
perforated copper tube, which extends entirely across the lysimeter, and
passing through one of the sides connects the box with an underground
vault in which the observations are taken. These tubes give an outlet to
the drainage water, as described further on. The lysimeters are made of
any required depth, the two which are shown in section being three and
two-thirds and six and two-thirds feet deep, respectively.
The following method is employed for filling them with soil: There are
first placed in the bottom of each lysimeter six inches of fine sand,
sifted and washed, which fills them up to the level of the drainage
tubes. The lysimeters are then filled with fine, sifted surface soil, to
the depth of three and six feet, respectively, making a complete pair of
lysimeters, and leaving two inches of the lysimeter boxes projecting
above the surface of the soil so that each one will receive exactly its
proper share of the rain-fall.
The lysimeters of the other pair, which are the same size as the first,
are filled in a different way. The lysimeters are first constructed and
placed over vertical columns of soil _in situ_, which are obtained by
digging away all the surrounding soil and leaving the columns standing.
The shorter lysimeter is sunk in this way to within two inches of its
entire length. It is then tipped over carrying the column of soil with
it. Six inches of the subsoil are then removed, when the drainage tube
and sand are put in, as in the first pair, and the bottom of the tube
soldered in place. The lysimeter is thus filled with the natural soil in
place. The longer box is in the same way filled, as far as possible,
with the soil in place, but a gravelly nature of the soil may render it
impossible to do the filling with a single column unbroken, so the
gravel and sand from the lower portion of the soil are to be filled in
separately. The drainage tube and bottom of sand are placed in the
longer lysimeter in the same way as in the shorter.
[Illustration:
FIGURE 23.
GROUND PLAN AND VERTICAL SECTION OF LYSIMETERS AND VAULTS SHOWING
POSITION OF THE APPARATUS.
1, 1, 1, 1, Lysimeters.
2, 2, 2, 2, Receiving bottles.
3, 3, Supplying apparatus.
4, 4, Skylights.
5, 5, 5, 5, Wall of vault.
6, 6, Brick walls.
7, Entrance Steps.
8, Vault.
]
The purpose of placing sand at the bottom of each lysimeter is to offer
a porous stratum in which free water may collect and rise to the level
of the perforated copper tube, which would prevent any further rise by
conveying the surplus above into the vault as drainage water. The soil
above the tube will therefore be constantly drained and the sand below
constantly saturated, unless the water be drawn up by the capillary
action of the soil as the result of evaporation from the surface.
By means of a proper arrangement within the vault, of a kind of
Mariotte’s bottle, the water may be caused to flow back through the
drainage tube into the lysimeter to take the place of that lost by
evaporation, and thus maintain the level of free water just below the
drainage tube. The water flowing back to the lysimeter, and the amount
of drainage water, are carefully measured by a system of graduated
tubes.
The lysimeters thus constructed represent tile-drained land; in one case
the tile being three feet below the surface and in the other six feet
below. The drainage waters collected in the receiving bottles can be
measured and analyzed from time to time, as occasion may require, to
determine the amount of plant food which is removed.
=179. Improved Method of Deherain.=[120]—Deherain’s earlier experiments
were made in pots containing about sixty kilos of soil. These vases
serve very well for some kinds of plants, but there are other kinds
which do not grow at all normally when their roots are imprisoned. For
instance, in pots, even of the largest size, wheat is always poor, beets
irregular, maize never acquires its full development, and the
conclusions which can be drawn from the experiments can not be
predicated of the action of the plant under conditions entirely normal.
It is necessary therefore to carry on the work in an entirely different
way, and to construct boxes so large as to make the conditions of growth
entirely normal. The arrangement of these boxes is shown in Fig. 24.
They are placed in a large trench, two meters wide, one meter deep, and
forty meters long. There are twenty boxes in this trench, the upper
surface of each containing four square meters area. The boxes are one
meter deep, and therefore can contain four cubic meters of soil. The
sides and bottoms of the boxes are made of iron lattice work, covered
with a cement which renders them impervious to water.
The bottom inclines from the sides towards the middle, and from the back
to the front, thus forming a gutter which permits of the easy collection
of the drainage. The drainage water is conveyed, by means of a pipe and
a funnel, into a demijohn placed in the ditch in front of the apparatus,
as shown in the figure. These receptacles stand in niches under the
front of the cases, and are separated by the brick foundations. Access
to them is gained by means of the inclined plane shown in the figure,
and this plane permits the demijohns in which the drainage water is
collected, to be removed with a wheelbarrow for the purpose of weighing.
This apparatus is especially suitable for a study of the distribution of
the nitrogen to the crop, the soil and the drainage waters. The loss in
drainage waters of potash and phosphoric acid is insignificant in
comparison with the loss in nitrogen.
The cases having been placed in position they are filled with the
natural soil, which is taken to the depth of one meter, in such a way
that the relative positions of the soil and subsoil are not changed.
While the soil is transferring to the cases it is carefully sampled in
order to have a portion representing accurately the composition of both
the soil and subsoil. These samples are subjected to analysis and the
quantities of nitrogen, phosphoric acid, and potash contained therein
carefully noted.
One or two cases should be left without crop or fertilizer to determine
the relations of the soil and subsoil to the rain-fall. Three or four
cases should be kept free of vegetation and receive treatment with
different fertilizer, in order to determine the influences of these on
the deportment of the soil to rain-fall. The rest of the cases should be
seeded with plants representing the predominant field culture of the
locality, and some of them should be fertilized with the usual manures
used in farm culture.
[Illustration:
FIGURE 24.
DEHERAIN’S APPARATUS FOR COLLECTING DRAINAGE WATER.
]
AUTHORITIES CITED IN PART THIRD.
Footnote 70:
Comptes rendus, Tome 112, p. 598.
Footnote 71:
Stockbridge, Rocks and Soils, p. 153.
Footnote 72:
Die Landwirtschaftlichen Versuchs-Stationen, Band 8, S. 40.
Footnote 73:
König, Untersuchung Landwirtschaftlich und Gewerblich Wichtiger
Stoffe, S. 48.
Footnote 74:
Methods of Swedish Agricultural Chemists, translated for author by F.
W. Woll.
Footnote 75:
Poggendorff’s Annalen, Fifth Series, Band 9, Ss. 102, et seq.
Footnote 76:
Pennsylvania Agricultural Experiment Station Report, for 1891, pp.
194, et seq.
Footnote 77:
Agricultural Science, Vol. 8, pp. 28, et seq. (Correction. For Fig.
13, second line from bottom of page 112, read Fig. 14.)
Footnote 78:
Haberland. Forschungen auf der Gebiete der Agricultur-Physik, 1878, S.
148.
Footnote 79:
Grundlagen zur Beurteilung der Ackerkrume, Weimar, 1882.
Footnote 80:
Vid. supra, 10.
Footnote 81:
These general principles are taken chiefly from a résumé of the
subject by Prof. H. A. Huston. Indiana Agricultural Experiment
Station, Bulletin 33, pp. 46, et seq.
Footnote 82:
Knop’s Agricultur Chemie, Abteil II.
Footnote 83:
Beiträge zur Frage der Bodenabsorption.
Footnote 84:
Henneberg’s Journal, 1859, S. 35.
Footnote 85:
Die Landwirtschaftlichen Versuchs-Stationen, Band 27, S. 107.
Footnote 86:
Die Bonitirung der Ackererde, S. 49.
Footnote 87:
Journal Chemical Society of London, 1868.
Footnote 88:
Landw. Central-Blatt, Band 11, S. 169.
Footnote 89:
bis Die Landwirtschaftlichen Versuchs-Stationen, Band 12, Ss. 21–50.
Footnote 90:
Jour. f. Landw., 1862, Band 3, Ss. 49–67.
Footnote 91:
Ann. d. Landw., Band 34, S. 319.
Footnote 92:
American Journal of Science, Vol. 14, p. 25.
Footnote 93:
bis (p. 122). Ms. communication to author.
Footnote 94:
Maryland Agricultural Experiment Station, Fourth Annual Report, p.
282.
Footnote 95:
Bulletin No. 4, U. S. Weather Bureau, p. 80.
Footnote 96:
bis (p. 125), Beiträge zur Agronomische Bodenuntersuchung, S. 31.
Footnote 97:
Zeitschrift für angewandte Chemie, 1889, S. 501.
Footnote 98:
Die Landwirtschaftlichen Versuchs-Stationen, Band 17, S. 85.
Footnote 99:
Ms. communication to author.
Footnote 100:
Proceedings of the Ninth Meeting of the Society for the Promotion of
Agricultural Science, p. 51.
Footnote 101:
Rocks and Soils, pp. 155 et. seq.
Footnote 102:
Anleitung zur Wissenschaftlichen Bodenuntersuchung, S. 137.
Footnote 103:
Analyse du Sol, p. 13.
Footnote 104:
Landwirtschaftliche Jahrbücher, Band 3, Ss. 771.
Footnote 105:
Forschungen auf dem Gebiete der Agricultur-Physik, 1885, Ss. 177, et
seq.
Footnote 106:
Vid. supra, S. 259.
Footnote 107:
Poggendorf, Annalen, Band 129, Ss. 437, et seq.
Footnote 108:
König, Untersuchung Landwirtschaftlich und Gewerblich Wichtiger
Stoffe, S. 59.
Footnote 109:
Vid. 37, S. 60.
Footnote 110:
Landwirtschaftliche Jahrbücher, Band 2, S. 383.
Footnote 111:
Forschungen auf dem Gebiete der Agricultur-Physik, 1880, S. 218.
Footnote 112:
Beurteilung der Ackerkrume, S. 222.
Footnote 113:
Wisconsin Agricultural Experiment Station, Seventh Annual Report, pp.
134, et seq.
Footnote 114:
Vid. supra, pp. 139, et seq.
Footnote 115:
Wisconsin Agricultural Experiment Station, Seventh Annual Report, p.
145.
Footnote 116:
Bulletin No. 4, Weather Bureau, pp. 13, et seq.
Footnote 117:
Philosophical Magazine, 1878.
Footnote 118:
Weather Bureau, Bulletin No. 4.
Footnote 119:
Forschungen auf dem Gebiete der Agricultur-Physik, 1891, S. 11.
Footnote 120:
Annales Agronomiques, Tome 16, p. 337; Tome 17, p. 49; Tome 18, p.
237; Tome 19, p. 69.
PART FOURTH.
MECHANICAL ANALYSIS OF SOILS.
THE FLOCCULATION OF SOIL PARTICLES.
=180. Relation of Flocculation to Mechanical Analysis.=—The tendency of
the fine particles of silt to form aggregates, which act as distinct
particles of matter, is the chief difficulty connected with the
separation of the soil into portions of equal hydraulic value by the
silt method of analysis. This tendency has been discussed fully by
Johnson[121] and Hilgard.[122]
=181. Illustration of Flocculation.=—A sediment, consisting of particles
of a hydraulic value, equal to one millimeter per second, is introduced
into an ordinary conical elutriating tube placed vertically, in which
the current of water entering below performs all the stirring which the
particles receive.
A current of water corresponding to a velocity below one millimeter per
second will, of course, not carry any of the particles out at the top of
the cylindrical tube, but will keep them moving through the conical
portion of the tube. If now the current be increased until its velocity
is greater than one millimeter per second after having run at the slower
velocity for fifteen or twenty minutes, very little of the sediment will
pass over, although theoretically the whole of it should. Even at a
velocity of five millimeters per second, much of the sediment will
remain in the tube. This, of course, is due to the coagulation of the
particles into molecular aggregates having a higher hydraulic value even
than five millimeters per second. These aggregates can be broken up by
violent stirring or moderate boiling, and the sediment reduced again to
its proper value. The conclusions which Hilgard derives from a study of
the above phenomena are as follows:
1. The tendency to coagulation is, roughly, in an inverse ratio to the
size of the particles. With quartz grains it practically ceases when
their diameter exceeds about two-tenths of a millimeter having a
hydraulic value of eight millimeters per second. The size of the
aggregates formed follows practically the same law as above. Sediment of
0.25 millimeter hydraulic value will sometimes form large masses like
snow-flakes on the sides of the elutriator tube.
2. The degree of agitation which will resolve the aggregates into single
grains is inversely as the size of the particles; or, more properly
perhaps, inversely as their hydraulic value.
3. The tendency to flocculation varies inversely as the temperature. So
much so is this the case that Hilgard at one time contemplated the use
of water at the boiling point in the mechanical analysis of soils, in
place of mechanical stirring.
4. The presence of alcohol, ether, and of caustic or carbonated
alkalies, diminishes the tendency to flocculation, while the presence of
acids and neutral salts increases it.
5. As between sediments of equal hydraulic value, but different
densities, the tendency to flocculation seems to be greater with the
less dense particles.
In regard to the mechanical actions which take place between the
particles, Hilgard considers them as irregular spheroids, each of which
can at best come in contact at three points with any other particle. The
cause of aggregation cannot therefore be mere surface adhesion
independent of the liquid, and the particles being submerged there is no
meniscus to create an adhesive tension.
Since experiment shows that the flocculative tendency is measurably
increased by the cohesion coefficient of the liquid, it seems necessary
to assume that capillary films of the latter interposed between the
surfaces of solids create a considerable adhesive tension even in the
absence of a meniscus.
=182. Effect of Potential of Surface Particles.=—Whitney suggests that
this is due to the potential of the surface particles of solids and
liquids.[123] The potential of a single water particle is the work which
would be required to pull it away from the surrounding water particles
and remove it beyond their sphere of attraction. For simplicity, it may
be described as the total force of attraction between a single particle
and all other particles which surround it. With this definition, it will
be seen that the potential of a particle on an exposed surface of water
is only one-half of the potential in the interior of the mass, as half
of the particles which formerly surrounded and attracted it were removed
when the other exposed surface of water was separated from it. A
particle on an exposed surface of water, being under a low potential,
will therefore tend to move toward the center of the mass where the
potential, _i. e._, the total attraction, is greater, and the surface
will tend to contract so as to leave the fewest possible number of
particles on the surface. This is surface tension.
If, instead of air, there is a solid substance in contact with the
water, the potential will be greater than on an exposed surface of the
liquid, for the much greater number of solid particles will have a
greater attraction for the water particles than the air particles had.
They may have so great an attraction that the water particle on this
surface, separating the solid and liquid, may be under greater potential
than prevails in the interior of the liquid mass. Then the surface will
tend to expand as much as possible, for the particles in the interior of
the mass of liquid will try to get out on the surface. This is the
reverse of surface tension. It is surface pressure, which may exist on a
surface separating a solid and liquid.
Muddy water may remain turbid for an indefinite time, but if a trace of
lime or salt be added to the water the grains of clay flocculate, that
is, they come together in loose, light flocks, like curdled milk, and
settle quickly to the bottom, leaving the liquid above them clear.
Ammonia and some other substances tend to prevent this and to keep the
grains apart if flocculation has already taken place.
If two small grains of clay, suspended in water, come close together
they may be attracted to each other or not, according to the potential
of the water particles on the surface of the clay. If the potential of
the surface particle of water is less than that of the particle in the
interior of the mass of liquid, there will be surface tension, and the
two grains will come together and be held with some force, as their
close contact will diminish the number of surface particles in the
liquid. If, on the other hand, the potential of the particle on the
surface of the liquid is greater than of the particle in the interior of
the mass, the water surface around the grains will tend to enlarge, as
there will be greater attraction for the water particles there than in
the interior of the mass of liquid, and the grains of clay will not come
close together and will even be held apart, as their close contact would
diminish the number of surface particles in the liquid around them.
=183. Influence of Surface Tension.=—Hilgard supposes that the surface
tension which is assumed to exist between two liquid surfaces must exert
a corresponding influence between the surfaces of solids and liquids,
apart from any meniscal action.
It is then to be expected that the adhesion of the particles
constituting one of these floccules will be very materially increased
whenever the formation of menisci between them becomes possible by the
removal of the general liquid mass. Suppose one of the floccules to be
stranded, it will, in the first place, remain immersed in a sensibly
spherical drop of liquid. As this liquid evaporates, the spherical
surface will become pitted with menisci forming between the single
projecting particles, and as these menisci diminish their radius by
still further evaporation, the force with which they hold the particles
together will increase until it reaches a maximum. As the evaporation
progresses beyond this point of maximum, the adhesion of the constituent
particles must diminish by reason of the disappearance of the smaller
menisci, and when finally the point is reached when liquid water ceases
to exist between the surfaces, the slightest touch, or sometimes even
the weight of the particles themselves, will cause a complete
dissolution of the floccule, which then flattens down into a pile of
single granules.
In regard to natural deposits from water, Hilgard supposes that they are
always precipitated in a flocculated state. The particles of less than
two-tenths millimeter diameter are carried down with those of a larger
diameter having much higher hydraulic value. Thus the deposition of a
pure clay can take place under only very exceptionable circumstances.
Whitney, on the other hand, suggests that grains of sand and clay carry
down mechanically the particles of fine silt and clay as they settle in
a turbid liquid in a beaker; and it is often difficult to wash out a
trace of fine material from a large amount of coarse particles, for this
reason, although there may be no trace whatever of flocculation.
=184. Destruction of Floccules.=—The destruction of the natural
floccules is seen in the ordinary process of puddling earth or clay. It
is also the result of violent agitation of water or of kneading or
boiling, or, finally, to a certain extent, of freezing. All these
agencies are employed by the workers in clay for the purpose of
increasing the plasticity which depends essentially upon the finest
possible condition of the material to be worked. As an illustration of
this, Hilgard cites the fact that any clay or soil which is worked into
a plastic paste with water, and dried, will form a mass of almost stony
hardness. If, however, to such a substance one-half per cent of caustic
lime be added, a substance which possesses in an eminent degree the
property of coagulating clay, the diminution of plasticity will be
obvious at once, even when in a wet condition. If now the mass be dried,
as in the previous case, it is easily pulverized. This is an
illustration of the effect of lime upon stiff lands, rendering them more
readily pulverulent and tillable. The conversion of the lime into a
carbonate in the above experiment by passing bubbles of carbonic acid
through the mass while still suspended in water does not restore the
original plasticity, thus illustrating experimentally the fact known to
all farmers that the effect of lime on stiff soil lasts for many years,
although the whole of the lime in that time has been converted into
carbonate.
=185. Practical Applications.=—The practical application of this is,
according to Hilgard, that the loosely flocculated aggregation of the
soil particles is what constitutes good tilth. For this reason the
perfect rest of a soil, if it is protected from the tamping influence of
rains and the tramping of cattle, may produce a condition of tilth which
cannot be secured by any mechanical cultivation. As an illustration of
this, the pulverulent condition of virgin soils protected in a forest by
the heavy coating of leaves may be cited. On the contrary, as pointed
out by Hilgard, there are some kinds of soil in which a condition of
rest may produce the same effect as tamping. These are soils which
consist of siliceous silt without enough clay to maintain them in
position after drying. In such a case, the masses of floccules collapse
by their own weight or by the least shaking, and fall closely together,
producing an impaction of the soil. This takes place in some river
sediment soils in which the curious phenomenon is presented of injurious
effects produced by plowing when too dry, which is the direct opposite
of soils containing a sufficient amount of clay and which are injured by
plowing too wet.
It is further observed that the longer a soil has been maintained in
good tilth, the less it is injured by wet plowing. This is doubtless,
according to Hilgard, due to the gradual cementation of the floccules by
the soil water which fixes them more or less permanently.
Whitney believes that the arrangement of the grains, or the condition of
flocculation in the soil, or the distance apart of the soil grains, is
determined, to a large extent, by the potential on the surface of the
grains; and he suggests that by changing this the exceedingly fine
grains of silt and clay can be pulled together or can be pushed further
apart, and so alter the whole texture of the land.
The action of alkaline carbonates in preventing flocculation, and thus
rendering tillage difficult or impossible, is pointed out by Hilgard in
the case of certain alkali soils of California. The soils which are
impregnated with alkaline carbonates are recognized by their extreme
compactness. The suggestion of Hilgard to use gypsum on such soils has
been followed by the happiest results. This gypsum renders any
phosphates present insoluble, and thus prevents loss by drainage, and
yet leaves the plant food in a sufficiently fine state as to be
perfectly available for vegetation.
=186. Suspension of Clay in Water.=—The suspension of clay in water and
the methods of producing or retarding flocculation and precipitation
have also been studied by Durham.[124] His experiment is made as
follows:
In a number of tall glass jars fine clay is stirred with water, and the
results of precipitation watched. In all cases it will be noticed that
the clay rapidly separates into two portions, the greater part quickly
settling down to the bottom of the jars, and the smaller part remaining
suspended for a greater or less length of time.
The power which water possesses of sustaining clay is gradually
destroyed by the addition of an acid or salt; a very small quantity, for
instance, of sulfuric acid, is sufficient to precipitate the clay with
great rapidity. In solutions of sulfuric acid and sodium chlorid of
varying strengths, suspended clay is precipitated in the order of the
specific gravity of the solutions, the densest solutions being the last
to clear up. This may be due to the greater viscosity of the denser
liquids.
The power which water possesses of sustaining clay is gradually
decreased by the addition of small quantities of certain salts and of
lime.
=187. Effect of Chemical Action=—Brewer[125] emphasizes the importance
of chemical action in the flocculation of clays. As expressed by him the
chemical aspects of the phenomena of sedimentation have either been
lightly considered or entirely ignored. Brewer is led to believe that
the action of clay thus suspended is analogous to that of a colloidal
body. Like a colloid, when diffused in water, the bulk of the mass is
very great, shrinking enormously on drying. He therefore concludes that
clays probably exist in suspension as a series of hydrous silicates
feebly holding different proportions of water in combination and having
different properties so far as their behavior to water is concerned.
Some of them he supposes swell up in water much as boiled starch does,
and are diffusible in it with different degrees of facility, and that
the strata observed on long standing of jars of suspended clay represent
different members of this series of chemical compounds which hold their
different proportions of combined water very feebly and are stable under
a very limited range of conditions.
These compounds are probably destroyed or changed in the presence of
acids, salts and various other substances, and are stable only under
certain conditions of temperature, those which exist at one temperature
being destroyed or changed to other compounds at a different
temperature.
=188. Theory of Barus.=—Brewer’s hypothesis, however, is not in harmony
with the demonstration of Barus, who proves that a given particle of
clay has the same density in ether as in water.
The physical and mathematical aspects of sedimentation have also been
carefully studied by Barus.[126] The mathematical conditions of a fine
particle suspended in a liquid and free from the influences of
flocculation are described by Barus in the following equations.
If P be the resistance encountered by a solid spherule of radius r,
moving through a viscous liquid at the rate x, and if k be the
frictional coefficient, then P = 6πkrx. Again, the effective part of the
weight of the particle is P´ = ⁴⁄₃πr³ (ρ-ρ´)g, where g is the
acceleration of gravity and ρ and ρ´ the density of solid particle and
liquid, respectively. In case of uniform motion P = P´. Hence x = 2/9kr²
(ρ-ρ´)g ... (1).
In any given case of thoroughly triturated material the particles vary
in size from a very small to a relatively large value; but by far the
greater number approach a certain mean figure and dimension. An example
of this condition of things may be formulated. To avoid mathematical
entanglement let y = Ax^{³⁄₂}e^{-x²} ... (2) where y is the probable
occurrence of the rate of subsidence x. If now the turbidity of the
liquid (avoiding optical considerations) be defined as proportional to
the mass of solid material particles suspended in unit of volume of
liquid, then the degree of turbidity which the given ydx particles add
to the liquid is, _caeteris paribus_, proportional to r³ydx, where r is
the mean radius. Hence the turbidity, T, at the outset of the experiment
(immediately after shaking), is T = T₀∫₀^∞r³ydx = T₀, where equations
(1) and (2) have been incorporated.
If the plane at a depth d below the surface of the liquid be regarded,
then at a time after shaking the residual turbidity is
(3) ... T_{d} = T₀∫^{d/t}₀r³ydx = T₀(1 − (1 + (d²/t² × e^{-d²/t²}))
The equation describes the observed occurrences fairly well.
The phenomena of stratification observed by Brewer are explained by
Barus from the above formula: In proportion as the time of subsidence is
greater, the tube shows opacity at the bottom, shading off gradually
upward, through translucency, into clearness at the top. If, instead of
equation (2), there be introduced the condition of a more abrupt
maximum, if, in other words, the particles be very nearly of the same
size, then subsidence must take place in unbroken column capped by a
plane surface which at the time zero coincided with the free surface of
the liquid. Again, suppose one-half of the particles of this column
differ in some way uniformly from the other half. Then at the outset
there are two continuous columns coinciding, or, as it were,
interpenetrating throughout their extent. But the rate of subsidence of
these two columns is necessarily different, since the particles, each
for each, differ in density, radius and frictional qualities, by given
fixed amounts. Hence the two surfaces of demarcation at the time zero
coincided with the free surface. In general, if there be n groups of
particles uniformly distributed, then at the time zero n continuous
columns interpenetrate and coincide throughout their extent. At the time
t, the free surface will be represented by n consecutive surfaces of
demarcation below it, each of which caps a column, the particles of
which form a distinct group.
From a further discussion of the mathematical condition under which the
subsidence of the particles takes place, Barus is of the opinion that
Durham’s theory of suspension being only a lower limit of solution is
rapidly gaining ground, yet without being attended with concise
experimental evidence which will account for the differences in the rate
of subsidence. On the contrary, Brewer’s hypothesis of colloidal
hydrates is more easily subjected to experimental proof. The test shows
that the particles retain their normal density, no matter how they are
suspended or circumstanced.
Further, in the explanation of the phenomenon of sedimentation, the
following principle may be regarded as determined; namely, if particles
of a comminuted solid are shaken up in a liquid, the distribution of
parts after shaking will tend to take place in such a way that the
potential energy of the system of solid particles and liquid, at every
stage of subsidence, is the minimum compatible with the given
conditions.
According to Barus it is necessary, in order to pass judgment on the
validity of any of the given hypotheses, to have in hand better
statistics of the size of the particles relatively to the water
molecule, than are now available. Inasmuch as the particles in pure
water are individualized and granular, it is apparently at once
permissible to infer the size of the particles from the observed rates
of subsidence. His observations show that the said rate decreases in
marked degree with the turbidity of the mixture. Hence the known formulæ
for single particles are not rigorously applicable, though it cannot be
asserted whether the cause of discrepancy is physical or mathematical in
kind. It follows that special deductions must be made for the subsidence
of stated groups of particles before an estimate of their mean size can
fairly be obtained.
Rowland[127] reaches a closer approximation for the fall of a single
particle by showing that the liquid, even at a large distance from the
particle, is not at rest.
In the case of water, however, it is noticed that despite the large
surface energy of the liquid, subsidence takes place in such a way that
for a given mass of suspended sediment the surfaces of separation are a
maximum. On the other hand, in case of subsidence in ether or in salt
solutions, the solid particles behave much like the capillary spherules
of a heavy liquid shaken up in a lighter liquid with which it does not
mix. In other words, the tendency here is to reduce surfaces of
separation to the least possible value, large particles growing in mass
and bulk mechanically at the expense of smaller particles; in other
words, exhibiting the phenomenon of flocculation.
=189. Physical Explanation of Subsidence.=—Whitney[128] thinks that the
phenomena of the suspension of clay in water may be explained on purely
physical principles, and that neither the partial solution nor hydration
hypotheses are necessary, or will explain the suspension of clay in
water, for the solution, or hydrated substance, would still have a
higher specific gravity than the surrounding liquid. He calls attention
in the first place to the fact, that in a turbid liquid, which has been
standing for weeks and which is only faintly opalescent, the grains in
suspension are still of measurable size, if properly stained as in
bacteriological examinations and viewed through an oil emersion
objective. He gives a value of 0.0001 millimeter, as the lower limit of
the diameters of these particles of “clay,” which are usually met with
in agricultural soils. He refers to the fact that fine dust and ashes,
and even filings of metals, may remain in suspension in the air for days
and even months in very apparent clouds, or haze, although they may be a
thousand times heavier than the surrounding air. Particles of clay, no
smaller than the limits which have been assigned, should remain in
suspension in the much heavier fluid, water, for an indefinite time, for
the volume or weight of the particles (⁴⁄₃)(πr³) decreases so much more
rapidly in proportion than the surface (4πr²), that there is,
relatively, a larger amount of surface area in these fine clay
particles, and a great deal of surface friction in their movement
through a medium, and they would settle very slowly. Under ordinary
conditions, however, the mean daily range of temperature is about twenty
degrees, the mean monthly range is fifty degrees, and the yearly range
100° F., and the ordinary convection currents, induced by the normal
change of temperature, would be sufficient of itself to keep these fine
particles in suspension in the liquid for an indefinite time, as it is
known that currents of air keep fine particles of dust and ashes in
suspension. If the volume or weight of a fine gravel, having a diameter
of one and five-tenth millimeters, be taken as unity, then for a
particle, having a diameter of 0.00255 millimeter, which is the mean
diameter for Whitney’s clay group, the volume decreases in the ratio
1:0.000000004853, and the surface decreases only in the ratio
1:0.000286.
=190. Practical Applications.=—The action of mineral substances in
promoting flocculence has been taken advantage of in later times in the
construction of filters for purifying waters holding silt in solution.
In these filters the introduction of a small quantity of alum, or some
similar substance, into the water usually precedes the mechanical
separation of the flocculent material. In the same way the action of
iron and other salts on sewage waters has been made use of in their
purification and in the collection of the sewage material for
fertilizing purposes.
=191. Separation of the Soil Into Particles of Standard Size.=—The
agronomic value of a soil depends largely on the relative size of the
particles composing it. The finer the particles, within a certain limit,
the better the soil. The size of the particles may be estimated in three
ways: (1) by passing through sieves of different degrees of fineness;
(2) by allowing them to subside for a given time in water at rest; (3)
by separating them in water moving at a given rate of speed. The first
method is a crude one and is used to prepare in a rough way, the
material for the second and third processes.
=192. Separation in a Sieve.=—The soil should be dry enough to avoid
sticking to the fingers or to prevent agglutination into masses when
subjected to pressure. It should not, however, be too dry to prevent the
easy separation of any agglutinated particles under the pressure of the
thumb or of a rubber pestle.
The sieve should have circular holes punched in a sheet of metal of
convenient thickness to give it the requisite degree of strength. Sieves
made of wire gauze are not so desirable but it is difficult to get the
finer meshes as circular perforations. Such sieves cannot give a uniform
product on account of the greater diagonal diameter of the meshes and
the ease with which the separating wires can be displaced. It is
convenient to have the sieves arranged _en batterie_; say in sets of
three. Such a set should have the holes in the three sieves of the
following dimensions; _viz._,
1st sieve 2 millimeters diameter.
2nd „ 1 millimeter „
3rd „ 0.5 „ „
Coarser single sieves may be used to separate the fragments above two
millimeters diameter if such a further classification be desired. Each
sieve fits into the next finer one and the separation of a sample into
three classes of particles may be effected by a single operation. In
most cases, however, it is better to conduct each operation separately
in order to promote the passage of agglutinated particles by gentle
pressure with the thumb or with a rubber pestle. In no case should a
hard pestle be used and the pressure should never be violent enough to
disintegrate mineral particles.
There is much difference of opinion concerning the smallest size of
particles which should be obtained by the sieve.
Most analytical processes prescribe particles passing a sieve of one
millimeter mesh (¹⁄₂₅ inch). There is little doubt, however, of the fact
that a finer particle would be better fitted for subsequent analysis by
the hydraulic method.
For this purpose a sieve of 0.5 millimeter circular mesh is preferred.
=193. Sifting with Water.=—In soils where the particles adhere firmly
the sifting should be done with the help of water. In such cases the
soil is gently rubbed with a soft steple or the finger in water. It is
then transferred to the sieve or battery of sieves which are held in the
water, and rubbed through each of the sieves successively until the
separation is complete. After the filtrate has stood for a few minutes
the supernatant muddy liquor is poured off, the part remaining on the
sieve is added to it and the process repeated until only clean particles
larger than 0.5 millimeter are left on the sieve. These particles can be
dried and weighed and entered on the note book as sand. The filtrate
should be evaporated to dryness at a gentle temperature and when
sufficiently dry be rubbed up into a homogeneous mass by a rubber
pestle.
The sieve recommended by the Association of Official Agricultural
Chemists[129] for the preparation of fine earth for chemical analysis
has circular openings ¹⁄₂₅ inch (one millimeter) in diameter.
Wahnschaffe[130] directs that a sieve of two millimeters mesh be used in
preparing the sample for silt analysis and that the residue after the
silt analysis is finished, which has not been carried over by a velocity
of twenty-five millimeters per second, be separated in sieves of one
millimeter and 0.5 millimeter meshes respectively.
Hilgard objects to leaving this coarse material in the sample during the
process of churn elutriation on account of the attrition which it exerts
and therefore directs that it be separated by sieve analysis before the
elutriation begins.
=194. Method of the German Experiment Stations.=[131]—In the method
recommended for the German Agricultural Stations an attempt is made to
secure even a finer sieve separation than that already mentioned.
Sieves having the following dimensions are employed; sieve No. 1, square
meshes 0.09 millimeter in size, diagonal measure 0.11 millimeter; sieve
No. 2, square meshes 0.14 to 0.17 millimeter in diameter, diagonal
measure 0.22 to 0.24 millimeter; sieve No. 3, square meshes 0.35 to 0.39
millimeter in diameter, diagonal measure 0.45 to 0.50 millimeter;
finally a series of sieves one, two and three millimeters circular
perforations.
Five hundred grams of the soil (in the Halle Station only 250) are
placed in a porcelain dish with about one liter of water and allowed to
stand for some time with frequent stirring, on a water bath. After about
two hours, when the soil is sufficiently softened so that with the help
of a pestle it can be washed through the sieves, the process of sifting
is undertaken in the following manner: Sieve No. 3 is placed over a dish
containing water, the moistened soil placed therein and the sieve
depressed a few centimeters under the water and the soil stirred by
means of a pestle until particles no longer pass through. After the
operation is ended the residue in the sieve is washed with pure water
and dried. The part passing the sieve is thoroughly stirred and then
washed with water into sieve No. 2 and treated as before. The product
obtained in this way is brought into sieve No. 1 and carefully washed.
All the products remaining on each of the sieves are dried at 100° and
weighed. The portion passing sieve No. 1 is either dried with its wash
water or estimated by loss by deducting from the total weight taken, the
sum of the other weights obtained. If a more perfect separation of the
first sieve residue be desired it can be obtained by passing it through
sieves of the last series which may have meshes varying in size, _viz._:
one, two, or three millimeters in diameter. Each sieve of the same class
should have holes uniformly of the same size.
The sieve products are characterized as follows: The part passing a
three millimeter sieve is called fine earth, while the part remaining is
called gravel. The fine earth is separated into the following products:
The part that passes through the three millimeters opening and is left
by the two millimeters opening is called _steinkies_. The product from
the two millimeters opening and the residue from the one millimeter
opening is called _grobkies_. The product from the one millimeter
opening and the residue on the sieve No. 3 is called _feinkies_. The
product from the sieve No. 3 and the residue from the sieve No. 2 is
called coarse sand. The product from sieve No. 2 and the residue from
sieve No. 1 is called fine sand. The product from sieve No. 1 is called
dust. The dust can be further separated into sand, dust, and clay. For
the examination of the clay the Kühn silt cylinder as modified by
Wagner, is recommended. The cylinder has a diameter of eight centimeters
and a height of thirty centimeters, and is furnished with a movable exit
tube reaching to its bottom.
=195. General Classification of the Soil by Sieve Analysis.=—The
classification recommended by the German chemists is satisfactory but
the following one is more simple. All pebbles, pieces of rock, etc.,
should first be separated by a two millimeters circular mesh sieve,
dried at 105° and weighed. The result should be entered as pebbles and
coarse sand.
The finer sand may be separated with a sieve of one millimeter circular
openings.
The still finer sand is next separated with the sieve of 0.5 millimeter
circular openings as indicated above.
The sample may now be classified as follows:
1. Coarse pebbles, sticks, roots, etc., separated by hand.
2. Pebbles and coarse sand not passing a two millimeters sieve.
3. Sand not passing a one millimeter sieve.
4. Fine sand not passing a 0.5 millimeter sieve.
5. Fine earth passing a 0.5 millimeter sieve.
=196. Classification of Orth.=[132]—As fine silt are reckoned those
particles which range from 0.02 to 0.05 millimeter; as fine sand the
groups from 0.05 to 0.2 millimeter; as medium sized sand those ranging
from 0.2 to 0.5 millimeter and for large grained sand those particles
ranging from 0.5 to 2 millimeters in diameter. Particles over two
millimeters form the last classification.
SEPARATION OF THE EARTH PARTICLES BY A LIQUID.
=197. Methods of Silt Analysis.=—The further classification of the
particles of a soil passing a fine sieve can best be effected by
separation in water. The velocity with which the current moves or with
which the particles subside will cause a separation of the particles
into varying sizes. The slower the velocity the smaller the particles
which are separated. There is, however, a large and important
constituent of a soil which remains suspended in water, or in a state of
seeming solution. This suspended matter would still be carried over by a
current of water moving at a rate so slow as to make a subclassification
of it impossible. This suspended matter passing off at a given velocity
may be classed as clay, and it consists in fact chiefly of the hydrated
silicate of alumina, or other particles of equal fineness. The laws
which govern its deposit have already been discussed.
The apparatus which have been used for silt analysis may be grouped into
four classes.
(1) Apparatus depending on the rate of descent of the particles of a
soil through water at rest. The apparatus for decanting from a cylinder
or a beaker belong to this class.
(2) Apparatus which determine the rate of flow by passing the liquid
through a vessel of conical shape. The system of Nöbel is a good
illustration of this kind of apparatus.
(3) Apparatus in which the elutriating vessel is cylindrical and the
rate of flow determined by a stop-cock or pressure feed apparatus. The
system of Schöne represents this type.
(4) Apparatus in which the above system is combined with a device for
mechanically separating the particles and bringing them in a free state
into the elutriating current. The system of Hilgard is the type of this
kind of apparatus.
In practice the use of cylindrical apparatus with or without mechanical
stirring and the method by decantation have proved to be the most
reliable and satisfactory procedures. Between the beaker and churn
methods, of separation there is little choice in regard to accuracy.
Which is the superior method, is a question on which the opinions of
experienced analysts are divided. The various processes will be
described in the order already mentioned.
=198. Methods Depending on Subsidence of Soil Particles.=—The simplest
method of effecting the further separation of the soil particles is
without doubt that process which permits them to fall freely in a liquid
sensibly at rest. The practical difficulties of this method consist in
the trouble of securing a perfect separation of the particles, in
preventing flocculation after division and in avoiding currents in the
liquid of separation.
For the separation of the soil particles for this method boiling and wet
pestling are the only means employed. The flocculation of the separated
particles may be partially prevented by adding a little ammonia to the
water employed. The author has also tried dilute alcohol as the
separating liquid but the results of this method are not yet
sufficiently definite to find a place in this manual. Evidently the
practical impossibility of avoiding convection currents prevents the use
of water at a high temperature for this separation, although the
tendency to flocculation almost disappears as the temperature approaches
100°. The general method of avoiding the errors due to flocculation in
the subsidence method consists in repestling the deposited particles and
thus subjecting them as often as may be necessary to resedimentation.
These principles are well set forth by Osborne,[133] who states that
when a soil is completely suspended in water by vigorous agitation,
particles of all the sizes present are to be found throughout the entire
mass of liquid. When subsidence takes place, the larger particles will
go down more rapidly than the smaller ones, but some of the small
particles that are near the bottom will be deposited sooner than some of
the larger ones which have a much greater distance to travel. Thus,
independently of the fact that the larger particles in their descent are
somewhat impeded by the smaller, the smaller being at the same time
somewhat hastened by the larger, the sediment that reaches the bottom at
any moment is a more or less complex mixture of all the mechanical
elements of the soil. The liquid, however, above this sediment at the
same moment will have completely deposited all particles exceeding
certain dimensions of hydraulic value, determined mainly by the time of
subsidence.
If now the aforesaid first sediment be suspended in pure water, and
allowed to subside for the same time as before, the larger part of it
will be again deposited, but some will remain in suspension, consisting
of a considerable part of the finer matter of the first sediment. By
pouring off these suspended particles with the water and agitating the
sediment again with clear water as before, another portion of fine
particles will be suspended and may be decanted from it. On continuing
this process of repeated decantations it will soon be found that the
soil has been separated into two grades.
It is evident that in this way a separation can be made, but it is
perhaps not so clear that such a separation would be sharp enough for
the purposes of a mechanical soil analysis. If, for instance, the
separation is to be made at 0.05 millimeter diameter, it is evident that
by repeated decantations all below 0.01 millimeter can be washed out of
that above 0.05 millimeter, but it may not appear so probable that all
below 0.045 millimeter can be removed without removing some above 0.055
millimeter.
Such a result may be easily attained, however, if the following
principle be adhered to:
Make the duration of the subsidence such that the liquid decanted the
first few times shall contain nothing larger than the desired diameter.
Then decant into another vessel, timing the subsidence so that the
sediment shall contain nothing smaller than the chosen diameter. This
can not be done without decanting much that is larger than the chosen
diameter, but the greater part of the particles greater and less than
the chosen diameter can be removed and an intermediate product obtained,
the diameters of whose particles are not very far from that desired.
If this intermediate portion be again subjected to the same process, two
fractions may be separated from it, one containing particles larger than
the chosen diameter and another containing particles smaller than this
diameter, while a new intermediate product will remain which is less in
amount than that resulting from the first operation. By frequent
repetitions of this process this intermediate product can be reduced to
a very small amount of substance the particles of which have diameters
lying close to the chosen limit and may then be divided between the two
fractions.
The principles of the separation described by Osborne set forth with
sufficient clearness the purposes to be achieved by the analysis. The
chief methods of manipulation practiced will be found below.
=199. Kühn’s Silt Cylinder.=—A simple form of apparatus for the
determination of silt by the sedimentation process is the one described
by Kühn.[134]
The cylinder should be about twenty-eight centimeters high with a
diameter of 8.5 centimeters. At the lower end of the cylinder five
centimeters from the bottom it carries a tube 1.5 centimeter in diameter
furnished with a pinch-cock and held in position by a rubber stopper.
In carrying out the process thirty grams of sifted soil (two millimeters
mesh sieve) are boiled with water for an hour and after cooling the soil
and water are washed into the separating cylinder. The cylinder is then
filled with water with constant shaking.
After standing for ten minutes the stop-cock is opened and the water
with its suspended matter allowed to flow into a porcelain dish.
The cylinder is then again filled with water and the process is
continued until the water drawn off is practically clear.
The fine particles having been separated in this way the next coarser
grade of particles is separated by repeating the process at intervals of
five minutes.
By these two operations it is considered that the clay is entirely
removed. The residue remaining in the cylinder is dried and weighed. The
relative proportions of clay and residue in the sample are thus
determined.
The residue is then separated into two portions by sieves of one
millimeter and 0.5 millimeter mesh.
The soil is thus separated into the following parts:
1. By the first sifting coarse quartz larger than two millimeters
diameter.
2. Fine quartz two millimeters, to one millimeter diameter.
3. Coarse sand one millimeter, to 0.5 millimeter diameter.
4. Fine sand finer than 0.5 millimeter diameter.
5. Silt, clay, humus, etc., separated by the water.
=200. Knop’s Silt Cylinder.=—The cylinder recommended by Knop[135] is
essentially that of Kühn being furnished with four lateral tubes instead
of one (Fig. 25).
[Illustration:
FIGURE 25.
KNOP’S SILT CYLINDER.
]
The sample of soil, twenty-five to thirty grams, after passing a two
millimeters mesh sieve, and long boiling, is washed through a series of
sieves of the following diameters of mesh respectively; _viz._, one
millimeter, 0.5 millimeter, 0.25 millimeter, and 0.1 millimeter. The
part which passes the finest sieve is placed in a Knop’s cylinder, the
cylinder filled with water one decimeter above upper tube and well
shaken. The cylinder is allowed to rest for five minutes when the upper
cock is opened and the water drawn off. After five minutes more the next
tube is opened and so on with equal intervals for the three upper tubes.
The operation is repeated with fresh water until the water drawn off is
clear. Finally the lowest tube is opened and all the water poured off of
the sandy residue. The space between each tube is one decimeter. The
dust remaining is dried and weighed and the weight of material carried
over as silt determined by difference.
=201. Siphon Silt Cylinder.=—Instead of the tubulated cylinder one
furnished with a siphon can be employed[136] (Fig. 26).
It should be about forty centimeters high and six centimeters internal
diameter. The cylinder first receives twenty-five to thirty grams of the
well boiled fine earth and then water until there is but a small space
between it and the stopper when the latter is inserted.
The cylinder is marked exactly 200 millimeters below the surface of the
water with a narrow strip of paper at _a_, stoppered, inverted and well
shaken. The cylinder being again placed in normal position the soil
particles under the influence of gravity tend to sink with greater or
less rapidity according to their size. The siphon _a b c_ is filled with
water, the cock at _c_ closed and the opened end _a_ placed in the
cylinder A just at the mark 200 millimeters below the surface of the
water, and the water thus transferred to B when desired. If the
suspended matter is allowed to stand for 100 seconds the particles of
more than two millimeters hydraulic value will have fallen below the
open end of the siphon. If allowed to stand 1,000 seconds the silt value
of the particles will be 0.2 millimeter per second. Whatever the number
of seconds may be, the operation is continued until the water removed is
practically clear. The open end of the siphon _a_ should be bent upwards
so that no disturbing current may bring the particles below the line
into the liquid discharged into B.
[Illustration:
FIGURE 26.
SIPHON CYLINDER FOR SILT ANALYSIS.
]
While the results obtained by this method are satisfactory as compared
with other similar processes it cannot be highly recommended because of
the time and trouble required to get a complete separation and by reason
of the difficulty of collecting the separated silt.
=202. Wolff’s Method.=[137]—As modified by Wolff the Knop process is
conducted as follows: Fifty grams of fine earth are boiled with water
and then the entire mixture is passed through three sieves with openings
of one millimeter, 0.5 millimeter, and 0.25 millimeter in diameter,
respectively. The finest part is mixed with water to a height of
eighteen centimeters in a bottle twenty centimeters high and having a
capacity of one liter and thoroughly agitated, after which it is left to
rest, and finally the turbid liquid is drawn off with a siphon, the
bottle refilled with water, agitated, and left to rest, and the process
repeated as long as the water carries any suspended matter after a
definite time.
Wolff proposes for the first three periods of rest one hour, for the
second three, a half an hour, for the third three, a quarter of an hour,
and for the fourth three, five minutes.
=203. Moore’s Modification of Knop’s Method.=[138]—The sample of soil is
first passed through a sieve having round perforations three millimeters
in diameter. The weight of the particles remaining on the sieve is then
determined, and likewise that of the portion passing through, which is
known as fine earth. The last named portion constitutes the material for
all subsequent operations of mechanical and chemical analysis.
Thirty grams of the fine earth are boiled rapidly with water until the
lumps are disintegrated and clayey portions separated from the sand. The
material is then successively washed through perforated metal sieves,
the holes of which are respectively 1, 0.5, and 0.25 millimeter in
diameter. The portions retained on the sieves are severally dried,
ignited and weighed, and the finest portion, or that passing through the
0.25 millimeter sieve, is then submitted to the following process of
separation:
The sediment and water passing through the 0.25 millimeter sieve are
placed in a glass cylinder fifty centimeters long and thirty-seven
millimeters in internal diameter. The cylinder is closed at the bottom
and is provided with a lateral tube inserted six centimeters above the
bottom. Three other lateral tubes are inserted at intervals of ten
centimeters above the first tube, and a ring is etched into the cylinder
ten centimeters above the uppermost tube. The lateral tubes are closed
with rubber tubes compressed by spring clips. The sediment being placed
in the cylinder, water is added to the mark or ring, the cylinder closed
with a rubber stopper, and vigorously shaken until the contents are
thoroughly mixed. It is then placed upright, the stopper removed, and
after standing undisturbed for five minutes the clip on the uppermost
tube is opened and the water allowed to flow into a beaker. After five
minutes further standing, the second clip is opened and the water drawn
off into the same beaker; in the same manner the water is drawn off from
the other tubes at intervals of five minutes until the level of the
lowest tube is reached. The cylinder is then refilled with water to the
mark, thoroughly shaken after inserting the stopper and the water again
drawn off at intervals of five minutes, as before; the operation being
repeated until the water drawn off is almost free from turbidity. The
sediment remaining in the cylinder from this process of washing by
subsidence is termed by Knop, fine sand, the material flowing off in
suspension in the wash waters, dust, and the process of separation by
Knop’s original method ends here.
In order to remedy the imperfect separation into definite particles
secured by the above method, Moore proposes the following device:
The fine sand from the first series of subsidences is placed in a
separate vessel, the washings are allowed to remain undisturbed for
twelve hours, the turbid liquid decanted and the sediment returned to
the cylinder. Water is then added to the mark, the whole shaken, and the
liquid drawn off at intervals of five minutes, as in the first series.
The sediment from this operation is placed in a separate beaker, the
washings returned to the cylinder, and again allowed to subside as
before; the sediment from this second subsidence is added to that from
the preceding operation, and the washings again returned to the
cylinder, the operation being repeated as long as any sediment can be
obtained from renewed treatment of the washings; the final washings are
then placed in a separate vessel for subsequent microscopic
measurements.
The collective sediments from the last series of operations are then
returned to the cylinder and allowed to subside with fresh additions of
water, as in the case of the first series; the fine sand thus obtained
being added to that from the first series, and the washings being
collected in a large beaker. The latter are left at rest for twelve
hours, and the sediment returned to the cylinder and treated as before
until no further separation can be effected. The fine sand resulting
from all of these operations is then dried, ignited and weighed; the
weight of the portion removed by the washing being determined by
difference, as it is, owing to its excessively slow rate of subsidence,
found impracticable to collect it for direct weighing. The size of the
particles of fine sand is then determined by micrometric measurement.
Similar measurements are made on the material obtained by long
subsidence from the washings from the foregoing operations. The average
diameter of the largest particles should not exceed 0.01 millimeter.
=204. Statement of Results.=—The results of the analyses on three soils
from the localities indicated in the table, and the method of stating
them, are given in the following table:
New Milford, Clarksville, Granville,
Conn., per Tenn., per N. C., per
cent. cent. cent.
Particles larger in diameter 8.55 0.32 0.23
than 3.0 millimeters
Particles of diameter from 3.0 4.96 0.45 15.04
millimeters to 1.0 millimeter
Particles of diameter from 1.0 4.43 0.96 33.43
millimeter to 0.5 millimeter
Particles of diameter from 0.5 11.86 1.25 18.82
millimeter to 0.25 millimeter
Particles of diameter from 0.25 60.54 61.58 23.59
millimeter to 0.01 millimeter
Particles smaller in diameter 9.66 35.44 8.89
than 0.01 millimeter
───────────────────────────────────────────────────────────────────────
Total 100.00 100.00 100.00
=205. Method of Bennigsen.=—The silt flasks recommended by
Bennigsen[139] are shown in Fig. 27.
The glass flask _b_ carries a long cylindrical neck _a_ the upper part
of which is graduated in cubic centimeters. Ten grams of the fine soil
are shaken with water in the flask, the neck of which is closed with a
rubber stopper. The flask is then inverted bringing the soil and water
into the neck. The flask is hung up and sedimentation is assisted by
imparting a pendulous motion to the neck for ten minutes. After an hour
the soil particles have separated into a coarse layer below and a fine
layer above. The relative volumes of the two layers are then read off in
cubic centimeters. While this method may be useful in helping to form a
speedy judgment concerning the character of a soil it can lay no claim
to being an accurate method of silt separation.
[Illustration:
FIGURE 27.
BENNIGSEN’S SILT FLASKS.
]
=206. Method of Gasparin.=—The method of Gasparin only gives a very
primitive separation of the various components of the earth according to
their fineness. It is conducted as follows:
Ten grams of sifted earth are put into a beaker, water is added and
strongly agitated; after five minutes the water is decanted into another
vessel, the first vessel is filled anew with water, agitated, decanted,
and this process is repeated until the liquid remains perfectly clear.
Only two portions are weighed, _i. e._, the pebbles which remain in the
sieve and the coarse sand which remains in the beaker; while the
argillaceous portion drawn off with the water is determined by the
difference.
=207. The Italian Method.=—The following modification of Gasparin’s
process is practiced by the Italian chemists:[140]
Twenty grams of earth are passed through a sieve having openings of one
millimeter in diameter, then the sifted part is mixed with 100 cubic
centimeters of water in a 200 cubic centimeters beaker and left to rest
for some hours, then strongly agitated and after ten seconds the turbid
liquid is poured into another vessel of half a liter capacity. This
manipulation is repeated until the liquid is clear.
The decanted liquid is thoroughly agitated, then left to stand until the
movement shall be completely arrested, after which the supernatant
liquid is poured into another vessel holding two liters. To the residuum
is added more water; it is agitated, decanted, and this process is
repeated until the water is no longer turbid.
=208. Method of Osborne.=—In the foregoing paragraphs the methods of
silt separation by subsidence as practiced in different countries have
been outlined. The good points of the various methods are combined in
the process as carried out by Osborne.[141] The details of this method
will be given with sufficient minuteness to make its practice possible
by all analysts.
_Selecting the Sample._—Several pounds of air-dried, fine earth are
secured by passing the soil through a sieve, the holes of which are
three millimeters in diameter.
_Sifting._—Thirty grams of the above fine earth are stirred with 300 to
400 cubic centimeters of water and then thrown successively upon sieves
with circular holes of 1, 0.5, and 0.25 millimeter diameter
respectively. By means of successive additions of water and the use of a
camel’s hair brush, all the fine material is made to pass through the
sieves and these at the last are agitated under water in a shallow dish
in such a way that the soil is immersed. The finest sieve should be well
wet with water on its lower surface just before using. The finest
particles which render the water turbid are easily washed through. The
turbid water is kept separated from the clear water which comes off with
the last portions that pass the sieves. The turbid water usually does
not amount to more than one liter.
_Elutriation._—The elutriation should be carried on so as to secure
three grades of silt; the diameters of the particles ranging in the
first grades from 0.25 to 0.05 millimeter, in the second grade from 0.05
to 0.01 millimeter, and in the third grade from 0.01 millimeter to the
impalpable powder. The term sand is applied to the first grade, silt to
the second, and dust to the third. After the turbid liquid from the
sifting has stood a short time it is decanted from the sediment and
after standing until a slight deposit is formed, is again decanted and
the sediment examined with a microscope. If sand be present, the
subsidence of the turbid liquid is continued until no more sand is
deposited. As the sand subsides rapidly there is no difficulty in
altogether freeing the liquid first decanted from this grade of
particles. The sediment thus obtained contains all the sand, a part of
the dust and much silt. As only dust and the finest silt render the
water turbid the sediment is stirred a few times with a fresh quantity
of water and decanted after standing long enough to let all the sand
settle. When the water decanted is free from turbidity, the last
portions of the soil passing through the sieve with clear water are
added to the sediment and the decantations continued so as to remove
most of the silt. When no more silt can be easily removed from the
sediment without decanting sand, the decantations are made into a
different vessel and the subsidences so timed as to remove as much of
the silt as possible. By using a little care, at least three-quarters of
the sand are thus obtained free from silt. The rest of the sand is mixed
with the greater part of the silt which has been decanted into the
second vessel. The size of the smallest particles in this vessel is
determined with the microscope, to make sure that its contents are free
from dust as they usually will be if, after settling for a few moments,
they leave the water free from turbidity.
The soil is thus separated into three portions, one containing sand, one
sand and silt, and the other silt, dust, and clay. The sand and silt are
separated from each other by repeating the subsidences and decantations
in the manner just described.
In this way there is removed from the sediment, on the one hand, a
portion of silt free from sand and dust, and on the other hand a portion
of sand free from silt. Thus is obtained a second intermediate portion
consisting of sand and silt, but less in amount than the first and
containing particles of diameters much more nearly approaching 0.05
millimeter. By repeating this process a few times, this intermediate
portion will be reduced to particles whose diameters are very near 0.05
millimeter and which may be divided between sand and silt, according to
judgment. The amount of this is usually very small. As soon as portions
are separated, which the microscope shows to be pure sand or pure silt,
they are added to the chief portions of these grades already obtained.
The same process is applied to the separation of silt from dust. When
all the silt has been removed from the dust and clay, the turbid water
containing the dust and clay is set aside and allowed to settle in a
cylindrical vessel for twenty-four hours. The vessel is filled to a
height of 200 millimeters. According to Hilgard, the separation of the
dust from clay during a subsidence of twenty-four hours, will give
results of sufficient accuracy, although the clay then remaining
suspended will not be entirely free from measurable fine particles up to
0.001 or 0.002 millimeter diameter.
Small beakers and small quantities of distilled water are used at first
for the decantations, as thus the duration of subsidence is less and
more decantations can be made in a given time than when larger
quantities of water are employed. Beakers of about 100 cubic centimeters
capacity are convenient for the coarser grades, but it is necessary to
use larger vessels for the fine sediments from which turbid water
accumulates that cannot be thrown away, as may be done with the clear
water, from which the coarse sediments settle out completely in a short
time.
It is best to keep the amount of water as small as possible in working
out the dust since loss is incurred in using too large quantities.
It is also necessary in most cases to subject the various fractions
obtained during elutriation, to careful kneading with a soft rubber
pestle so that the fine lumps of clay may be broken up and caused to
remain suspended in the water. This treatment with the pestle should be
done in such a way as to avoid as far as possible all grinding of the
particles, the object being merely to pulverize the minute aggregations
of clay and extremely fine particles which always form on drying a
sample of soil after removing it from the ground.
_Measurement of the Particles._—To determine the size of particles in
suspension, a small glass tube is applied to the surface of the liquid
in such a way as to take up a single drop which is transferred to a
glass slide. This drop will contain the smallest particles in the
liquid.
To obtain a sample of the coarsest particles the liquid is allowed to
stand long enough to form a very slight sediment and a portion of this
sediment is collected with a glass tube.
To determine the diameter of the particles in a sediment it is stirred
vigorously with a little water and the pipette at once applied to the
surface of the water. On decanting the greater part of the sediment, the
large particles remain at the bottom of the beaker and may be easily
examined.
_Time._—The time required to make the separations, above described, is
about two hours for each, so that an analysis including the sittings, is
made in five or six hours, exclusive of the time necessary for
collecting the dust and separating the clay, for which a subsidence of
twenty-four hours is allowed.
_Weighing the Sediments._—The sediments are prepared for weighing by
allowing them to subside completely, decanting the clear water as far as
possible, rinsing them into a weighed platinum dish and igniting. The
dish is cooled in a desiccator.
_Effect of Boiling._—The analyses show a very decided increase in the
particles smaller than 0.01 millimeter diameter at the expense of
coarser particles as the result of boiling. The surfaces of the coarser
particles are seen to be polished and of a lighter color than those not
boiled. The surfaces of the unboiled particles are coated with a film of
fine material probably cemented to them by clay. When these coarse
particles which have not been boiled, are violently stirred with water
for a short time, no fine particles are detached from them; and a
careful examination under the microscope fails to reveal in any of the
sediments more than an occasional grain exceeding the 0.05 millimeter
limit by so much as 0.01 millimeter, or the 0.01 limit by as much as
0.005 millimeter. It would, therefore, appear that these small particles
thus set free by long boiling are really a part of the larger ones and
should be treated as such in a mechanical analysis of these soils.
=209. The French Method.=—The Schloesing method[142] as practiced by the
French agricultural chemists[143] differs essentially from those already
described in attempting to first free the silt from carbonates and
organic matter. It is conducted as follows:
One kilogram of the soil previously dried in the air, is taken and
passed through a sieve of which the meshes are five millimeters. The
agglomerated particles of earth are broken up by the hand. The pebbles
are also taken out and weighed. The pebbles are then treated with
hydrochloric acid until all effervescence is over. The insoluble part is
dried and again weighed. The difference in weight gives the quantity of
calcium carbonate contained on the external surface of the pebbles. The
earth which passes the sieve of five millimeters mesh is next passed
through a sieve having ten meshes to the centimeter. The masses on the
sieve are broken up with the hand or with a pestle, in such a manner as
to separate the fine agglomerated particles. The material which remains
upon the sieve after being dried at 100°, is weighed. This gives the
coarse sand. This is treated with hydrochloric acid as were the pebbles
before, washed and the residue dried and weighed. The difference in
weight gives the quantity of calcium carbonate adhering to the surface
of the coarse sand.
The mechanical analysis is continued with the matter which has passed
the sieve with ten meshes to the centimeter and which consists of the
soil, properly so-called. Ten grams of this are taken, dried at 100°
until no further loss takes place and the moisture thus determined.
Another ten grams are taken and placed in a capsule with a flat bottom,
and from nine to ten centimeters in diameter. This is moistened with a
small quantity of water in such a way as to make a paste. This paste is
rubbed with the finger in fifteen cubic centimeters of water. Ten
seconds after the stirring is completed the supernatant liquid is poured
into a precipitating jar of about 250 cubic centimeters capacity, taking
great care not to allow any particles to pass over which have been
deposited during that time. This operation is repeated in the same way
waiting about ten seconds each time before decanting, until the decanted
liquor is almost perfectly clear. In this way the particles of different
fineness are separated. The decanted portions contain the fine sand and
clay. The remaining portion contains the sand and particles of medium
fineness. This last part is dried, being kept at 100° until it has a
constant weight. It is afterwards treated with dilute nitric acid to
dissolve the calcium carbonate. When the carbonate is abundant, it is
sufficient to determine it by difference which is done by washing the
material, drying and weighing. But when the proportion of carbonate is
very small and in consequence when its exact determination acquires a
greater importance, it is better to determine the lime directly. For
this purpose the part soluble in dilute nitric acid is collected,
treated with ammonia and acetic acid and precipitated with ammonium
oxalate. Details of this operation will be given in another part of this
manual.
In regard to the matter which is insoluble in nitric acid, it is
composed chiefly of silica or silicates, and sometimes also of vegetable
débris. The vegetable matter is determined by the incineration of the
material which has been previously dried. The loss of weight gives the
proportion of vegetable or organic débris contained in the soil and of
combined water.
The portion which has been decanted, the volume of which should not
exceed 500 cubic centimeters, is treated with nitric acid until
effervescence ceases. It is then left to digest for some time, in order
to permit the whole of the carbonate to dissolve. It is next thrown upon
a smooth filter about one decimeter in diameter. After filtration it is
washed to secure the complete elimination of the soluble lime salts. The
lime is determined in the filtered liquid.
The insoluble portion contains the fine sand, the clay and humus bodies.
In order to separate the three elements the precipitate which was
received upon the filter, is rubbed with water, the filter is broken and
all its contents washed through. The volume of wash water is made up to
200 cubic centimeters; two or three cubic centimeters of ammonia are
added and the whole left to digest for two or three hours. The volume of
the liquid is then made up to one liter with distilled water, vigorously
shaking in such a way as to put all the matter in suspension. It is then
left to settle for twenty-four hours. At the end of this time the
supernatant liquid is decanted by the aid of a siphon. To the residue
are added two cubic centimeters of ammonia and one liter of water. The
matter is again brought into suspension and allowed to settle for
twenty-four hours. The supernatant liquid is again decanted with a
siphon, and added to the liquid previously removed. For ordinary soils
two decantations are generally sufficient but when the soils contain a
large quantity of clay it is convenient to decant three or four times.
By an examination of the supernatant liquid it is easy to tell if the
washings have been sufficiently prolonged. The decanted liquors contain
the organic matter and that which it is convenient to call clay, which
is constituted of very fine particles of sand and colloidal clay which
play, in arable soil, a rôle somewhat like that of cement.
These matters are estimated in the following manner: The liquor is first
treated with nitric acid and the clay and the humic matters are
precipitated together. They are collected upon a smooth filter one
decimeter in diameter and washed with water. By means of a washing
bottle all the solid matters which have stuck to the sides of the filter
are finally collected in the bottom of it. Since the last washings pass
the filter very slowly, they can be removed after the complete
deposition of the matter they contain, by means of a pipette. When all
the liquid is removed the filter is placed upon blotting paper, great
care being taken to avoid desiccation, having in view only the
elimination of the excess of humidity. The folds in the filter are then
carefully smoothed out with the finger. The matter which has collected
upon the filter is then removed completely with a washing bottle, placed
in a dish and dried at 100° and weighed. After weighing, the mass is
incinerated in a muffle in order to destroy the humic bodies. The
difference in weight before and after incineration, gives the total
weight of the humic bodies and since the diminution in weight comprises
not only the weight of the humic bodies, but also the weight of the
combined water which is lost during the process of incineration, there
should be subtracted from the total loss of weight ten per cent of the
weight of the residual mineral matter, which represents the water of
composition of the hydrated silicate.
=210. Statement of the Analysis.=—Schloesing in his original paper[144]
recommends that the analysis be commenced with 1,000 grams of soil. The
data of the analysis and the method of arrangement are illustrated by
the following example.
The physical examination of the earth having been completed as above,
the results can be tabulated as follows: taken, 1,000 grams of dry
earth, digested in water, thoroughly worked by hand, sifted, and passed
through the meshes of the sieve by a stream of water, the meshes having
a diameter of one millimeter.
Dry residue, fifty-five grams, contains Pebbles │ 21 grams.
„ Gravel │ 33 „
„ Organic débris│ 1 „
Sifted earth by difference, 1000 − 55 = │ 945 „
│————
│1000 grams.
Humidity of the homogeneous paste, twenty-seven per cent. Then 945 grams
of the dry sifted earth correspond to (945)/(1.00 − .27) = 1294.5 of
paste.
The analysis, therefore, should be carried on upon this weight or some
aliquot part say 0.01 thereof; _viz._, 12.945 grams.
12.945 grams of the │1st.—Coarse sand dry │Noncalcareous 3.05 grams.
paste after successive│giving by treatment │sand
kneadings and │with acid and │
decantations furnish │ignition. │
dry: │ │
„ │ „ │Calcareous 1.19 „
│ │sand
„ │ „ │Organic 0.08 „
│ │débris
│2nd.—Fine elements decanted with the water,
„ │their weight calculated by difference, 9.45 −
│4.32 = 5.13 grams.
_Treatment of the Fine Elements._—Treated by nitric acid until a
complete decomposition of the calcareous matter is secured, filtered,
washed, the residual matter collected upon a filter, and the liquid
received in a two-liter flask, a little ammonia added, allowed to
digest, the flask filled with distilled water, left for twenty-four
hours at repose, and decanted:
The decantation furnishes│1st.—A deposit of fine calcareous│3.14 grams.
│sand weighing dry │
„ │2nd.—Clayey liquid giving after coagulation
│by acid, filtration and drying 0.85 grams of
│clay.
│ │
Then: Total fine elements │5.13 grams.
Fine elements determined │Fine calcareous sand 3.14│3.99 „
directly. │ │
„ │Clay 0.85│ „
│ │————
Fine calcareous sand by difference │1.14 „
Calculating these results to the original quantity of 1,000 grams the
following data are obtained:
RÉSUMÉ.
One thousand grams of dry earth contain:
Pebbles 21 grams.
Gravel 33 „
Organic débris 1 gram.
Fine earth 945 grams.
————
Total 1000 „
945 grams of │Coarse sand 432 gms. │Noncalcareous sand 305 gms.
fine earth │ │
contain: │ │
„ │ „ │Calcareous sand 119 „
„ │ „ │Organic débris 8 „
„ │
„ │Fine elements 513 gms.│Fine, noncalcareous sand 314 gms.
„ │ „ │Clay 85 „
„ │ „ │Fine calcareous sand 114 „
│ │ ——
│ │Total 1000 „
There are counted as clay all the elements which have remained in
suspension in the water after a period of repose of twenty-four hours.
In fact, these elements comprise a notable proportion of very fine sand
which is not deposited during that time. In order that the liquid should
become entirely freed from this sand it would be necessary to wait
several weeks and even several months. Such a prolongation of the
analysis is evidently inadmissible. The period of twenty-four hours of
repose therefore has been adopted. This is merely conventional, in the
same way that the period of ten seconds adopted for the precipitation of
the gravel is conventional. But this convention is justified by the fact
that the substance which is called clay presents, when it has a proper
degree of humidity and cohesion, a plasticity entirely analogous to that
property of natural clay. Moreover, as has already been said, that which
is chiefly important in these analyses is the employment of processes
always comparable among themselves in their results and generally
followed.
=211. The Belgian Method.=—The method of estimating the percentage of
sand and clay practiced at the Gembloux Station[145] is essentially that
recommended by Schloesing with a few minor modifications.
With the ball of the thumb or with the finger, 100 grams of fine earth
are rubbed with water in a porcelain capsule or mortar with a capacity
of about 250 cubic centimeters. The suspended particles are poured off
with the wash water and the process repeated five or six times, using in
all about 200 cubic centimeters of water.
The water containing the sediment is rendered slightly acid
(hydrochloric acid) adding the acid in minute particles with constant
stirring for about an hour in order to dissolve all the carbonate and to
separate the organic acids from the bases with which they are combined.
The liquid is allowed to remain at rest for five or six hours and a part
of the liquor decanted to remove any supernatant particles of organic
matter which may have passed the sieve in the original preparations of
the sample. Filter through a smooth filter about twelve centimeters in
diameter, wash until the chlorin has disappeared, and throw the filtrate
away.
Break the filter paper over the vessel in which the soil was treated
with hydrochloric acid and wash all the contents of the filter into this
vessel with as little water as possible (about 100 cubic centimeters),
add five cubic centimeters of strong ammonia water, allow to stand for
three hours, shaking from time to time and with distilled water make the
volume up to 250 cubic centimeters. Stir vigorously with a glass rod or
spatula, take this out and wash any adhering particles back, leave at
rest for twenty-four hours, siphon the turbid liquid into a two-liter
vessel. Make the volume up again to 250 cubic centimeters and treat as
above described and repeat the operation until the water becomes clear
after standing for twenty-four hours. Usually eight or ten washings are
necessary. Wash the residual sand into a weighed dish, evaporate to
dryness, ignite and weigh. The weight obtained divided by the weight of
the original sample gives the per cent of sand. The sand is separated by
sieves of varying fineness into coarse, fine, and pulverulent sand.
Add to the ammoniacal liquor collected in the two-liter flask some
powdered potassium chlorid (five grams per liter) to hasten the
coagulation and rapid deposit of the clay.
After twenty-four hours siphon the clear liquor, collect the deposited
clay in a smaller vessel, allow to remain at rest and decant as much of
the clear liquor as possible. Pass through a plain tared filter about
nine centimeters in diameter, dry at 150° and weigh the clay.
=212. The Italian Method.=—Schloesing’s method as carried out by the
Italian chemists[146] is as follows:
A kilo of earth dried in the air is passed through a sieve the threads
of which are separated a distance of five millimeters; and with this the
small pebbles are separated.
With another sieve having spaces of one millimeter, the coarse sand is
separated. The pebbles and sand are dried, weighed, treated with
hydrochloric acid and again weighed in order to find the quantity of
calcareous matter contained in them. In ten grams of this fine earth the
humidity is determined by drying at 100°.
Ten grams are mixed in a capsule with fifteen to twenty cubic
centimeters of water and after eight to ten seconds the supernatant
liquid is poured into a beaker having a capacity of 250 cubic
centimeters. The same operation is repeated until there are contained in
the beaker the fine sand and the clay, while the coarser sand remains in
the capsule.
This last is then dried and weighed and the quantity of calcium
carbonate determined by treating it with diluted nitric acid. By means
of calcination the organic matter is determined. The liquid decanted in
the beaker, the volume of which must not surpass 200 to 250 cubic
centimeters, is treated with nitric acid, filtered after some time,
washed and the calcium is directly determined by precipitating the
solution with ammonium oxalate.
The part in the filter which contains the fine sand, the clay, and the
humus material is mixed with water to a volume of about 200 cubic
centimeters; there are then added to it two to three cubic centimeters
of ammonia and after two or three hours it is diluted to a liter and
strongly agitated.
After twenty-four hours of rest it is decanted and the residuum is
treated a second time with diluted ammonia, decanting after twenty-four
hours. Ordinarily these two treatments suffice, if, however, the earth
is very argillaceous, this operation should be repeated three and even
four times.
The clay which is found in the liquid suspended in colloidal form
coagulates and is precipitated by adding thirty to forty cubic
centimeters of a saturated solution of potassium chlorid, while the
humus substance, under the influence of the ammonia remains dissolved.
Sestini found that the method of Schloesing was the only one which
indicated exactly the quantity of clay in the soil. He modified this
method by reducing the time of rest from twenty-four hours, as proposed
by Schloesing, to only twelve hours, a reduction which in his opinion
does not in the least impair the exactness of the method.
Sestini also proposes twelve treatments instead of six.
SEPARATION OF THE SOIL PARTICLES BY A LIQUID IN MOTION.
=213. General Principles.=—The laws, already discussed, applying to the
subsidence of a solid particle in a liquid, are equally applicable to
the separation of the particle by imparting a motion to the liquid at a
given rate. If a solid particle subside in a given liquid at the rate of
one millimeter per second it follows that this particle will remain at
rest if the liquid be set in motion upward with a like velocity. If the
velocity be greater the particle will be carried upward and eventually
out of the containing vessel. Such a particle is said to have a
hydraulic value of one millimeter per second. If there be a perfect
separation of a soil into its constituent particles and no subsequent
flocculation, all the particles of one millimeter hydraulic value and
less will be separated by a current of the velocity mentioned.
The general principles on which the separation rests, therefore, are the
securing of the proper granulation of the sample and the maintenance of
a fixed velocity of the current until the separation is finished. The
separation must be commenced with a period of subsidence so as to remove
first of all the suspended clay or impalpable particles. The velocity
can then be increased in a certain fixed ratio to secure a separation
into particles of any required hydraulic value.
=214. Nöbel’s Apparatus.=—One of the earliest methods of separating the
soil particles by a moving liquid is that of Nöbel.[147] The apparatus
is shown in Fig. 28. The four separating vessels 1, 2, 3, 4 are of
glass, pear shaped, and have a relative capacity of 1³, 2³, 3³, 4³, or 1
: 8 : 27 : 64. No. 4 has an outlet tube leading to the beaker B, of such
a capacity as to allow the passage of just nine liters of water in forty
minutes, constant pressure being maintained by means of a Mariotte’s
bottle or of the constant level apparatus A, _a_, _b_, which is
connected with the main water supply through the tube _a_ by means of a
rubber hose. The reservoir C should hold about ten liters. The sample of
soil to be separated should be previously boiled and passed through a
sieve having circular openings one millimeter in diameter. The flask in
which the sample is boiled is allowed to stand for some time when the
muddy supernatant liquid is poured into elutriator No. 2 and the
remaining sediment washed into No. 1. No. 1 is filled with water by
connecting it with the water supply and opening the pinch-cock _p_. The
water is carefully admitted until the air is all driven out and Nos. 1
and 2 connected. The cock _p_ is then opened and the vessels all filled,
and the water allowed to run into B for forty minutes, the level being
maintained uniformly at A.
[Illustration:
FIGURE 28.
NÖBEL’S ELUTRIATOR.
]
Of the water used, four liters are found in the elutriating vessels and
nine liters in the receiving vessel No. 5. The apparatus is left
standing for an hour until the liquid in the elutriators is clear and
the portions in each vessel are received on weighed filters dried at
125°, and the weight of each portion determined.
It is recommended that the loss on ignition of each part be also
determined. The separated particles thus secured are classified as
follows:
No. 1. Débris and gravel. No. 2. Coarse sand. No. 3. Fine sand. No. 4.
Clayey sand. No. 5. Finest parts or clay.
Although the method of Nöbel has been much used, the results which it
gives are entirely misleading. The convection currents produced in the
conical vessels by the passing water and the flocculation of the soil
particles prevent any sharp separation into classes of distinct
hydraulic value. The process may be useful for a qualitative test, but
its chief claim to a place in this manual is in its historic interest
arising from its use in the first attempts at silt analysis.
=215. Method of Dietrich.=[148]—The difficulties attending the silt
separation by the Nöbel method, led Dietrich to construct an apparatus
in which the sides of the elutriating vessels were parallel, but these
vessels, with the exception of the first, were not set in an upright
position.
[Illustration:
FIGURE 29.
DIETRICH’S ELUTRIATOR.
]
The apparatus (Fig. 29) consists of a series of cylindrical vessels
connected by rubber tubing.
The elutriators are of the following dimensions:
No. 1. Seventeen centimeters long, 2.8 centimeters in diameter,
position upright.
No. 2. Thirty-four centimeters long, four centimeters in diameter,
inclined 67°.5.
No. 3. Fifty-one centimeters long, 5.2 centimeters in diameter,
inclined 45°.
No. 4. Sixty-eight centimeters long, 6.4 centimeters in diameter,
inclined 22°.5.
The rubber tubes passing from one vessel to the other are furnished with
pinch-cocks so that each one of the elutriating vessels can be shut off
from the others and independently removed from the circuit.
The stream of water is made to pass through the apparatus under a
constant pressure of one meter.
Only the fine earth, boiled with water or hydrochloric acid, is to be
placed in the apparatus. The part coming through a sieve with a mesh
0.67 millimeters is to be used and placed in No. 1. About thirty grams
of soil, are employed for each elutriation. Before adding the soil, the
air is completely removed from all parts of the apparatus by connecting
it with the water supply and allowing it to be filled with water.
The rate of flow is controlled by the orifice of the last effluent tube
and the analyst is directed to continue the operation until the effluent
water collected in the beaker glass (5) is clear. The particles then
remaining in each of the vessels are collected separately.
The author of the method claims that in respect of likeness of particles
the results are especially gratifying and that duplicate analyses give
results fully comparable. The process, however, has not commended itself
to analysts, but it marks a distinct progress toward the principles of
later investigators. Had each of the elutriating vessels been placed
upright and the rate of flow determined, the apparatus of Dietrich would
have served, to a certain extent, for the more rigid investigations of
his successors.
=216. Method of Masure.=—The sifted earth, from ten to fifteen grams, is
carefully mixed with 200 cubic centimeters of water. It is then
introduced into a doubly conical elutriator B, Fig. 30, of about 250
cubic centimeters capacity. A current of distilled water is allowed to
flow from a Mariotte’s bottle, A, which secures a regular and constant
flow. The bottle A is joined to the elutriator B by means of a rubber
tube and the vertical glass tube D, the top of which is expanded into a
funnel for the purpose of receiving the water from the Mariotte flask.
The current of water flowing upward through the elutriator B carries in
suspension the most finely divided particles of clay, and these are
collected with the emergent water in the receiver C. The sand and
coarser particles of clay remain in the elutriator. The water flows out
by the tube F, the diameter of which should be less than that of D. When
the emergent water becomes limpid the operation is terminated. After the
apparatus is disconnected, the water is decanted from the sand in the
elutriator, and the whole residue is weighed after drying for two hours
at 110°.
[Illustration:
FIGURE 30.
MASURE’S SILT APPARATUS.
]
The fine soil collected in C may also be separated and weighed, for
control, after drying as above.
The pebbles and coarse sand separated by the sieves should also be
weighed. By this process the soil is separated into four portions;
_viz._,
(1) Pebbles. (2) Coarse sand. (3) Fine sand and other materials not
carried off by the current of water. (4) Fine soil, carried into the
receiver C.
=217. Method of Schöne.=—The method of Schöne[149] is based on the
combination of a cylindrical and conical separatory tube through which
the flow of water is regulated by a piezometer.
If, in the process of silt separation, the water move perpendicularly
upward with a given velocity, _e.g._ = v the separation is dependent:
(1) On the volume of the silt particles, (2) On their specific gravity,
and, (3) On their state of disintegration.
If it be assumed that the silt particle is a sphere with a diameter = d,
then according to Newton’s law of gravity, the following formula would
be applied: d = v² ((3Z)/(4g (S − 1))).
[Illustration:
FIGURE 31.
SCHÖNE’S ELUTRIATOR.
]
In the above formula Z = a coefficient which depends on the condition of
the surface against which the hydraulic pressure or resistance works, in
this case a sphere; g = the acceleration of gravity equivalent to 9.81
meter; and S = the specific gravity of the particle.
This expression signifies that in a given case, the velocity of the
current in the apparatus is just sufficient to counteract the tendency
of a given particle to sink. All particles of a smaller diameter, in
such a case, will be carried on by the current, while all of a greater
diameter would separate by sedimentation. These theoretical conditions
are not met with in practice where silt particles of all shapes and
degrees of aggregation abound. These particles, whatever their shape,
may be said to have the same hydraulic value when carried by the same
current. It is necessary, therefore, to secure some uniform standard of
expression to assume a normal form of particle and a normal specific
gravity. For the form, a sphere is evidently the normal which must be
considered and for specific gravity that of quartz is taken; _viz._,
2.65. The mean coefficient for Z may also be placed at 0.55, although
slightly different values are ascribed to it. Substituting these values
in the formula, it is reduced to the expression; d = v² × 0.0000255
millimeters. It can, therefore, be said that by this or that velocity of
the current, silt particles will be removed of this or that diameter, it
being understood that all particles of equal hydraulic value to
spherules of quartz of the given diameter are included in each class. In
order to have the theoretical formula agree with the results of analysis
it is necessary to modify it empirically to read d = v^{⁷⁄₁₁} × 0.0314
millimeters.
This formula is found to agree well with the results obtained for all
velocities between 0.1 millimeter and twelve millimeters per second, the
ordinary limits of silt separation.
_The Apparatus._—The conic-cylindrical elutriating vessel A, B, C, D, E,
F, G, Fig. 31, is of glass. The part B, C, is cylindrical, ten
centimeters in length and as nearly as possible five centimeters in
diameter.
The conical part C, D, is fifty centimeters in length. Its inner
diameter at D must not be greater than five centimeters nor smaller than
four centimeters.
The bend, D, E, F, should have the same diameter; _viz._, four to five
centimeters.
The part A, B, C, D, and D, E, F, G, may be made of separate parts and
joined by a rubber tube.
_Outflow Tube and Piezometer._—The outflow tube and piezometer, H, J, K,
L, is constructed as shown in Fig. 32. It should be made of barometer
tubing having an internal diameter of about three millimeters. The tube
is bent at J at an angle of forty to forty-five degrees. The knee J must
be as acute as possible not to interfere with the inner diameter. The
form and especially the magnitude of the outlet are of great importance.
It must be circular and nearly 1.5 millimeter in diameter. It must not
be larger than 1.67 millimeter nor smaller than 1.5 millimeter. The
opening should be so made as to direct the stream of outflowing liquid
in the direction shown by the arrow.
[Illustration:
FIGURE 32.
SCHÖNE’S ELUTRIATOR, OUTFLOW TUBE.
]
The piezometer L, K is parallel to the arm H, J, of the delivery tube.
Its graduation has its zero point in the center of the outlet K. It
commences with the one centimeter mark. From one to five centimeters it
is divided into millimeters, from five to ten centimeters into
one-fourth centimeter, from ten to fifty centimeters into one-half
centimeter, and from fifty to 100 centimeters into centimeters. The
dimensions given are those required for ordinary soils and for
velocities ranging from two-tenths millimeter to four millimeters per
second.
For greater velocities, a delivery tube with a larger outlet must be
used and the piezometer must be of greater internal diameter than
indicated.
[Illustration:
FIGURE 33.
SCHÖNE’S ELUTRIATOR, ARRANGEMENT OF APPARATUS.
]
_Arrangement of the Apparatus._—The apparatus is conveniently mounted as
shown in Fig. 33, giving front and side views of all parts of apparatus
in position ready for use. When numerous analyses are to be made much
time is saved by having a number of apparatus arranged _en batterie_.
_The Sieve._—The soil, before being subjected to elutriation, should be
passed through a sieve of which the meshes are 0.2 millimeter square.
_The Process._—To measure the diameter of the cylinder, two marks are
made with a diamond upon the glass which are distant from each other a
certain space, for instance, _h_ centimeters. The space between these
two marks is filled with water exactly measured. Suppose that a cubic
centimeters were used, then the diameter is determined by the formula:
D = √(4_a_)/(π_h_) centimeters.
In order to determine that the elutriating cylinder is strictly
comparable in all its parts this measurement should be made upon several
parts thereof.
The apparatus should now be tested in regard to the quantity of liquid
which it will deliver under a given pressure in the piezometer. By means
of the stop-cock H the flow of water is so regulated that the outflow at
_c_ can be measured at a given height of the water in the piezometer.
Suppose that _a_ cubic centimeters of water flow in _t_ seconds, then
the quantity which would flow in one second is determined by the
formula, Q = a/t cubic centimeters. Since according to the law of
hydraulic outflow the quantities are proportional to the square root of
the height of the column it is easy to compute from any given height the
quantity which will flow from any other one desired. For the retardation
due to capillary attraction, it is sufficient, in general, to take it in
a constant quantity; if this constant quantity be represented by C, the
observed height of the water in the piezometer by _h_, and the quantity
of water flowing out by Q, the data required for any given velocity can
be calculated from the following proportion:
√(_h_₁ − C) : √(_h_₂ − C) = Q₁ : Q₂.
It is necessary to compute the magnitude of this constant C which is to
be subtracted. This is accomplished by measuring the quantity of water
which flows out at two different heights of the column in the
piezometer. From the foregoing proportion, the value of C is as follows:
C = (Q₁² _h_₂ − Q₂² _h_₁)/(Q₁²) − Q₂²) centimeters.
The value of C can be the more exactly determined as _h_₁ is greater and
_h_₂ smaller. It is best to choose the lowest height from which an exact
reading can be made; that is, by which the regular rise and fall of the
level of the water in the piezometer (in consequence of the formation of
drops) just begins to disappear. This usually takes place when _h_₂ =
1.5 centimeter to 1.7 centimeter. For the higher value _h_₁ it is best
to take about 100 centimeters. Suppose, for example, the following
results are obtained:
Height of
column to be
Observed height. Observed quantity of subtracted
outflow. due to
capillary
attraction.
_h₂_ _h₁_ Q₁ cubic Q₂ cubic
centimeters. centimeters. centimeters. centimeters. centimeters.
80 1.6 5.53 0.406 1.21
100 1.6 6.13 0.484 1.17
80 1.8 5.53 0.406 1.19
100 1.8 6.13 0.484 1.19
The same quantity of water which flows out in a unit of time passes also
at the same time over a cross section of the elutriating cylinder. The
diameter of this cylinder being D the equation is derived
_v_ = Q(4)/(πD²) centimeters.
Since the velocity in the elutriating cylinder v is directly as the
quantity of water overflowing so is
_v_ : _v_ₙ = √(_h_ − C) : √(_h_ₙ − C);
then _v_ₙ = √(_h_ₙ − C) (_v_)/(√(h − C))
and _h_ₙ = _v_ₙ²((_h_ − C)/(_v_²) + C. The constant (_h_ − C)/(_v_²)
is obtained from the means of a number of estimations; for example as
illustrated in the following data:
Observed Corresponding velocity
quantity of in elutriating cylinder Constant.
Observed height, outflow, cubic of 4.489 centimeters (_h_ −
centimeters. centimeters. diameter, millimeters. C)/(2)
1.6 0.406 0.0257 621
1.8 0.484 0.0306 652
80.0 5.530 0.3490 647
100.0 6.130 0.3870 660
———
Mean 645
Then are obtained the following values of _h_ₙ and _v_ₙ:
_h_ₙ = 645(Vₙ²) + 1.19 centimeters.
and _v_ₙ = √(_h_ₙ − 1.19) × 0.0394 centimeters.
In order to be able easily and rapidly to judge under what pressure the
outflow has taken place in any particular instance, a larger number of
values are computed with the help of the formula given and placed
together in tabular form. As an example the following table may serve
which was computed for one of the apparatus used. Usually it will be
sufficient to test the apparatus for four different heights and then to
interpolate the values for all the others. The numbers marked with a
star in the table are those which were determined by experiment; the
others were calculated.
Height of column Velocity in the elutriating Corresponding diameter of
in piezometer. cylinder of 4.489 silt particles. _d_ =
_h_ centimeters diameter. _v_ v(⁷⁄₁₁)0.0314 millimeters.
centimeters. Observed, Calculated, millimeters.
millimeters. millimeters.
1.5 0.222 0.220 0.0120
1.6 0.257* 0.252 0.0131
1.7 0.284 0.281 0.0140
1.8 0.306* 0.307 0.0148
1.9 0.323 0.332 0.0155
2.0 0.346 0.355 0.0162
2.5 0.427 0.451 0.0185
3.0 0.531 0.530 0.0210
3.5 0.577 0.599 0.0227
4.0 0.650 0.660 0.0236
4.5 0.694 0.717 0.0254
5.0 0.751 0.769 0.0265
6.0 0.850 0.864 0.0286
7.0 0.942 0.950 0.0304
8.0 1.050 1.028 0.0320
9.0 1.120 1.101 0.0334
10.0 1.170 1.169 0.0347
15.0 1.490 1.460 0.0400
20.0 1.730 1.710 0.0441
25.0 1.940 1.920 0.0476
30.0 2.100 2.110 0.0506
35.0 2.310 2.290 0.0532
40.0 2.460 2.450 0.0556
45.0 2.610 2.610 0.0578
50.0 2.770 2.750 0.0598
60.0 3.030 3.020 0.0635
70.0 3.290 3.270 0.0667
80.0 3.490* 3.500 0.0697
90.0 3.710 3.710 0.0724
100.0 3.870* 3.920 0.0749
Suppose the problem is by means of the apparatus tested above, to
separate into a number of groups a mixture of silt particles, whose
hydraulic values are found between the following diameters: 0.01, 0.02,
0.03, 0.04, 0.05, 0.06, 0.07 millimeters. The table will show at once
under what pressure of water the piezometer must be placed in order to
give the values; _viz._, 1.4, 2.8, 7.0, 15.0, 29.0, 53.0, and 83.0
centimeters respectively.
The apparatus described above, is adapted for velocities in the
elutriating cylinder varying from two-tenths millimeter to four
millimeters per second. The largest silt particles which can be
separated by the velocities given above, have approximately a diameter
of 0.08 millimeter. For the separation of larger particles a sieve can
take the place of the silt apparatus. If, however, it be desired to
subject larger particles to silt analysis, the dimensions of the
elutriating cylinder and of the outlet of the delivery tube must be
changed accordingly.
_Preparation of Sample._—The conduct of silt analysis of natural soils
must, in certain cases, be preceded by a special treatment of the
sample. If the latter be rich in humus the organic substance must
previously be separated as completely as possible. With sandy soils this
can be accomplished by ignition. With clayey soils, on the contrary, it
is to be performed by boiling the soils at least one hour with water
which contains from one to two per cent of free alkali. Soils which
contain lime must also be subjected to treatment with dilute
hydrochloric acid, and the hydrochloric acid must be as carefully
removed, as possible before the sample is subjected to elutriation;
afterward follows the boiling of the sample in the ordinary way with
water. This, of course, can be omitted when it has already been treated
with boiling dilute alkali. It is also important to remove the larger
particles by a sieve before the elutriation begins. It is well to pass a
sample through a sieve after it has been boiled, by which all particles
of a larger diameter than 0.2 millimeter are removed. This will usually
require about one liter of water and this water should be allowed to
rest from one to two hours and poured off with the suspended material
which it contains. Only what subsides should be brought into the
apparatus. In rinsing the sample as much water must be used as will fill
the apparatus up to its cylindrical portion.
After the sample has been placed in the apparatus, the water is allowed
slowly to enter, being careful to avoid reaching more than the lowest
required velocity, until the outflow begins. The water then is so
regulated by the stop-cock as to bring it to the desired height in the
piezometer. This being accomplished, the different velocities which have
been decided upon for separating the particles of silt are used one
after the other, as soon as all the silt which can be removed at each
given velocity, has been secured. From three to five liters of water
will be required for the separation of each class of particles.
Sometimes the reading of the height of the water in the piezometer is
difficult; as, for instance, when foam or bubbles accumulate therein.
These bubbles can be removed by simply blowing into the tube, or
dropping into it a little ether. The outflow of water can be received in
vessels, beaker glasses, or cylinders, in which it is allowed to
subside. The finest particles which remain in suspension in the water
are best determined by difference. If it be desired to weigh them
directly, the water can be treated with ammonium bicarbonate until it
contains from one to two per cent thereof. The precipitation then takes
place in a few hours.
The collection and weighing of silt particles are accomplished in the
usual way. That which finally remains in the elutriating vessel is taken
out after the end of the operation by closing the stop-cock, removing
the stoppers with the piezometer tube, pouring the contents of the
elutriating vessel into a beaker glass and rinsing out carefully all
adhering particles. Examples of the working of the apparatus follow:
The soil was taken from the Imperial Russian Agricultural Experimental
Institute at Gorki. It was a fine clay sand and was carefully treated
with hydrochloric acid. The results of the analysis are given in the
following table:
Velocities Largest diameter of the Percentage of silt product
employed in collected particles in obtained in repeated
millimeters. millimeters. elutriations.
0.25 0.012 13.4 12.6 11.9
0.5 0.020 9.1 8.7 9.5
1 0.032 21.0 21.4 20.8
2 0.050 30.4 29.8 31.7
3 0.063 16.7 16.1 15.5
4 0.076 5.3 5.5 5.5
Residue 4.2 4.9 3.8
————— ———— ————
Total 100.0 99.0 98.7
Holthof modifies the apparatus of Schöne by putting into the lower mouth
of the elutriator a little mercury so that the particles of earth are
deposited upon its surface and are thus better agitated and washed by
the current of water.
=218. Mayer’s Modification of Schöne’s Method.=—An improvement of
Schöne’s apparatus in the direction of greater simplicity has been
tested by Mayer[150] with satisfactory results:
The apparatus, (Fig. 34), consists of a glass vessel having a glass
stop-cock at the bottom for admitting the water. For a distance of
twenty centimeters the sides of the tube are parallel and the diameter
about one centimeter. Next for a distance of fifty centimeters the tube
is conical expanding at a regular rate until the internal diameter
reaches five centimeters. For a distance of ten centimeters the vessel
is again strictly cylindrical and it is in this cylindrical portion that
the separation of the different constituents takes place. The vessel is
then rapidly narrowed until it carries the stopper A two centimeters in
diameter. This stopper carries two glass tubes, one F bent downward to
conduct the overflow into the receiving vessels, and one H for the
purpose of regulating the rate of overflow by the height of the column
of water therein. The orifice of the overflow tube F should be so
regulated that with a pressure of five centimeters water in H, one liter
shall pass over in ten minutes.
[Illustration:
FIGURE 34.
SCHÖNE’S APPARATUS
FOR SILT
ANALYSIS,
MODIFIED
BY MAYER.
]
If the separation be conducted in an apparatus thus mounted and
graduated with a pressure of two centimeters in H all that portion of
the soil which can properly be called clay will pass over. The fine
earth, that is, earth in which all coarse particles have been removed by
proper sifting, is used in ten-gram lots for each experiment. The
residue, after the separation is complete, consists of pure sand or at
least pure sand mixed with humus. Before the fine earth is placed in the
apparatus, the calcium carbonate therein is removed with hydrochloric
acid. The treatment with hydrochloric acid, however, is not to be
recommended in soil containing many undecomposed particles of calcium
carbonate or dolomite for then large additions to the silt output might
be made from these particles, which could not be regarded as coming from
the soil as it actually exists. For alluvial soil, however, previous
treatment with hydrochloric acid is recommended unconditionally.
=219. Schöne’s Method as Practiced by Osborne.=—The apparatus used by
Osborne[151] was obtained from Germany and was similar to that described
by Schöne in his original paper, except that it was furnished with a
second elutriating tube as suggested by Orth. The modification made by
Orth consists, essentially, of a second elutriating tube with straight
sides into which the bulk of the soil is introduced, only the final part
being carried over into the Schöne’s tube proper. Water is supplied to
the apparatus under constant pressure by means of a Mariotte’s bottle.
The preliminary treatment recommended by Schöne is omitted, as these
steps have been shown to be undesirable, on account of affecting the
accuracy of the results.
Twenty grams of the air-dried soil are passed, under water, through a
sieve of one-fourth millimeter mesh. That part of the soil which remains
in suspension after being sifted is placed at once in the Schöne’s tube
of the apparatus, the coarser portion being rinsed into the Orth tube.
The current is regulated so that the largest particles of quartz carried
off have an average diameter of 0.01 millimeter. When all is carried off
that can be removed at this rate the current is increased until the
largest quartz grains passing off have a diameter of 0.05 millimeter.
As noticed by Hilgard with Schulze’s apparatus, secondary currents are
formed during the process of elutriation which descend along the walls
of the conical portion of the Schöne’s tube and some distance along the
sides of the cylindrical portion. The tendency of these currents is to
produce globular aggregates of particles which fall to the bottom. They
are broken up from time to time by increasing the velocity of the
current but even this method fails to disintegrate a considerable
quantity of them.
=220. Statement Of Results.=—Two samples of soil from the garden of the
experiment station analyzed by Schöne’s method gave the following
proportions of sediment.
In the table the term clay is used to designate all that part of the
soil which has diameters less than 0.01 millimeter and which remains
suspended after twenty-four hours standing in water having a depth of
200 millimeters.
SOIL, FROM GARDEN OF THE EXPERIMENT STATION.—NOT
BOILED.
Analyses with the Schöne-Orth Elutriator.
_A._ _B._ _C._
Above 0.25 millimeter 48.82 48.82 48.82
0.25–0.05 27.36 29.94 22.37
0.05–0.01 millimeter 8.63 6.07 13.70
0.01 millimeter and less depos. 7.36 7.31 7.20
Clay (by difference) 1.00 1.03 1.08
Loss on ignition 6.83 6.83 6.83
—————— —————— ——————
100.00 100.00 100.00
The last column _C_ represents the average of three direct beaker
elutriations according to the method of Osborne.
The differences which these figures show are found to be due to
imperfect separation of the finer grades from the coarser and even when
the various fractions separated by the Schöne method are subjected to
beaker elutriation and the portions separated from them added to the
grades to which they properly belong the Schöne elutriator was found to
effect far less exact separations than the beaker method.
Samples of prairie soil from Mercer County, Ill., not boiled, were
examined by the two methods with the following results:
Schöne-Orth Beaker
elutriation. method.
Above 0.25 millimeter 0.76 0.62
0.25–0.05 millimeter 11.25 2.42
0.05–0.01 millimeter 52.65 43.58
0.01 millimeter and less deposited 14.84 31.58
Clay 4.44 5.81
Loss on ignition 14.49 14.49
————— —————
98.43 98.50
In this case it is seen that Schöne’s method varies considerably from
the beaker method and if the beaker method be regarded as correct the
Schöne method is evidently less reliable.
In the next table are given the data of the examination of brick clay
from North Haven, Conn., by the two methods.
BRICK CLAY FROM NORTH HAVEN, CONN.
Schöne-Orth elutriation. Beaker method.
Above 0.25 millimeter 1.02 1.02
0.25–0.05 millimeter 3.91 0.76
0.05–0.01 millimeter 29.63 20.95
0.01 millimeter and less 58.58 71.01
Loss on ignition 6.60 6.60
————— ——————
99.74 100.34
The failure of the Schöne method to give the results obtained by the
beaker method is ascribed to the fact that it is impossible for the
current of the strength used to disintegrate the clay and further that
the particles after they are once separated tend to coalesce by the
currents produced by the elutriating process.
=221. The Berlin-Schöne Method.=—Osborne has also made a study of the
Schöne method as modified by the Bodenlaboratorium of Berlin. The
directions for the analysis by this laboratory method are as follows:
Five hundred grams of the soil are sifted through a sieve with circular
holes two millimeters in diameter. Of the earth passing the sieve from
30 to 100 grams are boiled in water with constant stirring from one-half
to one hour or longer, according to the character of the soil. The finer
the texture of the soil the smaller the quantity taken and the longer
the time of boiling. Treatment with acids or alkalies is not practiced.
The finer portion of the soil remaining suspended in the water, after
boiling, is poured into the Schöne tube, the remaining coarse part is
rinsed into the Orth tube. The clay, together with the finest sand, is
collected in a separate vessel, the water in which it is suspended is
evaporated and the residue after drying in the air is weighed. The rest
of the operation is carried out as previously described except that the
products of elutriation are not ignited but weighed air dried, in order
that they may be further examined, chemically if desired. By proceeding
in this manner the following results were obtained:
SOIL, FROM GARDEN OF THE EXPERIMENT STATION,
BOILED FORTY-FIVE MINUTES.
Separations by the Berlin-Schöne method.
Air-dried. Ignited.
Above 0.05 millimeter 72.63 71.76
0.05–0.01 millimeter 14.17 12.53
0.01 millimeter and less 12.97 9.38
Loss on ignition 6.83
————— —————
99.77 99.50
For the sake of comparing the mechanical separation attainable by this
procedure with those yielded by other methods, the air-dried products
were ignited and again weighed and examined.
By subtracting from the ignited portion above 0.05 millimeter, 49.37 per
cent, the amount of this soil that remained on a 0.25 millimeter sieve,
the fraction between 0.25 millimeter and 0.05 millimeter is found, and
the separations in this analysis may be compared with those previously
obtained by the beaker method as follows:
SOIL FROM GARDEN OF EXPERIMENT STATION.
│
│ Beaker Method.
│ ———————————————————————————————————————
Berlin-Schöne,│ Boiled Pestled, not Not boiled
boiled │ twenty-three boiled. nor pestled,
forty-five │hours, average average of
minutes. │ of four three
│ analyses. analyses.
│
Above 0.25 49.37│ 47.77 48.82 48.82
millimeter │
0.25–0.05 21.39│ 20.75 22.44 22.37
millimeter │
0.05–0.01 12.53│ 11.18 12.55 13.70
millimeter │
<0.01 clay 9.38│ 13.47 9.36 8.28
included │
Loss on 6.83│ 6.83 6.83 6.83
ignition │
───────────────────────────┼───────────────────────────────────────────
99.50│ 100.00 100.00 100.00
Osborne concludes from the above facts that the Berlin-Schöne method,
while showing close agreement with the beaker method, does not give
results which are identical with that method. On subjecting portions
separated by the Berlin-Schöne method to the beaker analysis additional
separations were secured. In the case of heavy loams the inability of
the Berlin-Schöne method to effect even a rough or approximate
separation of the several grades becomes very conspicuous.
=222. Method of Hilgard.=—Two important principles lie at the foundation
of this method; _viz._, 1, the use only of separating vessels of true
cylindrical shape and 2, the employment of a mechanical stirrer to break
up the floccules formed during the process of separation. The points in
the apparatus to be considered are uniformity of the cross section of
the elutriator at every point, exact perpendicularity of position,
careful control of the rate of flow and continuous operation of the
mechanical stirrer. According to Hilgard’s observations the stirring due
to the current of water alone is not sufficient to break up the
floccules unavoidably formed during the separation, while any
inclination of the sides of the elutriating vessel from the
perpendicular due either to a conical shape or false position favors in
the highest degree the formation of floccules due to reflex currents
formed in the body of the liquid.
In order to carry out the idea suggested by Türschmidt of substituting
for the accidental and indefinite products usually appearing in the
statements of silt analyses sediments of known and definite hydraulic
value a constant head of water is used, secured by means of a Mariotte’s
bottle connecting with the tube delivering the current through a cock
provided with an arm moving on a graduated arc.
According to Hilgard the separation of sediments by the method of
subsidence does not possess the analytical accuracy of the moving liquid
method, especially when the latter is combined with mechanical stirring.
The subsidence method requires close and continuous attention and in the
case of fine sediments tending to flocculation the difficulties of the
method are greatly increased. The views of Hilgard in respect of the
laboriousness of the subsidence method lose, however, some of their
force since the modifications of Osborne have come into use. The
simplicity and cheapness of the apparatus required for subsidence give
it at the start many advantages over the more elaborate process with a
churn elutriator. For rigid scientific investigation, however, the
method of Hilgard is commended as a standard of comparison in all cases.
[Illustration:
FIGURE 35. HILGARD’S CHURN ELUTRIATOR.
]
=223. The Elutriator.=—The instrument devised by Hilgard[152] for the
purpose of breaking up these flocculent aggregates is shown in figure
35, together with the simpler form, a Schöne’s elutriator, figure 36,
which can serve for grain sizes above eight millimeters hydraulic value.
The latter is conveniently selected so as to have half the cross-section
of the former, so that with the same position of the index lever the
velocity will be just doubled. The cylindrical glass tube, of about
forty-five millimeters inside diameter at its mouth, and 290 to 300
millimeters high, has attached to its base a rotary churn consisting of
a brass cup, shaped like an egg with point down, so as to slope rather
steeply at base, and triply perforated; _viz._, at the bottom for
connection with the relay reservoir, and at the sides for the passage of
a horizontal axis bearing four grated wings. This axis, of course,
passes through stuffing boxes, provided with good thick leather washers,
saturated with mutton tallow. These washers, if the axis runs true, will
bear a million or more revolutions without material leakage. When a
beginning is noted additional washers may be slipped on without emptying
the instrument, until the analysis is finished. For the finest
sediments, from five to six hundred revolutions per minute is a proper
velocity, which may be secured by clock work, turbine or electric power.
The driving pulley should not be directly connected with the axis, both
because it is liable to cause leakage, and because it is necessary to be
able to handle the elutriator quickly and independently. This is
accomplished by the use of “dogs” on the pulley and churn axis. For the
grain sizes of one to eight millimeters hydraulic value lower velocities
are sufficient; too low a velocity causes an indefinite duration of the
operation and may be recognized by the increase of turbidity as the
velocity is increased.
As the whirling agitation caused by the rotation of the dasher would
gradually communicate itself to the whole column of water and cause
irregularities, a wire screen of 0.8 millimeter aperture is cemented to
the lower base of the cylinder.
The relay vessel should be a thick, conical test glass with foot; its
object is to serve as a reservoir for the heavy sediments not concerned
at the velocity used in the elutriator tube, and whose presence in the
latter or in its base, the churn, would only cause abrasion of the
grains and changes of current velocity, such as occur in the apparatus
of Schöne, and compel the current measurement of the water delivered. It
is connected above with the churn by a brass tube about ten millimeters
in clear diameter, so as to facilitate the descent of the superfluous
sediments, which the operator, knowing the proportion of area between
the connecting tube and elutriator, can carry to any desired extent;
thus avoiding the disturbance of the gauged current velocities, as well
as all material abrasion.
[Illustration:
FIGURE 36.
IMPROVED SCHÖNE’S APPARATUS
WITH
RELAY.
]
A glass delivery tube should extend quite half way down the sides of the
relay vessel, to insure a full stirring up of the coarse sediments when
required. By means of a rubber hose, not less than twenty inches in
length, this delivery tube connects with the siphon carrying the water
from near the bottom of the Mariotte’s bottle, a ten-gallon acid carboy.
A stop-cock provided with a long, stiff index lever, moving on an
empirically graduated arc, regulates the delivery of water through the
siphon. Knowing the area of the cross section of the elutriator tube,
the number of cubic centimeters of water which should pass through it in
one minute, at one millimeter velocity, is easily calculated, and from
this the lever positions corresponding to other velocities are quickly
determined and marked on the graduated arc. The receiving bottle for the
sediments, also shown in the figure, must be wide and tall, so as to
allow the sediment to settle while the water flows from the top into the
waste pipe. The receiving funnel tube must dip nearly to the bottom of
the bottle. Thus arranged, the instrument works very satisfactorily, and
by its aid soils and clays may readily be separated into sediments of
any hydraulic value desired. But in order to insure correct and
concordant results, it is necessary to observe some precautions; _viz._,
(1) The tube of the instrument must be as nearly cylindrical as possible
and must be placed and maintained in a truly vertical position. A very
slight variation from the vertical at once causes the formation of
return currents, and hence of molecular aggregates on the lower side.
(2) Sunshine, or the proximity of any other source of heat, must be
carefully excluded. The currents formed when the instrument is exposed
to sunshine will vitiate the results.
(3) The Mariotte’s bottle should be frequently cleansed, and the water
used be as free from foreign matters as possible. For ordinary purposes
it is scarcely necessary to use distilled water. The quantities used are
so large as to render it difficult to maintain an adequate supply, and
the errors resulting from the use of any water fit for drinking purposes
are too slight to be perceptible, so long as no considerable development
of the animal and vegetable germs is allowed. Water containing the slimy
filaments of fungoid growths and moss protonema, algae, vorticellae,
etc., will not only cause errors by obstructing the stop-cock at low
velocities, but these organisms will cause a coalescence of sediments
that defies any ordinary churning, and completely vitiates the
operation.
(4) The amount of sediment discharged at any time must not exceed that
producing a moderate turbidity. Whenever the discharge becomes so
copious as to render the moving column opaque, the sediments assume a
mixed character, coarse grains being, apparently, upborne by the
multitude of light ones whose hydraulic value lies considerably below
the velocity used, while the churner also fails to resolve the molecular
aggregates which must be perpetually reforming where contact is so close
and frequent. This difficulty is especially apt to occur when too large
a quantity of material has been used for analysis, or when one sediment
constitutes an unusually large portion of it. Within certain limits the
smaller the quantity employed the more concordant are the results.
Between ten and fifteen grams is the proper amount for an instrument of
the dimensions given above.
=224. Preparation of the Sample.=[153]—In some cases simple sifting will
be sufficient to prepare the air-dried soil for the elutriator. In most
cases, however, some mechanical aid must be invoked to secure particles
of sufficient fineness. Nothing harder than a rubber pestle should be
used and care must be taken not to break up any calcareous or
ferruginous masses which the particles of fine soil may contain. The use
of water in this mechanical attrition should be avoided, if possible,
but in some heavy clay and adobe soils wetting becomes necessary. In
this case the parts separated by the sieve are collected separately and
the turbid mass removed by water and dried for further examination.
A sieve of 0.5 millimeter mesh is recommended as the best because that
is almost exactly the diameter of the particles passing off at the
maximum velocity of sixty-four millimeters per second to which the
elutriator is adapted. The particles passing the 0.5 millimeter mesh are
called fine earth.
=225. Preparation by Boiling.=—The method of preparation by boiling may
be applied to all samples of fine earth. The fact pointed out by
Osborne, that diffusibility of some clays is diminished by long boiling,
renders it important to restrict the time of this operation as much as
possible. With most soils from eight to fifteen hours will be long
enough, occasionally extending to even twenty-four hours. A thin
long-necked flask of about one-liter capacity should be used; filled
three-quarters full with distilled water and the sample of soil added.
The flask is supported over the lamp on a piece of wire gauze at an
angle of 45°. It carries a cork with a long condensing tube. At first
the boiling goes on smoothly, but after a time violent bumping may
supervene, endangering the flask but promoting the object in view.
The contents of the flask are transferred to a beaker and diluted with
distilled water to one and a half liters, shaken and allowed to settle
for a time necessary to allow all particles of 0.25 millimeter hydraulic
value to reach the bottom. The supernatant turbid liquid is decanted and
the process repeated with smaller quantities of water until no further
turbidity is produced. The united decantations, of which there will be
from four to eight liters, are well shaken and a proper time allowed for
the 0.25 millimeter hydraulic value sediments to fall. This last step is
necessary to remove any such sediments which may have been carried over
mechanically in the first separation. The dilution being very great, a
fairly perfect separation is thus secured and the sediments are then
ready for the elutriator.
=226. Separation of Clay and Finest Silt.=—The property which pure clay
possesses, of remaining suspended almost indefinitely in pure water,
affords a ready means of separation from the silt particles of less than
0.25 millimeter hydraulic value. But the finest silt particles subside
so slowly that this method of separation is too long to become
practically applicable to secure a perfect demarcation between the
finest silt and so-called colloidal clay.
Hilgard recommends the following procedure: The clay water from the
previous separation is placed in a cylindrical vessel of such a diameter
as to allow the column of water to be 200 millimeters high where it is
allowed to settle for twenty-four hours. When the clay is very abundant
a longer time may be allowed; _viz._, from forty to sixty hours. The
line of separation between the dark silt below and the translucent clay
above is sharply defined. Finally the clay water is decanted and the
remaining liquid poured off leaving the sediment as sharply defined as
possible. The sediment is rubbed with a rubber pestle and a few drops of
ammonia water added. Distilled water is added, the beaker well shaken or
stirred to break up the floccules that may have formed and subsidence
permitted as before. This operation is repeated from six to nine times
until the water remains quite clear after subsidence or the decanted
turbid water fails to be precipitated by brine showing the suspended
matter to be fine silt and not clay.
The diameter of the particles of silt thus obtained is from 0.001 to
0.02 millimeter, and it is impossible to obtain it quite free from any
admixture with clay.
=227. Estimation of the Colloidal Clay.=—The importance of the colloidal
constituent of the clay is such as to make its direct determination
desirable. The volume of the clay waters at this stage of the analysis
may amount to twenty liters. One method of determination consists in
evaporating an aliquot portion and this method will yield good results
if the sample be free from soluble salts and the quantity taken be not
too small. At least 500 cubic centimeters should be used for this
purpose. A better method consists in precipitating the clay by means of
a saline solution. A saturated solution of salt is recommended for this
purpose of which fifty cubic centimeters are sufficient to precipitate
the clay from one liter of the clay water. The precipitation is hastened
by heating. Each portion of the clay water should be precipitated as
soon as obtained, the total volume of the precipitate at the end of
twenty-four hours is thus reduced to a minimum. The clay water from the
succeeding separations of the same analysis can be mixed with the
precipitate which diffuses therein, thus promoting the precipitation of
the rest of the clay inasmuch as the separation takes place more readily
where more clay is present. When all the clay is thus collected it can
be gathered on a tared filter and washed with weak brine. Pure water may
not be used because of the diffusibility of clay therein. After drying
at 100° and weighing it is washed with a weak solution of ammonium
chlorid until all sodium is removed. The filtrate is evaporated to
dryness, ignited at low redness, and weighed. The weight of the sodium
chlorid thus obtained plus the weight of the filter deducted from the
total weight gives the weight of the clay precipitate. Whenever the clay
collected as above will not diffuse in water it may be washed with water
and its weight directly obtained. An excess of iron in clay will usually
allow of the above treatment.
=228. Properties of Pure Clay.=—The percentage of pure clay as obtained
by the procedure described is about seventy-five in the finest natural
clays, forty-five in heavy clay soils, and fifteen in ordinary loamy
soils. When freshly precipitated by brine it is gelatinous resembling a
mixed precipitate of iron and aluminum oxids. Its volume greatly
contracts on drying, clinging tenaciously to the filter, from which it
may be freed by moistening. On drying, it becomes hard, infriable, and
often resonant. It usually possesses a dark brown tint due to iron oxid.
Under the action of water it swells up like glue, the more slowly as the
percentage of iron is greater. In the dry state it adheres to the tongue
with great tenacity. According to Whitney the finest particles of
colloidal clay have a diameter of 0.0001 millimeter. With a magnifying
power of 350 diameters, however, Hilgard states that no particles can be
discerned.
=229. Chemical Nature of the Fine Clay.=—The fine particles separated as
above consist essentially of hydrous aluminum silicate or kaolinite. It
doubtless contains, however, other colloids or hydrogels whose
absorptive powers are similar to those of clay. It appears also to
contain sometimes free aluminum hydroxid, and colloidal ferric hydroxid,
and amorphous zeolitic compounds.
While the most careful mechanical separation can give at best only
approximately the really plastic kaolinite substance, yet it is far
closer than that attained by determination of total alumina with boiling
sulfuric acid. By the latter treatment all the lime-kaolinite particles
are decomposed and the method does not lead to even an approximate
estimate of the soil’s plasticity.
=230. Separation of the Fine Sediments.=—The sediments remaining after
the separation of the clay and fine silt are ready for separation in the
churn elutriator. The apparatus mounted, as already described, is
brought into use by beginning with a low velocity of the water in the
upright tube. The rate of flow should be set at from 0.25 millimeter to
0.50 millimeter per second, and the churn put in motion.
When the elutriating tube is partly full of water the sediments should
be poured in from a small beaker which is perfectly cleaned by means of
a washing flask. The stopper and delivery tube of the elutriator are
then put in place. The rate of flow should be so regulated that the
sediments shall have had a few seconds of subsidence before the water is
within thirty millimeters of the top. At this point the required
velocity for the first sedimentation should be turned on; _viz._, 0.25
millimeter per second. At first the sediment passes off rapidly and the
water in the elutriator is distinctly turbid. This excess of turbidity
ceases in a few hours and then some attention is necessary in order to
determine when the process is complete. In fact it never is completely
finished, but where no more than one milligram of silt comes off with
one liter of water it may be said to be practically done. The time
required for the first operation varies from fifteen to ninety hours.
Downward currents in the elutriator are likely to form in spite of all
precautions, and floccules of silt adhere to its walls. These should be
detached from time to time with a feather in order to bring them again
in contact with the churn.
Hilgard has found that, practically, 0.25 millimeter per second is about
the lowest velocity available within reasonable limits of time, and that
by successively doubling the velocities up to sixty-four millimeters a
desirable ascending series of sediments is obtained; provided always,
that a proper previous preparation has been given to the soil or clay.
It would seem that according to the prescription given above for the
preliminary sedimentation, no sediment corresponding to 0.25 millimeter
velocity should remain with the coarser portion. That such is
nevertheless always the case, often to a large percentage, emphasizes
the difficulty, or rather impossibility, of entirely preventing or
dissolving the coalescence of these fine grain sizes by hand stirring,
as in beaker elutriation. It is only by such energetic motion as is
above prescribed that this can be fully accomplished, and the delivery
of 0.25 and 0.50 millimeter hydraulic value really exhausted.
It is desirable to run off the upper third of the column at intervals of
fifteen to twenty minutes by temporarily increasing the velocity. Recent
sediments, river alluvium, etc., are more easily separated than soils of
more ancient formation. The second, third, etc., separations are
naturally accomplished in much less time than the first. The respective
velocities of the separations should be 0.25 millimeter, 0.50
millimeter, one millimeter, two millimeters, four millimeters, eight
millimeters, sixteen millimeters, thirty-two millimeters, and sixty-four
millimeters a second. Below a velocity of four millimeters a second the
mechanical stirrer is indispensable. Above this velocity the current of
water in the conical base will be sufficient to bring the desired
particles into the ascending column. At this velocity also a smaller
elutriating tube having one-half or one-quarter the cross-section of the
first may be employed to hasten the operation and diminish the quantity
of water required. The quantity of water required for a complete
separation is from 100 to 120 liters. Any soft water free of organic
matter may be used, but distilled water is best. Hard water should be
avoided.
The mean time required for the different separations is as follows: 0.25
millimeter hydraulic value, thirty-five hours; 0.50 millimeter hydraulic
value, twenty hours; one millimeter hydraulic value, seven and a half
hours; two to sixty-four millimeters hydraulic value, eight hours. With
proper arrangements for night work, an analysis may be finished in three
or four days not counting the time required for the previous separation
of the clay.
=231. Weighing the Sediments.=—The sediments should be dried at the same
temperature used for drying the soils. Hilgard dries both at 100°. Great
care should be used in weighing the exceedingly hygroscopic clay
sediments. In the case of the sediment of 0.25 millimeter hydraulic
value it is allowed to subside as much as possible and after removing
the supernatant water the residue, twenty-five to fifty cubic
centimeters, is evaporated in a platinum dish and weighed therein. The
water can be completely decanted from the other sediments, and they can
be dried and weighed without any unusual precautions.
The loss in the separation of clays and subsoils containing but little
organic matter is usually from 1.5 to 2 per cent. This loss is partly
due to the fine silt which comes off during the whole of the process and
which is lost in the decanted waters of the sediments of 0.25
millimeters hydraulic value and above. The procedures indicated above
are not strictly applicable to soils rich in humus and other organic
matters, but the destruction of these matters by ignition leaves the
residual soil in a condition wholly unfit for sedimentary separation.
=232. Classification of Results.=—A convenient method of stating the
results of an analysis may be seen from the following classification.
The percentage obtained for each of the classes is to be entered in the
column provided for that purpose.
No. Names of Silt Diameter of Velocity of
Classes. grains in current Per
millimeters. millimeters cent.
hydraulic
value.
Sieves. 1. Grits 1 –3 2.07
„ 2. Fine grits 0.5–1 „
───────────────────────────────────────────────────────────────────────
Elutriator 3. Coarse sand 0.50 64 0.55
without churn.
„ 4. Medium sand 0.30 32 „
„ 5. Fine sand 0.16 16 „
───────────────────────────────────────────────────────────────────────
Elutriator 6. Finest sand 0.12 8 0.21
with churn.
„ 7. Coarse silt 0.072 4 1.21
„ 8. Large silt 0.047 2 2.92
„ 9. Medium silt 0.036 1 7.36
„ 10. Silt 0.025 0.5 8.86
„ 11. Fine silt separated 0.016 0.25 7.85
in elutriator
───────────────────────────────────────────────────────────────────────
Beaker 12. Fine silt separated 0.010 <0.25 35.22
sedimentation. from clay water
„ 13. Clay 0.0001 <0.0023 33.16
—————
Total 99.36
The measurements of diameters in the above table is of the best formed
quartz grains in each class. Naturally the actual size of the particles
may vary in each class within the extreme limits of the diameter next
above and below. It is not easy to indicate in popular language
distinctions not popularly made but the grades of particles designated
by the names grits, sand and silt, may serve, at least, to establish
uniformity of expression. The term grits is thus applied to all grains
above one millimeter in diameter up to gravel. Below one millimeter down
to 0.1 millimeter may be called sand and below that silt may designate
the particles down to an impalpable powder.
=233. Influence of Size of Tube.=—The diameter of the elutriating tube
exerts a sensible influence on the character of the sediments. The
friction against the sides of a small tube is comparatively greater than
in a large tube. Strictly speaking, no class of sediments strictly
corresponds to the hydraulic value calculated from the cross section of
the tube and the quantity of water supplied thereto. The sediments
correspond actually to higher velocities, due to the fact that the
lateral friction causes a more rapid flow in the center of the water
column. This may be demonstrated by slightly diminishing the velocity
while a sediment is copiously discharging. The turbid column then
remains stationary while clear water is running off.
=234. Statement of Results.=—A complete silt analysis of a soil,
conducted by the method of Hilgard, depends largely for its practical
value on an intelligible tabulation. The method of collating results is
illustrated in the table of analyses of Mississippi soils shown on page
237.
The character of the soils entering into the given analyses is as
follows:
Nos. 248, 206, 209, 397, 219, belong to the end of the drift period.
No. 230 is one of the two chief varieties of soils occurring in what is
known as the flat-woods, a level surface bordering on the cretaceous
area, having lower tertiary clays near the surface.
No. 165 is a light soil which occurs in the former in irregular strips
and patches, is easily tilled, absorbs rain water readily, but is
subject to drought and does not hold manure.
SILT ANALYSES OF MISSISSIPPI SOILS AND SUBSOILS.
──┬─────────────┬────────────┬───────────┬──────────
│ Designation │ Diameter. │ Velocity │ DRIFT
│of Materials.│Millimeters.│(Hydraulic │
│ │ │ value). │
│ │ │Millimeters│
│ │ │per second.│
──┼─────────────┼────────────┼───────────┼──────────
│ „ │ „ │ „ │ „
──┼─────────────┼────────────┼───────────┼──────────
│ „ │ „ │ „ │ „
──┼─────────────┼────────────┼───────────┼──────────
│ „ │ „ │ „ │ 238
│ │ │ │ White
│ │ │ │pipeclay.
│ │ │ │Tishomingo
│ │ │ │ Co.
│ │ │ │
──┼─────────────┼────────────┼───────────┼──────────
1│Coarse grits │ 1.0 to 3.0│ │
2│Fine „ │ 0.5 to 1.0│ │
3│Coarse sand │ 0.40│ 6│ 0.06
4│Medium „ │ 0.30│ 32│ „
5│Fine „ │ 0.16│ 16│ „
6│Finest „ │ 0.12│ 8│ 0.08
7│Dust „ │ 0.072│ 4│ 0.02
8│Coarsest silt│ 0.047│ 2│ 0.04
9│Coarse „ │ 0.036│ 1│ 0.08
10│Medium „ │ 0.025│ 0.5│ 0.08
11│Fine „ │ 0.015│ 0.25│ 2.00
12│Finest „ │ 0.008│ <0.25│ 21.15
13│Clay │ 0.0001│ <0.0023│ 74.65
──┴─────────────┴────────────┴───────────┼──────────
│ 98.16
Compactness (resistance to tillage) │ 97.80
Porosity │ 0.36
Hygroscopic Moisture (+7° to +21°) │ 9.09
Ferric Oxide │ 0.13
─────────────────────────────────────────┴──────────
──┬─────────────┬─────────────────────────────────────────────────────
│ Designation │ UPLAND.
│of Materials.│
│ │
│ │
│ │
──┼─────────────┼─────────────────────────────────────────────────────
│ „ │ YELLOW LOAM.
──┼─────────────┼─────────────────────────┬───────────────────────────
│ „ │ SANDY. │ LOAM.
──┼─────────────┼─────────┬─────────┬─────┼────────┬─────────┬────────
│ „ │ 248 │ 165 │ 206 │ 209 │ 397 │ 219
│ │Tallahoma│ Lt. │Pine │ Pine │ Oxford │ Table
│ │ subsoil │Flatwoods│Hill │ Hill │subsoil. │ Land
│ │ Jasper │ soil. │soil.│subsoil.│Lafayette│subsoil.
│ │ Co. │Chickasaw│Smith│ Smith │ Co. │ Benton
│ │ │ Co. │ Co. │ Co. │ │ Co.
──┼─────────────┼─────────┼─────────┼─────┼────────┼─────────┼────────
1│Coarse grits │ 6.94│ 2.90│ 0.36│ 0.36│ │ 0.23
2│Fine „ │ 17.65│ 6.96│ 2.98│ 0.83│ │ „
3│Coarse sand │ 18.81│ 2.81│ 6.62│ 6.21│ 0.79│ 1.47
4│Medium „ │ 10.16│ 4.41│ 7.75│ 3.38│ „ │ 2.33
5│Fine „ │ 2.66│ 3.13│ 3.01│ 3.85│ „ │ 1.17
6│Finest „ │ 1.66│ 2.02│ 1.59│ 1.49│ 0.18│ 0.78
7│Dust „ │ 1.02│ 2.23│ 1.19│ 0.64│ 0.78│ 0.76
8│Coarsest silt│ 0.88│ 5.06│ 3.56│ 2.63│ 3.56│ 9.79
9│Coarse „ │ 1.96│ 9.67│ 6.50│ 5.40│ 13.12│ 7.26
10│Medium „ │ 7.89│ 14.18│13.97│ 7.77│ 16.64│ 13.14
11│Fine „ │ 8.40│ 22.03│14.20│ 16.65│ 27.28│ 15.07
12│Finest „ │ 15.53│ 15.62│29.36│ 37.75│ 18.87│ 26.50
13│Clay │ 8.63│ 7.86│ 4.58│ 10.70│ 17.23│ 19.19
──┴─────────────┼─────────┼─────────┼─────┼────────┼─────────┼────────
│ 99.28│ 98.68│95.67│ 97.77│ 98.35│ 97.65
Compactness (res│ 32.56│ 45.33│48.14│ 45.10│ 63.38│ 60.82
Porosity │ 59.55│ 40.40│37.89│ 47.13│ 20.23│ 26.04
Hygroscopic Mois│ 1.80│ 3.36│ 2.48│ 7.69│ 8.79│ 7.21
Ferric Oxide │ 1.10│ 1.45[G]│ 1.25│ 4.15│ 2.53│ 5.11
────────────────┴─────────┴─────────┴─────┴────────┴─────────┴────────
──┬─────────────┬────────────────────────────────────
│ Designation │ UPLAND.
│of Materials.│
│ │
│ │
│ │
──┼─────────────┼────────┬───────────────────────────
│ „ │ YELLOW │ TERTIARY.
│ │ LOAM. │
──┼─────────────┼────────┴───────────────────────────
│ „ │ CLAY.
──┼─────────────┼────────┬─────────┬────────┬────────
│ „ │ 173 │ 230 │ 246 │ 196
│ │Prairie │ High │ Red │ Hog
│ │subsoil.│Flatwoods│ Hills │ Wallow
│ │ Monroe │ soil. │subsoil.│subsoil.
│ │ Co. │Pontotoc │ Attala │ Jasper
│ │ │ Co. │ Co. │ Co.
──┼─────────────┼────────┼─────────┼────────┼────────
1│Coarse grits │ 2.10│ 0.33│ 1.97│ 0.83
2│Fine „ │ „ │ 0.35│ „ │ 1.19
3│Coarse sand │ 0.62│ │ 0.72│ 1.96
4│Medium „ │ „ │ │ 2.32│ 1.64
5│Fine „ │ „ │ │ 2.09│ 0.88
6│Finest „ │ 0.20│ 0.23│ 0.70│ 0.26
7│Dust „ │ 1.26│ 0.18│ 1.29│ 0.19
8│Coarsest silt│ 2.92│ 1.61│ 1.81│ 2.49
9│Coarse „ │ 7.36│ 2.66│ 3.60│ 3.67
10│Medium „ │ 8.81│ 9.13│ 2.73│ 5.39
11│Fine „ │ 7.85│ 26.64│ 13.30│ 10.31
12│Finest „ │ 35.22│ 32.35│ 25.33│ 24.18
13│Clay │ 33.16│ 25.48│ 40.25│ 47.03
──┴─────────────┼────────┼─────────┼────────┼────────
│ 99.50│ 97.87│ 96.11│ 100.00
Compactness (res│ 69.77│ 84.47│ 78.88│ 81.52
Porosity │ 17.04│ 6.40│ 39.18│ 10.12
Hygroscopic Mois│ 11.35│ 9.33│ 18.60│ 14.48
Ferric Oxide │ 5.42│ 5.90[G]│ 10.50│ 4.00
────────────────┴────────┴─────────┴────────┴────────
──┬─────────────┬────────────────────────────────────────────────────────────────────────
│ Designation │ MISSISSIPPI BOTTOM.
│of Materials.│
│ │
│ │
│ │
──┼─────────────┼───────────────────┬────────────────────────────────────────────────────
│ „ │ Champlain. │ MODERN.
──┼─────────────┼───────────────────┼──────────────────────────────┬─────────────────────
│ „ │ Swamp River. │ RIVER DEPOSIT. │ DELTA.
──┼─────────────┼─────────┬─────────┼────────────┬─────────┬───────┼──────────┬──────────
│ „ │ 390 │ 237 │ 365 │ 377 │ 395 │ Southwest│ Southwest
│ │Buckshot │ Loess. │Tallahatchie│Frontland│Dogwood│ Pass.│ mudlump.
│ │ soil. │Claiborne│soil. Panola│subsoil. │ ridge │Plaquemine│Plaquemine
│ │Issaquena│ Co. │ Co. │Sunflower│ soil. │ Par.│ Par.
│ │ Co. │ │ │ Co. │Coahoma│ │
│ │ │ │ │ │ Co. │ │
──┼─────────────┼─────────┼─────────┼────────────┼─────────┼───────┼──────────┼──────────
1│Coarse grits │ 0.09│ 0.24│ 0.09│ │ │ │
2│Fine „ │ 0.05│ „ │ „ │ │ │ │
3│Coarse sand │ │ 0.37│ 0.04│ 0.32│ 0.15│ 0.18│ 0.10
4│Medium „ │ 0.36│ 0.61│ 0.05│ „ │ │ „ │ „
5│Fine „ │ │ 0.93│ 0.21│ 2.97│ │ 0.47│ 5.02
6│Finest „ │ 0.31│ 1.65│ 1.30│ 2.41│ 3.74│ 7.03│ 3.68
7│Dust „ │ 0.27│ 1.95│ 2.68│ 16.90│ 21.49│ 12.38│ 5.34
8│Coarsest silt│ 1.56│ 14.25│ 9.38│ 19.79│ 21.83│ 13.27│ 10.09
9│Coarse „ │ 2.23│ 16.20│ 9.88│ 13.90│ 14.01│ 15.87│ 5.58
10│Medium „ │ 3.68│ 20.08│ 20.37│ 4.27│ 9.93│ 8.25│ 9.54
11│Fine „ │ 8.97│ 5.59│ 19.79│ 1.89│ 9.58│ 7.26│ 8.01
12│Finest „ │ 38.19│ 33.38│ 25.30│ 30.08│ 8.65│ 19.67│ 34.46
13│Clay │ 44.30│ 2.51│ 9.64│ 5.51│ 10.35│ 12.20│ 18.18
──┴─────────────┼─────────┼─────────┼────────────┼─────────┼───────┼──────────┼──────────
│ 100.01│ 97.74│ 98.73│ 98.04│ 99.72│ 96.58│ 100.00
Compactness (res│ 89.46│ 41.48│ 54.63│ 37.48│ 28.57│ 39.13│ 60.65
Porosity │ 4.87│ 38.44│ 23.63│ 58.25│ 61.50│ 49.20│ 28.81
Hygroscopic Mois│ 14.31│ 4.18│ 6.12│ 5.68│ 3.95│ │
Ferric Oxide │ 5.82[G]│ 3.27│ 2.58│ 2.31│ 2.69│ │
────────────────┴─────────┴─────────┴────────────┴─────────┴───────┴──────────┴──────────
Footnote G:
Bog ore.
No. 248 is from a soil stratum three feet thick. The soil is so light
that the finer particles of it are carried away by high winds.
Nos. 206 and 209 are typical of the soils producing the long-leaf pine.
This soil is much improved by an admixture of the subsoil No. 209, which
enables it to hold manure.
No. 219 is a cotton upland soil of the best quality, found in Western
Mississippi and Tennessee.
No. 397 is the same soil of a second rate quality. These lands are
easily washed into gullies on account of their lack of perviousness to
water. They also easily swell up in contact with water, and become
thereby readily diffused. The denudations produced by heavy rains are
rapidly destroying the lands covered by these soils.
No. 173 is a sedimentary or residual subsoil of the cretaceous prairies
of Northeastern Mississippi, forming a stratum from three to seven feet
thick.
No. 230 is a residual soil which is formed by the disintegration of the
old tertiary clays. It yields good crops only in very favorable years,
and is easily injured both by wet and dry seasons.
No. 246 is a soil of the same origin, but is more easily tilled than the
foregoing, does not crack, but becomes very hard when dried slowly. Its
superiority to the former soil as regards tillage consists in the
presence of the large amount of iron and lime.
No. 196 is a typical heavy clay soil; is better suited for the potter
than the farmer. It cracks on drying, whence its popular name. On the
accession of rain the edges of these cracks crumble and fall, until
finally the lumpy surface is produced which is locally known as hog
wallows.
No. 390, the richest soil of the Yazoo Bottom, seems to have a physical
composition like the preceding one. Its superiority is due not only to
the increased quantity of plant food which it contains, but to its
property of crumbling on rapid drying. Even when plowed wet, on drying
each clod crumbles into a loose pile resembling buck-shot; whence its
name. It is strongly calcareous.
As comparative data, are added the soils 365, 377, and 395, representing
alluvial deposits, and two deposits from the Delta of the Mississippi.
=235. Comparison of Osborne’s Method with Hilgard’s Method.=[154]—The
comparative results obtained by Osborne’s method, beaker elutriation,
and Hilgard’s method, churn elutriation, are given in the following
tables:
SOIL FROM EXPERIMENT STATION GARDEN, NEW HAVEN, CONN.
SURFACE SOIL, BOILED TWENTY-THREE HOURS.
Beaker
elutriation.
Average of
four
Churn elutriation. analyses.
Diameter in
millimeters. per cent. per cent. per cent.
Removed by sieves 47.77 47.77 47.77
0.25–0.05 22.06 21.95 20.75
0.05–0.01 11.20 11.62 11.18
<0.01 9.82 9.14 10.72
Clay (difference) 2.32 2.69 2.75
Loss on ignition 6.83 6.83 6.83
—————— —————— ——————
100.00 100.00 100.00
SUBSOIL, BOILED TWENTY-THREE HOURS.
Churn elutriation. Beaker elutriation.
Diameter in
millimeters. per cent. per cent. per cent. per cent.
Removed by sieve 39.33 39.33 39.33 39.33
0.25–0.05 33.61 30.83 32.35 32.95
0.05–0.01 10.91 12.25 10.32 10.37
<0.01 7.05 8.11 8.29 7.64
Clay 5.02 5.40 5.63 5.63
Loss on ignition 4.08 4.08 4.08 4.08
—————— —————— —————— ——————
100.00 100.00 100.00 100.00
These analyses agree quite as well as could be expected from two such
different methods.
_Elutriation of Clayey Soils._—Hilgard found that by churn elutriation
no satisfactory results could be obtained on clay without long boiling
and subsequent kneading of the finer sediments. Osborne examined a
sample of clay by his method after previous boiling for twenty-three
hours. When the sediments were examined by the microscope they were
found to contain many aggregations of particles which broke into dust
under the pressure of the thin glass slide-cover. These sediments were
then gently crushed in the beaker with the help of a soft rubber stopper
with a glass rod for a handle, the grinding together of the particles
being, as much as possible, avoided. This pestling was continued with
clear water as long as it occasioned turbidity. Comparison of the
analyses shows that practically identical results were obtained on this
soil whether it was boiled or not and indicates that the sediments are
reduced to their elements by gentle pestling alone. For such soils,
therefore, it is demonstrated that pestling is a much safer treatment
than boiling. The same remark may be applied to the fertile prairie soil
of Mercer County, Illinois, where boiling proved quite insufficient and
in which the pestling process proved completely successful. The general
conclusions arrived at from the results obtained by Osborne are as
follows:
1. On sands and silts of pure quartz or similar resistant material
Hilgard’s method and beaker elutriation give practically identical
results.
2. With coarse sands and silts upon whose grains finer matter has been
cemented by silicates, etc., and with soils containing soft slaty
detritus, the churn elutriator with preliminary boiling may give results
too low for the coarse and too high for the finer grades. In these cases
beaker elutriation with pestling yields more correct figures.
3. Some loamy soils containing no large amount of clay or of extremely
fine silt, as well as prairie soils rich in humus, cannot be suitably
disintegrated by twenty-four hours’ boiling, but are readily reduced by
pestling.
4. Beaker elutriation preceded by sifting, gives results in five or six
hours with use of two to three gallons of pure water, which, in churn
elutriation, require several days and consume eight to ten gallons of
pure water.
5. Hilgard found that practically 0.25 millimeter is about the lowest
velocity of water current per second available within reasonable limits
of time in his elutriator. Such a current carries over particles up to
0.015 millimeter diameter and hence the silts of less dimensions cannot
be conveniently separated by churn elutriation. In beaker elutriation
there is no difficulty in making good separations at 0.01 millimeter and
at 0.005 millimeter.
6. Beaker elutriation requires no tedious boiling or preliminary
treatment and with careful pestling of the sediments gives, we believe,
as nearly as possible, a good separation of adhering particles and at
every stage of the process carries with it, in the constant use of the
microscope, the means of testing the accuracy of its work and of
observing every visible peculiarity of the soil. It is not claimed that
pestling may not easily go too far, but in any case a good judgment may
be formed of its effects and of the extent to which it is desirable to
carry it.
7. In beaker elutriation the flocculation of particles occasions little
inconvenience and does not impair the accuracy of the results.
=236. Comparison of the Osborne with the Schloesing
Method.=—Schloesing’s method has been compared by Osborne[155] with the
beaker method of elutriation with the following results:
SCHLOESING’S METHOD.
Per cent.
Calcium carbonate 4.20
Sand 64.91
Clay 22.65
Humus none
Loss on ignition 6.60
BY THE DIRECT BEAKER METHOD.
Per cent.
Above 0.25 millimeter diameter 1.02
0.25–0.05 millimeter diameter 0.76
0.05–0.01 millimeter diameter 20.95
Below 0.01 millimeter diameter 71.01
Loss on ignition 6.60
It is seen by the above that there is little agreement between the
results of the two methods.
With the prairie soil from Mercer County, Ill., the following results
were obtained working on the original sample and the sand separated by
the Schloesing process:
SCHLOESING’S METHOD.
Per cent.
Calcium carbonate 0.88
Humus 1.57
Loss at 150° C. 4.42
Sand 82.86
Clay 7.86
—————
97.59
BEAKER METHOD.
Original
Schloesing’s sand. soil.
Dried at
150°C., Ignited, Ignited,
per cent. per cent. per cent.
Above 0.25 millimeter diameter 0.12 0.10 0.92
0.25–0.05 millimeter diameter 3.58 3.55 2.89
0.05–0.01 millimeter diameter 42.69 41.87 42.86
0.01–0 millimeter diameter 23.66 20.47 } 31.44
Clay 12.81 10.14 }
Clay 7.40
Loss on ignition 6.73 14.49
————— ————— ——————
82.86 82.86 100.00
Osborne says the above figures indicate that the treatment with acid has
disintegrated the particles of less than 0.01 millimeter diameter so
that one-third of this portion appears as clay, according to the Hilgard
method of estimating clay, which is the one employed.
As to the humus it may be noted that loss in the analysis by
Schloesing’s method; _viz._, 2.41 per cent, plus loss at 150° = 4.42 per
cent, plus humus found = 1.57 per cent, plus carbon dioxid (⁴⁴⁄₅₆ of
0.88 =) 0.69 per cent amounts to 9.09 per cent, while the loss on
ignition which represents humus, carbon dioxid and water is 14.49 Per
cent. The 5.40 per cent difference must evidently be, for the most part,
humus which has escaped estimation by the Schloesing method, having been
distributed among the sand and clay.
=237. The Mechanical Determination of Clay.=—Schloesing’s method for the
separation of the clay as stated by Osborne[156] is essentially one of
subsidence for twenty hours from a volume of from 200 to 250 cubic
centimeters of water, but of no specified height. Hilgard’s conventional
method requires the same time and a height of solution of 200
millimeters.
Such methods of separation assume, first, that most of the sand and,
second, that little of the clay shall settle within the fixed time. That
both of these assumptions are fallacious, the following experiments
show. The clay obtained by twenty hours subsidence from thirty grams of
brick clay is suspended in four liters of distilled water and allowed to
settle out completely, which requires several days. The water is then
decanted so as to remove all soluble matters, the jar again filled with
distilled water, and the clay and fine sand allowed to settle again for
several days. The upper three-quarters of the liquid are then decanted
and made up to a volume of four liters, and this is allowed to stand
several days, when a considerable sediment forms. A decantation is again
made as before. The operations are repeated until the clay water has
been so far freed from the clay as to become opalescent; then it first
ceases to deposit any appreciable sediment. A microscopic examination of
the several sediments thus collected shows them all to contain particles
of sand. It appears, therefore, that only after the liquid containing
the clay has become opalescent does it cease to deposit fine particles
of sand as well as of clay.
Furthermore, the character of the true clay itself is so changed under
certain conditions that it loses the property of remaining in prolonged
suspension in water.
A sample of clay which has been freed from particles of sand exceeding
0.005 millimeter diameter is suspended in water and precipitated from it
by freezing. It is then washed by decantation with alcohol and dried in
the air. A portion of this clay is shaken with water and allowed to
stand a few hours, during which time the greater part of it has settled.
After decanting the water and suspended clay and repeating this process
a few times, a very considerable part of the clay is left which will
subside completely through 100 millimeters in a few hours. After
standing under water for several months, only a small part of the clay
has regained the quality of prolonged suspension. It has been found,
however, that if this clay be pestled, this quality of prolonged
suspension is restored to it to a very considerable degree.
It is evident, therefore, that conventional methods depending on simple
subsidence can give no accurate results because the ever varying amounts
of finest sand and clay in different soils yield variable mixtures of
the two when subjected to any simple course of treatment by elutriation
and subsidence.
The method of persistent pestling and repeated subsidences and
decantations continued until no further separation can be effected,
although extremely tedious, is the only one which has so far yielded
even approximately good separations on any of the clayey soils examined
by Osborne.
A single subsidence of the clay water for twenty-four hours will free it
from all particles of sand having a diameter greater than 0.005
millimeter, but in many cases a considerable amount of finer sand will
remain in suspension for many hours or days.
On the other hand, the sediment formed during the twenty-four hours
subsidence will not be free from clay, as may be easily seen by
suspending it in water a second time and allowing it to stand again for
twenty-four hours. Both Hilgard and Schloesing direct attention to these
defects, but assume that they do not usually influence the results to a
sufficient extent to deprive them of value. In many cases this is
undoubtedly true, as, for example, in such soils as that from the garden
of the Experiment Station, at New Haven, in which there is but little
clay and fine sand; but in soils of the opposite character, as in the
North Haven brick clay where exact separations are most desirable, a
very considerable error is thus inevitably encountered.
=238. Effect of Boiling on the Texture of Clayey Soils.=—Most
investigators who have worked upon mechanical soil analysis advise
boiling with water in order to detach clay and sand from each other and
make a good separation of the several mechanical elements practicable or
possible. In general, however, the instructions as to the time and
manner of boiling are rather indefinite, and no definite research as to
the effects of this treatment has been undertaken.
The practice of Hilgard, to boil twenty-four hours or even longer in
case of adhesive clays, according to Osborne[157] appears to be
objectionable in view of the dehydration and change of physical
properties known to occur in case of many hydroxids, especially those of
iron and aluminum, which may be present in the soil. It is a familiar
fact that the hydroxids above named and many other amorphous substances
when precipitated from cold solutions are more bulky and less easily
washed upon a filter than when thrown down hot. It is also well known
that their properties are considerably changed by warming or boiling
with water. Heating with water to boiling for some hours or days
gradually converts the bulky brown-red ferric hydroxid, which when
precipitated cold and air-dried for eighteen days, contains thirty-eight
per cent of water, into a much denser, bright red substance containing
but two per cent of water. St. Gilles has also observed the partial
dehydration of aluminum hydroxid from Al₂O₃.5H₂O to Al₂O₃.2H₂O by
prolonged boiling.
The hydrate of silica and the highly hydrated silicates are most
probably affected in a similar manner, and if such be the case, boiling
would evidently change the constitution of clay in a very essential
degree.
The following experiments throw light on this subject:
Ten grams of North Haven brick clay were boiled continuously for nine
days with about 700 cubic centimeters of distilled water, in a glass
flask of one liter capacity and furnished with a reflux condenser.
Fifteen grams were boiled in the same manner for eight and one-half
days. When the boiling was concluded, the soil was found to have assumed
a granular condition, the clay and fine sand being collected into a mass
of small grains resembling coarse sand and settling rapidly. One portion
thus boiled was elutriated by the beaker method, the other by Hilgard’s.
The pestle was not used on either of those portions as it was desired to
determine simply the effect of prolonged boiling. The separations thus
accomplished are here compared with the elutriations of the same soil
boiled twenty-three hours and of the pestled but unboiled soil.
Hilgard elutriation. Beaker elutriation.
Not pestled. Not pestled. Not Pestled. Pestled.
Boiled Boiled eight
twenty-three Boiled nine and a half
hours, days, days. Not boiled.
Diameter of
particles. per cent. per cent. per cent. per cent.
By sieves 3.36 3.24 3.63 3.49
0.25–0.05 1.21 1.11 1.91 1.29
0.05–0.01 28.27 33.04 33.61 27.02
0.01–0 56.29 48.85 54.78 52.21
Clay 4.92 3.05 1.97 10.15
Loss on
ignition 5.95 5.95 5.95 5.95
—————— —————— —————— ——————
100.00 95.24 101.85 100.11
Here it is observed that the eight to nine days boiling diminished the
clay as determined by Hilgard’s conventional method by seven to eight
per cent, increasing the dust by two to three per cent, and the silt by
about six per cent.
Under the microscope small, rounded, opaque, brown granules were seen in
large numbers, which when pressed under the cover glass, broke up into a
multitude of very fine particles.
From these experiments it would appear to be conclusively proved that
too long boiling precipitates clay and thereby defeats the very object
of the operation.
In these experiments the time of boiling was prolonged in order to bring
out unmistakably the effects of this operation. If ebullition for eight
or nine days reduces clay from ten to two per cent, increasing the
0.05–0.01 millimeter diameter grades by six per cent, it is evident that
boiling for one day or a shorter time becomes a questionable treatment.
Further experiments[158] made by boiling clay in a platinum vessel with
a platinum condenser showed that this precipitation of the clay was
largely if not wholly due to the salts extracted from the soil.
When the clay has once been converted into the granular condition,
considerable difficulty is experienced in restoring it to the state in
which it is capable of prolonged suspension in water.
The results of the studies herewith reported may be summed up as
follows:
1. The Berlin-Schöne method of elutriation gives fairly correct
separations with sandy soils containing little clay or matters finer
than 0.01 millimeter diameter, but on soils of fine texture, as loams
rich in humus and clays, it gives results which are grossly inaccurate,
the error on single grades amounting to from eight to fourteen per cent.
2. In respect of rapidity, economy of time, and ease of operation, the
Schöne elutriation has no advantage over the beaker method.
3. Schloesing’s method on its mechanical side makes no satisfactory
separations, and the chemical treatment it employs is liable to alter
seriously the texture of the soil.
4. The determination of clay from a single subsidence from any
conventional depth or volume of water, or for any conventional time, is
not a process certain to effect even a roughly approximate separation of
the finest quartz grains from true clay.
5. Boiling with water must be rejected as a treatment preliminary to
mechanical analysis, because it not only abrades and reduces the coarser
sediments, but may dehydrate and coagulate the true clay and thus alter
essentially the texture and grain of the soil.
=239. General Conclusions.=—The methods of Hilgard and Osborne have been
given in detail and largely in the descriptive language used by the
authors. The other methods of elutriation in use in other countries have
also been described. For practical use the methods of Hilgard and
Osborne are to be preferred to all others. For simplicity and speed the
Osborne method has the preference over the Hilgard. For rigid control of
the work the Hilgard method is to be preferred. The effect of long
boiling on clay pointed out by Osborne would suggest that the boiling
process preliminary to the Hilgard method be made as short as possible.
It would seem that the churn attrition in the Hilgard method might well
be regarded as a substitute for the soft pestling of the Osborne
process, and any prolonged boiling in the former method might be safely
omitted. When carefully carried out, the results of the Hilgard and
Osborne method are fairly comparable.
=240. Distribution of Soil Ingredients.=—The determination of the
distribution of the soil ingredients in the sediments obtained in silt
analysis is illustrated by the following table:[159]
────────────╥────────────╥───────────╥────────────╥────────────
Hydraulic ║ ║ ║ ║
value ║ Clay ║ <0.25mm ║ 0.25mm. ║ 0.5mm.
Per cent in ║ ║ ║ ║
soil ║ 21.64 ║ 23.56 ║ 12.54 ║ 13.67
════════════╬══════╤═════╬═════╤═════╬══════╤═════╬══════╤═════
║ │ ║ │ ║ │ ║ │
║ =A= │ =B= ║ =A= │ =B= ║ =A= │ =B= ║ =A= │ =B=
║ │ ║ │ ║ │ ║ │
Insoluble ║ │ ║ │ ║ │ ║ │
residue ║ 15.96│ 4.35║73.17│17.29║ 87.96│11.03║ 94.13│12.72
Soluble ║ │ ║ │ ║ │ ║ │
silica ║ 33.10│ 7.17║ 9.95│ 2.34║ 4.27│ 0.53║ 2.35│ 0.32
Potash ║ 1.47│ 0.32║ 0.53│ 0.12║ 0.29│ 0.04║ 0.12│ 0.01
Soda ║(1.70)│ ║ 0.24│ 0.06║ 0.28│ 0.04║ 0.21│ 0.02
Lime ║ 0.09│ 0.03║ 0.13│ 0.03║ 0.18│ 0.02║ 0.09│ 0.01
Magnesia ║ 1.33│ 0.29║ 0.46│ 0.11║ 0.26│ 0.03║ 0.10│ 0.01
Manganese ║ 0.30│ 0.06║ 0.00│ 0.00║ 0.00│ 0.00║ 0.00│ 0.00
Ferric oxid ║ 18.76│ 4.06║ 4.76│ 1.11║ 2.34│ 0.29║ 1.03│ 0.14
Alumina ║ 18.19│ 3.97║ 4.32│ 1.04║ 2.64│ 0.33║ 1.21│ 0.17
Phosphoric ║ │ ║ │ ║ │ ║ │
acid ║ 0.18│ 0.04║ 0.11│ 0.02║ 0.03│ 0.00║ 0.02│ 0.00
Sulfuric ║ │ ║ │ ║ │ ║ │
acid ║ 0.06│ 0.01║ 0.02│ 0.01║ 0.03│ 0.00║ 0.03│ 0.00
Volatile ║ │ ║ │ ║ │ ║ │
matter ║ 9.00│ 1.33║ 5.61│ 1.43║ 1.72│ 0.23║ 0.92│ 0.29
────────────╫──────┼─────╫─────┼─────╫──────┼─────╫──────┼─────
Total ║100.14│21.64║99.30│23.56║100.00│12.54║100.21│13.67
Total ║ │ ║ │ ║ │ ║ │
soluble ║ │ ║ │ ║ │ ║ │
matter. ║ 75.18│ ║20.52│ ║ 10.32│ ║ 5.16│
„ „ ║ │ ║ │ ║ │ ║ │
bases ║ 41.84│ ║10.44│ ║ 5.99│ ║ 2.76│
Soluble ║ │ ║ │ ║ │ ║ │
silica in ║ │ ║ │ ║ │ ║ │
crude ║ │ ║ │ ║ │ ║ │
substance.║ 0.38│ 0.01║ │ ║ │ ║ │
────────────╨──────┴─────╨─────┴─────╨──────┴─────╨──────┴─────
────────────╥───────────╥──────────┬──────────┬────────
Hydraulic ║ ║ │ │
value ║ 1.0mm. ║ │ │
Per cent in ║ ║ Other │ Total │Original
soil ║ 13.11 ║sediments.│sediments.│ soil.
════════════╬═════╤═════╬══════════╪══════════╪════════
║ │ ║ │ │
║ =A= │ =B= ║ │ │
║ │ ║ │ │
Insoluble ║ │ ║ │ │
residue ║96.52│12.74║ 13.76│ 71.89│ 70.53
Soluble ║ │ ║ │ │
silica ║ │ 0.36║ │ 10.36│ 12.30
Potash ║ │ „ ║ │ 0.49│ 0.63
Soda ║ │ „ ║ │ 0.12│ 0.09
Lime ║ │ „ ║ │ 0.09│ 0.27
Magnesia ║ │ „ ║ │ 0.44│ 0.45
Manganese ║ │ „ ║ │ 0.06│ 0.06
Ferric oxid ║ │ „ ║ │ 5.60│ 5.11
Alumina ║ │ „ ║ │ 5.51│ 8.09
Phosphoric ║ │ ║ │ │
acid ║ │ „ ║ │ 0.06│ 0.21
Sulfuric ║ │ ║ │ │
acid ║ │ „ ║ │ 0.02│ 0.02
Volatile ║ │ ║ │ │
matter ║ │ „ ║ │ 3.64│ 3.14
────────────╫─────┼─────╫──────────┼──────────┼────────
Total ║ │13.10║ │ 98.28│ 100.63
Total ║ │ ║ │ │
soluble ║ │ ║ │ │
matter. ║ │ ║ │ │
„ „ ║ │ ║ │ │
bases ║ │ ║ │ │
Soluble ║ │ ║ │ │
silica in ║ │ ║ │ │
crude ║ │ ║ │ │
substance.║ │ ║ │ │ 0.19
────────────╨─────┴─────╨──────────┴──────────┴────────
=A.= Calculated on the amount of sediment.
=B.= Calculated on the amount of soil.
It is seen from the above analyses that the clay is by far the richest
in mineral constituents, of all the ingredients separated in silt
analysis, the amount in the clay being more than twice that of all the
others combined. Its volatile matter is also the largest. The large
amount of soda, however, is probably in part due to the sodium chlorid
used in the precipitation of the diffused clay. The following points in
regard to the distribution of the different ingredients are instructive:
1. The iron and alumina exist in almost identical relative proportions
in each sediment, making it probable that they are in some way
definitely correlated.
2. Potash and magnesia also exist in almost the same quantities, and
their ratio to each other in all the sediments being almost constant
seems to indicate that they occur combined, perhaps in some zeolitic
silicate which may be a source of supply to plants.
3. Manganese exists only in the clay, a mere trace being found in the
next sediment.
4. The lime appears to have disappeared in the clay, having probably
been largely dissolved in the form of carbonate by the large quantity of
water used in elutriation. Its increase in the coarser portions may be
owing to its existence in a crystallized form not so readily soluble.
5. In a summary of the ingredients, it is seen that there is a loss in
potash, magnesia and lime in the sediments as compared with the original
soil; and this loss is doubtless due to the solution of these bodies in
the water of elutriation.
A noteworthy fact shown in this table is the rapid decrease of
acid-soluble matter in the coarser sediments; even what is dissolved
from so fine a sediment as 1.0 millimeter hydraulic value, equal to a
diameter of 0.04 millimeter, is in this case a negligible quantity. This
suggests forcibly the inutility of introducing into chemical soil
analysis, grains of as large a size as will pass a sieve of one
millimeter aperture. The hydraulic value of these grains would be
somewhere between 150 and 200 millimeters per second. While the exact
results of the above analysis may not be applicable to all soils, yet
the range is so wide that the systematic exclusion from chemical
analysis of inert material, by means of preliminary mechanical
separation, seems likely to lead to important improvements in the
interpretation of the results.
=241. Percentage of Silt Classes in Different Soils.=—The adaptation of
a soil to different crops depends largely on the sizes of the particles
composing it and consequently on the relative percentages of the silt
classes.
The following table gives the mechanical analysis of some markedly
different types of subsoils:[160]
Diameter, Conventional Truck
millimeters. names. and Grass
Early small and
truck. fruit. Tobacco. Wheat. wheat. Limestone.
1 2 3 4 5 6
2–1 Fine gravel 0.49 0.04 1.53 0.00 0.00 1.34
1–0.5 Coarse sand 4.96 1.97 5.67 0.40 0.23 0.33
0.5–0.25 Medium sand 40.19 28.64 13.25 0.57 1.29 1.08
0.25–0.1 Fine sand 27.59 39.68 8.39 22.64 4.03 1.02
0.1–0.05 Very fine
sand 12.10 11.43 14.95 30.55 11.57 6.94
0.05–0.01 Silt 7.74 4.95 28.86 13.98 38.97 29.05
0.01–0.005 Fine silt 2.23 2.02 7.84 4.08 8.84 11.03
0.005–0.0001 Clay 4.40 8.79 14.55 21.98 32.70 43.44
————— ————— ————— ————— ————— —————
99.70 97.52 95.04 94.20 97.63 94.23
Org. matter, water and
loss 0.30 2.48 4.96 5.80 2.37 5.77
=242. Description of the Soils.=—Number one represents the very early
truck lands of southern Maryland. It is a light yellow sand, belonging
to the Columbia terrace formation. Under an intense system of
cultivation and heavy manuring with organic matter, good crops of garden
vegetables are produced which mature very early, at least ten days or
two weeks before the crops from any other part of the state. Under the
prevailing meteorological and cultural conditions this soil maintains
about five or six per cent of moisture, while a heavier wheat and grass
soil maintains from twelve to twenty per cent. The truck soil is so
loose and open in texture that the rain-fall passes through it very
readily, and it is undoubtedly owing to this drier soil that the plant
is forced to the early maturity which secures it from competition from
other parts of the State and insures a good market price.
Number two represents the later truck and fruit lands of southern
Maryland. These lands contain rather more clay than those just
described; they are somewhat heavier and closer in texture, and are
rather more retentive of moisture. This land gives a larger yield per
acre than the one just described, and in every way crops make a more
vigorous growth and development, but the crop is about a week or ten
days later in maturing, and for this reason it brings a lower price in
the market. It is much better land than number one for small fruit and
peaches. These lands are altogether too light in texture for the
profitable production of wheat, and it would cost altogether too much to
improve them so that even a moderate yield of wheat could be obtained.
Number three is a tobacco land of southern Maryland. The finest tobacco
lands of this locality come between the truck and wheat lands in
texture, and contain from ten to twenty per cent of clay. The lighter
the texture of the soil and the less clay it contains, the less tobacco
it will yield per acre, but the finer the texture of the leaf. The
tobacco yields more per acre on the heavier wheat soils, but the leaf is
coarse and sappy and cures green and does not take on color. It brings a
very low price in the market and does not pay for cultivation. The crop
on the lighter lands is of much finer quality; there is a smaller yield
per acre but the leaf takes on a fine color in curing, and brings a much
better price per pound. Wheat is commonly raised on these tobacco lands
to get advantage of the high manuring, and because the rotation is
better for the land than where tobacco is grown continuously on the same
soil. The finest tobacco lands are, however, too light in texture for
the profitable production of wheat. These lands belong to the neocene
formation.
Number four is a type of the wheat lands of southern Maryland. These
lands represent soil of about the lightest texture upon which wheat can
be economically produced under the climatic conditions which there
prevail. They contain from eighteen to twenty-five per cent of clay, and
are much more retentive of moisture than the best tobacco lands. This
type is about the limit of profitable wheat production. These soils will
maintain about twelve per cent of water during the dry season. Garden
truck is so late in maturing on these lands that there is often a glut
in the market when the crop matures, and the crops often do not pay the
cost of transportation. The lands are too light in texture for a
permanent grass sod. They belong to the neocene formation.
Number five represents the heavier wheat lands of southern Maryland,
belonging probably to a different horizon of the neocene formation and
containing about thirty per cent of clay. This soil is much more
retentive of moisture and produces very much larger crops of wheat than
the last sample. It is strong enough and sufficiently retentive of
moisture to make good grass lands. It is too close in texture and too
retentive of moisture for the production of a high grade of tobacco, or
to be profitable for market truck.
Number six is from a heavy limestone soil of lower Helderberg formation.
It is a strong and fertile wheat and grass land.
=243. Interpretation of Silt Analysis.=—The primary conceptions upon
which the interpretation of the mechanical analysis is based may be
briefly stated as follows:[161] The circulation of water in the soil is
due to gravity, or the weight of water, acting with a constant force to
pull the water downward, and also to surface tension, or the contracting
power of the free surface of water (water-air surface), which tends to
move the water either up or down, or in any direction, according to
circumstances. There is a large amount of space between the grains in
all soils in which water may be held, ranging from about thirty per cent
in light sandy lands to sixty-five or seventy per cent in stiff clay
soils. The relative rate of movement of water through a given depth of
soil will depend upon how much space there is in the soil; upon how much
this space is divided up, _i. e._, upon how many grains there are per
unit volume of soil; upon the arrangement of the grains of sand and
clay; and upon how this skeleton structure is filled in and modified
with organic matter. It also appears that the ordinary manures and
fertilizers change this surface tension, or pulling power of water; that
they also change the arrangement of the grains, and consequently the
texture or structure of the soil may be changed and the relation of the
soil to water, through the effect of the ordinary manures and
fertilizers in causing flocculation or the reverse.
=244. Number of Particles in a Given Weight of Soil.=—The approximate
number of particles in the soil can be calculated from the results of
the mechanical analysis by the following formula:[162]
(_a_/((π(_d_)³ω)/6)) ÷ A
Where _a_ is the weight of each group of particles, _d_ the mean
diameter of the particles in the several groups in centimeters, ω is the
specific gravity of the soil, and A is the total weight of soil. For the
specific gravity of ordinary soils, the constant 2.65 may be used.
In using the formula the per cents are expressed as grams. Thus, if
there were twenty per cent of silt, this would be taken as twenty grams,
and if the results of the analysis added up ninety-seven per cent the
whole weight of soil would be taken as ninety-seven grams. The diameter
_d_ is taken as the mean for the extreme diameters taken for any group,
for instance, for the silt this would be 0.003 centimeter, which is
assumed to be the diameter of the particles in that group. This formula
can only give approximate values, as the number of separations in a silt
analysis must necessarily be small, amounting usually to not more than
eight or ten grades, on account of the time and labor required for
closer separations. There is relatively rather a wide range in the
diameters of grains within any one of these grades, and absolute values
could not be expected without a vast number of separations, so that all
the grains in each group would be almost exactly of the same size.
The clay group has relatively the widest limits, which is unfortunate,
as this is the most important of all the groups on account of the
exceedingly small size of the particles. The figure 0.0001 millimeter is
taken as the lowest limit of the diameter of the clay particles. These
particles have been heretofore assumed to be ultra-microscopic, but by
the use of a microscope of high power with oil-immersion objective and
staining fluids, it has been possible to define the clay particles in a
turbid liquid which has stood so long as to be only faintly opalescent.
Pending more exact measurements, the figure 0.00255 millimeter has been
used as the diameter of the average sized particle in the clay group.
The following table gives the approximate number of grains per gram in
the different types of subsoils calculated from the mechanical analysis
of the typical soils already given:
NUMBER OF PARTICLES OF EACH CLASS IN ONE GRAM OF SOIL.
─────────────────┬─────────────────┬─────────────────┬─────────────────
Silt classes. │ No. 1 │ No. 2│ No. 3
Diameter (_d_) in│ Early truck. │ Truck and small│ Tobacco.
centimeters. │ │ fruit.│
─────────────────┼─────────────────┼─────────────────┼─────────────────
0.15 │ 0│ 0│ 3
0.075 │ 85│ 34│ 102
0.0375 │ 5,511│ 4,011│ 1,900
0.0175 │ 37,230│ 54,610│ 11,890
0.0075 │ 207,500│ 199,700│ 267,900
0.003 │ 2,073,000│ 1,355,000│ 8,092,000
0.00075 │ 38,210,000│ 35,360,000│ 140,900,000
0.000255 │ 1,915,000,000│ 3,918,000,000│ 6,637,000,000
│ —————————————│ —————————————│ —————————————
│ 1,955,000,000│ 3,954,973,355│ 6,786,273,795
─────────────────┼─────────────────┼─────────────────┼─────────────────
Silt classes. │ No. 4 │ No. 5 │ No. 6
Diameter (_d_) in│ Wheat. │Grass and wheat. │ Limestone.
centimeters. │ │ │
─────────────────┼─────────────────┼─────────────────┼─────────────────
0.15 │ 0│ 0│ 12
0.075 │ 726│ 4│ 60
0.0375 │ 8,273│ 181│ 157
0.0175 │ 32,340│ 5,556│ 1,456
0.0075 │ 554,100│ 202,600│ 125,900
0.003 │ 3,962,000│ 10,670,000│ 8,231,000
0.00075 │ 73,990,000│ 154,900,000│ 199,900,000
0.000255 │ 10,150,000,000│ 14,570,000,000│ 19,430,000,000
│ ——————————————│ ——————————————│ ——————————————
│ 10,228,547,439│ 14,735,778,341│ 19,638,258,585
─────────────────┴─────────────────┴─────────────────┴─────────────────
=245. Estimation of the Surface Area of Soil Particles.=—The approximate
extent of surface area of the soil grains in one gram of soil can be
calculated from the foregoing by the following formula:[163]
π(_d_)²_n_
in which _d_ is the mean of the diameters of any group in centimeters,
and _n_ is the number of particles in the group.
The following table gives the approximate extent of surface area of the
particles in one gram of soil calculated from the preceding table:
APPROXIMATE EXTENT IN SQUARE CENTIMETERS, OF SURFACE AREA IN ONE GRAM
OF SOIL.
Soil number.
————— —————— —————— —————— —————— ——————
Diameter, millimeters. 1 2 3 4 5 6
1.5 0.0 0.0 0.4 0.0 0.0 0.1
0.75 1.8 0.6 1.8 12.8 0.1 0.1
0.375 24.3 17.7 8.4 36.5 31.0 0.7
0.175 35.8 52.6 11.4 31.1 5.3 1.4
0.075 21.3 35.3 47.3 97.9 35.8 22.2
0.03 218.8 38.3 228.9 112.0 301.4 232.7
0.0075 67.4 62.5 248.9 130.8 273.5 353.4
0.00255 390.8 800.5 1355.0 2072.0 2976.0 3965.0
————— —————— —————— —————— —————— ——————
Total 760.2 1007.5 1902.1 2493.1 3593.1 4575.3
=246. Logarithmic Constants.=—The following logarithmic constants have
been used in the calculation of the approximate number of grains per
gram and of the surface area, using 2.65 in all cases as the specific
gravity of the soil.
Diameter. (_d_) Approximate number of grains. Surface area.
log.(π(_d_)³_w_)/(6) log.(_d_)²π
0.15 centimeters \̅3.6703 \̅2.8493
0.075 „ \̅4.7674 \̅2.2473
0.0375 „ \̅5.8641 \̅3.6451
0.0175 „ \̅6.8711 \̅4.9831
0.0075 „ \̅7.7674 \̅4.2473
0.003 „ \̅8.5734 \̅5.4513
0.00075 „ \̅1̅0.7674 \̅6.2473
0.000255 „ \̅1̅1.3616 \̅7.3101
=247. Mineralogical Examination of the Particles of Soil Obtained by
Mechanical Analysis.=—The principal object of the mechanical analysis of
soils as has already been set forth is the separation of the soil into
portions, the particles of which have the same hydraulic value. It is
evident without illustration that particles of the same hydraulic value
do not necessarily have the same size. The rate of flow of a liquid
carrying certain definite particles does not imply that these particles
are of the same dimensions. Of two particles of the same size and shape,
that one which has the lower specific gravity, will be carried off at
the lower rate of flow. At the end of the operation, therefore, the
several portions of the soil obtained will be found composed of
particles of sizes varying within certain limits, and of these particles
the larger ones will tend to be composed of minerals of lower specific
gravity, and the smaller ones of minerals of higher specific gravity. Of
the same mineral substance, the particles which are most irregular,
exposing for a given weight the largest surface will be found to pass
over at a lower velocity than those of a more nearly spherical shape.
The same law holds good for particles falling through a liquid at rest,
_i. e._, the heavier and more spherical particles, weight for weight,
will sooner reach the bottom of the containing vessel. To complete the
value of a mechanical analysis, it becomes necessary to submit the
several portions of soil obtained not only to a chemical but also to a
mineralogical examination. Only the outlines of the methods of examining
silt separates for mineral constituents can be given here and special
works in petrography must be consulted for greater details.[164]
It is evident that the methods of separation and examination from a
mineralogical point of view about to be described can only be applied to
silts of the largest size. The finer silts can not be separated into
portions of different specific gravities by separating liquids of
varying densities on account of the slowness with which they subside,
thus tending to adhere to the sides of the separating vessels and to
form floccules which are not all composed of the same kind of mineral
particles. While, therefore, these processes are more appropriately
described in connection with the silts obtained by hydraulic
elutriation, they can be applied with greater success to the fine
particles passing the different sieves used in the preparation of the
soil for analysis or to the finely pulverized soil as a whole.
The minerals which have contributed to soil formation, moreover, are
better preserved in the larger silt particles and therefore more easily
identified. While the desirability of securing like determinations in
the finer silts is not to be denied, in the present state of the art the
analyst must be content with the examination of the larger particles.
=248. Methods of Investigation.=—The chief points to be observed in the
examination of the fine particles of soil are the following: (1) the
size and shape of the particles; (2) measurement of crystal angles; (3)
separation into classes of approximately the same specific gravity; (4)
separation by means of the magnet; (5) determination of color and
transparency; (6) determination of refractive index; (7) examination
with polarized light; (8) examination after coloring; (9) chemical
separation. For many of the optical studies above noted, it is first
necessary to prepare thin laminae of the mineral particles and properly
mount them for examination. For the purposes of this manual only those
processes will be described which are essentially connected with a
proper understanding of the nature of the soil particles. For the more
elaborate methods of research the analyst will consult the standard
works on mineralogy and petrography.
=249. Microscopical Examination.=—The direct examination of the silt
particles with the microscope should attend the progress of separation.
Unless the particles obtained have the same general appearance, the
separation is not properly carried on. Especially is the microscope
useful to determine that the value of the silt separation is not
impaired by flocculation. Unless flocculation be practically prevented
during the separation of the finest particles, many of these will be
left as aggregates to be brought over subsequently with particles of far
different properties. No special directions are necessary in the use of
the microscope. The silt particles are removed with a few drops of water
by means of a pipette, a drop of the liquid with the suspended particles
is placed on the glass, covered and examined with a convenient
magnification. A micrometer scale should be employed in order that the
approximate sizes of the particles may be determined. A _camera lucida_
may also be conveniently used for the purpose of delineating the form of
particles of peculiar interest.
=250. Petrographic Microscope.=—Any good microscope furnished with
polarizing apparatus may be used for the examination of the silt
particles and sections. For directions in manipulating microscopes the
reader is referred to works on that subject. A special form of
microscope for petrographic work is made by Bausch and Lomb of
Rochester. The stand of this instrument is shown in Fig. 37. The base,
upright pillars and arm are made of japanned iron. The stage is made in
two forms, first, plain revolving, having silvered graduates at right
angles and second, a mechanical stage with silvered graduations on the
edge with vernier and graduations for the rectangular movements. The
mirror bar is adjustable and graduated and the mirror is of large size,
plane and concave. The double chambered box in the main tube carries the
upper Nicol prism (analyzer). The lower Nicol prism (polarizer) is
mounted in a cylindrical box beneath the stage to which it is held by a
swinging arm. It is adjustable also up or down and is provided with a
compound lens for securing converged polarized light. In revolving the
prism a distinct click shows the position of the crossed Nicols.
[Illustration:
FIGURE 37.
]
=251. Form and Dimensions of the Particles.=—In order to study the
contour of the fine silt particles, it is well to suspend them in a
liquid whose refractive index is markedly lower than that of the
particles themselves, and for this purpose pure water is commonly used.
Care must be taken that not too many particles are found in the drop of
water which is to be placed on the object holder and protected with a
thin, even glass. The tendency to flocculation in these fine particles
will make the study of their form difficult if they are allowed to come
too close together. The size of the particles, or linear diameter, is to
be determined by means of an eye-micrometer. This consists of a glass
plate on which a millimeter scale is engraved with a diamond, or
photographed. The millimeter scale is the one usually employed, each
millimeter being divided into tenths. On microscopes designed especially
for photographic work the micrometer is fastened to the eyepiece, and so
adjusted as to read from left to right, or at right angles thereto.
Sometimes an eyepiece-micrometer has two scales at right angles so that
dimensions may be read in two directions without change. With an
eyepiece-micrometer, not the dimensions of the object, but those of its
magnified image are read, and the degree of magnification being known,
the actual size of the object is easily calculated. The actual
measurements may also be obtained by placing in the field of vision, a
stage-micrometer and determining directly the relation between that and
the eyepiece-scale. If, for example, the stage-micrometer is ruled to
0.01 millimeter, and the eye-micrometer to 0.1 millimeter, and one
division of the stage-rule should cover three divisions of the eye-rule,
then the one division of the eye-micrometer would correspond to an
actual linear distance of 0.0033 millimeter in the object. If the two
lines of division in the two micrometers do not fall absolutely
together, the calculation may be made as follows: suppose that six
divisions, 0.6 millimeter, in the eyepiece correspond to nearly five
divisions, 0.25 millimeter, in the stage piece. To get at the exact
comparison, take ninety-six divisions of the eye-scale and they will be
found to be somewhat longer than eighty-one and somewhat shorter than
eighty-two divisions of the stage-scale. It follows therefore that
one division of the eye-scale >0.008438 millimeter, and
„ „ „ „ „ „ <0.008541 „ ;
and, hence, one division of the eye-scale corresponds almost exactly to
0.008489 linear measure.
=252. Illustrations of Silt Classes.=—In figure 38 are shown the
relative sizes and usual forms of a series of silt separates made by the
Osborne beaker method. The photomicrographs were made by Dr. G. L.
Spencer from specimens furnished by Prof. M. Whitney.
The soil represented by the separates is from a truck farm near Norfolk,
Virginia.
The particles represented in each class are not all strictly within the
limits of size described. For instance, in the largest size (No. 1) are
two particles at least which show a diameter of more than one
millimeter. The particles in general, however, are within the limits of
the class; _viz._, one-half to one millimeter, and this general
observation is true of all the classes. In the case of the finer
particles, especially of clay, the tendency to flocculation could not be
overcome in the preparation of the slides for the photographic
apparatus. The clay particles are so fine as to present but little more
than a haze at 150 diameters of magnification. The particles seen are
clearly, in most cases, aggregates of the finer clay particles. The
larger particles show the rounded appearance due to attrition and
weathering. It would have been more instructive to have had the
particles of the different classes all photographed on the same scale,
but this is manifestly impossible. The lowest power which shows any of
the clay particles to advantage is at least 150 diameters, and with the
larger particles such a magnification would have been impracticable.
=253. Measurement of Crystal Angles.=—The fine silt particles rarely
retain sufficient crystalline shape to permit of the measurement of
angles and the determination of crystalline form thereby. The rolling
and attrition to which the silt particles have been subjected have, in
most cases, given to the fragments rounded or irregular forms which
render, even in the largest silts, the measurement of angles impossible.
For the methods of mounting minute crystals and the measurement of
microscopic angles, the analyst is referred to standard works on
mineralogy and petrography.
=254. Determination of the Refractive Index.=—For a study of the theory
of refraction, works on optics should be consulted. The general
principles of this phenomenon which concern the determination of the
refractive power of fine earth particles are as follows: if a
transparent solid particle is observed in the microscope imbedded in a
medium of approximately the same refractive power and color, its
outlines will not be clearly defined, but the imbedded particle will
show in all of its extent the highest possible translucency. If,
therefore, the form or perimeter of the particle is to be studied with
as much definiteness as possible, it should be held in a medium
differing as widely from it as possible in refractive power. For
minerals, water is usually the best immersion material. On the other
hand, when the internal structure of the particles is the object of the
examination, it should be imbedded in oil, resin (Canada balsam), etc.,
or in some of the liquids mentioned below.
If particles of different refractive powers and the same character of
surface be studied in the same medium, they will not all appear equally
smooth on the field of the microscope. Some of the surfaces will seem
smooth and even, others will appear rough and wrinkled. Those particles
whose refractive index is equal to or less than that of the liquid
appear smooth, because all the emergent light therefrom can pass at once
into the environing medium. On the other hand, the surfaces of those
particles which have a higher refractive power than the medium will
appear roughened, because, on account of the unavoidable irregularities
on the surface, many of the emergent rays of light must strike at the
critical angle and so suffer total reflection, and consequently those
portions of the surface will be less illuminated, producing the
phenomenon of apparent roughness above noted. In the case of any given
particle, liquids of increasing refractive power can be successively
applied until the change in the appearance of the surface of the
particle is noticed. The refractive index of the liquid being known,
that of the particle is in this way approximately to be determined.
The following liquids, having the indexes mentioned, are commonly
employed:
Substance. Refractive index.
Water 1.333
Alcohol 1.365
Glycerol 1.460
Olive oil 1.470
Canada balsam 1.540
Oil of cinnamon 1.580
Oil of bitter almonds 1.600
Oil of Cassia 1.606
Concentrated solution of potassium and mercuric iodid 1.733
Concentrated solution of barium and mercuric iodid 1.775
The solution of potassium and mercuric iodid may also be used for all
refractive indexes from 1.733 to 1.334 by proper dilution with water.
The mineral particle may also be imbedded in Canada balsam and over it a
drop of a liquid of known refractive power placed. By a few trials one
of the liquids will be found having practically the refractive index of
the particle under examination.
=255. Examination with Polarized Light.=—The internal structure of a
mineral particle can often be determined by its deportment with
polarized light. The theory of polarization is fully set forth in works
on optics and will not be discussed here. The principle on which the
utility of polarized light in the examination of soil particles rests is
found in the information it may give in respect of crystalline
structure. The structure of mineral particles which make up the bulk of
an ordinary soil is, as a rule, so thoroughly disintegrated that all
trace of its original form is lost. Some particles may exist, however,
in which there is no determinable element of shape and which yet possess
an internal crystalline structure which the microscope with polarized
light may be able to reveal.
=256. Staining Silt Particles.=—The finer silts and clays before
microscopic examination should be colored or stained. The methods used
in staining bacteria may be employed for the clay particles.
Evaporation to dryness with a solution of magenta will often impart a
color to the clay particles which is not removed by subsequent
suspension in water. The harder and larger silt particles are not easily
stained, especially if they be firm and undecomposed. On the other hand,
if the particles be broken and seamed, and well decomposed, the stain
will be taken up and held firmly in the capillary fissures. Valuable
indications are thus obtained respecting the nature of the silt
particles. Particles of mica, chlorite and talc are easily distinguished
in this way from the firmer and less decomposed quartz grains.
The staining of the particles after ignition and treatment with acids
gives better results than the direct treatment. Particles of carbonate
which are stained with difficulty before ignition take the stain easily
afterwards on account of the decomposition produced by the loss of
carbon dioxid. This is the case also with particles containing water of
composition or crystallization.
=257. Cleavage of Soil Particles.=—A microscopic examination of the
cleavage of soil particles may be useful in determining their mineral
origin. The course followed by cleavage lines and their mutual position
is dependent on the direction in which the separation of the mineral
fragment takes place. The character of the microscopic fragments
produced by crushing a soil particle is determined primarily by the
system of crystallization to which it belongs. Perhaps the most
distinguishing cleavage marks in soil particles will be found in
fragments of mica and orthoclase. These characteristic forms are shown
in Figs. 39 and 40. The first (Fig. 39) shows the pinacoidal cleavage in
a fragment of mica. Fig. 40 illustrates the appearance of the cleavage
lines in a fragment of orthoclase. Figs. 41 and 42 show the
characteristic cleavage lines in fragments of epidote and titanite.
=258. Microchemical Examination of Silt.=—The methods of quantitative
chemical examination of silts will be given in another part of this
manual. Certain qualitative and microchemical tests, however, are useful
in identifying silt particles. For instance, any soluble iron mineral
will be detected, even in minute quantity, by the blue coloration of the
solution produced by the addition of potassium ferrocyanid. Manganese
will be revealed by fusion with soda and saltpeter on platinum foil, in
the oxidizing flame, producing the well-known green coloration due to
the sodium manganate formed.
More valuable indications of the character of the fragments examined are
obtained by microchemical processes. The best method of decomposing the
silt particles for this purpose is by treatment with hydrofluosilicic
acid. When the particles are composed of silicates, pure hydrofluoric
acid is to be preferred.
The method of treatment is essentially that of Boricky.[165] The slide
used is protected by a film of Canada balsam, and a few of the silt
particles are placed thereon, and fixed in place by slightly warming the
balsam. Each particle is then treated with a drop of hydrofluosilicic
acid, care being taken not to let the drops flow together. The acid must
be pure, leaving no residue on evaporation. The acid should be prepared
by the analyst from a mixture of barium fluorid, sulfuric acid and
quartz powder, or the commercial article should be purified by
distillation before using. The acid should be kept in ceresin or
gutta-percha bottles and must be applied with a ceresin or gutta-percha
rod. Each particle should be as completely dissolved as possible by the
acid, and the rate of solution may be hastened by gentle warming,
provided the heat is not great enough to remove the balsam and allow the
acid to attack the glass. The bases present in the silt particles
crystallize on drying as fluosilicates. In case of a too rapid
crystallization, the mass may be dissolved in a drop of water or of very
dilute hydrofluosilicic acid, and allowed to evaporate more slowly. Some
fragments need more than one treatment with acid to secure complete
solution, and particles of mica may even resist repeated applications.
In such a case the decomposition may be made in a platinum crucible with
hydrofluoric acid, adding afterwards an excess of hydrofluosilicic acid
and evaporating to dryness. The crystals may then be dissolved in a
little water and a drop of the solution allowed to crystallize on the
slide.
=259. Special Reactions.=—The number of microchemical reactions is very
great, but there will be given here only some of the more important for
silt identification.
_Sodium._—Sodium mineral fragments dissolved in hydrofluosilicic acid
and dried give the combinations shown in Fig. 43. With sodium and
aluminum the forms shown in Figs. 44 and 45 are obtained. With an
increasing amount of lime in the mineral, the crystals tend to become
longer. For microscopic work it is not advisable to try to produce the
tetrahedral crystals of the double uranium sodium acetate because the
commercial uranium acetate often contains sodium and even the pure
article will often take up sodium from the bottles.
_Potassium._—Fragments containing potash give isotropic clear cubes, or
octahedra of low refracting power, or combinations of these forms with
each other and with rhombic dodecahedra. These crystals have the
composition K₂SiF₆. Their forms are shown[166] in Figs. 46 and 47. In
case much sodium be present, the first crystals obtained may be strongly
double refractive rhombohedra, but on dissolving in water and allowing
to recrystallize, the normal forms will be obtained. If the crystals be
dissolved in hydrochloric or sulfuric acids, and treated with platinum
chlorid, the characteristic yellow octahedral crystals of K₂PtCl₆ will
be obtained. Ammonium and cesium compounds also give this reaction.
_Lithium._—When fragments containing lithium are treated with the
solvent mentioned, monoclinic crystals are produced on drying. These
crystals dissolved in sulfuric acid and freed from calcium sulfate by
treatment with potassium carbonate give aggregates of lithium carbonate
resembling a snowflake. At a high temperature lithium solutions treated
with sodium phosphate give spindle-shaped crystals of lithium phosphate.
The double lithium aluminum silicofluorid is shown in Fig. 48. The ease
with which traces of lithium may be detected by the spectroscope renders
unnecessary any further description of its microchemical reactions.
_Calcium._—Nearly all mineral particles, save quartz grains, contain
calcium. When these particles are dissolved by treatment with
hydrofluosilicic acid, they form on drying hydrated monoclinic crystals
of calcium silicofluorid (CaSiF₆ + 2H₂O). These crystals assume many
forms, some of which are shown in Figs. 49 and 50. These crystals are
easily decomposed by sulfuric acid, the well-known long prismatic
crystals of gypsum taking their place. On treatment of silt particles
containing lime with hydrofluoric and sulfuric acids, only a part of the
lime passes into solution if the content thereof be large. Where but
little lime is present and the sulfuric acid is in large excess, all the
lime passes into solution and the characteristic gypsum crystals appear
as in Fig. 51.
_Magnesium._—Rhombohedral crystals of magnesium silicofluorid separate
from the solution of particles containing magnesium in hydrofluosilicic
acid. They have the composition MgSiF₆6H₂O and their common forms are
shown in Fig. 52. Quite characteristic also are the crystals of struvite
(NH₄MgPO₄ + 6H₂O), which are produced in a very dilute solution of the
magnesium compound first obtained by carefully adding ammonium hydroxid
and chlorid until a faint alkaline reaction is produced, and then
placing a drop of dilute sodium phosphate at the edge of the solution.
The crystals should be allowed to form slowly in the cold. Their form is
shown in Fig. 54.
[Illustration:
FIGURE 38. PHOTOMICROGRAPHS OF SILT PARTICLES.
]
No. Diameter in mm. Name. Magnification. Diameters.
1 1.0–0.5 coarse sand ×10
2 0.5–0.25 medium sand ×10
3 0.25–0.1 fine sand ×10
4 0.1–0.05 very fine sand ×30
5 0.05–0.01 silt ×30
6 0.01–0.005 fine silt ×150
7 0.005–0.0001 clay ×150
[Illustration:
Figures 39–42, show examples of the various degrees of perfection and
relative positions of cleavage lines.
Figure 39, illustrates pinacoidal cleavage in mica from granite.
Magnified thirty diameters.
Figure 40. A cleavage of orthoclase from augite syenite magnified
twenty-seven diameters.
Figure 41. Cleavage of epidote magnified sixty diameters.
Figure 42. Cleavage of titanite magnified seventy-five diameters.
Figure 43. Sodium fluosilicate crystals magnified seventy-two
diameters.
Figure 44. The same with aluminum fluosilicate magnified twenty-seven
diameters.
Taken from Rosenbusch, Mikroskopische Physiographie.
]
[Illustration:
Figure 45. Sodium and aluminum silicofluorid crystals magnified 100,
140 and 160 diameters.
Figure 46. Potassium silicofluorid crystals magnified 130 diameters.
Figure 47. Another preparation of the same magnified 140 diameters.
Figure 48. Lithium and aluminum silicofluorid crystals magnified 100
diameters.
Figure 49. Calcium silicofluorid crystals magnified 45 diameters.
Figure 50. Another preparation of the same magnified 42 diameters.
]
[Illustration:
Figure 51. Calcium sulfate crystals magnified twenty diameters.
Figure 52. Magnesium silicofluorid crystals magnified thirty
diameters.
Figure 53. Cesium aluminum sulfate crystals magnified twenty
diameters.
Figure 54. Ammonium magnesium phosphate crystals magnified ten
diameters.
Figure 55. The same crystallized from dilute solution magnified thirty
diameters.
Figure 56. Ammonium phosphomolybdate crystals magnified 140 diameters.
]
_Barium._—From solution of barium bearing minerals in hydrofluosilicic
acid fragments, no characteristic crystals, are obtained. Treated with
hydrofluoric and sulfuric acids the barium is left as sulfate. If this
salt be dissolved in boiling oil of vitriol and a drop of the solution
placed on the slide, a mixture of rectangular tablets and St. Andrew’s
cross-shaped growths will be separated before any crystals of gypsum
which may be present appear. When strontium is present, the barium
sulfate residue obtained by treatment with hydrofluoric and sulfuric
acids should be fused with sodium and potassium carbonate, washed with
water until the sulfuric acid is removed, the residue dissolved in
hydrochloric or nitric acids, and the solution treated with potassium
chromate. Pale yellow crystals of barium chromate are thus obtained,
which resemble in form those secured by dissolving the barium sulfate in
oil of vitriol. Strontium is not precipitated by this treatment. If
potassium ferrocyanid be used instead of barium chromate with the
hydrochloric acid solution, crystals of barium potassium ferrocyanid are
formed of a bright yellow color and rhombohedric shape.
_Strontium._—From a hydrofluosilicic acid solution, strontium
crystallizes in columns or tablets of the monoclinic system as strontium
silicofluorid, SrSiF₆. On treating these with sulfuric acid, rhombic
plates of strontium sulfate are formed, which serve to distinguish this
element from calcium. On treatment of the particles of the original
mineral with hydrofluoric and sulfuric acids, the strontium remains in
the insoluble residue. When this residue is treated with boiling oil of
vitriol, rhombic plates of celestine are separated. If the residues
above mentioned be dissolved by fusion with the alkaline carbonates,
washed with water, dissolved in hydrochloric acid and treated with
oxalic acid, octahedral crystals of strontium oxalate are formed.
_Iron._—Mineral particles containing iron give crystals, when treated as
is first described above, which are fully isomorphous with those
obtained from magnesium. By moistening the crystalline mass with
potassium ferrocyanid, the presence of iron is at once revealed by the
blue coloration produced.
_Aluminum._—No crystals containing aluminum are formed from the mineral
particles containing this substance when dissolved in the solvent
already mentioned. If, however, the gelatinous mass be dissolved in a
little sulfuric acid and a fragment of a cesium salt added, beautiful
crystals of cesium alum are obtained, illustrated in Fig. 53.
_Phosphorus._—When a mineral fragment containing phosphorus is treated
according to the usual analytical methods for securing the ammonium
magnesium phosphate, crystals are obtained of the form shown in Figs. 54
and 55. A phosphatic fragment of silt may be identified when soluble by
treatment with nitric acid and ammonium molybdate. On slowly drying,
rhombohedral crystals are produced, yellow by reflected, and green by
transmitted light. Their form is shown in Fig. 56.
=260. Petrographic Examination of Silt Particles.=—The larger silt
particles and the minute fragments of minerals in the soil can best be
studied in thin sections. For this purpose the following plan, proposed
by Thoulet, may be used. Mix the soil minerals in considerable
proportion—Thoulet recommends ten per cent, but a greater percentage is
often better—with zinc oxid and make into a paste with sodium silicate.
The paste should be worked to the consistence of putty and then rolled
into little tablets about one-eighth of an inch thick and an inch in
diameter. After drying a day or two without heating, the tablets become
hard enough to mount and grind like rock sections. These tablets are
mounted in Canada balsam on glass slides and ground as thin as possible
with fine emery on the turn-table or glass plate, as rock sections are
treated. As these tablets are not as strong as rock sections usually
are, they require care in this treatment. Some of the grains also are
apt to be torn out in the process of grinding and to compensate for this
loss a number of slides should be prepared with each lot of soil
minerals. When this operation has been successful, the optical
properties of the various minerals can be studied as in rock sections.
As the iron oxid contained in the soils obscures the transparency of the
minerals, it is well to treat a portion of the material under
examination with hot hydrochloric acid for a short time to remove this
oxid and then prepare slides with the cleansed material and compare
results with the untreated. As the acid will dissolve phosphates and
carbonates, and will partly or wholly decompose some other minerals, the
operator must be guided by his judgment in its use.
=261. Machine for Making Mineral Sections.=—A convenient apparatus for
this purpose has been described by Williams[167] and is represented in
Fig. 57. It is supported on a substantial table provided underneath with
electric batteries and a motor for driving the cutting disks seen on the
top. The table is three feet six inches square and two feet nine inches
high.
[Illustration:
FIGURE 57.
MACHINE FOR MAKING MINERAL SECTIONS.
]
The grinding apparatus consists of two circular disks of solid copper,
nine inches in diameter, and three-eighths inch thick, which may be used
alternately as different grades of emery are required. They are attached
either by a screw or square socket to a vertical iron spindle which
revolves smoothly in a conical bearing. The grinding disk is surrounded
when in use by a large cylindrical pan of tin, which is not shown in the
cut, which has an opening in its center to allow of the passage of the
spindle.
The sawing apparatus consists of a horizontal countershaft placed on a
different part of the table and connected with the motor by a separate
belt. It carries at one end a vertical wheel of solid emery, and at the
other an attachment, level-table and guide for the diamond-saw. A small
water-can with spout, not shown in the cut, is suspended over the edge
of the table to keep the saw wet when it is in use.
The machine is very conveniently driven by a storage battery when street
circuits cannot be drawn on.
For the details of making mineral sections, the works on petrography may
be consulted.
=262. Separation of Silt Particles by Specific Gravity Solutions.=—In
silt separates the specific gravity of the different mineral particles
present may vary from graphite (1.9–2.3) to hematite (5.2–5–3).
The following list gives the specific gravities of some of the more
common minerals which may be met with in soils:
Gypsum 2.31
Albite 2.56–2.63
Quartz 2.65
Talc 2.74
Chlorite 2.78
Muscovite 2.85
Calcite 2.5–2.78
Dolomite 2.90
Tourmaline 2.94–3.3
Biotite 3.01
Apatite 3.16
Pyroxenes 3.22–3.5
Epidote 3.39
Titanium Minerals 3.48–4.75
Iron oxids 5.2–5.3
The finest particles of silt are separated by gravity with great
difficulty, inasmuch as they tend to remain suspended in the solutions
for an indefinite period. With the coarser silts, however, useful data
are often obtained by this method. The separation is preceded by
extraction of the particles with hydrochloric acid to remove encrusted
soluble matter, and by ignition to destroy any traces of organic matter.
Those mineral matters which are soluble in acid or are changed by
ignition must, of course, be sought for in separate portions of the
silt,
=263. Thoulet’s Solution.=[168]—The standard solution is of such a
density that particles of 2.65 specific gravity-will just float thereon,
using for this purpose a solution of about 2.7 specific gravity. The
solution from which the above standard is prepared is made as follows:
One part of potassium iodid is weighed and placed in a beaker and one
and one-quarter part of mercuric iodid is placed on top of it. Then
water is added in the proportion of ten cubic centimeters to 100 grams
of the mixture, and after some time (twelve to twenty-four hours), with
occasional stirring, the salts will nearly completely dissolve. Filter
from the undissolved residue and evaporate in a porcelain dish until
crystals form on the surface of the liquid. Allow to cool, pour off the
liquid from the crystals and evaporate the liquid for another crop. The
first solution, after cooling, has a specific gravity between 3.10 and
3.20, the second a specific gravity of 3.28, practically the limit of
density of the solution. The solution of 2.7 specific gravity and other
densities are made by cautiously adding a few drops of water at a time
and ascertaining the specific gravity by the Westphal balance or other
convenient method.
The strong solution, according to Goldschmidt,[169] may be prepared
directly by using potassium iodid and mercuric iodid in the ratio of 1 :
1.24. Twenty-five cubic centimeters of water, 210 grams of potassium
iodid, and 280 grams of mercuric iodid afford a solution of 3.196
specific gravity at 15°, on which fluorspar fragments will float.
=264. Klein’s Separating Liquid.=—A solution of cadmium borotungstate,
of the composition 2H₂O,2CdO,B₂O₃,9WO₃ + 16H₂O, has been proposed by
Klein[170] for separating silt particles. This salt is obtained by
dissolving pure sodium tungstate in five times its weight of water,
adding one and a half parts of boric acid and boiling until, complete
solution takes place. On cooling; the borax is separated in crystalline
form. The mother-liquor after the removal of the crystals is carefully
concentrated by boiling. By stirring the cold solution, there is a
further separation of sodium borate and polyborate. This operation is
continued until glass will float on the mother-liquor. The salt in
solution then has the following composition: 4Na₂O,12WO₃,B₂O₃. To this
boiling concentrated solution, is added a boiling saturated solution of
barium chlorid, in the proportion of one part of the chlorid to three
parts of the original double tungstate. An abundant pulverulent
precipitate is formed, making the whole mass mushy. The mass is filtered
under pressure and well-washed with hot water. The residue is then
suspended in hot water containing one part in ten of hydrochloric acid
of 1.18 specific gravity. It is then evaporated to dryness in the
presence of an excess of hydrochloric acid and decomposed, by which
process hydrated tungstic acid is separated. The boiling mass is taken
up with water and the boiling continued for two hours with occasional
addition of water to take the place of that evaporated, and the tungstic
acid separated by filtration.
From the solution, beautiful quadratic crystals separate having the
composition 9WO₃,B₂O₃,2BaO₂H₂ + 18H₂O. These are purified by several
recrystallizations and freed from any scales of boric acid by washing
with alcohol. Any reducing action, revealed by a violet coloration of
the crystals, can be avoided by adding a few drops of nitric acid. From
a boiling solution of these crystals, the cadmium salt desired is
obtained by treatment with the proper amount of cadmium sulfate solution
to precipitate the barium. The barium sulfate is separated by
filtration. The cadmium borotungstate is soluble in less than ten parts
by weight of water. From this solution it is obtained in pure form by
evaporation under a vacuum, or by carefully concentrating on a
water-bath and cooling. A saturated solution of these crystals at 15°
has a bright yellow color and a specific gravity of 3.28.
If a dilute solution of the above salt be carefully evaporated on a
water-bath, any violet color which may be present disappears when the
specific gravity reaches 2.7. If the evaporation be continued until a
crystal of augite will float on the hot liquid, crystals may be obtained
on cooling which, dissolved in as little water as possible, make a
solution which will almost support olivine. If the two solutions be
united, the specific gravity of the mixture is 3.30–3.36. The highest
attainable specific gravity; _viz._, 3.6, is produced by continuing the
evaporation on a water-bath until the liquid will support olivine, and
then allowing to stand in a closed place for twenty-four hours. The
crystals of cadmium borotungstate thus obtained are freed as much as
possible from the mother-liquor by drainage and then melted at about 75°
in their own water of crystallization. A liquid is thus obtained on
which spinel will float. The same concentration may also be obtained by
careful heating on a water-bath. At its highest specific gravity this
solution has an oily consistence and this renders its practical use in
the separation of fine particles somewhat restricted. By filtering the
liquor when a crystalline crust begins to form during evaporation, a
cold solution of 3.360–3.365 specific gravity is obtained which is found
practically useful. It has a higher specific gravity than Thoulet’s
mixture, is not injurious to any of the mineral particles, not even of
iron with which it is brought into contact, but the trouble of preparing
it is far greater than that of the mixture of mercuric and potassium
iodids.
=265. Rohrbach’s Solution.=—The solution of barium mercuric iodid
recommended by Rohrbach[171] for this purpose was originally prepared by
Suchsin. The solution must be rapidly prepared on account of the
tendency of the barium salt to decomposition. The solution is prepared
by weighing rapidly 100 grams of barium and 130 grams of mercuric iodid,
mixing the two salts well in a dry flask and adding twenty cubic
centimeters of water. The mixture is raised to a temperature of
150°–200° on an oil-bath. The formation and solution of the double salt
are promoted by constant stirring.
After solution, the liquor is boiled for a few minutes and then
evaporated on a water-bath until it will bear a crystal of epidote. On
cooling, a small quantity of a yellow double salt is separated by
crystallization and the resulting mother-liquor is dense enough to carry
a fragment of topaz. Inasmuch as the liquor is filtered with difficulty,
the clear mother-liquor should be separated by decantation after
standing for several days. This solution has the disadvantage of not
being dilutable with water, the addition of which causes a separation of
red mercuric iodid. Were this solution not so easily decomposed, it
would prove of high value in silt separation.
=266. Braun’s Separating Liquid.=—In many respects the separatory
solution proposed by Braun[172] is superior to those already mentioned.
It is the commercial methylene iodid, CH₂I₂, which has at 16° a specific
gravity of 3.32, at 5° of 3.35, and at 25° of 3.31. It is a strongly
refractive liquid having a refractive index of 1.7466 for the yellow
ray.
As a separating medium the liquid is open to two objections; _viz._,
first, it cannot be diluted with water and, second, it turns brown on
heating or on long exposure to the sunlight.
When dilution is necessary, it should be accomplished with benzene or
xylene. To bring the diluted liquor again to its maximum density, the
benzene must be removed by evaporation, which causes a considerable loss
in the liquid. When this substance becomes opaque, the transparency may
be restored by removing the separated iodin by shaking with potash lye,
washing with pure water, drying by the addition of pieces of calcium
chlorid and filtering. The same result may also be reached by freezing
and separating the liquid portion. The frozen portion on melting will
have the density of the original liquid.
=267. Method of Bréon.=—Instead of a solution of a salt, Bréon[173] has
proposed to use salts in a fused state for separating mineral particles.
Lead and zinc chlorids may be used for this purpose in a melted state,
having the specific gravities of 5.0 and 2.4, respectively. By mixing
the molten salts in different proportions, any desired specific gravity
between the extremes mentioned may be secured. The fusion is
accomplished at 400° in a test-tube. The silt is added gradually with
constant stirring until a sharp separation is secured between the
sinking and floating particles. After cooling, the tube is broken, the
two parts separated, and the silt recovered by dissolving the mixed
salts in hot water containing a little nitric acid. Only the coarser
silts can be separated by this method. Fused silver nitrate, melting
point 198°, specific gravity 4.1, has also been used for separation.
=268. The Separation.=—Forty cubic centimeters of the solution in the
Thoulet process are placed in the separatory tube A, Fig. 58, together
with from one to two grams of the silt and the stopper F inserted. The
tube G is connected with a vacuum apparatus by means of which any air
particles adhering to the mineral fragments are removed. The silt which
sinks in the solution is removed after G has been disconnected by
opening the cock C and sucking through B at I. The cock C is closed and
the separated particles washed into a beaker at H after opening D. Water
is next added to the materials left in A in quantities previously
determined to secure a given specific gravity and thus a second, a
third, etc., separation secured. An intimate mixture of the solutions in
A can be effected by closing D, opening C, and blowing through B in such
a way that no liquid is allowed to pass through C.
[Illustration:
FIG. 58.
THOULET’S SEPARATING
APPARATUS.
]
The quantity of water to be added in each case to secure a given
specific gravity is determined by the formula _v_₁ = (_v_(D − _d_))/(_d_
− 1), in which _v_ is the volume of the solution, D its specific
gravity, and _d_ and _v_₁ the specific gravity desired and volume of the
water to be added.
_Example._—Let the specific gravity of the original solution be 3.2, its
volume thirty cubic centimeters, and the desired specific gravity of the
new solution 2.85.
Then _v_₁ = (30(3.2 − 2.85))/(2.85 − 1) = 5.68.
The desired specific gravity is therefore secured by adding 5.68 cubic
centimeters of water, which is easily accomplished by means of the
graduations on the tube.
According to Rosenbusch,[174] the calculated specific gravity as made
above is not wholly reliable on account of the contraction which takes
place. An empirical process is rather to be commended which consists in
introducing a fragment of mineral of known or desired specific gravity
and then adding water drop by drop until the fragment remains suspended
in the mixture. Should too much water be added, the necessary increase
in density can be secured by adding a little of the strong solution.
=269. Method of Packard.=—A separatory funnel, according to
Packard,[175] may be safely used to hold the solution while separation
is going on. As the lighter minerals form the bulk of soils, the heavier
constituting only a small percentage, it is well to use a wide funnel
holding as much as one-half liter for quantitative separations, because
a large quantity of soil, say 100 grams, is necessary from which to
recover the small quantity of heavy particles satisfactorily. The soil
is introduced into the solution contained in the funnel, agitated,
stirred with a glass rod, and allowed to stand some time. This operation
may be repeated as often as desired. Separation is not absolute by this
operation, the heavy and light particles being sometimes so united that
they sink or float together according as one or the other preponderates.
There are also particles having so nearly the same specific gravity as
the solution that they remain indifferent to its action in any position.
After separation has been effected, the heavy portion is drawn off
through the stop-cock of the funnel and the lighter is skimmed off the
top. Both must be thoroughly washed from the adhering heavy solution for
further examination with the microscope, and by chemical, microchemical,
and blow-pipe tests. One who has familiarized himself with the
appearance of minerals in minute fragments under the microscope, in
ordinary and polarized light, will be able to determine some minerals in
that way. But for certain identification it is necessary to ascertain
their optical properties as is done in the case of the minerals in thin
sections of rocks.
_Illustration._—The following example from the work of Packard will
serve to illustrate the results of separating a soil by the specific
gravity method:
One hundred grams of soil, residual clay from the Trenton limestone,
were placed in the Thoulet’s solution contained in the large separatory
funnel. The heavy portion, after washing and drying, weighed 0.6886
gram, or 0.69 per cent. Of this, the magnet removed 0.1635 gram, or 0.16
per cent. This heavy material consisted of rounded yellowish and brown
grains up to twenty-five millimeters in diameter, mingled with lustrous
angular black grains which were seen under the microscope to be cubes
with striated faces, cubes penetrating each other and aggregations of
cubes. Combinations of cubes with octahedra and instances of the
pentagonal dodecahedron were also observed. These forms, characteristic
of pyrites, were also seen in the fine sand obtained as a residue on
elutriating the same soil. As these crystals dissolved in hydrochloric
acid, giving a strong iron solution, they were regarded as pseudomorphs
of iron oxid after pyrites. The yellowish grains on treatment with acid
left a grayish residue which contained some grains of quartz but was not
wholly quartz. The lighter portion of the soil, over ninety-nine per
cent, which floated in the Thoulet’s solution of 2.8 was next examined.
It was colored red by the iron oxid which coated and adhered to the
other minerals. It contained all the quartz, the feldspars if present,
and the other minerals whose specific gravity is less than 2.8. It was
examined by the microscope and found to consist largely of irregular
grains of a mineral which acted on polarized light, obscured somewhat by
the iron oxid, and was apparently quartz; and another mineral which was
yellowish-brown in color and seemed to be dull and not transparent.
Besides there was a large quantity of indistinguishable amorphous
material. To clean these minerals the material was treated with
hydrochloric acid to remove the iron oxid and other matter soluble in
acid, when the quartz grains appeared transparent and gave interference
colors in polarized light. But mingled with these were grains of the
other mineral which now appeared grayish, dull, and without action on
polarized light. The character of this mineral substance could only be
determined by chemical analysis.
=270. Harada’s Apparatus.=—Although it has been affirmed by some
analysts that in the subsidence of small particles it is advisable that
the containing vessels have parallel sides, yet in the method just
given, and in those about to be described, good results are obtained in
a funnel or pear-shaped holder.
[Illustration:
FIGURE 59.
HARADA’S APPARATUS.
]
In the apparatus of Harada,[176] Fig. 59, the separating vessel _a_ is
made of thick glass furnished with a glass stopper above and a glass
stop-cock _h_ below. The separating liquid and silt are placed in the
pear-shaped vessel _a_, the stopper inserted, and the whole well-shaken.
As soon as a ring of clear liquid is seen between the sinking and
floating silt, the lower end of the apparatus is brought near the bottom
of a conical glass _b_, the cock _h_ opened and the heavy silt allowed
to fall out. Very little of the liquor will flow out because of the air
pressure. Should an air bubble enter the apparatus and be held at the
stop-cock, it should be made to ascend by gently tapping. When all the
heavy silt has passed into the conical glass, the cock _h_ is closed and
some water poured over the solution and silt in _b_. The separatory
apparatus is now raised until the beveled end of it is in the water
layer, when the water at once rises to _h_ and thus washes all the silt
particles adhering to the glass into _b_. The liquid in _a_ may then be
diluted by inverting the apparatus, adding the required amount of water
through _h_, again shaken after closing _h_, and another separation
secured as before.
This apparatus is somewhat easier to manipulate than Thoulet’s but does
not admit of the same quantitative dilution of the separating liquid.
[Illustration:
FIG. 60 a. FIG. 60 b. FIG. 60 c.
BRÖGGER’S APPARATUS.
]
=271. Apparatus of Brögger.=—All silt separations in narrow tubes are
open to the objection of permitting more or less flocculation. Some of
the lighter particles are thus carried down by the heavier, and, on the
other hand, some of the heavier float with the lighter. This disturbing
action Brögger[177] seeks to avoid by the following device, Fig. 60, a,
b, c. The length of the apparatus is forty-six centimeters, and its
greatest diameter 3.5 centimeters. The opening in the large stop-cock A
is the same diameter as that of the apparatus at that point. The cubical
content of the apparatus with A open and B closed is about seventy-five
cubic centimeters. In conducting the separation the cock B is closed,
the separating liquid and silt introduced, A being open, the stopper K
inserted and the whole well-shaken. In the first separation, the silt S,
lying over B is contaminated with some of the lighter particles S′₂,
while the lighter particles above A, S₂, are mixed with some of the
heavier particles, S′₁. After closing A the apparatus is again
well-shaken and inverted as in Fig. 60 b. The two parts of the silt will
now undergo another separation as indicated. The apparatus is now
carefully inclined as in c, when the various grades of silt will flow in
the directions indicated by the arrows, but without mixing, passing each
other on opposite sides of the apparatus. When the movement is complete,
A is carefully opened, the apparatus still being held as in c, and the
light silt formerly between A and B will flow above A, while the heavy
silt above A will flow down and join the silt collected over B. This
operation may be repeated until a perfect separation is effected.
Finally B is opened and the heavy silt collected in a beaker, and the
lighter silt then removed from the upper part of the apparatus.
[Illustration:
FIGURE 61.
APPARATUS OF WÜLFING.
]
=272. Method of Wülfing.=—A somewhat more convenient method of purifying
the silt segregates and freeing them of mechanically occluded particles
of differing specific gravities has been proposed by Wülfing.[178] An
elliptical ring of heavy glass tubing carries glass stop-cocks A and B,
Fig. 61, at the two extremities of the ellipse, each arm of which is
provided with a lateral glass-stoppered neck. The perforation in the
stop-cocks has the same diameter as the sides of the ellipse. The
apparatus has an interior cubical content of about forty cubic
centimeters. Thirty cubic centimeters of the separating fluid are
introduced through one of the lateral apertures and brought to the same
height in the two arms by opening the cock B. The silt is then
introduced in equal quantities into each of the arms. The stoppers
having been inserted, the whole is well-shaken. At the beginning of the
separation, the apparatus being held in position 1, the lighter soil
above and the heavier soil below are somewhat mixed by reason of
flocculation and mechanical entanglement. At this point B is opened and
the apparatus placed in the inclined position 2. The heavier particles S
+ l, on the right arm, are thus united with the same class of particles
in the left arm making 2S + 2l. This operation is hastened by opening A
and allowing the higher column of liquid in the right arm to pass into
the left. The liquid in the left arm is allowed to rise to A. After all
of S + l in the right arm has passed into the left B is closed, the
apparatus then placed back in position 1 and inclined in the opposite
direction until L + s in the top of the left arm has been transferred to
the L + s in the top of the right, and the same quantity of liquid is
found in each arm. The operation is then repeated and this continued
until all S + s is found in the bottom of the left arm and all L + l in
the top of the right arm.
=273. Separation with a Magnet.=—Particles of magnetic iron oxid are
easily separated from the fine soil particles by means of a magnet. A
strong bar or horseshoe magnet may be used. Electro-magnets are rarely
necessary except for the separation of particles of feeble magnetic
power. Particles of iron which may be found would owe their origin to
the mortars in which the soil had been pulverized, or they might come
from a recently crushed meteorite. Some minerals, as limonite, after
ignition are attracted by the magnet and it is advisable to subject a
part of the sample to this treatment. The best method of separation
consists in spreading the particles evenly on paper and gradually
bringing the magnetic particles to one side by moving the magnet
underneath.
=274. Color and Transparency.=—But little can be learned from the color
and transparency of the smallest silt particles, but these properties in
the larger grains have considerable diagnostic value. Many minerals of
distinct color appear wholly colorless in petrographic sections or in
silt particles, as for instance, highly-colored quartz. On the other
hand, even the smallest particle of chlorite will show its distinctive
tint. The colors in some minerals are due to occluded matter not
essential to their structure, and these foreign bodies would naturally
escape when the crystal mass is reduced to an almost impalpable powder.
=275. Value of Silt Analyses.=—As in the case of chemical analyses a
silt analysis of a soil which is not typical or representative has
little value. On the other hand, a systematic separation of soils into
classes of particles can not fail to reveal a definite correspondence of
mechanical composition to soil properties. The production of a crop is
the result of certain functions, chief among which are temperature,
moisture, and plant food. In a given soil the temperature is markedly
affected by its physical state. It has been demonstrated in previous
paragraphs that the circulation of moisture in the soil and its capacity
to be held therein are chiefly functions of the state of aggregation of
the soil itself. The availability of plant food in a soil is not
measured by its quantity alone, but rather by its state of subdivision.
It is not therefore a matter of surprise that the fertility of a soil is
found, _caetèris paribus_, to be commensurate to a certain limit with
the percentage of fine silt and clay which it contains. It is true that
two soils quite different in fertility, may have approximately the same
silt percentages, but in such a case it is demonstrable that even in the
poorer soil the measure of fertility is largely the percentage of fine
particles and not its actual content of plant food. In other words,
almost all soils, even the poorest, have still large quantities of plant
food, but these stores, owing to certain physical conditions, are not
accessible to the rootlets of plants. An illustration of this is seen in
the use of concentrated fertilizers. It might seem absurd to suppose
that the addition of 100 pounds of sodium nitrate would prove useful to
a plat containing already many tons of nitrogen; but the nitrate is at
once available and its beneficial influences are easily seen.
The full value of silt analysis will only be appreciated when many
typical soils from widely separated areas are carefully studied in
respect of their chemical and physical constitution and the character of
the crops which they produce.
AUTHORITIES CITED IN PART FOURTH.
Footnote 121:
Annual Report, Connecticut Agricultural Experiment Station, 1887.
Footnote 122:
American Journal of Science, March 1879, p. 205.
Footnote 123:
Bulletin, No. 4, United States Weather Bureau, p. 19.
Footnote 124:
Chemical News, Vol. 30, August 7, 1874, p. 57.
Footnote 125:
American Journal of Science, Vol. 29, 1885, p. 1.
Footnote 126:
American Journal of Science, Vol. 37, (1889), p. 122.
Footnote 127:
Proceedings National Academy of Science, Baltimore Meeting, 1892.
Footnote 128:
Manuscript communication to author.
Footnote 129:
Division of Chemistry, Bulletin 38, p. 200.
Footnote 130:
Anleitung zur Wissenschaftlichen Bodenuntersuchung, S. 23.
Footnote 131:
Die Landwirtschaftlichen Versuchs-Stationen, Band 38, Ss. 309, et seq.
Footnote 132:
Berichte der deutschen chemischen Gesellschaft, Band 15, S. 3025.
Footnote 133:
Connecticut Agricultural Experiment Station, Annual Report, 1886, pp.
141, et seq.
Footnote 134:
König, Untersuchung Landwirtschaftlich und Gewerblich Wichtiger
Stoffe, S. 7.
Footnote 135:
Wahnschaffe, Anleitung zur Wissenschaftlichen Bodenuntersuchung, S.
25.
Footnote 136:
Vid. 15, S. 24.
Footnote 137:
König, op. cit. 14, S. 13.
Footnote 138:
Tenth Census of the United States, Vol. 3, pp. 872–3.
Footnote 139:
Wahnschaffe, op. cit. 15, S. 26.
Footnote 140:
Le Stazioni Sperimentali Agrarie Italiane, Vol. 17, pp. 672, et seq.
Footnote 141:
Vid. 13.
Footnote 142:
Encyclopedie Chimique, Tome 4, pp. 155, et seq.
Footnote 143:
Annales de la Science Agronomique, 1891, Tome 1, Seconde Fasicule, pp.
250, et seq.
Footnote 144:
Vid. 22.
Footnote 145:
Petermann, L’Analyse du Sol., p. 15.
Footnote 146:
Vid. 20.
Footnote 147:
Zeitschrift für analytische Chemie, Band 3, Ss. 89, et seq.
Footnote 148:
Zeitschrift für analytische Chemie, Band 5, Ss. 295, et seq.
Footnote 149:
Bulletin de la Société des Naturalistes de Moscou, Tome 40, pp. 324,
et seq.
Footnote 150:
Journal für Landwirtschaft, Band 38, Theil 2, S. 162.
Footnote 151:
Connecticut Agricultural Experiment Station, Annual Report, 1887, pp.
145, et seq.
Footnote 152:
Division of Chemistry, Bulletin No. 38, pp. 60, et seq. The figures
are from original drawings under the direction of Prof. Hilgard.
Footnote 153:
Op. cit. supra, pp. 65–69.
Footnote 154:
Op. cit. 13, p. 150.
Footnote 155:
Op. cit. 31, p. 152.
Footnote 156:
Op. cit. 31, p. 157.
Footnote 157:
Op. cit. 31, p. 159.
Footnote 158:
Connecticut Agricultural Experiment Station, Annual Report, 1888, p.
154.
Footnote 159:
Loughridge, Proceedings American Association for the Advancement of
Science, Vol. 22, p. 81.
Footnote 160:
Whitney, United States Weather Bureau, Bulletin No. 4.
Footnote 161:
Whitney, op. cit. 40.
Footnote 162:
Vid. 40.
Footnote 163:
Vid. 40.
Footnote 164:
Vid. Anleitung zur Mineralogischen Bodenanalyse von Franz Steinreide;
and Mikroskopische Physiographie von H. Rosenbusch.
Footnote 165:
Elemente einer neuen Chemisch-Mikroskopischen Mineral und
Gesteinsanalyse, 1877.
Footnote 166:
Rosenbusch, Mikroskopische Physiographie, Plate 11, Fig. 3. The
figures of crystals of potassium, sodium, calcium, magnesium, etc.,
are taken from the same work, Plates 10, 11, and 12.
Footnote 167:
Williams, American Journal of Science, February 1893, p. 203.
Footnote 168:
Op. cit. 46, S. 231.
Footnote 169:
Op. et., loc. cit. 48.
Footnote 170:
Op. cit. 46, S. 233.
Footnote 171:
Op. cit. 46, S. 235.
Footnote 172:
Op. cit. 46, S. 236.
Footnote 173:
Op. cit. 46, S. 237.
Footnote 174:
Op. cit. 46, S. 232.
Footnote 175:
Manuscript Communication from R. L. Packard.
Footnote 176:
Op. cit. 46, S. 241.
Footnote 177:
Op. cit. 46, S. 242.
Footnote 178:
Op. cit. 46, S. 243.
NOTE.—The analyses on page 237 are by Hilgard and Loughridge from
Proceedings American Association for the Advancement of Science,
Portland Meeting, 1873.
PART FIFTH.
ESTIMATION OF GASES HELD IN SOILS.
=276. Relation of Soil Composition to Gases.=—The power of a soil for
occluding gases rests primarily on its composition as determined by silt
analysis. The discussion of this part of the subject is so nearly
related to that of the physical properties of the soil that it might
properly have been included in that part of the work. Since, however, we
deal in this part more with the determination of the gas constituents of
the soil, it was deemed preferable to place it after the silt analysis
and as introductory to the general estimation by more strictly
analytical processes of the chemical constituents of the soil.
=277. Occurrence of Carbon Dioxid.=—The amount of organic matter in the
soil, according to Wollny,[179] is no indication of the quantity of
carbon dioxid when the organic matter is in excess. The percentage of
carbon dioxid is only proportional to the amount of organic matter when
this is in small quantities. Large quantities of organic matter do
increase the amount of carbon dioxid, but the increase is not a
proportional one, since a larger quantity of this gas in the air of a
soil reduces the activity of the organisms which produce oxidation.
Water and temperature have a greater influence on the oxidation, and act
in an opposite direction to that of the organic matter. The amount of
free gas in the soil affords no indication either of the intensity of
the action of oxidation or of the amount of organic matter.
The addition of liquid manure to the soil results in a reduction of the
decomposition of the organic matter when the quantity of the salts
therein contained is greater than that already present in the soil. But
if the liquid manure is dilute, and the absorptive power of the soil for
salts is great, then the decomposition is promoted.
=278. Absorption of Aqueous Vapor.=—The power of a soil to resist
drought depends largely upon its coefficient of absorption for aqueous
vapor. Hilgard has shown[180] that at temperatures between 7° and 21°,
the amount of aqueous vapor absorbed by a thin layer of a clay or soil
not unusually rich in humus, in a saturated atmosphere, is sensibly
constant. In general, clay soils are more absorbent than sandy ones, yet
there is no direct connection between the amount of clay present and the
absorbent power of the soil. Evidently the hygroscopic coefficient is
largely controlled by the presence with the clay of the powdery
ingredients which determine its looseness of texture, and it is found
that the finer silts themselves possess a considerable absorbing power.
According to Whitney this is largely dependent upon the extent of the
surface area of the soil grains and upon the size and arrangement of
these grains. Again, the presence of hydrated ferric oxid materially
influences this power, so that the amount of iron present must always be
taken into consideration.
=279. Methods of Study.=—The study of the deportment of a soil with
vapors or gases may be divided into two general classes. The first
depends on the subjection of a sample of soil to the saturating
influence of a given vapor or gas and measuring the amount thereof
absorbed, either directly by increase of weight, or by the diminution in
the amount of gas originally supplied. The maximum absorbent capacity of
a soil under given conditions for a gas or vapor is in this way
determined.
In the second class the determination consists in accurately estimating
the amount of gas which is absorbed by a soil in natural conditions or
_in situ_, thus giving the natural percentages of the gaseous
constituents of the soil.
In the first case in general, the principle of the method depends upon
the exposure of the soil for a given time under given conditions, to an
atmosphere of the gas to be absorbed. The principle of the second class
of determinations depends upon the extraction, usually by means of
suction, from a given mass of soil of the gaseous matters therein
contained. The general details of the methods of procedure for the first
class are found in the following directions for manipulation:
=280. Determination of the Maximum Hygroscopic Coefficient.=—The fine
earth, in Hilgard’s method, is exposed to an atmosphere saturated with
moisture for about twelve hours at the ordinary temperature (60° F.) of
the cellar in which the box should be kept. The soil is sifted in a
layer of about one millimeter thickness upon glazed paper, on a wooden
table, and placed in a small water-tight covered box, twelve by nine by
eight inches, in which there is about an inch of water; the interior
sides and cover of the box should be lined with blotting paper, kept
saturated with water, to insure the saturation of the air.
Air-dried soil yields results varying from day to day to the extent of
as much as thirty to fifty per cent, nor have we any corrective formula
that would reduce such observations to absolute measure. Knop’s law,
that the absorption varies directly as the temperature, while applicable
to low percentages of saturation, is wide of the truth when saturation
is approached. The ordinary temperature of cellars will serve well in
these determinations without material correction.
After eight to twelve hours the earth is transferred as quickly as
possible, in the cellar, to a weighed drying tube and weighed. The tube
is then placed in a paraffin bath; the temperature gradually raised to
200° C. and kept there twenty to thirty minutes, a current of dry air
passing continually through the tube. It is then weighed again and the
loss in weight gives the hygroscopic moisture in saturated air.
The reason for adopting 200° C. as the temperature for drying instead of
100° is that water will continue to come off from most soils at the
latter temperature for an indefinite time, a week or more, before an
approach to constancy of weight is attained; and that up to 200° only an
arbitrary limit can be assigned for the expulsion of hygroscopic
moisture. Moreover, the great majority of soils, especially those poor
in humus, will reabsorb moisture from a saturated atmosphere to the full
extent of that driven off at 200° C.
=281. Estimation of the Absorption Power of Soils for Aqueous
Vapors.=[181]—_Method A._—The fine earth, ten to twenty grams, is spread
out on a surface of about twenty-five square centimeters, and left for
several days with the observation of the temperature of the air and the
loss of weight determined from time to time. This evaporation is
continued until the weight remains practically constant. Afterwards by
drying the sample at 100° the amount of hygroscopic moisture is
determined. A similar result can be reached if the sample is first dried
at 100°, or over sulfuric acid at ordinary temperatures, and then the
increase in weight observed which the sample acquires on being exposed
for several days to the atmosphere under ordinary conditions. Soils with
about the same content of humus show variations in the power to absorb
aqueous vapors which are almost proportional to the amount of clay which
they contain. With the increase of humus substance, the power of the
soil for absorbing moisture is increased, so that a sandy soil which is
rich in humus often will retain as much moisture in an air-dried state
as a clay soil which is poor in humus. If the experiment is carried on
by drying over sulfuric acid instead of at 100°, the sample should be
left from four to seven days in order that a constant weight may be
reached. Even after this time the loss in weight is 0.2 to 1.5 per cent
less than when the sample is dried at 100°.
_Method B._—In order to determine the amount of aqueous vapor which a
soil will absorb in an atmosphere saturated with the vapor the following
method is used:
The sample of air-dried soil in a flat dish of given surface; _viz._,
about twenty grams of soil to twenty-five square centimeters surface is
placed in a vessel over water without contact with the water, and the
whole of the apparatus is covered with a glass bell-jar. The sample is
weighed at intervals of six or eight hours until no appreciable increase
of weight is observed. An empty vessel of the same size and character as
that containing the soil is kept under the bell-jar, also in the same
conditions, so that any increase in weight by the deposition of moisture
on this vessel may be determined. This increase in weight is to be
deducted from the total increase in weight of the vessel and the soil.
Sandy and loamy soils become saturated in this manner in the course of
the first twenty-four hours and remain after that unchanged in weight.
Very clayey soils, and also those which are very rich in humus, require
a much longer time, three or four days even. In this case it is better
to take a smaller sample of the soil; _viz._, ten grams. The temperature
of the air within the glass vessel, of course, must be taken into
consideration.
_Method C._—The same flat dish and the same quantity of soil as in the
other methods are taken in this one. The sample is left out over night
where it can be fully saturated with dew. The amount of dew which
appears on the bushes should be noted and also the temperature of the
air and the percentage of clouds in the sky. An experiment should also
be made on spots of earth which are entirely free from vegetation in
order that the difference in the amount of water absorbed in places
practically devoid of dew and in places where the dew is abundant may be
observed.
_Method D._—Deeper flat dishes should be used for this determination so
that the depth of soil contained in them shall be from one to three, or
even six centimeters. The sample of soil should be completely air-dried
and in a state of fine subdivision. The vessels containing the soil
should be placed in a locality saturated with aqueous vapor or in the
open air during the night where they are subjected to the influence of
the cooling of the atmosphere and the deposition of dew. Note should be
made of the different amounts of moisture absorbed by the layers of
earth of different thicknesses in a given time. Observation should also
be made of the depth to which the moisture sinks in the sample of soil
under consideration.
=282. Estimation of the Absorption Power of the Soil for Oxygen and
Atmospheric Air.=[182]—From fifty to one hundred grams of air-dried soil
are placed in a glass vessel of about 500 cubic centimeters capacity,
and the flask closed with a stopper after the addition of enough water
to make the percentage of moisture in the soil about twenty. After from
eight to fourteen days the air contained in the vessel is analyzed for
oxygen, nitrogen, and carbon dioxid, with special reference to the
determination of how much oxygen has disappeared and how much the carbon
dioxid has been increased. As an alternative method, twenty-five grams
of the soil may be moistened with tolerably concentrated potash lye in a
small glass vessel, which is itself joined with air-tight connections to
an azotometer in which a known volume of air is confined by quicksilver.
The glass vessel is frequently shaken during the progress of the
experiment. The diminution of the volume of air in the apparatus after
from one to four days gives approximately the quantity of oxygen
absorbed.
=283. General Method of Determining Absorption.=—This method, due to
Freiherrn von Dobeneck,[183] is as follows: The soil, in a state of fine
powder, is dried at 100° to 105° to a constant weight. It is then placed
in an absorption tube of the following construction:
The absorption tube consists of a =ᥩ= shaped wide glass tube, both ends
of which are supplied with small glass tubes sealed upon the end of the
=ᥩ= tube, and those are furnished with tightly-ground glass stop-cocks.
Above these stop-cocks these small tubes are bent in opposite directions
at right angles. On the bend of the =ᥩ= is sealed another tube which is
furnished with a ground glass stopper. Through this opening the =ᥩ= tube
can be filled with the sample of soil. When the tube is filled, the
glass stopper inserted, and the two stop-cocks on the small tubes
closed, the contents of the tube are completely excluded from the
external atmosphere. Many of these tubes can be used at once so as to
hasten the progress of the work.
The tubes after being filled are placed in a drying oven with the
stop-cocks open. The stop-cocks are then closed before the tubes are
removed, when they are placed in a desiccator for cooling preparatory to
weighing. The weighed tubes are held in a tin box which can be placed in
a water-bath which is kept at a given temperature by means of a
thermostat. The top of the tin box should be hinged and made of a thick
non-conducting material so as to prevent any rapid change of temperature
within. On the inner side of the box a small thin-walled glass tube is
carried around four times. One end of this tube passes through an
opening in the side of the box by means of which it can be connected
with the gas apparatus outside. The other end of it is connected
directly with the absorption tubes.
The absorption tubes are so connected among themselves that when ammonia
or carbon dioxid is employed the gas passes through one of the tubes
before it can reach the next, and so on. For experiments with water-gas,
however, that is, air charged with aqueous vapor, the arrangement must
be different. While in the case of ammonia and carbon dioxid the
composition of the gas is not changed by passing through the samples of
soil, the case is quite different when air charged with aqueous vapor
passes through. In the latter case the amount of aqueous vapor in the
air would be notably lessened in passing from sample to sample on
account of the retention of a part of the aqueous vapor by the soil. In
this case, therefore, the saturated air, after it has passed through the
glass tube around the inside of the box in order to reach the proper
temperature, is conducted into a receptacle of glass which has a number
of connections equal to the number of absorption tubes so that the
saturated air can pass directly into each one of them.
The gases which are to be used for the experiments are prepared in
proper apparatus and are forced through the samples of soil, either by
pressure as in the case of ammonia or carbon dioxid, or by means of
aspirators as in the case of air saturated with aqueous vapor.
The carbon dioxid employed is purified by passing over sodium carbonate
and calcium chlorid.
The ammonia is prepared by the action of finely powdered lime on
ammonium chlorid, and is dried by passing over lime and sticks of
potassium hydroxid.
The air which is to be saturated with aqueous vapor, in order to purify
it from dust, carbon dioxid, and ammonia, is passed through two flasks
in which are contained respectively, diluted sulfuric acid and potash
lye. It is afterwards thoroughly saturated with aqueous vapor at the
temperature desired.
Various kinds of soil material may be employed as follows:
(1) Pure quartz sand.—Freed from all fine particles by subjection to
silt analysis, afterwards boiled with hydrochloric acid and washed with
water to free it from all clayey materials. The sand prepared in this
way should be passed through different sieves in order to prepare it in
different states of fineness.
(2) Quartz powder.—Prepared from pure quartz crystals by grinding in an
iron mortar.
(3) Kaolin.—Material such as is used in the manufacture of the finest
porcelain which, after being freed of all foreign matter, is rubbed to a
fine powder in a porcelain mortar.
(4) Humus.—Washed with ether and alcohol, boiled with hydrochloric acid,
washed, dried and reduced to a state of fine powder.
(5) Iron oxid.
(6) Calcium carbonate.—Precipitated, washed, and dried.
(7) Soil mixtures.—Prepared artificially by mixing the kaolin, quartz,
and humus, above mentioned.
The quantity of gas absorbed by each of these materials is determined by
filling the tubes, as above mentioned, with the dried material. The
content of each tube is previously determined by filling with mercury
and weighing. Having determined the weight of the substance to the
exclusion of the air contained within its pores, it is treated with the
gas in the apparatus described above and weighed from time to time until
no further increase of weight takes place.
The method of calculating the results is shown in the following scheme:
V = content of the absorption tube obtained by filling with mercury and
weighing.
P′ = weight of the empty tube filled with air at 100°.
pl = weight of the air in the tube (pl = V × specific gravity of the air
at 100°).
pt′ = weight of the tube (pt′ = P′ − pl).
P² (second weighing) = weight of the tube filled with the substance with
the included air at 100°.
v^s = volume of the substance calculated according to the formula
v^s = (P² − P′)/(s^s − specific gravity of air).
s^s = specific gravity of the substance.
vl = volume of the air in the flask filled with the substance (vl = V −
v^s).
pl′ (weight of this included air) = vl × specific gravity.
p^s = weight of the substance (pl = p² − pt′ − pl)
P³ = weight of the apparatus at the end of the experiment.
sg = specific gravity of the gas employed for saturation.
pg (weight of the gas remaining over the substance) = vl × sg.
pa (weight of the absorbed gas) = P³ − pt′ − p^s − pg.
p^s gram of substance absorbs pa gram of the gas and 100 grams of
substance would absorb (100 × pa)/(p^s) grams.
The specific gravities of the gases employed are calculated from the
tables given by Landolt and Börnstein in “Physical and Chemical Tables,”
page 5.
The specific gravity of the quartz sand employed was 2.639; of the
quartz powder, 2.622; of the kaolin, 2.503; of the humus, 1.462; of the
iron hydroxid, 3.728; and of the calcium carbonate, 2.678.
One liter of ammonia, at a pressure of 760 millimeters of mercury and a
temperature of 0°, weighs 0.7616 gram; one liter of carbon dioxid,
1.9781 grams; one liter of aqueous vapor, 0.8064 gram; and one liter of
dried air, 1.2931 grams.
At a pressure of 720 millimeters, and at 20° temperature, a liter of air
saturated with aqueous vapor at 0° weighs 1.1383 grams; saturated at
8.6°, 1.1362 grams; saturated at 10°, 1.1358 grams; saturated at 14°,
1.1340 grams; saturated at 18.2°, 1.1330 grams; saturated at 20°, 1.1321
grams; saturated at 30°, 1.1313 grams.
The general results of the experiments are as follows:
ABSORPTION AT 0°.
Aqueous vapor Ammonia. Carbon dioxid.
from saturated
air.
Grams. Cubic Grams. Cubic Grams. Cubic
cm.[H] cm.[H] cm.[H]
100 grams quartz 0.159 197 0.107 145 0.023 12
„ „ kaolin 2.558 3,172 0.721 947 0.329 166
„ „ humus 15.904 19,722 18.452 24,228 2.501 1,263
„ „ Fe₂(OH)₆ 15.512 19,236 4.004 5,275 6.975 3,526
„ „ CaCO₃ 0.224 278 0.256 320 0.028 14
Footnote H:
Reduced to 0° and 760 millimeters pressure mercury.
The foregoing methods will suffice to show the procedures to be followed
in estimating the maximum amount of any common gas or vapor a given
quantity of soil may be made to absorb. We pass next to consider the
quantities of gases or vapor soils _in situ_ may hold.
=284. Method of Boussingault and Lewey.=[184]—This method is the oldest
and most simple procedure for estimating the nature of the gases held in
a soil _in situ_.
For the purpose of collecting the sample of gas from the soil a hole,
thirty to forty centimeters in depth, is dug, and a tube placed in it in
a vertical position, having on its lower extremity a bulb perforated
with fine holes. The hole is filled and the earth closely packed around
the tube which is left for twenty-four hours. At the end of that time
the tube is slowly aspirated until a volume of gas approaching from five
to ten liters is obtained.
_Estimation of Carbon Dioxid._—The carbon dioxid in the sample of gas is
estimated by allowing it to bubble through a solution of barium
hydroxid.
_Estimation of the Oxygen._—The oxygen is estimated in a separate sample
of the gas by means of potassium pyrogallate.
The chief objection to this simple process is the uncertainty of being
able to obtain an average sample of the occluded gas. In digging the
hole and refilling, there must evidently be a considerable disturbance
of the original distribution of the gas or vapor.
The methods of Pettenkofer[185] and Aubry[186] are essentially like that
just described. Pettenkofer found the largest quantities of carbon
dioxid in the earth gases in July, August, and September, and the
smallest quantities in the winter months.
No greater detail concerning these methods of the direct aspiration of
the air is considered necessary inasmuch as the methods about to be
described, while more elaborate, are superior in accuracy to the older
methods mentioned. In general, in these experiments, it is deemed
sufficient to determine the carbon dioxid only.
[Illustration:
FIGURE 62.
SCHLOESING’S SOIL-TUBE FOR COLLECTING GASES.
]
=285. Method of Schloesing.=—The apparatus used by Schloesing[187] in
the collection of the soil gases consists of a steel tube (Fig. 62) a
little over one meter in length, ten millimeters in external diameter,
and one and one-half to two millimeters in internal diameter. The end
which penetrates the soil is made slightly conical for a distance of
twenty-five to thirty centimeters. By reason of the shape of the tube,
when it is driven into the soil all connection between the orifice in
the point of the tube and the external air is prevented. The obstruction
of the internal canal of the tube is prevented by introducing a thread
of steel which penetrates the whole length of the tube. This thread,
represented by A, B, C, D, is flush with the interior extremity of the
tube at D. It extends for about three centimeters above the upper end of
the tube in order to be easily handled when it is to be removed.
For the purpose of driving the tube into the soil its upper part is
covered with a cylindrical piece of steel, EF, in the interior of which
are freely engaged H and A. This head piece rests upon a ring of steel,
K. This ring is fastened solidly into the tube. On striking the piece EF
the tube and the steel wire in the center are driven together into the
soil. The tube is flattened at L and L′ in order to be embraced by the
key MM, the employment of which is necessary in order to revolve the
tube around its axis when it is being driven into the soil. When the
tube has been driven to the depth desired, the steel wire is withdrawn
and it is immediately connected at H with the rubber tube N (Fig. 63)
belonging to the system PQT, and furnished with a pinch-cock X. The
system PQT comprises the following elements: PQT made of a capillary
glass tube in the form of a T. The lower end of the tube P is closed by
the larger glass tube O, sealing the end of P with a little mercury. O
is held to P by the cork S, which is attached firmly enough to prevent O
from dropping off, but is furnished with a canal in order to allow the
air to flow in or out freely. This system is connected with the system
UV by the rubber connection T. U is a glass vessel having the
constrictions as indicated in its stem above and below the bulb. V is a
glass vessel of convenient size connected with U by the rubber tubing as
indicated. The capacity of the cylindrical portion of U should be from
fifteen to eighteen cubic centimeters.
[Illustration:
FIGURE 63.
SCHLOESING’S APPARATUS FOR COLLECTING GASES FROM SOIL.
]
To take a sample of soil gas, V is lifted above U. The air is driven
from U and escapes through O, which acts as a true valve. When the
mercury has completely filled U the pinch-cock X is opened and V
depressed gradually. The gas coming from the soil is thus collected in
U. A few cubic centimeters of the soil gas are collected in this way,
the pinch-cock X is again closed and V is raised in order to drive the
whole of the contents of U again through O. In this way the whole of the
air which the capillary vessel originally contained is removed and all
parts of it remain filled with soil gas. Two or three operations, using
from five to ten centimeters of soil gas in all, will be sufficient to
completely free the apparatus from its original content of air. U is
then entirely filled by depressing V, and it is then hermetically sealed
at the two constricted points by means of an alcohol lamp. The sealed
tube can then be transported to the laboratory and its contents
subjected to eudiometric analysis.
Without displacing the tube from the soil, several samples of gas can be
taken from the same spot. A sufficient number of the bulbs V should be
at hand to hold the required number of samples. Instead of submitting
the sample to eudiometric analysis it is usually sufficient to determine
the quantity of carbon dioxid which it contains, inasmuch as numerous
experiments have shown that in 100 parts of soil gas the oxygen and
carbon dioxid together constitute twenty-one parts. No appreciable trace
of marsh gas, or other combustible gas, has yet been detected in
ordinary arable soils. These gases have only been found in special soils
from marshes, in the neighborhood of gas wells, etc., and not in arable
soils.
[Illustration:
FIGURE 64.
SCHLOESING’S APPARATUS FOR DETERMINATION OF CARBON DIOXID.
]
=286. Apparatus for Estimating the Carbon Dioxid.=—The apparatus used
for determining the carbon dioxid in Schloesing’s work consists of the
apparatus shown in Fig. 64. A represents a glass vessel surrounded by a
jacket of glass, full of water, and sealed on its lower part to the tube
BC of about six millimeters internal diameter. On its upper part it is
sealed to the capillary tube D. The tube BC is graduated from C in
hundredths of the volume of DAC, which volume is about twelve cubic
centimeters. On its lower part it is connected by a rubber tube with a
reservoir F which is capable of being raised or lowered. GHK are
capillary tubes connected together by the rubber tubes L and M, which
are furnished with pinch-cocks. The tube G is connected to a vacuum by
the rubber tube N. The rubber tube should be of very small internal
diameter and from forty to fifty centimeters in length. To the tube H
are sealed, at right angles, the branch D and another branch O. This
last dips into a little mercury which the tube P contains. It serves as
a valve, permitting the exit of the gases but not their entrance. The
tube K carries some lines engraved on its inferior part and is sealed to
the system of the two bulbs Q and R. The bulb Q contains a concentrated
solution of potash. It carries a number of pieces of glass tubing for
the purpose of increasing the surface of the potash solution.
All the parts of the apparatus are fixed upon a rectangular board,
nineteen centimeters broad by twenty centimeters long. This forms one of
the faces of a wooden box to which it is hinged and which serves for the
transportation of the apparatus in a vertical position. The graduation
of the tube BC is recorded behind this tube upon a card fixed upon the
board. By means of these two graduations, the height of the mercury in
the tube BC is most easily read, even when the tube is not perfectly
vertical. Each one of the pinch-cocks L and M, on its upper part is
fixed in a sort of guard which prevents it from being displaced
laterally during the processes of the manipulation, thus avoiding all
danger of breakage.
After the operation is finished a little air is sent into Q in such a
manner as to sensibly lower the level of the solution of potash, and the
upper extremity of R is closed with a rubber stopper. Afterward, the
apparatus can be transported without any danger of the potash becoming
engaged in the tube K and reaching the measuring tank A.
To proceed to the analysis, a stake is driven into the soil to which all
of the apparatus can be fixed. At the side of the stake the apparatus
for taking the sample, already described, is driven into the soil and
this apparatus is connected by the tube N with the apparatus for
determining the carbon dioxid. The pinch-cocks L and M being closed, F
is lifted until the mercury which runs from it fills A and approaches D.
During this time the air which the apparatus contains has been driven
out through O. The tube NGD is freed from air by opening the pinch-cock
L, lowering F and drawing into A the gas coming from the soil; afterward
closing L and driving out the gas through O. After two or three rinsings
of this kind, which employ altogether only ten to twelve cubic
centimeters, the gas which is to be analyzed is sucked into A. For this
purpose F is lowered until the mercury in the tube BC is very near C.
The pinch-cock L is closed and M opened. The reservoir F is displaced
little by little by pressing lightly against the rectangular board in
order to give it greater firmness in such a way as to fix the level of
the mercury exactly at C, and the line is noticed where the solution of
potash in K stands. The gas contained in the apparatus is under a
pressure, the difference of which from the external pressure is
represented by the column of the potash solution between the mark just
noticed and the level of the same solution in the bulb R. In order to
absorb the carbon dioxid, F is lifted until the mercury stands between D
and E. The gas thus passes from A into Q. It gives up immediately its
carbon dioxid to the potash solution. It is then made to come again into
A, and afterward a second time into Q in order to free it from the last
trace of dioxid. Finally it is made to return to A and F is kept at such
a height that the potash solution maintains in the tube K the same level
as at the commencement of the operation. The gas is then at the same
pressure to which it was subjected before absorption. The level of the
mercury is then read on BC. At the time the apparatus is used, the
measuring tube A should be slightly moist. If it is not so, a small
quantity of water should be introduced which is afterward rejected, but
which leaves a sufficient quantity of moisture upon the internal walls
of A. In this way the gas will always, before or after absorption of
carbon dioxid, be saturated with vapor of water, and the figure read in
the last place upon the tube BC represents the percentage of carbon
dioxid in 100 parts of the gas extracted from the soil supposed to be
saturated with vapor at the temperature of the experiment.
During the course of the analysis, the temperature of the measuring
flask, which is almost entirely surrounded with water, does not vary
sensibly, but in a series of experiments which are executed at different
times, the temperature of the measuring apparatus, which is that of the
ambient air, may change much. It may oscillate between 10° to 25°, and
exceptionally between 0° and 30°, whence there are notable variations in
the tension of the vapor of the gas measured. If it should be desired to
calculate to 100 parts of dry gas the observations made at 30° upon 100
parts of saturated gas, it would be necessary to increase the percentage
of carbon dioxid by about ¹⁄₂₅ of its value. It is noticed that with the
apparatus described above, the gas upon which the estimation is really
conducted comprises not only that which the measuring apparatus contains
from E to C before the absorption of the carbon dioxid, but also the
small quantity which remains in the capillary tube KME at the moment
when closing the pinch-cock M, after the second rinsing, the gas from
the soil is aspired into EAC. On the other hand, there is left in the
same tube KME, when the final reading is made, some gas which belongs to
that which has been measured at the end. These two small gaseous
portions which we consider in the tube KME to be sensibly equal, do not
contain any carbon dioxid and may be left out of consideration. That is
why the volume of the measuring apparatus is limited to E and the
graduation of the tube BC is in hundredths of the volume comprised from
E to C. In reality the two portions are not absolutely equal because the
two successive levels of the potash solution, which limit them in the
tube K, are not absolutely identical. These two levels can differ in
such a manner as to correspond to a volume of about ¹⁄₁₀₀₀ of the
measuring apparatus. Thus the estimation is really made upon a volume of
gas which may be greater or less by ¹⁄₁₀₀₀ than the volume of EAC;
whence there might result an error of ¹⁄₁₀₀₀ in the estimation of the
carbon dioxid, an error which is wholly negligible.
As a result of numerous analyses it is concluded, first, that the oxygen
exists normally in the atmosphere of soils in large proportion; second,
very probably the gaseous atmosphere of arable soils, to a depth of
sixty centimeters, contains scarcely one per cent of carbon dioxid and
about twenty per cent of oxygen; third, the highest percentages of
carbon dioxid correspond to epochs of highest temperature and periods of
greatest calm; fourth, the proportion of carbon dioxid increases
ordinarily with the depth at which the samples are taken. This
disposition of the carbon dioxid would appear almost necessary, since
near the surface the internal atmosphere is almost constantly diluted by
external air by virtue of diffusion. Fifth, from one epoch to another
the composition of the atmosphere of the soil can undergo considerable
variation.
=287. Determination of Diffusion of Carbon Dioxid in Soil.=—The method
proposed by Hannén[188] is a convenient one to use in studying the rate
of diffusion of carbon dioxid in soils. A large Woulff’s bottle with
three necks serves for the reception of the gas. The two smaller outer
necks of the bottle carry two glass tubes bent outwards and provided
with stop-cocks. One of these passes to near the bottom of the bottle
and the other just through the stopper. The middle tubule of the bottle
is of a size to give in section an area of about twenty-two square
centimeters. It is made with a heavy rim two centimeters wide and plane
ground. This rim carries a plane-ground glass plate with a circular
perforation in one-half of it, of the size of the opening in the central
tubule of the bottle. A glass cylinder, carrying a fine wire-gauze
diaphragm near the lower end, fits with a ground-glass edge air-tight,
over this aperture, being held in position by a brass clamp. The
ground-glass plate moves air-tight between the cylinder and the bottle,
so that the cylinder can be brought into connection with the bottle or
cut off therefrom without in any way opening the bottle to the air. The
plate and all ground movable surfaces should be well lubricated with
vaseline.
The experiment is carried on as follows: The glass cylinder is filled
with the soil to be tested, closed above with a rubber stopper carrying
a gas tube, and then by moving the perforated-glass plate brought into
connection with the bottle. The side tube, with short arm inside the
bottle, is then closed, and carbon dioxid introduced through the other
lateral tube until the gas passing from the tube at the top of the
cylinder is pure carbon dioxid.
The lateral tube is then closed and the bottle is placed in a water-bath
and kept at a constant temperature of 20°. When the temperature within
and without the apparatus is the same the reading of the barometer is
made, the stopper removed from the top of the cylinder, and the process
of diffusion allowed to begin. After from six to ten hours the glass
plate is moved so as to break the connection between the cylinder and
bottle. The carbon dioxid remaining in the bottle is driven out by a
stream of dry, pure air. The air is allowed to pass through the
apparatus for about ten hours. The carbon dioxid driven out is collected
in an absorption apparatus and weighed. The absorption apparatus should
consist of a series of Geissler potash absorption bulbs and finally a
=ᥩ= form soda-lime tube. In front of the absorption apparatus is placed
a drying bulb containing sulfuric acid. Inasmuch as the temperature and
pressure can be readily determined, the weight of carbon dioxid obtained
is easily calculated to volume.
The weight of 1,000 cubic centimeters of carbon dioxid at 0° and 760
millimeters pressure is 1.96503 grams. Therefore one milligram is
equivalent to 0.5089 cubic centimeter of the gas. The volume of the
bottle should be carefully determined by calibration with water. The
results should be calculated to cubic centimeters per square centimeter
of exposed surface in ten hours. The depth of the soil layer is
conveniently taken at twenty centimeters.
=288. Statement of Results.=—
THE SOIL PACKED LOOSELY IN THE DIFFUSION TUBE.
DIFFUSION TIME, TEN HOURS.
Diameter of Weight of Pure carbon Carbon dioxid Cubic
soil soil taken, dioxid at at end of centimeters
particles, grams. beginning of experiment, of carbon
millimeters. experiment, cubic cm. dioxid
cubic cm. diffused for
each square
cm.
0.01–0.071 520 2549.4 1230.3 59.9
0.071–0.114 550 2545.9 1269.2 58.0
0.114–0.171 590 2556.4 1354.2 54.6
0.171–0.250 620 2538.9 1336.1 54.6
0.250–0.500 660 2532.0 1374.5 52.6
0.500–1.000 680 2528.2 1440.2 49.5
1.000–2.000 690 2496.6 1396.9 50.0
Mixture of 720 2514.3 1572.5 42.8
the above
In greater detail the calculation and statement of the results may be
illustrated by the following data:
In the first experiment given in the above table the diameter of the
soil particles varied from 0.010 to 0.071 millimeter. The weight of soil
in the diffusion tube was 520 grams. The volume of gas, at 0° and 760
millimeters, before the diffusion began was 2549.4 cubic centimeters.
The volume of carbon dioxid under standard conditions remaining after
ten hours of diffusion was 1230.3 cubic centimeters. This volume is
calculated from the weight of carbon dioxid obtained in the potash
bulbs, each milligram being equal to 0.5089 cubic centimeter of carbon
dioxid. The volume of carbon dioxid diffused is therefore 2549.4 −
1230.3 = 1319.1 cubic centimeters. The per cent of carbon dioxid
diffused is 1319.1 ÷ 2549.4 = 51.74. The volume of carbon dioxid
diffused for each square centimeter of cross section of the diffusion
tube is 1319.1 ÷ 22 = 59.9 cubic centimeters.
The carbon dioxid should be passed long enough to secure complete
expulsion of the air before the determination is commenced.
=289. General Conclusions.=—The general results of the experiments with
the diffusion apparatus to determine the effect of the physical
condition of the soil upon the rate of diffusion are as follows:
1. The diffusion of carbon dioxid through the soil is, at a constant
temperature, chiefly dependent upon the pores in the cross section of
the column of soil. Therefore, the absolute quantity of the diffused gas
is greater the larger the total volume of the pores and _vice versa_.
2. Every diminution of the volume of the pores, whether secured by
pressure of the soil or by an increase in the moisture thereof, is
followed by a decrease in the volume of diffused gas. The giving up of
the carbon dioxid present in the soil atmosphere to the upper atmosphere
by the method of diffusion is therefore the less the finer the soil is,
the more compressed the soil particles are, and the larger the water
capacity of the sample and _vice versa_.
3. The quantity of diffused carbon dioxid is diminished according to the
measure of compression to which the soil is subjected but is not
strictly proportional to the height of the soil layer.
4. In soils in which rain water percolates slowly the diffusion of the
carbon dioxid on account of this property is depressed to a greater or
less extent.
AUTHORITIES CITED IN PART FIFTH.
Footnote 179:
Proceedings of the American Association for the Advancement of
Science, 1872, p. 328.
Footnote 180:
Die Landwirtschaftlichen Versuchs-Stationen, 1889, S. 197.
Footnote 181:
König, Untersuchung Landwirtschaftlich und Gewerblich Wichtiger
Stoffe, Ss. 64–66.
Footnote 182:
König, op cit. supra.
Footnote 183:
Forschungen auf dem Gebiete der Agricultur-Physik, Band 15, S. 190.
Footnote 184:
Annales de Chimie et de Physique, Tome 37, 1853; Encyclopedie
Chimique, Tome 4, p. 154.
Footnote 185:
Zeitschrift für Biologie, Band 7, S. 395 and Band 9, S. 250.
Footnote 186:
Jahresbericht für Agriculturchemie, Band 1, S. 160.
Footnote 187:
Annales de Chimie et de Physique, 1891, Sixième Série, Tome 23, pp.
362, et seq.
Footnote 188:
Op. cit. 5, 1892, Ss. 8, et seq.
PART SIXTH.
=290. Preliminary Considerations.=—The sample of soil intended for
chemical analysis should consist of the fine earth which has passed at
least a one-millimeter mesh sieve and subsequently been completely
air-dried. According to Petermann the air-drying of a soil should
continue for about four days for an ordinary arable soil, and about six
days for one very rich in organic matter. With peat and muck soils I
have found that ten or twelve days with frequent stirring, even when in
thin layers, are necessary to attain approximately a constant weight.
The soil is conveniently spread on a zinc or other metal sheet of
sufficient area so that the layer will be only one or two centimeters in
thickness. The weight before and after desiccation will give the
percentage of moisture lost on air-drying, which, of course, will depend
chiefly on the degree of saturation of the sample when taken and the
atmospheric conditions prevailing during drying.
If samples of soil are taken in very dry times it is often necessary to
moisten them with distilled water in order to prepare them properly for
air-drying.
The quantity of hygroscopic water which the sample loses at 100°–105°
should be determined, and all subsequent calculations of the percentages
of the various constituents be based on the water-free material. When a
soil which has been dried at 100°–105° to a constant weight is heated to
140°–150° it loses additional weight not due to loss of water of
constitution. A part of this loss may be due to hygroscopic moisture
which is not given off at 100°–105°, and a part may be hydrocarbons, or
other easily volatile organic or inorganic bodies. Before estimating the
total loss on ignition it is recommended by most chemists to dry at
140°–150°. The samples of soil, however, intended for chemical
examination should never be dried beyond the point which is reached by
exposure in thin layers at ordinary room temperatures. The state of
aggregation, degree of solubility, and general properties of a soil, may
be so changed by absolute desiccation as to render the subsequent
results of chemical investigation misleading. In the methods which
follow the actual processes employed have been given, which in some
instances transgress the general principle stated above, but in all
cases standard and approved methods are given in detail, even if some of
their provisions seem unnecessary or imperfect.
=291. Order of Examination.=—First of all in a chemical study of the
soil should be determined, its reaction (with litmus), its water-holding
power in the air-dried state (hygroscopicity), its content of combined
water (giving hydrous silicates of alumina), its organic matter (humus
and organic nitrogen), its content of carbon dioxid (carbonates of the
alkaline earths), and the part of it soluble in acids. A determination
of these values gives the analyst a general view of the type of soil
with which he is engaged, and leads him to adopt such a method of more
extended analysis as the circumstances of the case may demand.
For this reason those operations which relate to the above
determinations are placed first in the processes to be performed, while
the estimation of the more particular ingredients of the soil is left
for subsequent elaboration.
Next follows a description of the standard methods of estimating the
more important elements passing into solution on treatment of a soil
sample with an acid. The method of treating the insoluble residue, and
the detection and estimation of rare or unimportant soil constituents,
closes the analytical study of the soils.
With respect to the determination of nitrogen as nitric or nitrous acid
in the soil and drainage waters, it has been thought proper to collect
all standard methods relating particularly thereto into one group, and
they will appear separate from the methods under nitrogen analysis in
fertilizers.
The question of the utility of chemical soil analyses is one which has
been the subject of vigorous discussion, a discussion which finds no
proper place in a work of this character. Unless, however, intelligent
soil analysis be productive of some good it would be a thankless task to
collect and arrange the details of the processes employed. An accurate
determination of the constituents of a soil may not enable the chemist
to recommend a proper course of treatment, but it will help in many ways
to develop a rational soil diagnosis which will permit the physician in
charge of the case, who last of all is the farmer, to follow a rational
treatment which in the end will be productive of good.
The analyst will find in the methods given all that are approved by
bodies of official or affiliated chemists, or by individual experience,
and among them some method may be found which, it is hoped, will be
suited, in the light of our present knowledge, to each case which may
arise.
=292. Reaction of the Soil.=—In soils rich in decaying vegetable matter
the excess of acid is often great enough to produce a distinct acid
reaction.
On the contrary, in arid regions the accumulation of salts near the
surface may produce the opposite effect.
The reaction of the soil may be determined with a large number of
indicators among which, for convenience, sensitive litmus paper, both
red and blue, stand in the front rank. A sample of the soil, from
fifteen to thirty grams, is mixed with water to a paste and allowed to
settle. The litmus paper is then dipped into the supernatant liquid.
=293. Determination of Water in Soil.=—The following problems are
presented:
(a) _The Determination of Water in Fresh Samples taken in situ._—The
content of water in this case varies with the date and amount of
rain-fall, the capacity of the soil for holding water, the temperature
and degree of saturation of the atmosphere, and many other conditions,
all of which should be noted at the time the samples are taken.
(b) _The Determination of Water in Air-Dried Samples._—In this case the
soil is allowed to remain in thin layers, and exposed to the air until
it ceases to lose weight. The quantity of water left is dependent on the
capacity of the soil to hold hygroscopic water and to the temperature
and degree of saturation of the air.
(c) _The Determination of the Total Water by Ignition._—This process not
only gives the free and hygroscopic moisture, but also combined water
present in the hydrous silicates and otherwise. The estimation is
complicated by the presence of carbonates and organic matter.
=294. Determination of Water in Fresh Samples.=—This determination
requires that the sample, when taken in the field, should be so secured
as to be weighed before any loss of moisture can take place. For this
purpose it can be sealed up in tubes or bottles and preserved for
examination in the laboratory.
According to Whitney, the relations of soils to moisture and heat are
such prominent factors in the distribution and development of
agricultural crops, that the determination of the actual moisture
content of soils in the fields should be considered a necessary part of
the meteorological observations, and of far more importance, indeed, or
having far more meaning to the agriculturist than the simple record of
the rain-fall.
In order to determine the relation of the soil to moisture, uninfluenced
by the varying conditions of cultivation and of the different size of
crop, he recommends that a small plot of ground be reserved at each
station, adjacent to the soil thermometers, where the samples may be
taken for the moisture determinations. No crops should be allowed to
grow on this area and the soil is not to be disturbed, except that weeds
and grass are carefully removed by hand when necessary. Samples of the
soil should be taken every morning at 8 o’clock, by correspondents in
the principal soil formations from the different parts of the area under
observation, and sent by mail to the laboratory.
The samples should be taken as described in paragraph =65=. The locality
and date are written on a label attached to the tube. The tube contains
about sixty or seventy grams of soil, and the moisture determination is
made on this in the laboratory in the usual way.
It would be desirable to have this sample represent a depth of from six
to nine inches, thus rejecting the surface three inches which are more
liable to sudden and accidental changes. These tubes are very
inexpensive, and a sufficient number should be purchased to keep each
station supplied. The sample represents a definite depth, and it does
not have to be subsampled or even transferred in the field. This record
of the moisture of the soil will show the amount of moisture which the
different soils can maintain at the disposal of the plants, which,
together with the temperature of the soil, is believed to be a most
important factor in crop distribution and development.
=295. Method of Berthelot and André.=—The estimation of the water
according to Berthelot and André[189] should be made under three forms;
_viz._,
1. Water eliminated spontaneously at ordinary temperatures.
2. Water eliminated by drying to constant weight at 110°.
3. Water eliminated at a red heat.
The water may be determined directly on a sample weighed at the time of
taking and afterwards dried in the open air, and finally, if necessary,
in a desiccator. For a general idea the desiccation should be made on a
sample of 100 grams, for exact work on ten grams. The dish in which the
drying takes place should be shallow, and during the time the sample
should be frequently stirred and thoroughly pulverized with a spatula
which is weighed with the dish. The drying in the air should continue
several days. The data obtained are not fixed since they depend on the
temperature and the degree of saturation of the air with aqueous vapor.
The variations due to these causes, however, are not very wide. The
process may be regarded as practically finished when successive weights
sensibly constant are obtained. In this state the soils contain very
little water eliminable at 110°.
=296. Estimation of Water Remaining after Air-Drying.=—The sifted sample
is placed in quantities of five or ten grams in a flat-bottomed dish and
dried at 110° to constant weight. This treatment not only removes the
moisture, but all matters volatile at that temperature.
Petermann,[190] in the Agricultural Station, at Gembloux, practices
drying the sample to constant weight at 150°.
It is further recommended by Petermann to determine total volatile and
combustible matters by igniting to incipient redness, allowing to cool,
moistening with distilled water, and drying at 150°.
The German experiment stations[3] estimate hygroscopic moisture for
analytical calculations by drying to constant weight at 100°. In
determining loss on ignition, however, the preliminary drying is made at
140°, with the exception of peaty samples where so high a temperature is
not admissible.
The Official Agricultural Chemists[191] place five grams of air-dried
soil in a flat-bottomed and tared platinum dish; heat in an air-bath to
110° for eight hours; cool in a desiccator, and weigh; repeat the
heating, cooling, and weighing, at intervals of an hour till constant
weight is found, and estimate the hygroscopic moisture by the loss of
weight. Weigh rapidly to avoid absorption of moisture from the air.
In the German laboratories, according to König,[192] from ten to twenty
grams of the fine earth, properly prepared by air-drying and sifting,
for analysis, are heated at 100° to constant weight. For control, five
grams are placed in a desiccator over sulfuric acid for two or three
days.
Wolff directs that a small portion of the well-mixed earth, for example,
twenty grams, be spread out on a flat zinc plate, and its changes in
weight observed through several days. These observations are continued
until the variations are so slight that the means can be determined with
sufficient exactness from the last weighings. The soil is then dried at
125° in a hot air-chamber. The loss in weight will give the mean
hygroscopic moisture in the soil under the conditions in which the
experiment is made.
=297. Drying in a Desiccator.=—The sample dried as indicated previously
by the method of Berthelot and André is placed in a desiccator over
sulfuric acid. It is better to have the sample traversed by a current of
perfectly dry air, and in this case it should be placed in a tube, which
is closed while weighing, to prevent absorption of moisture. Much time
is also required for this operation, and it does not possess the
practical value of the method of drying in the free air.
=298. Water Set Free at 110°.=—This is determined by Berthelot and André
on a weight of five to ten grams of soil. The sample which has been
employed for the preceding determination may be used. While this is
going on in an air-bath heated to 110°, about ten times as much soil
should be dried for the same time at the same temperature, and this
should be preserved in a well-stoppered flask. All subsequent
determinations are to be made with the soil dried at 110°.
The loss of weight in a soil increases with the temperature to which it
is exposed. The apparent quantity of water, therefore, determined at
140° or 180° is always greater than that obtained at 110°. But when the
temperature exceeds 110° there is danger of decomposing organic bodies
with the loss of a part of their constituent elements. Carbon dioxid and
ammonia may also be lost, as well as acetic acid and other volatile
bodies.
=299. Loss on Ignition.=—The loss on ignition represents any hygroscopic
moisture not removed by previous drying, all water in combination with
mineral matters as water of constitution, all organic acids and
ammoniacal compounds, all organic matter when the ignition is continued
until the carbon is burned away, all or nearly all of the carbon dioxid
present in carbonates, and, finally, some of the chlorids of the
alkalies, if the temperature have been carried too high or been
continued too long.
The loss of carbon dioxid in carbonates may be mostly restored by
moistening the ignited mass two or three times with ammonium carbonate,
followed by gentle ignition for a few minutes to incipient redness, to
remove excess of the reagent. The apportionment of the rest of the loss
justly among the remaining volatile constituents of the original sample
is a matter of some difficulty but may be approximately effected by the
methods to be submitted.
=300. Determination of Loss on Ignition.=—_Method of the Official
Agricultural Chemists._ The platinum crucible and five grams of soil
used to determine the hygroscopic moisture may be employed to determine
the volatile matter. Heat the crucible and dry soil to low redness. The
heating should be prolonged till all organic material is burned away,
but below the temperature at which alkaline chlorids volatilize. Moisten
the cold mass with a few drops of a saturated solution of ammonium
carbonate, dry, and heat to 150° to expel excess of ammonia. The loss in
weight of the sample represents organic matter, water of combination,
salts of ammonia, etc.
According to Knop[193] the total loss on ignition is determined as
follows: About two grams of the fine earth are carefully ignited until
all organic matter is consumed. The sample is then mixed with an equal
volume of finely powdered, pure oxalic acid, and again heated until all
the oxalic acid is decomposed. After cooling, the sample is weighed,
again mixed with oxalic acid, ignited, cooled, weighed, and the process
continued until the weight is constant.
The method recommended by König consists in igniting about ten grams of
the fine earth at the lowest possible temperature until all the humus is
destroyed. Thereafter the sample is repeatedly moistened with a solution
of ammonium carbonate and ignited after drying at 100°, until constant
weight is obtained. In soils rich in carbonates some carbon dioxid may
be lost by the above process. For this a proper correction can be made
by estimating the carbon dioxid in the sample, both before and after the
execution of the above described process.
The method described by Frühling as much used in the German
laboratories, consists in igniting ten grams of the fine earth,
previously dried at 140° in a crucible placed obliquely on its support
and with the cover so adjusted over its mouth as to give a draft within
the body of the crucible. The ignition, at a gentle heat is continued
until on stirring with a platinum wire no evidence of unconsumed carbon
is found. The moistening with solution of ammonium carbonate, should not
take place until the contents of the crucible are cool. Subsequent
ignition, at a low heat for a short time, will remove the excess of
ammonium salt.
=301. Method of Berthelot and André.=[194]—The earth dried at 110°
contains still a greater or less quantity of combined water. This is the
water united with alumina, silica and certain salts, but not the water
of constitution belonging to organic bodies. The exact estimation of
this water offers many difficulties. The determination of loss obtained
at a red heat embraces:
(1) The water combined with zeolitic silicates, with alumina and with
organic compounds.
(2) The water produced by the combustion of the organic compounds.
(3) The carbon dioxid resulting from the partial decomposition of the
calcium and magnesium carbonates.
(4) The carbon burned and the nitrogen lost during ignition. The measure
of the loss of weight in an earth heated to redness in contact with the
air is not therefore, an exact process of estimating water or even
volatile matters.
A better defined result is obtained in carefully burning a known weight
of earth either in a current of free oxygen, or with lead chromate. The
water produced in such a combustion is secured in a =ᥩ= tube filled with
pumice stone saturated with sulfuric acid, the carbon dioxid being
absorbed afterwards in potash bulbs and by solid potash. The weight of
earth burned is chosen so as to furnish a convenient weight of both
water and carbon dioxid. In general about five grams are sufficient.
When the combustion is made with oxygen, the soil is contained in a boat
and the products of the combustion are carried over a long column of
copper oxid heated to redness. The residue left in the boat is weighed
at the end of the operation, and in this residue it is advisable to
determine any undecomposed carbonate. Should the sample burn badly and
be mixed with carbonaceous matter at the end of the operation, it will
be necessary to substitute the lead chromate method. In this case, of
course, the residue left after combustion is not weighed. Whichever
method is employed gives a quantity of water originally combined with
the soil, plus the quantity arising from the combustion of the hydrogen
of the organic matter. The details of the processes for organic
combustion, will be given in a subsequent part of this manual. It is not
possible to divide the water between these two sources directly, but
this can be done by calculation, which gives results lying within the
limits of probability. The method follows:
The organic nitrogen, determined separately, by soda-lime, the method of
Kjeldahl, or volumetrically, is derived from proteid principles
resembling albuminoids containing about one-sixteenth of their weight of
nitrogen. The nitrates contained in the earth are in such feeble
proportion, as to be negligible in this calculation. The total weight of
these nitrogenous principles in the soil is therefore easily calculated.
The carbon contained in the proteids is then calculated on a basis of 53
per cent of their total weight, and the hydrogen on a basis of 7.2 per
cent. From the weight of the total organic carbon (determined as
described further on) is subtracted the carbon present in the proteids.
The remainder corresponds to the organic carbon present as
carbohydrates, (ligneous principles) containing 44.4 per cent carbon and
6.2 per cent hydrogen. By adding together the weight of the hydrogen
contained in the ligneous principles, and the hydrogen contained in the
proteids, and multiplying the sum by 9, the weight of water formed by
the combustion of all the organic matter in the sample is obtained. This
is subtracted from the weight of the total water obtained by direct
determination as described above. The difference represents the weight
of water combined with the silicates, etc., as well as with organic
matters.
=302. Method of Von Bemmelén.=[195]—According to the view of Von
Bemmelén, the soil contains colloidal humus and colloidal silicate,
which complicate the determination of water. The colloids retain water
in varying quantities, depending upon the following conditions:
(1) Upon their composition and state of molecular equilibrium.
(2) Upon the pressure of the aqueous vapor of the room.
(3) Upon the temperature.
At each degree of temperature, the quantity of absorbed water which a
colloid can retain in a room saturated with aqueous vapor, is different.
The quantity of water which air-dried earth gives off at 100°, has
therefore, no special significance unless all conditions are known.
In addition to the estimation of the quantity of water which soils, in
their natural condition, are capable of taking up and holding, at
ordinary temperatures, the estimation of the quantity of water which
they can take up in different temperatures in rooms saturated with
aqueous vapor should be of interest. It follows, therefore, that there
is no special value in data obtained by drying earth at 100° or 110°.
For the purpose of comparison, he prefers to select that point at which
the soil is dried over sulfuric acid, the point at which the tension of
the water vapor in the earth, at a temperature of plus or minus 15°,
approaches zero. The water which still remains in the earth under these
conditions is characterized as firmly combined water.
Von Bemmelén truly observes that only in soils which contain no
carbonates and no chlorids and sulfids, can the loss on ignition be
regarded as the sum of the humus and water content. By moistening with
ammonium carbonate, the correction for lime or carbon dioxid cannot be
correctly made as has been the custom up to the present time. In the
first place, ignited magnesia, when it has lost its carbon dioxid, does
not take this up completely on moistening with ammonium carbonate; in
the second place, reactions with the chlorids may take place; and in the
third place, the lime which is in the humus will be converted into
calcium carbonate. Chlorids on ignition may be volatilized or oxidized.
The sulfuric acid formed from the sulfids, on ignition, can expel carbon
dioxid; further than this the iron of pyrites takes up oxygen on
ignition. All these influences make the numbers obtained from loss on
ignition extremely variable.
With sea soils, Von Bemmelén has weighed the soil after the elementary
analysis and estimated, in addition to the carbon dioxid, both chlorin
and sulfuric acid therein. The comparison of these estimations with
those of CO₂, Cl, SO₃ and S, made in the original soil, gave the
necessary corrections; _viz._, for the increase in the weight through
oxidation of sulfur and iron, and for the decrease in weight through the
volatilization of sodium chlorid, sulfur, and carbon dioxid. A trace of
chlorin was evolved as ferric chlorid, nevertheless, the molecular
weight of sodium chlorid, 58.5, is scarcely different from the
equivalent quantity of ferric chlorid 54.1. For this reason the
estimation of loss of water, on ignition, of sea soils is less exact
than that of soils which are free from carbonates and sulfids and which,
as is usually the case with tillable soils, contain only small
quantities of chlorids and sulfates.
_The Strongly Combined Water._—Water which, at a temperature of plus or
minus 15°, in a dry room, still remains in the soil, is chiefly combined
according to Von Bemmelén with the colloidal bodies therein. Its
estimation, presents, naturally, difficulties and is not capable of any
great exactness. The quantity of strongly combined water, on the one
hand is determined from the difference between the loss on ignition and
the quantity of humus present, calculated from the content of carbon; on
the other hand, from the difference between the water obtained by
elementary analysis and the water which corresponds to the calculated
quantity of humus. If the hydrogen content of humus is correctly taken
and no appreciable error is introduced through the factor 1.724, both of
these differences must agree. On the other hand the hydrogen content of
the humus can be computed from the difference between the water found
and the calculated content of the firmly combined water.
The hydrogen content of humus bodies, dried at 100°, varies between four
and five per cent. Eggertz has found the content from 4.3 to 6.6 per
cent of hydrogen in thirteen soils which he first treated with dilute
hydrochloric acid then extracted with ammonia or potash lye and
precipitated this alkaline extract with acid. The method of applying
these principles to soil analysis is indicated in the following scheme:
A volcanic earth from Deli gave, on elementary analysis:
Per cent.
Carbon 2.94
Water 14.78
Nitrogen 0.28
Loss on ignition 17.54
FIRST CALCULATION.
Per cent.
Loss on ignition 17.54
Humus = carbon, 2.94 × 1.724 = 5.07
Difference = firmly combined water 12.47
Assuming that a humus dried over sulfuric acid contains five per cent of
hydrogen, the second calculation is made as follows.
SECOND CALCULATION.
5.00 humus × 5 per cent = 0.25 per cent of
hydrogen in humus corresponding to 2.28 per
cent of water.
Per cent.
Water found 14.79
Difference = firmly combined water 12.51
THIRD CALCULATION.
Per cent.
Firmly combined water 12.47
Water from the hydrogen in humus 2.28
Total water 14.75
Found 14.79
In this way, in three other volcanic earths and in an ordinary alluvial
clay from Rembang, there were found by analysis and by calculation the
following percentages of water:
1. 2. 3. 4. 5.
Percentage of water calculated 14.75 7.74 8.06 4.90 6.01
„ „ „ found 14.97 7.63 8.05 4.70 6.00
On the contrary, when the calculation is made from sea-slime taken from
under the water a higher content of hydrogen must be assumed; _viz._,
about six per cent. In two samples of sea-slime calculated in this way
the following numbers were obtained:
Percentage of water calculated 8.61 3.71
„ „ „ found 8.53 3.57
It is, therefore, quite evident that the organic compounds of soil taken
from under the sea-water are richer in hydrogen than those exposed to
the air or in cultivation.
=303. General Conclusions.=—In the foregoing paragraphs have been
collected the most widely practiced methods of determining moisture in
soil in both a free and combined state. The following conclusions may
serve to guide the analyst who endeavors to determine the water in any
or all of its conditions:
(1) In determining water in fresh samples the method of Whitney is
satisfactory. Although the samples taken by this method are small they
may be easily secured in great numbers over widely scattered areas, and
can be easily transported without change. These samples should be dried
at 100° to 110° for rapid work, or where time can be spared may be
air-dried.
(2) For a simple determination of the water left in the soil after
air-drying (hygroscopic water) the method of the Association of Official
Agricultural Chemists may be followed. There is much difference of
opinion in respect of the proper temperature at which this moisture is
to be determined. Much here depends on the nature of the soil. An almost
purely mineral soil may safely be dried at 140° or 150°. A peaty soil,
on the contrary, should not be exposed to a temperature above 100°. For
general purposes the temperature chosen by the official chemists is to
be recommended.
(3) Water of composition can only be determined by ignition. As has been
fully shown, this process not only eliminates the water, but also
destroys organic matter, decomposes carbonates and sulfids, and, to some
extent, chlorids. Subsequent repeated treatment with ammonium carbonate
may restore the loss due to carbon dioxid, but in many cases not
entirely. The water which comes from organic matter may be approximately
calculated from the humus content of the sample, but as will be seen
further on the methods of estimating humus itself are only approximate.
Nevertheless, in distributing the losses on ignition properly to the
several compounds of the soil there is no better method now known than
that of taking into consideration the humus content and carbonates
present. The principles of procedure established by Berthelot and André,
and Von Bemmelén, are to be applied in all such cases, modified as
circumstances may arise according to the judgment of the analyst.
=304. Estimation of the Organic Matter of the Soil.=—The organic matter
in the soil may be divided into two classes. First, the undecayed roots
and other remains of plant and animal life, and the living organisms
existing in the soil. The study of the organisms which are active in the
condition of plant growth will be the subject of a special chapter.
Second, the decayed or partially decayed remnants of organic matter in
the soil known as humus. Such matter may be present in only minute
traces, as in barren sand soils, or it may form the great mass of the
soil under examination, as in the case of peat, muck, and vegetable
mold. It is with the investigation of the second class of matter that
the analyst has chiefly to do at present. The problems which are to be
elucidated by the analytical study of such bodies are the following: (1)
The total quantity of such matter in the soil. (2) The determination of
the organic carbon and hydrogen therein. (3) The determination of total
nitrogen. (4) The determination of the availability of the nitrogen for
plant growth. (5) The estimation of the humic bodies (humus, humic acid,
ulmic acid, etc.).
The importance of humus in the promotion of plant growth is sufficient
excuse for the somewhat extended study of the principles which underlie
the analytical methods, and the methods themselves, which follow.
=305. Total Quantity of Organic Matter.=—The total approximate quantity
of organic matter in the soil can be determined by simple ignition, in
the manner noted in paragraphs =294= and =295=. The proper correction
for free and combined water being applied by the further copper oxid or
lead chromate combustion of the sample, and for carbonates and volatile
chlorids, the approximate total of the organic matter of all kinds is
obtained.
=306. Estimation of the Organic Carbon.=—To estimate the organic carbon
in an earth the sample may be burned in a current of oxygen, or after
mixing with lead chromate.
_In a Current of Oxygen._—When burned in a current of oxygen the sample
is held in a boat and the gases arising from the combustion directed
over copper oxid at a red heat. The carbon thus disappears as carbon
dioxid and is absorbed and weighed in the usual way.
_With Lead Chromate._—The lead chromate employed should be previously
tested since it often contains other compounds, especially lead acetate
and nitrate, furnishing in the one case both carbon dioxid and water,
and in the other hyponitric acid.
From two to ten grams of earth are employed, according to its richness
in organic matter. The total carbon dioxid is obtained in this process
both from carbonates and organic bodies. The water and carbon dioxid are
secured and weighed in the usual manner.
The oxygen method should be used in all cases possible. Although it does
not always give the whole of the carbon dioxid present as carbonates,
the rest can be easily estimated by treating the residue in the boat
with hydrochloric acid, in an apparatus for estimating that gas.
_Calculation of Results._—The whole of the carbon dioxid is determined
either by direct combustion with lead chromate, or by taking the sum of
the amounts by burning in a stream of oxygen and treating the residue in
a carbon dioxid apparatus.
The carbon dioxid contained in the original carbonates should be
determined by direct treatment of the sample in the usual way.
The carbon in organic compounds is determined by subtracting the carbon
present as carbonates from the total.
From the organic carbon contained in the soil the humus is calculated by
Wolff on the supposition that it contains fifty-eight per cent of
carbon. It is, therefore, only necessary to multiply the percentage of
carbon found by 1.724, or the carbon dioxid found by 0.471, to determine
the quantity of humus in the dried soil.
=307. Details of the Direct Estimation of Carbon in Soils by Various
Methods.=—(1) _Oxidation by Chromic Acid._—The method of Wolff by
oxidation with chromic acid has been worked out in detail by Warington
and Peake.[196] It consists in treating the soil with sulfuric acid and
potassium bichromate, or by preference with a mixture of sulfuric and
chromic acids, the carbon dioxid evolved being estimated in the usual
way. This method is recommended by Fresenius as an alternative to a
combustion of the soil with copper oxid or lead chromate. It is
apparently the method which has been most generally employed in
agricultural investigations.
Ten grams of the finely powdered soil are placed in a flask of about 250
cubic centimeters capacity, provided with a caoutchouc stopper, through
which pass two tubes, one for the supply of liquids, the other for the
delivery of gas. The soil is treated with twenty cubic centimeters of
water and thirty cubic centimeters of oil of vitriol; and the whole,
after being thoroughly mixed, is heated for a short time in a
water-bath, the object in view being the decomposition of any carbonates
existing in the soil. Air is next drawn through the flask to remove any
carbon dioxid which has been evolved. The stopper is next removed, and
coarsely powdered potassium bichromate introduced. In the case of a soil
containing three per cent of carbon, six grams of bichromate will be
found sufficient, a portion remaining undissolved at the end of the
experiment. The stopper is then replaced, its supply-tube closed by a
clamp, and the delivery-tube connected with a series of absorbents
contained in =ᥩ= tubes. The first of these tubes contains solid calcium
chlorid; the second, fragments of glass moistened with oil of vitriol;
the third and fourth are nearly filled with soda-lime, a little calcium
chlorid being placed on the top of the soda-lime at each extremity. The
last named tubes are for the absorption of carbon dioxid, and have been
previously weighed. The series is closed by a guard-tube containing
soda-lime, with calcium chlorid at the two ends.
The flask containing the soil and bichromate is now gradually heated in
a water-bath, the contents of the flask being from time to time mixed by
agitation. A brisk reaction occurs, carbon dioxid being evolved in
proportion as the soil is rich in organic matter. The temperature of the
water-bath is slowly raised to boiling as the action becomes weaker, and
is maintained at that point till all action ceases. As bubbles of gas
are slowly evolved for some time, it has been usual in these experiments
to prolong the digestion for four or five hours. When the operation is
concluded the source of heat is removed, an aspirator is attached to the
guard-tube at the end of the absorbent vessels, and air freed from
carbon dioxid is drawn through the flask and through the whole series of
=ᥩ= tubes. The =ᥩ= tubes filled with soda-lime are finally weighed, the
increase in weight showing the amount of carbon dioxid produced. The
object of the calcium chlorid placed on the surface of the soda-lime is
to retain the water which is freely given up when the soda-lime absorbs
carbon dioxid. The second =ᥩ= tube filled with soda-lime does not gain
in weight till the first is nearly saturated; it thus serves to indicate
when the first tube requires refilling. The same tubes may be used
several times in succession.
No increase in the carbon dioxid evolved is obtained by substituting
chromic acid for potassium bichromate.
The organic matter of the soil appears to the eye to be completely
destroyed by the digestion with sulfuric acid and potassium bichromate;
the residue of soil remaining in the flask when washed with water is
perfectly white, or the dark particles, if any, are found to be
unaltered by ignition, and therefore to be inorganic in their nature.
Under these circumstances considerable confidence has naturally been
felt in this method. The complete destruction of the humic matter of the
soil does not, however, necessarily imply that the carbon has been
entirely converted into carbon dioxid as has been pointed out by
Wanklyn. According to his demonstration of the action of chromic acid on
organic matter the oxidation frequently stops short of the production of
carbon dioxid. While oxidation with chromic acid apparently leads to a
complete reaction when the carbon is in the form of graphite, it would
probably yield other products than carbon dioxid when the carbon exists
as a carbohydrate. The doubt thus raised as to the correctness of the
results yielded by the chromate method makes it desirable to check the
work by the use of other methods for the determination of carbon. For
this purpose Warington and Peake recommend:
(2) _Oxidation with Potassium Permanganate._—In the trials with this
method ten grams of soil are digested in a closed flask with a measured
quantity of solution of caustic potash containing five grams of potash
for each twenty cubic centimeters, and crystals of potassium
permanganate. Seven grams of the permanganate are found to be sufficient
for a soil containing 3.3 per cent of carbon. The flask is heated for
half an hour in boiling water, and then for one hour in a salt-bath. The
flask during this digestion is connected with a small receiver
containing a little potash solution, to preserve an atmosphere free from
carbon dioxid; distillation to a limited extent is allowed during the
digestion in the salt-bath.
The first part of the operation being completed a rubber stopper,
carrying a delivery and supply-tube, is fitted to the flask, which is
then connected with the system of =ᥩ= tubes already described. Dilute
sulfuric acid is then poured down the supply-tube, a water-bath
surrounding the flask is brought to boiling, and maintained thus for one
hour, after which air, free from carbon dioxid, is drawn through the
apparatus, the =ᥩ= tubes containing soda-lime being finally disconnected
and weighed.
In the first stage of this method the carbon of the organic matter is
converted into carbonate, and probably also into potassium oxalate.[197]
In the second stage the oxalate is decomposed by the sulfuric acid and
permanganate, and the carbon existing, both as oxalate and carbonate, is
evolved as carbon dioxid, and absorbed by the weighed soda-lime tubes.
Both F. Schulze and Wanklyn have employed potassium permanganate for the
determination of organic carbon, but they have preferred to calculate
the amount of carbon from the quantity of permanganate consumed, as,
however, by so doing everything oxidizable by permanganate is reckoned
as carbon, it seems better to make a direct determination of the carbon
dioxid formed.
From the amount of carbon dioxid found, is to be subtracted that
existing as carbonates in the soil, and in the solution of potash used.
For this purpose an experiment is made with the same quantities of soil
and potash previously employed, but without permanganate, and the carbon
dioxid obtained is deducted from that yielded in the experiment with
permanganate. If the potash used contains organic matter two blank
experiments will be necessary, one with potash and permanganate, and one
with soil alone.
A further difficulty arises from the presence of chlorids in the
materials, which occasions an evolution of free chlorin when the
permanganate solution is heated with sulfuric acid. This error occurs
also with the chromic acid method, but in that case the quantity of
chlorid is merely that contained in the soil, which is usually very
small; in the permanganate method we have also the chlorid present in
the caustic potash, and this is often considerable. Corrections for
chlorin by blank experiments are unsatisfactory, the amount of chlorin
which reaches the soda-lime tubes depending in part on the degree to
which the calcium chlorid tube has become saturated with chlorin. It is
better therefore to remove the chlorin in every experiment by the plan
which Perkin has suggested, by inserting a tube containing silver foil,
maintained at a low red heat, between the flask and the absorbent =ᥩ=
tubes.
The amount of carbon dioxid yielded by oxidation with potassium
permanganate is found to be considerably in excess of that obtained by
oxidation with chromic acid; to ascertain whether these higher results
really represented the whole of the carbon present in the soil, trials
were next made by actual combustion of the soil in oxygen.
(3) _Combustion in Oxygen._—The most convenient mode of carrying out the
combustion of soil is to place the soil in a platinum boat, and ignite
it in a current of oxygen in a combustion tube partly filled with cupric
oxid. A wide combustion tube is employed, about twenty inches long, and
drawn out at one end; the front of the tube is filled for eight inches
with coarse cupric oxid, the hind part is left empty to receive the
platinum boat. The drawn out end of the combustion tube is connected
with a series of absorbent =ᥩ= tubes, quite similar to those employed
for the estimation of carbon dioxid in the chromic acid method. Between
these absorbent vessels and the combustion tube is placed a three-bulbed
Geissler tube filled with oil of vitriol. The oil of vitriol is quite
effective in retaining nitrous fumes. The wide end of the combustion
tube is connected with a gas-holder of oxygen; the oxygen gas is made to
pass through a =ᥩ= tube of soda-lime before entering the combustion
tube, to remove any possible contamination of carbon dioxid.
In starting a combustion the part of the combustion tube containing the
cupric oxid is brought to a red heat, and oxygen is passed for some time
through the apparatus. Ten grams of soil, previously dried, are placed
in a large platinum boat, which is next introduced at the wide end of
the combustion tube. The combustion is conducted in the usual manner, a
current of oxygen being maintained throughout the whole operation. It is
very useful to terminate the whole series of absorbent vessels with a
glass tube dipping into water; the rate at which the gas is seen to
bubble, serves as a guide to the supply of oxygen from the gas-holder,
the consumption of oxygen varying, of course, with different soils, and
at different stages of the combustion. At the close of the combustion,
oxygen, or air freed from carbon dioxid, is passed for some time through
the apparatus to drive all carbon dioxid into the absorbent vessels. One
experiment can be followed by another as soon as the hind part of the
combustion tube has cooled sufficiently to admit a second platinum boat.
The same combustion tube can be employed for several days, if packed in
the usual manner in asbestos.
The presence of carbonates in the soil occasions some difficulty in
working the combustion method, as a part of this carbon dioxid will, of
course, be given up on ignition, and be reckoned as carbon. The simplest
mode of meeting this difficulty is to expel the carbon dioxid belonging
to the carbonates before the combustion commences. The method of
Manning; namely, treatment with a strong solution of sulfurous acid, may
be employed for this purpose. The ten grams of soil taken for combustion
are placed in a flat-bottomed basin, covered with a thin layer of
sulfurous acid, and frequently stirred. After a time the action is
assisted by a gentle heat. When the carbonates have been completely
decomposed the contents of the basin are evaporated to dryness on a
water-bath; the dry mass is then pulverized, and removed to the platinum
boat for combustion in oxygen. For the action of the sulfurous acid to
be complete it is essential that the carbonates should be in very fine
powder, since even chalk is but imperfectly attacked when present in
coarse particles.
=308. Comparison of Methods.=—A considerable number of soils analyzed by
the chromic acid method and by the combustion, method, by Warington and
Peake, with the assistance of Cathcart, shows the following comparisons:
PERCENTAGE OF CARBON FOUND BY TWO METHODS IN SOILS DRIED AT 100°.
Chromic acid method. Combustion method.
No. Kind of Exp. 1. Exp. 2. Mean. Exp. 1. Exp. 2. Mean. Per
soil. cent.
yielded
by
chromic
acid.
1. Old pasture 2.85 2.79 2.82 3.58 3.55 3.57 79.0
2. „ „ 2.83 2.79 2.81 3.57 3.53 3.55 79.1
3. „ „ 2.76 2.76 2.76 3.46 3.46 3.46 79.7
4. „ „ 2.74 2.76 2.75 3.37 3.38 3.38 81.4
5. „ „ 2.64 2.54 2.59 3.31 3.36 3.34 77.5
6. „ „ 2.51 2.43 2.47 3.15 3.15 3.15 78.4
7. „ „ 2.40 2.44 2.42 3.09 3.13 3.11 77.8
8. New pasture 1.92 1.93 1.93 2.41 2.40 2.41 80.1
9. „ „ 1.66 1.81 1.74 2.39 2.43 2.41 72.2
10. Arable soil 1.78 1.78 1.78 2.14 2.13 2.14 83.2
11. „ „ 1.21 1.14 1.18 1.40 1.43 1.42 83.1
12. Subsoil 0.28 0.27 0.28 0.37 0.38 0.38 73.7
Of the above soils the arable soils, Nos. 10 and 11, were the only ones
containing carbonates in any quantity exceeding a minute trace. The two
soils in question were treated with sulfurous acid before combustion,
the others not.
All the determinations by the chromic acid method were made by Mr. P. H.
Cathcart, with the exception of Nos. 9 and 12, which were executed by
another experimenter, and are seen to give distinctly lower results.
Excluding these two analyses the relation of the carbon found by the two
methods is tolerably constant, the average being 79.9 of carbon found by
oxidation with chromic acid for 100 yielded by combustion in oxygen. The
results obtained by the chromic acid method thus appear to be very
considerably below the truth.
Four typical soils were analyzed by the permanganate, as well as by the
chromic acid and combustion methods. The results obtained were as
follows:
PERCENTAGE OF CARBON FOUND BY THREE METHODS IN SOILS DRIED AT 100°.
Permanganate method.
Kind of Chromic Yielded by
soil. acid permanganate
Combustion method. if carbon by
method. Mean. Exp. 1. Exp. 2. Mean. combustion =
Mean. Per Per Per Per Per 100. Per
cent. cent. cent. cent. cent. cent.
Old pasture 3.55 2.81 3.26 3.30 3.28 92.4
New pasture 2.41 1.93 2.29 2.30 2.30 95.4
Arable soil 1.42 1.18 1.28 1.33 1.31 92.3
Subsoil 0.38 0.28 0.34 0.34 0.34 89.5
Oxidation by permanganate thus gives a much higher result than oxidation
with chromic acid; but even the permanganate fails to convert the whole
of the carbon into carbon dioxid, the product with permanganate being on
an average of the four soils 92.4 per cent of that yielded by combustion
in oxygen.
Wanklyn states that a temperature of 160°–180° is necessary in some
cases to effect complete oxidation with permanganate and caustic potash.
Such a temperature is found impracticable when dealing with soil, from
the action of the potash on the silicates present; hence possibly the
low results obtained.
Combustion in oxygen appears from these experiments to be the most
satisfactory method for determining carbon in soil, nor is this method,
on the whole, longer or more troublesome than the other methods
investigated.
Warington and Peake have further determined the loss on ignition of the
four soils mentioned above, with the view of comparing this loss with
the amount of organic matter calculated from the carbon actually
present. In making this calculation they have taken as the amount of
carbon in the soil, that found by combustion in oxygen, and have assumed
with Schulze, Wolff, and Fresenius, that fifty-eight per cent of carbon
will be present in the organic matter of soils. The four soils were
heated successively at 100°, 120°, and 150°, till they ceased to lose
weight; the loss on ignition in each of these stages of dryness is shown
in the following table:
PERCENTAGE LOSS ON IGNITION COMPARED WITH ORGANIC MATTER CALCULATED
FROM CARBON.
Organic
matter at
fifty-eight
Between 100° Between 120° Between 150° per cent
and ignition. and ignition. and ignition. carbon.
Kind of soil. Per cent. Per cent. Per cent. Per cent.
Old pasture 9.27 9.06 8.50 6.12
New pasture 7.07 6.88 6.55 4.16
Arable soil 5.95 5.70 5.61 2.44
Clay subsoil 5.82 5.39 4.76 0.65
The loss on ignition is seen to be in all cases very considerably in
excess of the organic matter calculated from the carbon, even when the
soil has been dried at as high a temperature as 150°. The error of the
ignition method is least in soils rich in organic matter, as, for
instance, the old pasture soil in the above table. The error reaches its
maximum in the case of the clay subsoil, which contains very little
carbonaceous matter, but is naturally rich in hydrated silicates, which
part with their water only at a very high temperature.
The above methods of Warington and Peake have been given in detail, and
in almost the verbiage of the authors for the reason that the working
directions are clearly set forth, and may serve, therefore, as guides to
the previous methods where only general indications of manipulation have
been given.
=309. Estimation of Organic Hydrogen.=—The estimation of the total
hydrogen is made without difficulty either by burning the sample in a
current of oxygen or with lead chromate, and weighing the water
produced. This water comes from two sources, the pre-existing water and
organic hydrogen. There is no direct method of distinguishing one from
the other. They may, however, be estimated indirectly. The method of
calculating the organic hydrogen has already been given (paragraph
=299=). Experience shows that the hydrogen thus calculated is a little
greater than is necessary to form water with the whole of the oxygen
found in the organic matters.
=310. Estimation of Organic Oxygen.=—The determination of this oxygen
cannot be made directly. It is obtained by calculation, according to
Berthelot and André,[198] from the oxygen in the proteid and ligneous
matters.
Let p represent the weight of the proteid bodies in a sample of soil.
Then O = (p × 33.5)/(100)
Let p′ = weight of ligneous bodies.
Then O′ = (p′ × 49.4)/(100)
The total oxygen = O + O′.
An approximate result is thus obtained, very useful to have when account
is taken of the oxidizing processes which go on in the soil during
agricultural operations.
=311. Estimation of Humus (Matière Noire).=—The original method of
determining this substance is due to Grandeau.[199] It is carried on as
follows: Ten grams of the fine earth are mixed with coarse sand
previously washed with acids and ignited. The mixture is placed in a
small funnel, the bottom of which is filled with fragments of glass or
porcelain. The mass is moistened with ammonia diluted with an equal
volume of distilled water, and allowed to digest for three or four
hours. The ammonia dissolves the dark matter without attacking the
silica. The ammoniacal solution is displaced by treating the mass with
pure water, or water to which some ammonia has been added, and the whole
of the dark matter is thus obtained in a volume of twenty to fifty cubic
centimeters of filtrate. The filtrate is evaporated to dryness in a
weighed platinum dish, and the weight of residue is determined and the
percentage of _matière noire_ calculated therefrom. The residue is
incinerated, and when in sufficient quantity the phosphoric acid is
determined in the ash. In soils poor in humus a larger quantity than ten
grams may be taken. If the soil be previously treated with hydrochloric
acid, Grandeau recommends that the phosphoric acid be determined always
in the ash of the dark matter.
The method has undergone various modifications and, as given by Hilgard,
is now practiced as follows:
About ten grams of soil are weighed into a prepared filter. The soil
should be covered with a piece of paper (a filter) so as to prevent it
from packing when solvents are poured on it. It is now treated with
hydrochloric acid from five-tenths per cent to one per cent strong
(twenty-five and one-third cubic centimeters of strong acid and 808
cubic centimeters of water) to dissolve the lime and magnesia which
prevent the humus from dissolving in the ammonia. Treat with the acid
until there is no reaction for lime; then wash out the acid with water
to neutral reaction. Dissolve the humus with weak ammonia water,
prepared by diluting common saturated ammonia water (178 cubic
centimeters of ammonia to 422 cubic centimeters of water). Evaporate the
humus solution to dryness in a weighed platinum dish at 100°; weigh,
then ignite; the loss of weight gives the weight of humus.
The residue from ignition is carbonated with carbon dioxid, heated and
weighed, thus giving the ash. It is then moistened with nitric acid and
evaporated to dryness. The residue is treated with nitric acid and
water, allowed to stand a few hours, and the solution filtered from the
insoluble residue, which is ignited and weighed, giving the silica.
The soluble phosphoric acid is determined in the solution by the usual
method, as magnesium pyrophosphate. It usually amounts to a fraction,
varying from one-half to as little as one-tenth of the total in the
soil. While the phosphoric acid so determined is manifestly more soluble
and more available to vegetation than the rest of that found by
extraction with stronger acid, it is clearly not as available as that
which, when introduced in the form of superphosphates, exerts such
striking effects even though forming a much smaller percentage of the
whole soil. Nevertheless, very striking agreement with actual practice
is often found in making this determination.
The estimation of humus by combustion, in any form, of the total organic
matter in the soil, gives results varying according to the season, and
having no direct relation to the active humus of the soil. The same
objection lies against extraction with strong caustic lye.
=312. Modification of Grandeau’s Method for Determining Humus in
Soils.=—According to Huston and McBride[200] the function of the
vegetable matter in the soil has long been a matter of contention among
those interested in the science of agriculture. Two factors have
contributed to the uncertainty existing in this matter: First, the very
complex and varying nature of the compounds resulting from the
decomposition of vegetable matter in the soils; and second, the lack of
uniformity in the methods of determining either the total amount of
organic matter present in a soil, or the amount that has been so far
decomposed as to be of any immediate agricultural value. Prominent among
these methods are the methods in which a combustion is resorted to, the
substance being either burned in air or in a combustion tube with some
agent supplying oxygen. The loss on ignition is no measure of the amount
of organic matter present since it is practically impossible to remove
all the water from the soil previous to ignition, and neither of the
methods gives information regarding the extent of the decomposition of
the organic matter. Pure cellulose and the black matter of a fertile
soil are of very different agricultural value.
Determinations of carbon in soils by oxidation with chromic and sulfuric
acid, and with alkaline permanganate have been used. The method with
alkaline permanganate agrees fairly well with combustion with copper
oxid or lead chromate, but the chromic sulfuric acid method gives only
about eighty per cent of the carbon found by combustion processes.
However valuable these processes may be for determining the total carbon
in the soil, they furnish no information regarding the condition of the
carbonaceous soil constituents, and as the determination is really one
of carbon, the organic matter must be calculated by using an arbitrary
factor. Generally the organic matter of the soil is considered to have
fifty-eight per cent carbon; yet different values are given from forty
to seventy-two per cent.
There is a general opinion that the black or dark brown material of the
soil, resulting from the decay of vegetable matter, has a much higher
agricultural value than the undecomposed vegetable matter. No very sharp
dividing line can be drawn, for changes in the soil are continually
going on, and material may be found in almost every stage between pure
cellulose and carbon dioxid. The character of the intermediate products
will vary according to the conditions of tillage and the supply of air
and water.
For agricultural purposes some means of determining the amount of
decomposed matter is very desirable. Several solvents have been tried
for this purpose. The earlier attempts were made by treating the soil
with successive quantities of boiling half-saturated solution of sodium
carbonate until the soil appeared to yield no more coloring matter to
the solvent. The solutions were then united, rendered acid with HCl,
which precipitated the humic acid, which was then washed, dried, and
weighed. This was considered the more soluble portion of the humic acid.
The soil was afterward treated with caustic potash solution in the same
manner, and the humus thus extracted was called insoluble humus. This
last process was really more in the nature of manufacturing humus, for
sawdust treated with caustic potash yields humic acid, and the inert
organic matter in the soil was decomposed to some extent by the caustic
alkali. Neither of the processes provided for the separation of the
humic acid from the lime, magnesia, alumina, and iron with which it is
usually combined in the soil.
In case results of different workers are to be compared, it is of the
greatest importance that methods should be used that are of such a
nature that errors resulting from difference of manipulation, and from
difficulty of reproducing duplicate work can be reduced to a minimum.
Hence, a simple modification of the Grandeau method has been tried which
has the advantage of keeping a definite amount of the soil in contact
with a definite volume of ammonia for a fixed time, the strength of the
ammonia remaining constant.
The process is as follows: The soil is washed with acid and water as
usual. It is then washed into a 500 cubic centimeter cylinder with
ammonia, the cylinder closed and well shaken and allowed to remain for a
definite time, usually thirty-six hours. The material is shaken at
regular intervals. The cylinder is left inclined as much as possible
without having the fluid touch the glass stopper, thus allowing the soil
to settle on the side of the cylinder and exposing a very large surface
to the action of the ammonia. During the last twelve hours the cylinder
is placed in a vertical position to allow the soil to settle well before
taking out the aliquot part of the solution.
The process of washing the soil with hydrochloric acid, water and
ammonia, is very tedious when performed in the usual way with the
wash-bottle. A simple automatic washing apparatus was devised by which a
fixed volume of the washing fluid can be delivered at regular intervals,
giving ample time for the thorough draining between each addition of the
fluid, and requiring no attention. By this apparatus work can be
continued day and night. Instead of washing on the usual form of filter
paper in funnels, it is preferable with this apparatus to hold the soils
on a disk of filter paper resting on a perforated porcelain disk in the
bottom of the funnel. This removes the necessity of washing out the
filter papers, does not permit of the accumulation of humus on the edge
of the filter paper when the Grandeau process is used, and insures that
all the washing fluids pass through the soil and not around it. This
form of apparatus reduces the labor to a minimum and permits many
determinations to be carried on at once.
This form of apparatus was only lately devised and has only been used
long enough to test it and to show its advantages. The reported results
were obtained by the ordinary methods of washing.
In all the work reported, five grams were used, as the soils contained
so much humus that this amount gave enough humus for good work in the
final weighings. The results obtained so far appear in the following
tables:
TABLE I.
COMPARISON OF METHOD OF GRANDEAU WITH HUSTON’S MODIFICATION AND OF
INFLUENCE OF STRENGTH OF AMMONIA SOLUTION. TIME OF DIGESTION IN MODIFIED
METHOD THIRTY-SIX HOURS.
Two per cent Four per cent
NH₃. NH₃.
Grandeau. Huston. Grandeau. Huston.
1. Peat soil, 16.40 20.06
Bogus „ 13.98 20.80
„ „ 17.43
————— —————
Mean 15.94 20.43
2. Peat subsoil, 13.98 19.38
Bogus „ 13.85 20.30
————— —————
Mean 13.92 19.84
3. Peat soil, 9.05 15.60 14.71 21.24
Good „ 10.27 15.88 15.34 20.20
————— ————— ————— —————
Mean 9.61 15.74 15.03 20.72
4. Peat subsoil, 16.75 24.34
Good „ 18.60 23.52
————— —————
Mean 17.68 23.93
5. Black soil, A 3.90 6.90 (1.86) 7.42
„ „ „ (1.67) 6.98
„ „ B 3.88 7.00 4.42
„ „ „ 4.20
————— ————— ————— —————
Mean 3.99 6.95 (3.05) 7.20
„ 4.31
6. Clay loam, 1.86 4.20 2.40 4.26
West side, A 4.28
„ „ B 1.76 4.36 2.48 (3.40)
„ „ „ (3.10)
————— ————— ————— —————
Mean 1.81 4.28 2.44 (3.76)
„ 4.27
7. Clay loam, A 1.90 4.12 (1.60) (4.59)
Lysimeter soil, B 1.61 4.22 (1.41) (4.58)
„ „ C 1.80 4.12
„ „ D 1.95 4.04
„ „ E 1.92 3.85
„ „ F 1.95 4.08
„ „ G 1.90 3.93
„ „ H 1.90 3.80
————— ————— ————— —————
Mean 1.76 4.17 (1.80) (4.12)
„ 1.90 3.97
Seven per cent Eight per cent
NH₃. NH₃.
Grandeau. Huston. Grandeau. Huston.
1. Peat soil,
Bogus „
„ „
Mean
2. Peat subsoil,
Bogus „
Mean
3. Peat soil, 19.77 21.70 16.05 21.42
Good „ 19.85 21.90 15.40 21.80
————— ————— ————— —————
Mean 19.81 21.80 15.73 21.61
4. Peat subsoil,
Good „
Mean
5. Black soil, A
„ „ „
„ „ B
„ „ „
Mean
„
6. Clay loam, 2.14 4.02 1.85 4.12
West side, A
„ „ B 2.13 4.48 1.90 4.40
„ „ „
————— ————— ————— —————
Mean 2.14 4.25 1.88 4.26
„
7. Clay loam, A
Lysimeter soil, B
„ „ C
„ „ D
„ „ E
„ „ F
„ „ G
„ „ H
Mean
„
NOTE.—Numbers in parentheses indicate results, generally the earliest
ones, which the authors do not consider strictly comparable with the
rest of the work. They are given solely for the purpose of exhibiting
all the work that has been done to date. When a mean is included in
parentheses it indicates that it is calculated from all the results
obtained, including those not considered strictly comparable. Bogus is
a name given to a peaty soil which is very sterile.
TABLE II.
INFLUENCE OF TIME OF DIGESTION. FOUR PER CENT OF AMMONIA USED
THROUGHOUT. HUSTON’S METHOD.
Thirty-six Forty-eight Sixty-eight Ninety-eight
hours. hours. hours. hours.
Peat Soil, 21.24 22.28 24.04
Good „ 20.20 21.70 23.94
————— ————— —————
Mean 20.72 21.99 23.99
Clay loam, 4.28 4.00 4.40
„ 4.26 4.01 4.85
West side (3.40)
„ „ (3.05)
—————— ————— ————
Mean 4.27 4.01 4.63
TABLE III.
INFLUENCE OF TIME OF EXTRACTION. TIME, TEN DAYS. GRANDEAU’S METHOD,
FOUR PER CENT AMMONIA. PEAT SOIL.
A. B. Mean. Remarks.
Per Per Per
cent. cent. cent.
1st extraction, 750 cc 16.90 18.96 17.93
2nd „ 250 „ 2.80 2.38 2.59
3rd „ 250 „ 1.77 1.10 1.44
4th „ 250 „ 1.34 1.30 1.32 Stood over night.
5th „ 250 „ 0.89 0.85 0.87
6th „ 250 „ 1.41 1.65 1.53 Stood overnight.
7th „ 250 „ 2.10 1.80 1.95 Washed again with HCl for Ca.
Trace found. HCl washed out,
but trace of chlorids found in
ash. Probably HCl absorbed
from air as humus showed small
quantity of a white volatile
solid on evaporation.
8th „ 250 „ 0.67 0.65 0.66
9th „ 250 „ 0.57 0.50 0.53
———— ————— ————— —————
Total 2750 „ 28.45 29.19 28.82
=313. Summary of Results.=—1. The modified method gives much higher
results than the original method of Grandeau.
2. In the Grandeau method marked irregularities follow a change in the
strength of the ammonia solution. These differences in results bear no
relation to the strength of the solution used. They seem to be errors
due to the difficulty of securing uniform and complete washing of the
soil by the ammonia solution.
In the modified method the change in the strength of the ammonia
solution makes practically no difference in the amount of humus
extracted, except in the case of the peat soil where two per cent
ammonia failed to extract all the humus. But the results show no
considerable increase when the strength is increased to over four per
cent.
3. The factor of time has not been fully investigated, but the results
so far obtained indicate that the time exerts less influence in the
modified than in the Grandeau method.
4. Table III shows that considerable quantities of the peat soil are
still passing into solution in the Grandeau method at the end of ten
days. With ordinary soils this is not true; but in the case of soil No.
5, a black soil, the solutions were colored at the end of a week. On the
peat soil the modified method extracted from ten to fifty per cent more
than the Grandeau, and on the ordinary soil from two to three times as
much humus.
5. In comparing duplicate results by both methods it is found that with
soil No. 3, peat soil, the following differences appear calculated to
percentage of the total amount involved in the determination:
Per cent. Per cent. Per cent. Per cent.
Strength of ammonia 2. 4. 7. 8.
Modified 1.7 5. 1.0 1.8
Grandeau 13.0 4.3 0.5 3.4
Special attention was paid to this point in case of soil No. 7, an
ordinary soil; taking all results into consideration the greatest
difference in percentage of total amount involved was, by the modified
method, nineteen per cent, and by the Grandeau, thirty per cent. In the
set of six special determinations made by both methods to test this
point and which are strictly comparable with each other, the maximum
range was by the modified method 7.8 per cent and by the Grandeau 8.3
per cent of the total amount involved in the determination. From which
it appears that the modified method is on the whole capable of yielding
rather more concordant results than the Grandeau.
=314. Estimation of Free Humic Acids.=—This process, due to Müntz[201]
is essentially that of Huston and McBride. Twenty grams of the soil are
reduced to a fine powder and saturated with fifty cubic centimeters of
concentrated ammonia and allowed to digest two or three days in a warm
place. The volume is then made up to one liter with water, well shaken,
and set aside for one day in order to permit the subsidence of the solid
matter. At the end of this time 500 cubic centimeters of the supernatant
liquor are taken and acidified with hydrochloric acid in order to
precipitate the humic bodies. The humus is collected on a filter, dried
and weighed. It is then ignited and the weight of ash deducted from the
first weight thus giving the actual weight of the humus obtained, free
from mineral matter. This process gives the free humic acids. By
previous treatment of the sample with hydrochloric acid as in the
process of Huston and McBride, the total humus is obtained. The
estimation of the free humic acids is of importance in determining the
quantity of lime or marl which should be added to acid lands.
=315. Humus Method of Von Bemmelén.=[202]—Von Bemmelén obtains the
content of humus by the multiplication of the content of carbon in the
soil by the factor of Wolff; _viz._, 1.724. The estimation of carbon,
water, and of the loss on ignition is conducted in combustion tubes in a
current of oxygen. The nitrogen estimation is carried on according to
the method of Dumas.
In soils containing calcium carbonate the carbon content is derived from
the carbon dioxid taken up by the potash bulbs during combustion (a);
from other carbonates not decomposed on ignition and which are
subsequently determined in the residue by treatment with hydrochloric
acid in a carbon dioxid apparatus, (b) and the total carbon dioxid
derived from the carbonates in the soil (c).
For each estimation from three to five grams of the soil are taken,
because with smaller quantities the errors of analysis too strongly
influence the results. The carbon is then calculated according to the
formula:
Carbon = ³⁄₁₁ (a + b − c).
_The Carbon Dioxid of Carbonates._—It is necessary to expel the carbon
dioxid at ordinary temperatures, because on heating to boiling, carbon
dioxid would be formed from the humus. In a flask, as small as possible,
the soil is treated at ordinary temperature, with dilute sulfuric or
citric acid, the escaping gas dried over sulfuric acid and taken up with
soda-lime. Behind the soda-lime is a small tube filled with pieces of
glass and moistened with sulfuric acid, which retains any moisture taken
out of the soda-lime. A stream of about one liter of air, free from
carbon dioxid, is sufficient to drive out all of the carbon dioxid when
the estimation is made at ordinary temperatures.
A volcanic earth from Deli, which contained five per cent of humus,
gave, at a temperature plus or minus 15°, 0.01 per cent CO₂. At boiling
temperature two analyses gave 0.54 and 0.56 CO₂. This soil contained no
carbonate, and the carbon dioxid found at the boiling temperature, must
have come from the humus substances under the influence of the dilute
acids.
A heavy clay containing 6.9 per cent of humus gave, at plus or minus
15°, 3.60 per cent CO₂; at 100° without boiling, it gave an additional
0.53 per cent, and with boiling an additional 0.11 per cent, or a total
of 4.24 per cent CO₂. A light clay containing 3.2 per cent of humus,
gave, at 15°, 5.09 per cent CO₂; at a boiling temperature an additional
0.43 per cent, and by continued boiling an additional 0.27 per cent.
=316. Estimation of Humus by the German Method.=—The German experiment
stations follow the method of Loges,[203] depending on the oxidation of
the humic bodies with copper oxid after evaporation of the sample with
phosphoric acid. The object of the preliminary evaporation is to set the
humic acids free in order that they may be better and more easily
oxidized than when burned in the combined state.
The sample of soil is placed in a Hoffmeister dish (Schälchen),
moistened with dilute phosphoric acid and evaporated to complete
dryness. The dish and its contents are rubbed up with pulverized copper
oxid and placed in a combustion tube of sixty centimeters in length,
open at both ends. There is then placed in the tube, and held in place
by asbestos plugs, granular copper oxid to a length of twenty
centimeters.
The combustion tube is placed in a proper furnace and one end connected
with two washing-flasks, the first containing potash lye, and the other
a solution of barium hydroxid. These flasks are to free the aspirated
air from carbon dioxid. The other end of the combustion tube is
connected with an appropriate apparatus for absorbing the carbon dioxid.
Loges recommends the Pettenkofer absorption tube and a Fresenius drying
cylinder.
Between the absorption apparatus and the aspirator, is also placed a
washing-flask containing barium hydroxid solution, serving to detect any
unabsorbed carbon dioxid. The layer of granular copper oxid is first
heated, the air being slowly aspirated through the apparatus meanwhile,
but not through the absorption bulbs. All the carbon dioxid is thus
removed from the apparatus.
The absorption system being connected, the tube is heated slowly from
the front, backwards, and after the tube is well heated a slow current
of air is drawn through and continued until the combustion is complete,
which is usually in about three-quarters of an hour.
After the tube is cool the powdered copper oxid and residue of
combustion are removed, and for this reason the tube is stopped with a
cork at both ends instead of being drawn out and sealed at one end. The
tube can thus be refilled without disturbing the granular layer of
copper oxid.
The drying cylinder used between the combustion tube and the absorption
system has its upper part filled with cotton to avoid the deleterious
effects of the nitric oxid produced in the combustion. With this
arrangement the use of metallic copper in the combustion tube to reduce
the nitric oxid can be dispensed with, the moist cotton holding back the
acid fumes. The per cent of humus is obtained by multiplying the per
cent of carbon found by 1.724.
=317. Method of Raulin for the Estimation of Humus.=[204]—The volumetric
estimation of humus in soil by a solution of potassium permanganate
would be convenient and practical if the combustion of the organic
matter were complete, and if the browning of the liquor did not render
the end of the reaction uncertain. The process of Schmidt, modified as
below, has given satisfactory results.
In a small flask, with flat bottom, containing about 250 cubic
centimeters, are introduced ten cubic centimeters of a solution of
manganese sulfate containing sixteen grams of the anhydrous salt per
liter, and ten cubic centimeters of a ten per cent solution of potassium
permanganate. The solution is heated for a few minutes, the liquor is
decolorized and manganese bronze is precipitated. One hundred cubic
centimeters of water are added, and four cubic centimeters of sulfuric
acid containing 150 cubic centimeters of monohydrated acid per liter.
There is now added an exactly measured volume of the humic liquid
properly prepared, so that in oxidizing completely it destroys at most
only half of the manganese dioxid. The mixture is submitted to gentle
ebullition for eight hours, the water being kept at a constant volume.
The excess of manganese dioxid remaining is dissolved hot by a measured
portion of decinormal oxalic acid in slight excess, and the excess of
oxalic acid is removed by a solution of potassium permanganate
containing one gram per liter. The volume of oxalic acid not destroyed
by manganese dioxid is calculated from the amount of permanganate
consumed. The volume of oxalic acid, which corresponds to the same
quantity of dioxid as the introduced humus, is also calculated by taking
the difference between the volume of oxalic acid necessary to destroy
all the dioxid formed by ten cubic centimeters of the ten per cent
permanganate solution, and the volume of the oxalic acid which has
destroyed the dioxid remaining after the action of the humus. The first
volume of oxalic acid, that is to say, that which destroys the dioxid
formed by ten cubic centimeters of ten per cent permanganate is
determined in a preliminary titration.
In regard to the humic liquor, it is prepared by treating ten grams of
earth with soda solution in the usual manner. It will be easy to
calculate the volume of the oxalic solution equivalent to the total
volume of the humic solution, of which a determined fraction has been
assayed, and consequently the volume of oxalic solution equivalent to
the humus in ten grams of the dry earth. This number of cubic
centimeters of the decinormal oxalic solution multiplied by 0.8 will
express in milligrams the weight of oxygen necessary to burn the humus
from ten grams of dry earth. Humus not being a definite compound, but a
residue of complex organic matters partially oxidized, it will require
as much more oxygen to complete the combustion as the previous oxidation
has been less pronounced. This weight of oxygen necessary to burn the
humus from ten grams of dry earth may serve to detect the total value as
well as the weight of the humus itself. However, if we wish to have
directly the weight of the humus, resource can be had to a table which,
without being rigorous, can be regarded as sufficiently exact when the
variability of the constitution of humus is taken into account.
Volume of decinormal oxalic acid Corresponding humus, directly
for ten grams of earth. determined.
Cubic centimeters. Milligrams.
50 80
100 150
200 280
300 400
400 510
500 610
600 705
700 790
800 885
900 975
1,000 1,060
1,200 1,225
1,400 1,390
1,600 1,560
1,800 1,720
2,000 1,890
2,500 2,315
3,000 2,735
3,500 3,170
4,000 3,605
4,500 4,035
5,000 4,460
5,500 4,890
6,000 5,310
6,500 5,745
=318. Pasturel’s Method.=—According to Pasturel[205] the process of
Raulin does not furnish figures that are rigorously exact only with soil
of which the humus contains forty-five per cent of carbon. When the
richness in organic carbon is less, the results of the estimation are
too high. Pasturel modifies the process as follows:
_Manganese Sulfate._—Dissolve sixteen grams of the pure anhydrous
manganese sulfate in distilled water and make the solution up to one
liter.
_Potassium Permanganate._—Make a solution of ten grams of potassium
permanganate in one liter of water; 100 cubic centimeters of the liquor
just mentioned are diluted to one liter and constitute the potassium
permanganate solution one to ten.
_Oxalic and Sulfuric Acids._—A solution of oxalic acid is prepared
containing 6.3 grams of the acid in one liter of water, and a dilute
solution of sulfuric acid, by dissolving 150 grams of the monohydrated
acid in one liter of water.
_Humus Solution._—The solution of humus is prepared by the following
process: Ten grams of fine earth are freed from all their carbonates by
dilute hydrochloric acid. After washing, the filter is broken and the
dirt is washed into a small flask. Not more than twenty or thirty cubic
centimeters of water should be employed for this purpose. Twenty cubic
centimeters of a liquor containing two grams of caustic soda are added,
and the flask is placed upon a sand-bath and maintained at a boiling
temperature for six hours. It is then diluted with water, filtered and
washed as long as the waters are colored. The liquor is treated with
dilute sulfuric acid until almost the whole of the soda is saturated. It
is indispensable, however, to maintain a slight alkalinity in order that
the organic matter may rest totally dissolved. The precipitation of
silica which is almost always produced is without inconvenience.
Afterward the volume is completed to 500 cubic centimeters and the humus
solution is then ready for use.
_Estimation of the Humus._—Ten cubic centimeters of the manganese
sulfate are placed in a flask and ten cubic centimeters of the
permanganate added, and the whole is then slightly heated, and afterward
100 cubic centimeters of water and four cubic centimeters of sulfuric
acid are added. The humic liquor is now introduced in such proportion
that the humus which it contains dissolves at the greatest, a half of
the precipitated manganese and the rest of the process is continued as
described by Raulin.
=319. Estimation of Carbonates in Arable Soil.=—The principle of the
determination depends on the liberation of the carbon dioxid from its
compounds in the soil by acting on them with strong acid, and the
desiccation, absorption, and weighing of the evolved gas. Any of the
ordinary forms of apparatus for estimating carbon dioxid may be used in
this determination.
The apparatus of Knorr[206] has been used with satisfaction for many
years in the laboratory of the Department of Agriculture.
[Illustration:
FIGURE 65.
KNORR’S APPARATUS FOR THE DETERMINATION OF CARBON DIOXID.
]
The apparatus consists of a flask A, Fig. 65, in which the carbon dioxid
in the soil is liberated. A condenser, D, fits by means of a
ground-glass joint into the neck of the flask in which the liberated
gas, together with any air or aqueous vapor which may be carried
forward, is cooled. This prevents any excess of vapor of water from
entering the absorbing bulbs, which could easily happen at the end of
the experiments when the contents of A are raised to the boiling point.
The bulb B contains the acid, usually hydrochloric, which is employed
for decomposing the carbonates. It is provided with a guard bulb-tube,
C, which serves to absorb any carbon dioxid which might enter the
apparatus with the air during aspiration at the close of the
determination. The carbon dioxid is dried in the bulb-tube, E, in oil of
vitriol, and absorbed in the potash solution in F. It is advisable to
aspirate a slow current of air through the apparatus by means of the
tube G during the whole of the operation. The quantity of the sample to
be taken depends on its richness in carbonates. Many soils are so poor
in carbonates as to render any attempt at exact determination nugatory.
On the other hand, a comparatively small sample of marls will be
sufficient. A preliminary qualitative test will indicate, in a general
way, the quantity of the sample to be taken. The sample of soil, five to
fifty grams, having been transferred to A, which should be perfectly
dry, is made into a batter with freshly boiled distilled water. When all
the parts of the apparatus are properly connected gas-tight, the cock
between B and A is slowly opened and the hydrochloric (nitric) acid in B
allowed to flow into A at such a rate as will secure a moderate
evolution of gas.
When the carbonate is entirely decomposed, a lamp is brought under A and
its contents gradually raised to the boiling point. The aspiration of
air, free from carbon dioxid, is meanwhile continued until all the
liberated gas has been absorbed in F. Usually about fifteen minutes will
be sufficient to accomplish this purpose.
=320. Bernard’s Calcimeter.=—For a rapid and approximately accurate
method of determining the amount of carbonate in the soil, estimated as
calcium carbonate, Bernard makes use of the well-known method of the
volumetric estimation of carbon dioxid. The sample to be examined should
not be powdered in any way. The sample in a natural state, but well
air-dried, is gently broken up by the fingers and passed through a sieve
having ten meshes to the centimeter. Of the fine earth thus obtained,
one gram is taken, for the determination. If the percentage of carbonate
in the soil exceeds fifty then only half a gram is taken.
[Illustration:
FIGURE 66.
BERNARD’S CALCIMETER.
]
The apparatus employed is one well known. The small erlenmeyer C is
fitted with a rubber stopper carrying an exit tube for the gas and a
small thermometer. This flask is connected by means of a rubber tube and
small glass tube to the measuring burette B. This burette is graduated
from 0 to 100 cubic centimeters. Below, by means of a rubber tube, it is
connected with the open bulb A, which, by means of a cord about its
neck, can be suspended by the hook as shown in the figure. The measuring
tube is filled with water through A until the level of the liquid in B
is slightly above the zero mark. Meanwhile the one gram of earth has
been placed in C, together with the tube D three-fourths filled with an
equal mixture of water and strong hydrochloric acid. The greatest care
must be taken that no part of the acid be spilled.
The rubber stopper is now forced into C until the level of the water in
B is just at the zero mark. Grasping C in the right hand and A in the
left, the operator inclines C until the contents of D are emptied.
Meanwhile as the gas is evolved, A is lowered at such a rate as to
always keep the level of the water in B and A on the same plane. In a
few moments the evolution of gas is complete, and the volume given off
is read at once without correction. This volume multiplied by 0.4 gives
the percentage of carbonate in the sample examined. It is understood
that the determination is made at ordinary temperatures; _viz._, 17° to
22°. Example:
One gram of a soil treated as above, gave of carbon dioxid
(uncorrected) 65 cubic centimeters. 65 × 0.4 = 26.00 = per cent
calcium carbonate in sample.
The above method is useful in the classification of soils and in
determining approximately the quantity of calcium carbonate which they
contain. The practical use of this method is of great value in
determining the character of fertilizer to be applied. It is well to
know the percentage of carbonate in selecting mineral fertilizers.
=321. Soils Deficient in Carbonates.=—When a soil contains but a small
quantity of carbonates, Müller[207] has called attention to the fact
that the carbon dioxid absorbed by the water in which the soil is rubbed
up may vitiate the result. Instead of water a titrated solution of
sodium carbonate is employed. The apparatus is composed of a flask
containing the mixture of the sodium carbonate and the soil on which the
hydrochloric acid is to act. The hydrochloric acid is contained in a
small tube, as in Scheibler’s apparatus. The gas is received in a rubber
tube 1.5 meters long and three to four millimeters interior diameter,
and connected with a burette, the open mouth of which dips into the
water of a cylinder of proper length. The volume of gas is read when the
burette is raised or lowered in the cylinder until the liquid within and
without stands at the same level.
During the action of the acid on the carbonates the flask is constantly
shaken.
Several readings of the volume of gas are made, the evolution flask
being vigorously shaken before each one. Finally, in order to allow for
the variations in temperature and pressure of the exterior air which may
take place between the beginning and the end of the reaction, a second
flask containing air is placed by the side of the evolution flask and
communicating with a narrow =ᥩ= tube half filled with water. Any
variations in the volume of the air in the flask will be shown by
variations in the height of the liquid in the two arms of the =ᥩ= tube,
and the volume of the variation can be easily determined by having the
=ᥩ= tube calibrated.
If now _a_ equals the volume per cent of carbon dioxid in the atmosphere
of the evolution flask at the end of the reaction, _v_ the volume of gas
disengaged, and V the volume of the atmosphere in the evolution flask,
the per cent of carbon dioxid contained in a given length of the rubber
tube will be equal to _a_/2. This arises from the fact that the first
gas which passes into the rubber tube is composed solely of air, while
the last contains a per cent of carbon dioxid. By reason of the shaking
of the flask the mean richness of the contents of the tube in carbon
dioxid, will be sensibly _a_/2.
From the above data the following equations are derived:
1. _v__a_/2 + V_a_ = _v_.
2. _a_ = _v_/(_v_/2 + V)
If the weight of the carbon dioxid dissolved in V′ cubic centimeters of
the liquid in the evolution flask be represented by _q_, the coefficient
of the solubility of pure carbon dioxid in this liquid will be,
according to the law of the solubility of a gas, equal to _k_ =
(_q_)/(V′_a_)
The volume of _k_ has been determined for various strengths of the
sodium carbonate solution, using five cubic centimeters of hydrochloric
acid containing 1.6 grams pure hydrochloric acid. For solutions
disengaging from five to fifty milligrams of carbon dioxid, the mean
value of _k_ was found to be 1.8 milligrams in the absence of calcium
chlorid. When calcium chlorid was present in quantities varying from
0.03 to 0.07 gram per cubic centimeter of liquid in the evolution flask,
the value of _k_ was 1.4 milligrams.
By adopting, according to circumstances, the one or the other of the
above numbers and multiplying it by V_a_, as determined by experiment,
results are obtained differing only 0.2 to 0.3 milligram from those
secured by direct weighing of the evolved gas.
Dietrich[208] has called attention to the necessity of adding the volume
of the dissolved gas to the measured volume in such determinations, and
this volume or weight is easily determined by the above formulas.
=322. Belgian Method.=—The method pursued at the Gembloux Station[209]
consists in taking from five to fifty grams of the sample of soil,
according to its content in carbonate, rubbing it up in a porcelain dish
with distilled water in order to make a thin paste. The mass is worked
to drive out all the air, the whole washed into a flask of 300 cubic
centimeters capacity, and the amount of carbon dioxid estimated by
setting free with an acid, and collecting the carbon dioxid evolved in
potash bulbs.
DIGESTION OF SOILS WITH SOLVENTS.
=323. General Considerations.=—There are two points in connection with
the determination of mineral matters in the soil which must always be
kept in view; _viz._, first, the estimation of the total quantities of
material in the soil, and second, the study of those materials which are
more easily brought into solution and thus made available for the food
of plants.
It is well understood that the soil particles do not give up entirely to
the plant the food materials which they contain. The practical value
therefore of an analysis of a soil depends more upon the exact
determination of the plant food available than upon its total quantity.
From a mineral and geological point of view, on the other hand, an idea
of the total composition of the soil is the object to be attained.
For the determination of the available plant food, various solvents have
been proposed, none of which, perhaps, imitates very accurately the
natural solvent action of organic life and moisture on the soil
materials. A description of the standard methods of preparing soil
extracts will be the subject of a few succeeding paragraphs.
=324. Estimation of the Quantity of Materials Soluble in
Water.=[210]—Five hundred grams of the air-dried soil are treated in a
flask with 1,500 cubic centimeters of water, less the quantity of water
already contained in the air-dried soil, which is volatile at 125°. The
mass is frequently shaken and, after seventy-two hours, 750 cubic
centimeters of the liquid filtered. The filtrate is evaporated to
dryness in a platinum dish, dried at 120° and weighed. This is then
incinerated and, after treatment with ammonium carbonate and gentle
ignition, is again weighed. The further examination of the residue for
acids and bases is made by some of the methods hereafter described.
=325. Treatment with Water Saturated with Carbon Dioxid.=—Two thousand
five hundred grams of the air-dried soil are treated with 8000 cubic
centimeters of distilled, and afterwards with 2000 cubic centimeters of
water, which have previously, at room temperature, been saturated with
carbon dioxid. The mixture is left in a closed flask for seven days,
frequently shaken, after which 7,500 cubic centimeters of the liquid are
filtered. The clear filtrate, after treatment with a little hydrochloric
acid and a few drops of nitric acid, is evaporated to dryness. After the
separation of the silica the traces of iron, alumina, lime, sulfuric
acid, magnesia, potash, and soda, are estimated in the liquid in the
manner hereinafter to be described. Phosphoric acid is always present in
such a case, in such small quantities as to make its estimation
unnecessary.
=326. Treatment with Water Containing Ammonium Chlorid.=—In the flask
containing the residue from the last experiment; _viz._, the soil with
2,500 cubic centimeters of liquid, are added 1,500 cubic centimeters of
water saturated with carbon dioxid, and 8,000 cubic centimeters of pure
water in which five grams of ammonium chlorid are dissolved. The mixture
is then left for seven days, with frequent shaking, and 7,500 cubic
centimeters of the liquid are then filtered, and the substances
dissolved, determined in the filtrate. In addition to the usual
quantities of lime and magnesia, from two to four times as much alkali
is dissolved by this treatment as is found in the solution from the
water containing carbon dioxid alone.
=327. Treatment with Water Containing Acetic Acid.=—The acetic acid
should be of such a strength that after it has fully acted on the soil
it should still contain twenty per cent of free acid. 1000 grams of the
soil dried at 100° are taken and the acid added in proper proportions
and treated in the manner to be described for determining the solvent
action of hydrochloric acid.
=328. Treatment with Citric Acid Solution.=—In ascertaining the
quantities of soil materials soluble in a solution of citric acid,
Dyer[211] recommends the use of a carefully prepared citric acid
solution. The digestion is carried on as follows: Place in a flask or
bottle, holding about three liters, 200 grams of air-dried soil and two
liters of distilled water, in which are dissolved twenty grams of pure
citric acid. The soil is left, at room temperature, in contact with the
one per cent acid for seven days, with thorough shaking several times a
day. At the end of the digestion the solution is filtered and 500 cubic
centimeters of the filtrate, corresponding to fifty grams of the soil,
are taken for analysis for each ingredient to be determined.
The digestion in citric acid is especially recommended by Dyer because
of its supposed near resemblance to the methods of solution of plant
food practiced by the rootlets of plants. It is evident, however, that
this process is in no sense an imitation of natural methods. The
solution is to be used exclusively for the estimation of potash and
phosphoric acid. Dyer concludes, from a comparison of the action of a
solution of citric acid on soils of known fertility, that when as little
as 0.01 per cent of phosphoric acid is dissolved from a soil by this
treatment it is justifiable to assume that it stands in immediate need
of phosphatic manure. The methods used by Dyer to determine the
phosphoric acid and potash in the citric acid solution will be given in
their appropriate place.
=329. Treatment with Hydrochloric Acid.=—The solutions of soils usually
subjected to chemical analysis are those obtained by long treatment with
hot mineral acids, among which the most common is hydrochloric.
It has long been assumed by soil analysts, perhaps not with justness,
that such treatment removed from the soil, all those elements of plant
food which could possibly be available for the needs of the growing
crop. In this connection, however, the analyst must not forget that
nature, in a series of years, with her own methods may easily accomplish
what he in five days, even with the help of a hot mineral acid, may not
be able to secure. Since, however, this method of solution has been so
long practiced it is not the place here to throw doubt on its
effectiveness without being able to suggest a better way. Of the mineral
acids available no one possesses solvent powers for soils in a higher
degree than hydrochloric. A somewhat detailed description will therefore
be given of the methods of its use.
=330. Strength of Acid to be Employed.=—The fact that hydrochloric acid
of nearly constant strength; _viz._, specific gravity 1.115, equivalent
to 22.9 per cent hydrochloric acid, may be obtained by distillation, led
Owen to use acid of this density in his classic work on soil analysis.
Hilgard has lately reviewed the conditions of constant strength in the
solvent with results confirming the statements of Owen.[212] He
evaporated on a steam-bath, to one-half its bulk, fifty cubic
centimeters of hydrochloric acid, specific gravity 1.116, obtained by
using the distillate from a stronger acid after rejecting the first
third. The same operation was conducted with similar acid diluted with
ten per cent of water. The acid used contained 22.96 per cent
hydrochloric acid. The residual acid contained 21.49 per cent
hydrochloric acid. These results lead Hilgard to believe that the
changes arising from evaporation in hydrochloric acid during soil
digestion are insignificant, compared with those due to its action on
the soluble matters, and that evaporation during digestion is effective
in maintaining a definite strength in the solvent. For this reason it is
contended that evaporation in a porcelain beaker covered by a
watch-glass is more effective in constancy of conditions than digestion
in a closed flask under pressure.
=331. Influence of Time of Digestion and Strength of Acid.=—Loughridge
has made an interesting study of the influence of the strength of acid
and time of digestion on the extraction of soils.[213] The method of
preparing the soil for the determination of the above points is as
follows:
The soil, having been passed through the appropriate number of sieves to
obtain the fine earth is pulverized with a wooden pestle and thoroughly
mixed. The hygroscopic moisture is determined, after exposing it in a
place saturated with vapor, in a layer not exceeding one millimeter in
thickness for twelve hours, and subsequently drying at 200° in a
paraffin-bath. Of this dried substance, from two to three grams are used
in the general analysis, the methods employed being in general those
adopted by Peter.[214]
The quantities of materials dissolved by acids of different densities
are shown below. The determinations were made by methods hereafter to be
described.
Specific gravity of
acid.
Ingredients. 1.00 1.115 1.160
Insoluble residue 71.88 70.53 74.15
Soluble silica 11.38 12.30 9.42
Potash 0.60 0.63 0.48
Soda 0.13 0.09 0.35
Lime 0.27 0.27 0.23
Magnesia 0.45 0.45 0.45
Manganese oxid 0.06 0.06 0.06
Ferric oxid 5.15 5.11 5.04
Alumina 6.84 8.09 6.22
Sulfuric acid 0.02 0.02 0.02
Volatile matter 3.14 3.14 3.14
—————— —————— —————
Total 100.02 100.69 99.29
Amount of soluble matter 24.00 27.02 22.27
„ „ „ bases 13.50 14.70 12.83
From the above table it is seen that the strongest acid exerts the least
soluble effect upon the substances present in the soil, while the
greatest degree of solution was obtained by the acid of 1.115 specific
gravity. This result indicates that while lime and magnesia are probably
present chiefly as carbonates, potash as well as alumina, and to some
extent lime, are present as silicates, and for that reason are not as
fully extracted by acid of low strength as by that of medium
concentration.
In regard to the influence of the time of digestion, the acid of
specific gravity 1.115 being used, the data obtained are given in the
following table:
Number of days digested.
Ingredients. 1. 3. 4. 5. 10.
Insoluble residue 76.97 72.66 71.86 70.53 71.79
Soluble silica 8.60 11.18 11.64 12.30 10.96
Potash 0.35 0.44 0.57 0.63 0.62
Soda 0.06 0.06 0.03 0.09 0.28
Lime 0.26 0.29 0.28 0.27 0.27
Magnesia 0.42 0.44 0.47 0.45 0.44
Manganese oxid 0.04 .06 0.06 0.06 0.06
Ferric oxid 4.77 5.01 5.43 5.11 4.85
Alumina 5.15 7.38 7.07 7.88 7.16
Phosphoric acid 0.21 0.21
Sulfuric acid 0.02 0.02 0.02 0.02 0.02
Volatile matter 3.14 3.14 3.14 3.14 3.14
————— —————— —————— —————— —————
Total 99.63 100.68 100.55 100.69 99.80
Amount of soluble matter 19.67 24.88 25.57 27.02 24.87
„ „ „ bases 11.05 13.68 13.91 14.49 13.68
From this table it appears that the amount of dissolved ingredients
increases up to the fifth day, the increase becoming, however, very slow
as that limit is approached. It is also found that the ingredients
offering the greatest resistance to this action are the same as those
whose amounts were sensibly affected by the strength of the acid;
namely, silica, potash, and alumina.
In regard to lime and magnesia, one day’s digestion not being sufficient
for full extraction, it is evident that they do not exist in the soil as
carbonates or hydric oxids only, as has been supposed, but also as
silicates. A comparison of the results of the five and ten days’
digestion shows that the solvent action of the acid has substantially
ceased at the end of five days, there being no further increase of the
amount of dissolved matter.
=332. Digestion Vessels.=—Hilgard prescribes that the digestion of the
sample of soil with acid be conducted in a small porcelain beaker
covered with a watch-glass.[215] Kedzie, however, prefers beakers of
bohemian glass, and shows that hydrochloric acid attacks the porcelain
with greater energy than the glass.[216] Platinum would be the ideal
material for the digestive vessels, but its great cost would exclude its
general use. In most cases it will be found that the error introduced
into the analysis by the use of porcelain or bohemian glass beakers is
quite small and not likely to affect the quantitative estimation of
soluble soil ingredients to any extent.
In this laboratory some comparative tests made by Mr. W. D. Bigelow have
shown that vessels of hard glass of special manufacture are less soluble
in hot hydrochloric acid of 1.115 specific gravity than porcelain, thus
confirming the observation of Kedzie. Following are the data showing the
weights of material dissolved in fifty hours:
Berlin porcelain 2.8 milligrams
Bohemian glass 1.7 „
Kaehler and Martini glass 1.2 „
In each case twenty-five cubic centimeters of the acid were used. The
vessels all had approximately a capacity of 200 cubic centimeters.
=333. Processes Employed—Hilgard’s Method.=—The sample of soil sifted
through a 0.5 millimeter mesh sieve and thoroughly air-dried, is
conveniently preserved in weighing tubes. The actual content of
hygroscopic and combined moisture may be previously made on a separate
sample of soil.
In determining the amount of material to be employed for the general
analysis regard must be had to the nature of the soil. This is necessary
because of the impracticability of handling successfully such large
precipitates of alumina as would result from the employment of as much
as five grams in the case of calcareous clay soils; while in the case of
very sandy soils even that quantity might require to be doubled in order
to obtain weighable amounts of certain ingredients. For soils in which
the insoluble portion ranges from sixty to eighty per cent, two and a
half to three grams are about the right measure for general analysis,
while for the phosphoric acid determination not less than three grams
should be employed in any case. It has been alleged that larger
quantities must be taken for analysis in order to secure average
results. It is difficult to see why this should be true for soils and
not for ores, in which the results affect directly the money value,
while in the case of soils the interpretation of results allows much
wider limits in the percentages. Correct sampling must be presupposed to
make any analysis useful; but with modern balances and methods it is
difficult to see why five grams should be employed instead of half that
amount, which in some cases is still too much for convenient
manipulation of certain precipitates.
The weighed quantity, usually of two to two and a half grams, is brought
into a small porcelain beaker, covered with a watch-glass, treated with
eight to ten times its bulk of hydrochloric acid of 1.115 specific
gravity, and two or three drops of nitric acid, and digested for five
days over the laboratory steam-bath. At the end of this time it is
evaporated to dryness, first on the water-bath and then on the
sand-bath. By this treatment all the silica set free is rendered
insoluble.
=334. Provisional Method of the Official Agricultural Chemists.=—Place
ten grams of the air-dried soil in a round bottom 150 to 200 cubic
centimeter bohemian flask, add 100 cubic centimeters of pure
hydrochloric acid of specific gravity 1.115, insert the stopper, wire it
securely, place in a steam-bath, and digest for thirty-six hours at the
temperature of boiling water. Pour the contents of the flask into a
small beaker, wash with distilled water, add the washings to the
contents of the beaker and filter through a washed filter. The residue
is the amount insoluble in hydrochloric acid. Add a few drops of nitric
acid to the filtrate, and evaporate to dryness on the water-bath; take
up with hot water and a few drops of hydrochloric acid, and again
evaporate to complete dryness. Take up as before, and filter into a
liter-flask, washing with hot water. Cool and make up to the mark. This
is solution A. The residue represents the silica originally dissolved.
In comparing the two preceding methods it is found that the former;
_viz._, digestion in flasks covered only with a watch-glass gives a
larger quantity of dissolved matter in five days than the digestion
under pressure does in thirty-six hours. In comparative tests in this
laboratory made by Mr. W. D. Bigelow the respective quantities of
soluble and insoluble matter obtained by the two methods in two soils
are as follows:
Soil No. 1. Soil No. 2.
Per cent. Per cent.
Method of Digestion. Insoluble. Soluble. Insoluble. Soluble.
Open flask 75.62 24.38 79.62 20.38
Closed flask 76.81 23.19 80.48 19.52
=335. The German Station Method.=—The method recommended by the German
Stations[217] is greatly different from that described above, both in
temperature and time of digestion. To one part of the soil are added two
parts by volume of a twenty-five per cent hydrochloric acid solution,
the quantity being increased to correspond to any excess of carbonates.
The mixture is left for forty-eight hours with frequent shaking. As an
alternate method, one part of soil is treated with two parts by volume
of ten per cent hydrochloric acid, and heated on the water-bath, with
frequent shaking, for three hours.
The soluble materials are determined in the filtrate by some of the
methods usually employed.
=336. The Gembloux Method.=—The method of making the acid extract of the
soil at the Gembloux Station does not differ greatly from some of those
already described.
The quantity of air-dried material taken is such that it may weigh
exactly 300 grams exclusive of the moisture which it contains. It is
dried at 150° for at least six hours. The drying is necessary in order
to obtain an extract in hydrochloric acid of exactly 1.18 specific
gravity. The dry earth is placed in a flask of two or three liters
capacity to which one liter of hydrochloric acid of 1.18 specific
gravity is added, being careful to take precautions to prevent frothing
if much carbonate be present. The acid is allowed to act for twenty-four
hours, it being frequently shaken meanwhile. After settling it is
decanted and filtered upon a double folded filter, the apex of which
rests upon a small funnel covered with a plain filter of strong paper.
Five hundred cubic centimeters of the filtrate are taken for the
estimation, and in this filtrate are estimated the silica, phosphoric
and sulfuric acids, potash, soda, iron, alumina, lime, and magnesia.
The filtrate is evaporated to dryness in a porcelain capsule, a few
drops of nitric acid added and the liquid kept well stirred. The residue
should be taken up with water, and if not perfectly bright a second and
even a third evaporation with nitric acid should take place, until all
the organic matter is destroyed, which will be indicated by the clear
yellow or reddish-yellow color of the liquid, caused by the iron oxid.
After the last evaporation the material is dried in a drying oven one
hour at 110°.
=337. Treatment with Cold Hydrochloric Acid.=—According to the digestion
method of Wolff[218] the soil sample is treated with cold concentrated
hydrochloric acid. The process is as follows:
Four hundred and fifty grams of the soil dried at 100° are placed in a
glass flask and treated with 1,500 cubic centimeters of hydrochloric
acid of 1.15 specific gravity, corresponding to thirty per cent of
gaseous hydrochloric acid. For every five per cent of calcium carbonate
which the soil may contain, an additional fifty cubic centimeters of
hydrochloric acid are added. With frequent stirring, the soil is left in
contact with the acid for forty-eight hours and then 1,000 cubic
centimeters of liquid, as clear as possible, are poured off, which
corresponds to 300 grams of the soil. After dilution with water it is
filtered and the filtrate treated with a few drops of nitric acid and
evaporated to dryness. After the separation of the silica the solution
is again made up with water to 1,000 cubic centimeters.
Two hundred cubic centimeters of this solution, corresponding to sixty
grams of the soil, are taken for the estimation of iron, alumina, lime,
manganese, and magnesia.
Four hundred cubic centimeters of the solution, corresponding to 120
grams of the soil, are left for the estimation of sulfuric acid and
alkalies. This method gives from five to six times less alkalies and a
much smaller quantity of iron than the treatment with hot acid. In the
use of hot acid, therefore, Wolff reduces the quantity of soil acted on
to 150 grams.
=338. Treatment with Nitric Acid.=—For the purpose of estimating
phosphoric acid Grandeau[219] directs that the soil be extracted with
nitric acid. For this purpose 100 grams of the air-dried fine earth are
placed in a bohemian flask and treated cautiously with nitric acid in
small quantities at a time. If the soil be calcareous in its nature it
should be previously moistened with water, and the acid so added as to
avoid undue effervescence, the flask being inclined during the
operation. Sufficient acid is added to strongly saturate the sample and
it is then digested on the sand-bath for two hours; or at least until
the organic matters are destroyed, which will be indicated by the
cessation of evolution of nitrous vapors. When the supernatant liquid
has become clear it is decanted. The residue is washed with distilled
water and separated on a filter, and washed until the wash-water is
colorless. The decanted portion is united with the filtrate and the
whole made up to a volume of one liter. The determinations are made in
portions of 200 cubic centimeters each.
=339. Digestion with Hydrofluoric and Sulfuric Acids.=—When a complete
disintegration of the siliceous substances in soils is desired as in
analysis in bulk, the decomposition is easily accomplished by digestion
with the above named acids in a platinum dish. The fine earth is
saturated with a concentrated aqueous solution of hydrofluoric acid to
which a few drops of sulfuric acid are added. It is then digested until
nearly dry. If any undecomposed particles remain, the treatment is
continued until complete decomposition is secured. The silica is thus
all volatilized as hydrofluosilicic acid and the bases pre-existing in
the soil are left as sulfates. This method of treatment is especially
recommended when it is desired to estimate the whole quantity of any of
the soil constituents with the exception of silica. The silica may,
however, be determined in the distillate. Instead of using the solution
of hydrofluoric acid, ammonium fluorid may be employed. In this process
the sample of earth reduced to an impalpable powder by grinding in an
agate mortar is mixed with four or five times its weight of the ammonium
fluorid in a platinum dish and thoroughly moistened with sulfuric acid
and allowed to stand at room temperature for several hours. It is then
gently heated until all fumes of hydrofluosilicic acid have been driven
off, but is not raised to a red heat. If any undecomposed particles
remain, the above treatment is repeated.
DETERMINATION OF THE QUANTITY OF DISSOLVED MATTER.
=340. Substances in Solution.=—By treatment with solvents as indicated
in the preceding paragraphs, greater or less quantities of the original
constituents of soil are brought into solution. The total quantity of
dissolved matters is determined by drying and weighing the insoluble
residue and the percentages of soluble and insoluble matters should be
noted; and each portion saved for further examination. In this country
the common practice of soil analysis is to digest the sample with
hydrochloric acid. The following paragraphs, therefore, will be devoted
to the general methods of determining the matters dissolved by that
treatment, leaving for later consideration the special methods of
analysis. The fundamental principle on which the treatment with
hydrochloric acid rests is based on the belief that such treatment
practically extracts from the soil all those elements which are likely
to become, immediately or in the near future, available for plant food.
=341. Provisional Methods of the Official Agricultural
Chemists.=[220]—(1) _The Analytical Operations_ are conducted with
solution A, paragraph =334=.
(2) _Ferric Oxid, Alumina, and Phosphoric Acid._—To 100 or 200 cubic
centimeters, according to the probable amount of iron present, of the
solution A, add ammonium hydroxid to alkaline reaction to precipitate
ferric and aluminum oxids and phosphates. Expel the excess of ammonia by
boiling, allow to settle, decant the clear solution through a filter;
add to the flask fifty cubic centimeters of hot distilled water, boil,
settle, and decant as before. After pouring off all the clear solution
possible, dissolve the residue with a few drops of warm hydrochloric
acid and add just enough ammonium hydroxid to precipitate the oxids.
Wash by decantation with fifty cubic centimeters of distilled water, and
then transfer all the precipitate to the filter and wash with hot
distilled water till the filtrate becomes free from chlorids. Save the
filtrate and washings which form solution B. Dry the filter and
precipitate at 110°, transfer the precipitate to a tared platinum
crucible, burn the filter and add the ash to the precipitate, heat the
whole red hot, cool in a desiccator, and weigh. The increase of weight,
minus the ash of filter and the phosphoric acid (found in a separate
process), represents the weight of the ferric and aluminum oxids.
(3) _Ferric Oxid._—Precipitate 100 cubic centimeters of solution A, as
under (2), except that only one precipitation is made; wash with hot
water; dissolve in dilute sulfuric acid; reduce with zinc and estimate
as ferrous oxid by a standard solution of potassium permanganate.
To prepare the potassium permanganate solution, dissolve 3.156 grams of
pure crystallized potassium permanganate in 1,000 cubic centimeters of
distilled water, and preserve in a glass-stoppered bottle, shielded from
the light. Standardize this solution with pure ferrous sulfate, ammonium
ferrous sulfate or oxalic acid.
(4) _Alumina._—The calculated weight of ferric oxid deducted from that
of ferric oxid and alumina with corrections for filter ash and
phosphoric acid, will give the weight of alumina in two grams of
air-dried soil.
(5) _Phosphoric Acid._—This may be estimated in the above iron solution,
if the soil is sufficiently rich, by the molybdate method, given under
fertilizers; or if the quantity of soil represented in the iron solution
is not sufficient, a fresh portion of solution A may be taken, and the
phosphoric acid determined directly by the molybdate method.
(6) _Manganese._—Concentrate the filtrate and washings (solution B) to
200 cubic centimeters or less; add ammonium hydroxid to alkalinity; add
bromin water and heat to boiling, keeping the beaker covered with a
watch-glass; as the bromin escapes, the beaker is allowed to cool
somewhat, ammonia and bromin water again added, and heated as before.
This process is continued until the manganese is completely
precipitated, which requires from thirty to sixty minutes, and the
solution filtered while still warm; the precipitate is washed, dried,
ignited and weighed; estimate as manganese protosesquioxid.
(7) _Lime._—If no manganese is precipitated, add to solution B, or the
filtrate and washings from (6) twenty cubic centimeters of a strong
solution of ammonium chlorid and forty cubic centimeters of saturated
solution of ammonium oxalate to completely precipitate all the lime as
oxalate and convert the magnesia into soluble magnesium oxalate. Heat to
boiling and let stand for six hours till the calcium oxalate settles
clear, decant the clear solution on a filter, pour fifty cubic
centimeters of hot distilled water on the precipitate and again decant
the clear solution on the filter, transfer the precipitate to the
filter, and wash it free from all traces of oxalates and chlorids. Dry
and ignite the precipitate over the blast-lamp until it ceases to lose
weight, weigh and estimate as calcium oxid; carefully moisten with
sulfuric acid, heat the inclined covered crucible gently to avoid loss,
then intensely, and weigh as calcium sulfate.
(8) _Magnesia._—Concentrate the filtrate and washings (from 7) to 200
cubic centimeters, place in a half-liter erlenmeyer, add thirty cubic
centimeters of a saturated solution of sodium phosphate and twenty cubic
centimeters of concentrated ammonium hydroxid, cork the flask, and shake
violently at intervals of a few minutes till crystals form, then set the
flask in a cool place for twelve hours. Filter the clear liquid through
a tared gooch, transfer the precipitate to the filter, and wash with
dilute ammonium hydroxid (1 : 3) till the filtrate is free from
phosphates; dry and ignite the crucible, at first gently and then
intensely, to form magnesium pyrophosphate. The increase of weight ×
0.36024 = MgO. By using an erlenmeyer free from scratches and marks, and
shaking violently instead of stirring with a glass rod, the danger is
almost entirely avoided of crystals adhering to the sides of the vessel;
but if crystals do adhere they are readily removed by a rubber-tipped
glass rod.
(9) _Sulfuric Acid._—Evaporate 200 cubic centimeters of solution A (1)
nearly to dryness on a water-bath to expel the excess of acid; then add
100 cubic centimeters of distilled water, heat to boiling and add ten
cubic centimeters of a solution of barium chlorid, and continue the
boiling for five minutes. When the precipitate has settled, pour the
clear liquid on a tared gooch, treat the precipitate with fifty cubic
centimeters of boiling water, and transfer the precipitate to the filter
and wash with boiling water till the filtrate is free from chlorids. Dry
the filter and ignite strongly. The increase in weight is barium
sulfate, which multiplied by 0.34331 = SO₃ in two grams of air-dried
soil.
(10) _Potash and Soda._—To another portion of 200 cubic centimeters of
solution A, add barium chlorid in slight excess, and make alkaline with
ammonia to precipitate sulfuric and phosphoric acids, ferric oxid, etc.
Then precipitate the calcium and barium by ammonium oxalate. Evaporate
the filtrate and washings to dryness, heat to a low red heat to
decompose oxalates and expel ammonia salts, dissolve in twenty-five
cubic centimeters of distilled water, filter and wash the precipitate;
add to the filtrate and washings ten cubic centimeters of baryta water,
and digest for an hour. Filter and wash the precipitate, add ammonium
carbonate to the filtrate to complete precipitation of baryta, filter
and wash this precipitate. Evaporate the filtrate and washings in a
tared platinum dish, gently ignite the residue to expel ammonia salts,
cool and weigh. The increase of weight represents the potassium and
sodium chlorids in two grams of air-dried soil.
=342. Hilgard’s Methods.=[221]—(1) _Soluble Silica._— The acid filtrate
obtained by the process given in paragraph =333= is employed for the
following determinations. After the solution obtained has been
evaporated to dryness to render silica insoluble, it is moistened with
strong hydrochloric acid and two or three drops of nitric acid. The mass
is warmed, and after allowing to stand for a few hours on a steam-bath
is taken up with distilled water. After clearing, it is filtered from
the insoluble residue, which is strongly ignited and weighed. If the
filtrate should be turbid the insoluble residue which has gone through
the filter can be recovered in the iron and alumina determination.
The insoluble residue is next boiled for fifteen or twenty minutes in a
concentrated solution of sodium carbonate, to which a few drops of
caustic lye should then be added to prevent reprecipitation of the
dissolved silica. The solution must be filtered hot. The difference
between the weight of the total residue and that of undissolved sand and
mineral powder is recorded as soluble silica, being the aggregate of
that set free by the acid treatment and that previously existing in the
soil. The latter, however, rarely reaches five per cent.
(2) _Destruction of Organic Matter._—The acid filtrate from the total
insoluble residue is evaporated to a convenient bulk. In case the
filtrate should indicate by its color, the presence of any organic
matter, it should be oxidized by aqua regia, otherwise there will be
difficulty in separating alumina.
(3) _Precipitation of Iron and Alumina._—The filtrate thus prepared is
now brought to boiling and treated sparingly with ammonia, whereby iron
and alumina are precipitated. It is kept boiling until the excess of
ammonia is driven off, and then filtered hot. (Filtrate A.) The previous
addition of ammonium chlorid is usually unnecessary. If the boiling is
continued too long, filtration becomes very difficult and a part of the
precipitate may redissolve in washing. Filtration may be begun as soon
as the nose fails to note the presence of free ammonia; test paper is
too delicate. Failure to boil long enough involves the contamination of
the iron-alumina precipitate with lime and manganese.
(4) _Estimation of Iron and Alumina._—The iron and alumina precipitate
with filter of (3) is dissolved in a mixture of about five cubic
centimeters of hydrochloric acid and twenty cubic centimeters of water.
Then filter and make up to 150 cubic centimeters. Take fifty cubic
centimeters for the determination of iron and alumina together by
precipitation with ammonia, after oxidizing the organic matter (filter)
with aqua regia; also fifty cubic centimeters for iron alone; keep fifty
cubic centimeters in reserve. Determine the iron by means of a standard
solution of potassium permanganate after reduction; this latter is done
by evaporating the fifty cubic centimeters almost to dryness with strong
sulfuric acid, adding water and transferring the solution to a flask,
and then reducing by means of pure metallic zinc in the usual way. The
alumina is then determined by difference. This method of determining the
two oxids in their intermixture is in several respects more satisfactory
than the separation with alkaline lye, which, however, has served for
most determinations made, until within the last ten years. It is,
however, much more liable to miscarry in unpracticed hands than the
other.
(5) _Estimation of Lime._—The filtrate A from iron and alumina is
acidified slightly with hydrochloric acid, and if too bulky is
evaporated to about twenty-five cubic centimeters, unless the soil is a
very calcareous one, and the lime is precipitated from it by
neutralizing with ammonia and adding ammonium oxalate. The precipitation
of the lime should be done in the hot solution, as the precipitate
settles much more easily. It is allowed to stand for twelve hours, then
filtered (filtrate B), washed with cold water, and dried. By ignition
the lime precipitate is partially converted into the oxid. It is then
heated with excess of powdered ammonium carbonate, moistened with water,
and exposed to a gentle heat (50°–80°) until all the ammonia is
expelled. It is then dried below red heat and weighed as calcium
carbonate. When the amount of lime is at all considerable, the treatment
with ammonium carbonate must be repeated till a constant weight is
obtained.
(6) _Estimation of Sulfuric Arid._—The filtrate B from the calcium
oxalate is put into a bohemian flask, boiled down over the sand-bath,
and the ammoniacal salts destroyed with aqua regia. From the flask it is
removed to a small beaker and evaporated to dryness with excess of
nitric acid. This process usually occupies four to five hours. The
residue should be crystalline-granular; if white-opaque, ammonium
nitrate remains and must be destroyed by hydrochloric acid. The dry
residue is now moistened with nitric acid, and the floccules of silica
usually present separated by filtration from the filtrate, which should
not amount to more than ten or fifteen cubic centimeters; sulfur trioxid
is then precipitated by treatment with a few drops of barium nitrate,
both the solution and the reagent being heated to boiling. If the
quantity of sulfuric acid is large it may be filtered after the lapse of
four or five hours (filtrate C). If very small let it stand twelve
hours. The precipitate is washed with boiling water, dried, ignited, and
weighed. Care should be taken in adding the barium nitrate to use only
the least possible excess, because in such a small concentrated acid
solution the excess of barium nitrate may crystallize and will not
readily dissolve in hot water. Care must also be taken not to leave in
the beaker the large heavy crystals of barium sulfate, of which a few
sometimes constitute the entire precipitate, rarely exceeding a few
milligrams. Should the ignited precipitate show an alkaline reaction on
moistening with water, it must be treated with a drop of hydrochloric
acid, refiltered and weighed. The use of barium acetate involves
unnecessary trouble in this determination.
(7) _Estimation of Sodium and Potassium._—Filtrate C is now evaporated
to dryness in a platinum dish; the residue is treated with an excess of
crystallized oxalic acid, moistened with water, and exposed to gentle
heat. It is then strongly ignited to change the oxalates to carbonates.
This treatment with oxalic acid must be made in a vessel which can be
kept well covered, otherwise there is danger of loss through spattering.
As little water as possible should be used, as otherwise loss from
evolution of carbon dioxid is difficult to avoid. Spatters on the cover
should not be washed back into the basin until after the excess of
oxalic acid has been volatilized. The ignited mass should have a
slightly blackish tinge to prove the conversion of the nitrates into
carbonates. White portions may be locally retreated with oxalic acid.
The ignited mass is treated with a small amount of water, which
dissolves the alkaline carbonates and leaves the magnesium carbonate,
manganese protosesquioxid, and the excess of barium carbonate behind.
The alkalies are separated by filtration into a small platinum dish
(filtrate D), and the residue is well but sparingly washed with water on
a small filter. When the filtrate exceeds ten cubic centimeters it may,
on evaporation, show so much turbidity from dissolved earthy carbonates
as to render refiltration on a small filter necessary, since otherwise
the soda percentage will be found too large and magnesia too small. If,
on dissolving the ignited mass, the solution should appear greenish from
the formation of alkaline manganates, add a few drops of alcohol to
reduce the manganese to insoluble dioxid. The residue of barium,
magnesium, and manganese compounds is treated on the filter with
hydrochloric acid, and the platinum dish is washed with warm nitric acid
(not hydrochloric, for the platinum dish may be attacked by chlorin from
the manganese oxid) dissolving any small traces of precipitate that may
have been left behind.
The filtrate D, which should not be more than ten or fifteen cubic
centimeters, containing the carbonates of the alkalies, is evaporated to
dryness and gently fused, so as to render insoluble any magnesium
carbonate that may have gone through; then redissolved and filtered into
a small weighed platinum dish containing a few drops of dilute
hydrochloric acid, to change the carbonates into chlorids; evaporated to
dryness, exposed to a gradually rising temperature (below red heat), by
which the chlorids are thoroughly dried and freed from moisture, so as
to prevent the decrepitation that would otherwise occur on ignition.
Then, holding the platinum basin firmly by forceps grasping the clean
edge, pass it carefully over a very low bunsen flame, so as to cause,
successively, every portion of the scaly or powdery residue to collapse,
without fully fusing. There is thus no loss from volatilization, and no
difficulty in obtaining an accurate, constant weight. The weighed
chlorids are washed by means of a little water into a small beaker or
porcelain dish, treated with a sufficient quantity of platinum chlorid,
and evaporated to dryness over the water-bath. The dried residue is
treated with a mixture of three parts absolute alcohol and one part
ether, leaving the potassium platinochlorid undissolved. This is put on
a filter, and washed with ether-alcohol. When dried, the precipitate and
filter are put into a small platinum crucible and exposed to a heat
sufficiently intense to reduce the platinum chlorid to metallic platinum
and to volatilize the greater part of the potassium chlorid. This is
easily accomplished in a small crucible, which is roughened by being
constantly used for the same purpose (and no other), the spongy metal
causing a ready evolution of the gases. The reduced platinum is now
first washed in the crucible with hot acidulated water, then with pure
water; then all moisture is driven off and it is weighed. From the
weight of the platinum, is calculated the potassium chlorid and the oxid
corresponding; the difference between the weights of the total alkaline
chlorids and potassium chlorid gives the sodium chlorid, from which may
be calculated the sodium oxid. When the heating of the platinum
precipitate has not been sufficient in time or intensity, instead of
being in a solid spongy mass of the color of the crucible itself, small
black particles of metallic platinum will obstinately float on the
surface of the water in the crucible, and it becomes difficult to wash
without loss.
(8) _Estimation of Manganese._—The solution containing the magnesium and
manganese chlorids is freed from barium salts by hot precipitation with
sulfuric acid, and the barium sulfate, after settling a few hours, is
separated by filtration. The filtrate is neutralized with ammonia, any
resulting small precipitate (of iron) is filtered, and the manganese
precipitated with ammonium sulfid, let stand twelve hours and filtered
(filtrate E); wash with cold water, dry, ignite, and weigh as manganese
protosesquioxid, Mn₃O₄. If preferred the manganese may be precipitated
with chlorin or bromin water as dioxid; but the process requires a
rather longer time and may fail in inexpert hands more readily than the
other.
(9) _Estimation of Magnesium._—The filtrate E from the manganese is now
freed from sulfur by acidulating with hydrochloric acid, evaporating, if
necessary, and filtering. From the filtrate the magnesia is precipitated
by adding an equal bulk of ammonia water and then sodium phosphate.
After standing at least twenty-four hours, the magnesium salt may be
filtered, washed with ammoniacal water, dried, ignited, and weighed as
magnesium pyrophosphate.
=343. Examination of Acid Extract by the Methods of
Petermann.=—_Estimation of the Silica._—The Gembloux method of
estimating silica consists in taking up the dry extract obtained from
the treatment of the earth, in the manner described in paragraph =336=,
with water and a few drops of hydrochloric acid, heating for a short
time on a sand-bath to facilitate the solution, and filtering, washing,
drying, igniting, and weighing the residue obtained as silica.
_Estimation of the Sulfuric Acid._—The method employed consists in
heating the filtrate obtained in the estimation of silica for half an
hour with a few drops of nitric acid and making the volume up to 500
cubic centimeters. One hundred cubic centimeters of this are
precipitated with barium chlorid, diluted to double its volume, heated
for some time, the precipitate of barium sulfate collected and weighed,
and the quantity of sulfuric acid calculated therefrom.
_Potash and Soda._—Potash and soda are estimated at the Gembloux Station
by heating the filtrate obtained in the estimation of the sulfuric acid
and precipitating the excess of barium in the hot solution after the
addition of ammonia by ammonium oxalate and carbonate. The whole is
allowed to digest for six hours at a gentle heat and then allowed to
remain at rest for twenty-four hours, filtered, washed, and the filtrate
evaporated to dryness in a large platinum dish and the ammoniacal salts
driven off at a low temperature. At the end, the temperature is carried
a little higher until it reaches low redness. The residue is taken up by
distilled water, filtered into a weighed platinum dish, a few drops of
hydrochloric acid added, evaporated, dried, heated with great care and
the sodium and potassium chlorids obtained weighed together. The
respective quantities of potash and soda in the earth are estimated in
the usual way by precipitating the potash with platinum chlorid.
_Estimation of the Iron and Aluminum Oxids._—The iron and aluminum oxids
are estimated by taking twenty-five cubic centimeters of the primitive
solution obtained with hydrochloric acid and adding ammonium carbonate
almost to complete neutralization, that is to say until the precipitate
formed is just redissolved in the feeble excess of hydrochloric acid
which remains. Dilute with distilled water and precipitate with a little
excess of ammonium acetate, and boil for a moment; after boiling, the
basic iron and aluminum acetate and the small quantity of iron and
aluminum phosphate present are easily deposited, and the supernatant
liquid should be completely limpid and colorless. Wash the precipitate
by decantation, boiling each time, filter, wash the filter with boiling
water to which a little ammonium acetate has been added, dry, ignite,
and weigh. The material obtained consists of ferric oxid, aluminum oxid,
and iron and aluminum phosphates. Deduct from the whole, the phosphoric
acid determined in another portion. The residue will be the sum of the
iron and aluminum oxids.
_Estimation of the Lime._—The filtrate from the portion used for the
estimation of the iron and alumina is treated with ammonium oxalate. The
mixture is kept at a low temperature for at least twelve hours, after
which it is filtered, washed with hot water, dried, and ignited over a
blast-lamp to constant weight and weighed as calcium oxid.
_Estimation of the Magnesia._—For the estimation of the magnesia the
filtrate obtained in the estimation of lime is evaporated to dryness in
a platinum dish, the ammoniacal salts driven off, the residue taken up
with water slightly acidified with hydrochloric acid, filtered, the
filtrate saturated with ammonia and heated some time to the boiling
point to precipitate any traces of iron and alumina which may have
remained in solution. Filter, wash, allow to cool and precipitate the
magnesia by the addition of sodium phosphate. It is then allowed to
stand for twelve hours, collected on a filter, ignited, and weighed as
pyrophosphate, and the quantity of magnesia calculated from the weight
of salt obtained.
_Estimation of the Phosphoric Acid._—The phosphoric acid is estimated by
taking 100 cubic centimeters of the original solution obtained by the
treatment of the soil with hydrochloric acid and evaporating it to
dryness on the water-bath. The residue is taken up with water to which a
few drops of nitric acid have been added and filtered. The total
phosphoric acid is then obtained by precipitation with ammonium
molybdate in the usual way.
=344. Analysis of the Insoluble Residue.=—The insoluble residue left
after digestion with hydrochloric acid is not without interest from an
agricultural and analytical point of view. While it is true that the
plant food, therein contained, is not immediately available, yet it must
not be forgotten that the method of the chemist may not fix a limit to
nature’s method of collecting nutriment for plants. In however
refractory a state they may exist, it is possible that all nutritive
elements may eventually become available for assimilation. For the
completion of an estimate of the total nutritive power of a soil,
therefore a further examination of the insoluble residue should be made.
The methods of securing this are essentially those of making a bulk
analysis of the soil.
The principle of the method depends on the reduction of the sample to an
impalpable powder and the subsequent decomposition of the insoluble
portions by treatment with hydrofluoric and sulfuric acids or by fusion
with the alkalies.
=345. Method of Wolff for Treating Residue Insoluble in Hot Acid.=—The
well-washed residue is dried with the filter, then separated therefrom,
the filter burned and the ash weighed with the whole of the residue.
About eight grams of the residue are ignited and serve for the
estimation of the insoluble mineral matter. Another portion of ten grams
of the dried, but not ignited, residue is boiled with a concentrated
solution of sodium carbonate with the addition of caustic soda, and the
quantity of dissolved silicic acid estimated. A third portion of about
fifteen grams is treated with about five times its weight of pure
concentrated sulfuric acid, and is evaporated until the mass has taken
the form of a dry powder. After moistening with concentrated
hydrochloric acid the mass is boiled with water, filtered, and the
filtrate examined according to the ordinary methods for silicic acid,
alumina, iron, lime, magnesia, and alkalies. The residue after treatment
with concentrated sulfuric acid is dried, but not ignited, and boiled
with a concentrated solution of sodium carbonate with the addition of a
little caustic soda, filtered, heated, and the silicic acid separated
from the solution. After thorough washing, the residue, after ignition,
is weighed and represents the material insoluble in concentrated
hydrochloric and sulfuric acids. The silicic acid found as before,
together with the small quantity dissolved in the hydrochloric acid
extract, gives, in connection with the alumina contained in the sulfuric
acid extract, approximately the quantity of pure water-free clay
contained in the soil.
In six samples of soils of very different compositions which were
examined by the above process, it was found that the clay had the
following mean composition: Silicic acid, 55.1 to 61.5 per cent,
alumina, 38.6 to 44.9 per cent; as a mean 58.05 per cent silicic acid
and 41.95 per cent alumina.
Finally, four or five grams of the residue, after treatment with
sulfuric acid and sodium carbonate, are rubbed up in an agate mortar and
completely separated into silt by water. The silt mass is dried, lightly
ignited, and three grams of it spread in a flat platinum dish moistened
with sulfuric acid, and subjected to the action of hydrofluoric acid in
a lead oven at 60°, until a complete decomposition of the material is
accomplished. In the solution all the different bases can be determined.
=346. Method of the Belgian Chemists.=—The method employed by
Petermann[222] at the Gembloux Station in the examination of the part of
the soil insoluble in hydrochloric acid consists in washing the
insoluble portion by decantation with distilled water until all acid
reaction is removed. Place the contents of the flask and of the filter
in a porcelain dish and dry. After a careful mixing of the mass take out
about fifty grams and wash upon the filter until all reaction for
chlorin has disappeared, dry, detach the mass from the filter, and
incinerate. Place in a platinum crucible two grams of the ground and
ignited residue and mix it, using a platinum stirring rod, with twelve
grams of ammonium fluorid; heat slightly over a bunsen burner in a
muffle with a good draught and regulate the flame in such a way that the
operation shall continue for about one hour. After complete
decomposition add about two cubic centimeters of sulfuric acid in such a
way as to moisten completely the residue, drive off the sulfuric acid
carefully at a low red heat and take up the residue with water slightly
acidulated with hydrochloric acid and wash the whole into a flask of 500
cubic centimeters capacity. Oxidize by heating for an hour with nitric
acid, make up to the mark and filter. The percentages of potash, soda,
lime, magnesia, and the silicates are determined exactly as in the
hydrochloric acid extract.
=347. Bulk Analysis.=—It is frequently desirable to determine the total
composition of a soil sample as well as the nature of that part of it
soluble in any of the solvents usually employed. The latest methods for
this purpose have been well studied by Packard[223] who finds that the
variations which occur between duplicates are probably due to the small
quantities of material taken for analysis, it being difficult to obtain
average samples of a material which is not very finely powdered when
small quantities are taken. Moreover, as it is likely to become of
importance to know whether the proportions of lime and magnesia vary by
as much as one-tenth per cent, and such small variations are within the
limits of error of an analysis, and as the total proportion of lime and
magnesia in highly siliceous soils, probably does not exceed one-tenth
per cent, it is deemed best to take a large quantity of soil for the
bulk analysis in each case. The amount adopted for the highly siliceous
soils containing much quartz is ten grams. This quantity, taken after
quartering down the entire sample, is ground to an impalpable powder and
used for the determination of the lime, magnesia, and alkalies, the
silica, iron oxid, alumina, and loss on ignition, being determined in
one gram samples. The ten grams are decomposed by hydrofluoric and
sulfuric acids in a large platinum dish, the solution evaporated, at
first on the water-bath until all water is removed and then at a higher
temperature until all the free sulfuric acid is driven off, when the
residue is heated in a muffle at a low red heat for several hours. At
this temperature the sulfuric acid combined with the iron oxid and
alumina is driven off, leaving the remaining sulfates unchanged and the
iron oxid and alumina are in the form of a powder of no great volume
which is easily and quickly washed. This operation is usually successful
at first but in some cases the decomposition is not complete as is shown
by the appearance of a precipitate on adding ammonia to the filtrate
from the aluminum and iron oxids. In such cases the precipitate is
dissolved in hydrochloric acid, reprecipitated by ammonia and removed by
filtration. In the filtrate from the thoroughly washed aluminum and iron
oxids, lime is precipitated as oxalate and separated by filtration; the
filtrate is evaporated to dryness and the ammonia salts driven off by
heat; the magnesia in the unfiltered watery extract of this residue is
precipitated by baryta water, which also removes the sulfuric acid with
which the bases had been combined. In the filtrate from this precipitate
baryta is precipitated by ammonium carbonate and removed by filtration,
leaving the alkalies to be determined in the usual way after conversion
into chlorids. The mixed precipitate of magnesia and barium sulfate is
treated with hydrochloric acid, filtered, the baryta present removed as
sulfate, and the magnesia precipitated in the filtrate from the latter
as phosphate. The advantages of this method are that the large quantity
of material employed gives some assurance that an average sample has
been operated on, and all the bases present in small proportions are
estimated in the same sample. The objection to it is the time consumed
both in grinding the samples and in determining all the bases in one
solution. As a small quantity of material is generally used for
determining the silica, iron oxid, alumina, and loss by ignition, and a
larger quantity for the remaining bases, slight differences in the
unground samples are unavoidable, especially when the quartz grains are
somewhat large, it being practically impossible to take two small
samples of such a soil which would have the same number of quartz
grains. Consequently tedious grinding of large quantities of the soils
for the bulk analysis is necessary. This objection does not apply to the
official analysis or assay of soils in which considerable quantities are
extracted by acid and the solution analyzed, and silica is not
determined. In any case, it may be said, when it becomes an object to
know whether a soil contains a total of 0.1 or 0.2 per cent of lime or
magnesia, of 0.7 or 0.5 per cent of potash, one analysis even of the
large quantity of ten grams would be insufficient to decide the point,
and at least the mean of two determinations should be taken.
SPECIAL METHODS OF DETERMINATION OF SOIL CONSTITUENTS.
=348. Preliminary Considerations.=—In the foregoing paragraphs the
general outline of the chemical methods of soil examination have been
given. There are often occasions, however, which demand a special study
of some particular soil constituent. It has been thought proper,
therefore, to add here some of the best approved methods of special
determinations which have been approved in this and other countries.
In the main, the final determination of any particular element of the
soil, and its previous separation from accompanying elements, are based
on the general processes already given. The variations in many
instances, however, seem to require special mention.
=349. Condition of Potash in Soils.=—Potash exists in the soil in very
different states. That part of it which is combined with the humus
material, or with the hydrated silicates, is easily set free from its
combinations and is to be regarded as the more assimilable portion.
The potash in the soil is found chiefly in combination with silicates,
and particularly with the hydrated aluminum silicates, forming clay. As
the particles with which it is combined are found in a state of greater
or less fineness, the potash itself is set free under the influences of
the agents which are active in the soil, with greater or less rapidity,
passing into a form in which it can be utilized by plants. In silicates
which are very finely divided, such as clay, the potash becomes active
in a relatively short time, while in the débris of rocks in a less
advanced state of decomposition it may rest for an indefinite period in
an inert state. The estimation of the potash which is assimilable in the
clay is quite as important for agricultural purposes as to determine
that which may be present in the soil in firmer combination. Treating
the sample of soil with water does not furnish any useful information in
regard to the potash which it contains. Indeed, the absorbing properties
of the soil tend to prevent the elimination of the potash in this way,
even when it is found in the soluble state. It is therefore, necessary
to employ an acid to set the potash free, but variable results are
obtained, according to the employment of acids of greater or less
concentration and for longer or shorter periods of contact.
=350. Estimation of the Potash Soluble in Concentrated Acids.=—In the
method of the French agricultural chemists[224] twenty grams of the
earth are placed in a dish with a flat bottom, eleven centimeters in
diameter, and rubbed up with twenty to thirty cubic centimeters of
water. There is added carefully, and in small quantities, some nitric
acid of 36° Baumé until all effervescence has ceased, the mass meanwhile
being thoroughly stirred. When the carbonates have been decomposed,
which can be told by the cessation of the effervescence, twenty cubic
centimeters more of the same acid are added. The dish is heated on the
sand-bath for five hours, regulating the heating in such a way that
there still remains some acid at the end of the operation and the mass
is not thoroughly dry. The acid mass is then taken up with hot water,
filtered, and washed with hot water until the amount of filtrate is
about 300 cubic centimeters. The filtrate should be received in a flask
of about one liter capacity. The filtrate will contain the dissolved
potash, soda, magnesia, lime, iron and aluminum oxids, and traces of
sulfuric and phosphoric acids. For the elimination of the other
substances, with the exception of potash, soda and magnesia, a few drops
of barium nitrate are added, afterwards sufficient ammonia to render the
solution alkaline, and finally an excess of ammonium carbonate in powder
added in small portions. These materials are added successively and the
whole is left to stand for twenty-four hours. By this operation the
sulfuric acid is separated in the form of barium sulfate; the iron and
aluminum oxids are precipitated, carrying down with them the phosphoric
acid, and the lime is thrown down in the form of carbonate. The mass is
now filtered and washed several times with hot water. The filtrate
contains in addition to potash, soda, magnesia, and the ammoniacal salts
which have been introduced. The ammonium salts are destroyed by adding
aqua regia and evaporating the liquid to a very small volume, as
described in the method for the estimation of magnesia. The mass is now
evaporated in a porcelain dish with a flat bottom, of about seven
centimeters diameter, and an excess of perchloric acid added. The
evaporation is carried to dryness on a sand-bath, and the heating
prolonged until the last white fumes of perchloric acid are disengaged.
The mass is now left to cool. There are then added five cubic
centimeters of alcohol, of 90° strength. The mass is triturated by a
stirring rod, the extremity of which is flattened, in such a manner as
to reduce it all to an impalpable powder. It is then left to settle and
the supernatant liquid is decanted upon a small filter. The treatment
with alcohol of the kind, quantity, and strength described, is continued
four or five times. Afterwards, as there may still remain a trace of the
sodium and magnesium perchlorates in the interior of the crystals of
potassium perchlorate, there are added to the capsule in which all of
the alkaline residue has been collected, two or three cubic centimeters
of water, and it is evaporated again to dryness and taken up twice with
small quantities of alcohol. There are thus removed the traces of sodium
and magnesium perchlorates. By means of a jet of boiling water the
stirring rod and the filter, which contains the small quantities of
potassium perchlorate, are washed, and the liquid passing through is
received in the capsule which contains the larger part of the salt. It
is then evaporated to dryness and weighed.
When there is very little magnesia present, as is generally the case,
the estimation of the potash is made without any difficulty by the
process just mentioned, but when the proportion of magnesia is high it
is found useful to separate it before the transformation into
perchlorates. The magnesia is separated by carbonating the residue as
indicated in the method for the estimation of magnesia, by treatment
with oxalic acid and ignition. By extracting the carbonates formed with
very small quantities of water, and filtering, the alkalies are obtained
free from magnesia.
It is advisable to test the purity of the potassium perchlorate formed
which sometimes contains a little silica. For this purpose it is
dissolved in boiling water, and any residue which remains is weighed,
and that weight deducted from the total weight of perchlorate. By
multiplying the weight of potassium perchlorate found by the coefficient
0.339, the quantity of potash contained in the twenty grams of earth
submitted to analysis is obtained.
_Estimation of the Potash Soluble in Cold, Dilute Acids._—(Method of
Schloesing.) Introduce 100 grams of the soil into a one or one and a
half liter flask with 600 to 800 cubic centimeters of water. A little
nitric acid, of 30° Baumé, is added until the carbonate is decomposed
and a slight acid reaction is obtained. Afterwards five cubic
centimeters of the same acid are added and it is left to digest for six
hours, shaking every fifteen minutes. Instead of taking the whole of the
wash-water for the examination, it is better to extract only a portion
of it and so dispense with washing. This process is conducted in the
following manner:
The weight P of the full flask having been determined, as much as
possible of the solution, is decanted by means of a very small siphon,
of which the flow is moderated by fixing a rubber tube with a pinch-cock
to its lower extremity. After the decantation is complete, the flask is
again weighed, giving the weight of P′; the weight of liquid taken,
therefore, is equal to P − P′. To determine the total weight of the
liquid, throw upon a filter the earthy residue insoluble in the acid,
and after washing and drying it determine its weight _r_. The weight of
the empty dry flask _p_ is also determined. The total weight of the soil
will be, therefore, P − _r_ − _p_. The part of the liquid which was
extracted from the flask, and upon which the analytical operation is to
be conducted is represented by the fraction (P − P′)/(P − _r_ − _p_).
This method avoids washing and evaporation which would be of very long
duration. It rests upon the supposition that the solid matter from which
the liquor is separated has no affinity for the dissolved substances,
and that the total of these substances has passed into the liquor, and
that the solution is homogeneous.
In the liquor first decanted as described before, the potash is
estimated. This liquor contains in addition to potash, soda, lime,
magnesia, iron and aluminum oxids, as well as phosphoric, sulfuric, and
hydrochloric acids. There is first added to it a little barium chlorid
to precipitate the sulfuric acid. It is then heated to about 40° in a
glass flask and some ammonium carbonate added in a solution containing
an excess of ammonium hydroxid. By this process the lime and baryta are
precipitated in the form of carbonates; the alumina and iron as oxids,
and the phosphoric acid in combination with the last two bases. The
magnesium carbonate is not precipitated because it is soluble in the
ammonium carbonate with which it forms a double salt.
The employment of a gentle heat favors the formation of the precipitate
of calcium carbonate in a granular form which lends itself easily to
filtration. The contents of the flask are now thrown upon a filter and
the insoluble residue washed. The filtrate contains the potash, soda,
magnesia, ammonia, and nitric and hydrochloric acids. It is concentrated
as rapidly as possible by heating in a flask, and afterwards the
ammoniacal salts are destroyed by weak aqua regia and the whole is then
transferred to a porcelain dish and evaporated to dryness. There is thus
obtained a mixture of potassium, sodium, and magnesium nitrates, from
which the potash is separated by means of perchloric acid in the manner
already described.
_Estimation of the Total Potash._—Beside the potash which can be
dissolved by the boiling concentrated acids the soil contains potash
combined with silicates, which becomes useful for plant life with
extreme slowness. It is often of great interest to estimate the total
potash contained in a soil, that is to say, the reserve for the future.
In this case it is necessary to free entirely this base from its
combinations by means of hydrofluoric acid. The operation is conducted
upon two grams of earth previously ignited and reduced to an impalpable
powder. The decomposition is conducted in a platinum capsule by
sprinkling the sample with a few cubic centimeters of hydrofluoric acid,
or solution of ammonium fluorid, and adding a few drops of sulfuric
acid. It is then evaporated to dryness and dissolved in boiling
hydrochloric acid. The part which remains insoluble is treated a second
time by hydrofluoric and sulfuric and afterwards by hydrochloric acid.
All of the potash is thus brought into solution. The estimation of the
potash, after having obtained it in a soluble state, is conducted in the
manner previously described.
_Estimation of the Potash as Platinochlorid._—Instead of estimating the
potash as perchlorate it can also be transformed into platinochlorid.
This process gives as good results as the preceding one but it is
necessary in all cases, to separate the magnesia. After having treated
the soil as indicated in the case of the estimation of the potash as
perchlorate, the separation of the sulfuric and phosphoric acids, of
alumina and iron, of magnesia, and the destruction of the ammoniacal
salts in the manner already described, there are finally left the
alkalies potash and soda in the form of carbonates. These are
transformed into chlorids by adding hydrochloric acid; afterwards they
are evaporated to dryness and the mixture of the two chlorids weighed in
order to determine what quantity of platinum chlorid it is necessary to
add, in order that it be in excess. The quantity of chlorid to be added
is calculated so as to be in sufficient quantity to saturate the whole
of the chlorids weighed, whether they may be composed wholly of sodium
or potassium. In this way there is a certainty of having an excess of
platinum. The solution of platinum chlorid used should contain in 100
cubic centimeters seventeen grams of platinum. Each cubic centimeter of
this solution will be sufficient for a decigram of the sodium and
potassium double chlorids. After the addition of the platinum chlorid
the mixture is evaporated in a capsule with a flat bottom, on a
water-bath. It is important that the temperature should not exceed 100°.
If the temperature should go above this there would be a tendency to
form some platinum subchlorids insoluble in alcohol.
The evaporation is continued until the contents of the dish are in a
pasty condition and form a rather solid mass on cooling. It is necessary
to avoid a complete desiccation. After cooling, the residue is taken up
by alcohol of 95° strength. It is allowed to digest with alcohol of this
strength for some time, after having been thoroughly mixed and shaken
therewith in order to obtain a complete precipitation of the
platinochlorid. This digestion should take place under a small bell-jar
resting upon a piece of ground glass. The evaporation of the alcohol is
thus prevented. The mass is then washed by means of alcohol of the same
strength and the liquors decanted upon a small filter placed within
another filter of identical weight, which serves as a tare for it on the
balance. The washing is prolonged until the filtrate becomes colorless.
All of the particles in the dish should be brought upon the filter by
means of a hair-brush. The filters are now dried at a temperature not
exceeding 95° and the platinochlorid received upon the interior filter
is weighed. The precipitate may also be washed from the small filter
into the capsule in which it was formed by means of a jet of alcohol.
The alcohol is evaporated and the precipitate weighed in the capsule.
The weighing should be made rapidly on account of the hygroscopicity of
the material. The weight obtained multiplied by 0.193 gives the
corresponding quantity of potash in the soil.
_Purification of the Oxalic Acid._—The commercial oxalic acid used in
separating the magnesia, often contains lime, magnesia, and potash. When
this reagent is used in a sufficiently large quantity in the estimation
of the above substances, it is indispensable to free it entirely from
them. This is secured by submitting the oxalic acid to successive
recrystallizations which are obtained by dissolving it in warm water,
filtering and leaving to cool. The mother waters are thrown away. After
two or three successive crystallizations the traces of potash and
magnesia have disappeared and the oxalic acid obtained after ignition
leaves no trace of residue.
The purification may also be secured in the following manner: At a
temperature of 60° a saturated solution of oxalic acid is made; the
liquid is decanted, carried to the boiling point and filtered. Five per
cent of nitric acid are added and it is allowed to cool. The crystals
which are deposited are collected upon a funnel in which a plug of
cotton has been placed, and are washed with a little cold water.
_Purity of the Ammonium Carbonate._—The ammonium carbonate employed
should not leave any residue whatever after volatilization. In general,
it may be said of all the reagents employed in analyses and especially
of those employed in large quantities, that it is indispensable to be
sure that they contain no traces of the substances which are to be
estimated. The acids, ammonia, etc., should always be examined with this
point in view.
_Estimation of the Soda._—It is often of interest to estimate the soda
in the soil, not that it is an element of any great fertility but rather
because it is hurtful when in excess. It is determined in the residue
obtained in the estimation of potash and is estimated by difference. The
weight of the mixture of sodium and potassium chlorids being known when
the potash is determined, the weight of its chlorid is to be deducted
from the weight of the two chlorids and thus the direct weight of the
sodium chlorid is obtained.
A better way is to make a direct estimation. The soda is found entirely
dissolved in the alcoholic solution obtained by washing the potash salt
as before described, for the separation of the potassium platinochlorid.
This alcoholic liquor is evaporated to dryness on a water-bath, in a
bohemian flask of about 100 cubic centimeters capacity. The residue
obtained consists of sodium platinochlorid and a little platinum
chlorid. There is now fitted to the bohemian flask a cork stopper
carrying two tubes. The apparatus is placed upon a water-bath and kept
at about 100°. Through the tube which reaches to the bottom of the
bohemian flask, a current of pure hydrogen is passed. The hydrogen
passes off through the second tube. The hydrogen completely reduces the
salts of platinum. In order that the decomposition may go on more
rapidly a few drops of water are added. When the whole mass in the flask
has become black owing to the separation of the platinum, it is shaken,
evaporated to dryness and hydrogen passed through a second time. This
operation is repeated three or four times, being stopped when the water
no longer shows a yellow color. There is then in the flask only a
mixture of reduced platinum and sodium chlorid. No trace of sodium
chlorid has been lost because the temperature has never exceeded 100°.
The sodium chlorid is dissolved by washing with water and filtered. The
liquor, which must be absolutely colorless, is evaporated to dryness in
a platinum capsule and weighed. There is thus obtained the weight of the
sodium chlorid. For verification, the sum of the weight of potassium
chlorid calculated from the platinochlorid and the weight of the sodium
chlorid should be equal to the initial weight of the mixture of the two
chlorids.
=351. Potash Methods of the German Experiment Stations.=[225]—_a._ To
one volume of air-dried fine earth which is obtained by sifting through
a three millimeter sieve, two volumes of twenty-five per cent
hydrochloric acid are added, or more if the soil contains much
carbonate. The acid is allowed to act with frequent stirring for
forty-eight hours at room temperature.
_b._ To one volume of the soil, as above prepared, are added two volumes
of hydrochloric acid and allowed to stand for three hours with frequent
shaking, at the temperature of boiling water.
_c._ (Halle method.) One hundred grams of the fine earth are treated
with 500 cubic centimeters of forty per cent hydrochloric acid, made up
to one liter with water and allowed to stand for forty-eight hours with
frequent shaking. After filtering, a large aliquot part of the filtrate
is evaporated for the estimation of the potash. The evaporated residue
is washed into a half-liter flask in which the sulfuric acid is
precipitated with barium hydroxid the flask filled to the mark and an
aliquot part of the filtrate in a half-liter flask, treated with
ammonium carbonate, filtered and the potash estimated as platinochlorid
by the usual method.
=352. Method of Raulin for the Estimation of Potash in Soils.=[226]—The
process rests upon the very feeble solubility in aqueous solution of
potassium phosphomolybdate, while sodium, magnesium, calcium, iron, and
aluminum phosphomolybdates are more or less soluble. The process does
not require complicated separation and permits of the treatment of a
small quantity of soil, since the weight of the phosphomolybdate
obtained is equivalent to nineteen times that of the potash.
The reagent is prepared by dissolving 100 grams of crystallized ammonium
molybdate in as little water as possible and adding six and a half grams
of neutral crystallized ammonium phosphate dissolved in a little water.
Aqua regia is now added cold and some ammonium phosphomolybdate is
precipitated. The mixture is heated, adding a little aqua regia from
time to time, until the solution of the precipitate is accomplished. The
whole is then evaporated to dryness, the final temperature of
evaporation not being carried above 70°. Four hundred cubic centimeters
of water are now added and five cubic centimeters of nitric acid, and
the contents of the dish heated and filtered. The reagent is then ready
for use.
The liquid to be used for washing the potassium phosphomolybdate is
prepared by dissolving twenty grams of sodium nitrate in one liter of
water, two cubic centimeters of pure nitric acid, and a mixture of about
twenty cubic centimeters of the phosphomolybdic reagent and one and a
half cubic centimeters of a solution of potassium nitrate containing
eighty grams per liter, slightly heated in order to saturate the liquid
with potassium phosphomolybdate. The solution is shaken, allowed to
rest, and the liquid decanted.
For the preparation of the solution in which the potash is to be
estimated, a sample of soil is carefully weighed of such magnitude as to
contain about fifteen milligrams of anhydrous potash. The potash salts
are dissolved by the usual processes and are separated from the largest
part of the calcium, iron, and aluminum salts, and converted into
nitrates. The solution is reduced to a volume of a few cubic centimeters
and slightly acidulated with nitric acid. Four cubic centimeters of the
phosphomolybdic reagent are added for every ten milligrams of anhydrous
potash supposed to be present. The solution is evaporated to dryness at
50° and immediately brought upon very small weighed filters, of which
each one is double, by using sixty cubic centimeters of the washing
liquor mentioned above. The tared filter is likewise washed with the
same liquid at 50° and weighed. The weight multiplied by 0.052 gives the
anhydrous potash. This method for a direct precipitation of the potash
salts does not have the merits of the perchlorate process and both are
inferior in accuracy to the usual platinochlorid procedure.
=353. Russian Method for Estimating Potash in Soils.=[227]—Ten grams of
the soil are digested with 100 cubic centimeters of ten per cent
hydrochloric acid on a steam-bath for twenty-four hours. After adding
five cubic centimeters of nitric acid to the filtrate it is evaporated
to dryness, taken up with dilute hydrochloric acid, filtered, the
filtrate saturated with ammonia, the excess of ammonia driven off, again
filtered, and the lime separated by ammonium oxalate.
The filtrate is treated with a little barium chlorid for the removal of
sulfuric acid and afterwards with ammonium carbonate in excess, and
digested for twenty-four hours. After filtering, the solution is
evaporated in a platinum dish, the excess of ammonia driven off, the
residue taken up with water, filtered, treated with hydrochloric acid,
evaporated to dryness, and ignited at low heat. The residue is again
dissolved in water, filtered, and the potash precipitated with platinum
chlorid and estimated in the usual way.
=354. Potash Method of the Italian Stations.=[228]—The potash in the
soil should be determined in three forms; _viz._,
1. Assimilable potash.
2. Potash soluble in concentrated acid.
3. Total potash.
For determination of the first, 100 grams of earth are put into a retort
holding a liter and digested with dilute nitric acid.
For the analysis, an aliquot portion of the clear liquid is taken or
weighed, and the determination of the potash is made by the common
methods.
For an alternate method, from twenty to fifty grams of earth are put
into a retort of 500 cubic centimeters, moistened with water, and nitric
acid is gradually added. After one or two hours there are added from 200
to 300 cubic centimeters of water; the liquid is poured without
filtering into a retort and the residue washed by decantation.
In the liquid, after the elimination of the other substances with barium
chlorid, ammonium carbonate, etc., the potash is determined by the
ordinary methods.
In the second case, by using warm concentrated acids, a portion of the
insoluble silica is decomposed, but this decomposition is always partial
and the quantity of the potash extracted depends upon the temperature,
upon the concentration, upon the duration of the action, and upon the
nature of the acid.
The method of moistening twenty to fifty grams of earth with water and
adding, thereto, concentrated nitric acid of 1.20 density, in such a
manner that the earth shall be completely saturated, may also be
employed. Then the temperature is kept at 100° during two hours. In the
solution, the potash is determined as usual.
In the third case the soil is to be decomposed by hydrofluoric and
sulfuric acids, or by fusion with alkaline carbonates, and the total
potash determined by one of the standard methods.
If it is desired to adopt a general method for the determination of the
potash the following points must be carefully considered:
1. The quantity of the earth to be examined.
2. The state of humidity or dryness of the same.
3. The quantity, nature, and concentration of the acid.
4. The quantity of the water.
5. The duration of the treatment.
=355. Method of J. Lawrence Smith for Potash.=—This method, designed
especially for mineral analysis, has been fully approved by the general
experience of analysts.
The principle of the method[229] depends upon the decomposition of
silicates on ignition with calcium carbonate and ammonium chlorid. The
object of this mixture is to bring into contact with the mineral,
caustic lime in a nascent state at a red heat, the caustic lime being
soluble to some extent in calcium chlorid at a high temperature. Pure
calcium carbonate, made by precipitation of marble, should be used.
The ammonium chlorid should be prepared by taking crystals of pure,
sublimed sal ammoniac, dissolving in water, and filtering, and
evaporating the solution until small crystals are deposited, the
solution being well-stirred until one-half or two-thirds of the whole
has crystallized. The mother-liquor is poured off while still hot, and
the crystals dried on an asbestos filter at ordinary room temperature.
A special platinum crucible should be used in the Smith method, but the
common crucible, especially if very deep, can be employed. The special
crucible is of about double the usual length. Smith recommends a
crucible ninety-five millimeters in length, diameter at top twenty-two
millimeters, at bottom sixteen millimeters, and weighing thirty-five to
forty grams. The object of the long crucible is to have the part of the
bottom containing the silicate subjected to a high heat, while the top
of the crucible is at a much lower temperature, thus preventing the loss
of alkalies by volatilization.
_Method of Analysis._—The samples of soil or silicate containing the
alkalies are well pulverized in an agate mortar, and from one-half to
one gram of the finely pulverized material taken for analysis. This is
carefully mixed with the same weight of finely powdered sal ammoniac and
the mineral and sal ammoniac rubbed well together in a mortar. Eight
parts by weight of calcium carbonate are next added in three or four
portions, and the whole intimately mixed after each addition. The
contents of the mortar are emptied on a piece of glazed paper and then
introduced into the crucible, which is tapped gently upon the table
until the contents are well settled. It is then fixed in the furnace
which is used for heating, and a small bunsen burner is placed beneath
the crucible, and the heat applied just about at the top of the mixture
and gradually carried toward the lower part until the sal ammoniac is
completely decomposed, which requires from four to five minutes. The
heat is then applied by means of a blast-lamp and the crucible kept at a
bright red heat for from forty to sixty minutes. The crucible is allowed
to cool, the contents detached and placed in a platinum or porcelain
dish of about 150 cubic centimeters capacity, and sixty to eighty cubic
centimeters of distilled water added. The solution of the flux may be
hastened by heating the water to the boiling point. The crucible and its
cover are also well washed with hot water until all matter adhering to
them is dissolved. After the slaking of the mass it is best to continue
the digestion with hot water for six or eight hours, although this is
not absolutely necessary. The contents of the crucible are filtered and
washed well with about 200 cubic centimeters of water. The filtrate
contains in solution all the alkalies of the mineral, or soil, together
with calcium chlorid and caustic lime. A solution of pure ammonium
carbonate containing about one and one-half grams of the pure salt is
added to the filtrate. This precipitates the lime as carbonate. The dish
containing the material is placed on a water-bath and its contents
evaporated to about forty cubic centimeters. Two additional drops of
ammonium carbonate are added, and a few drops of caustic ammonia, to
precipitate any lime which may be redissolved by the action of the
ammonium chlorid solution on the calcium carbonate. Filter on a small
filter and wash with as little water as possible and collect the
filtrate in a small beaker. The filtrate contains all the alkalies as
chlorids, together with a little ammonium chlorid. Add a drop of
ammonium carbonate solution to be sure all the lime is precipitated,
evaporate on a water-bath in a deep platinum dish, in which the alkalies
are to be weighed. The dish should have from thirty to sixty cubic
centimeters capacity, and during the evaporation should never be more
than two-thirds filled. After the evaporation has been completed the
dish is slowly heated and then gently ignited over a gas-flame to drive
off any ammonium chlorid which may be present. During this process the
platinum dish may be covered with a thin piece of platinum to prevent
any possible loss by the spitting of the salt after the ammonium chlorid
has been driven off. The heat should be gradually increased until it is
brought to a point a little below redness, leaving the cover off. The
platinum dish is again covered, and when sufficiently cooled placed on a
balance and weighed.
If lithium chlorid be present it is necessary to weigh it quickly as the
salt being very deliquescent takes up moisture rapidly. The alkalies may
now be separated in the usual way.
If the sample under examination contains magnesia the residue in the
capsule should be dissolved in a little water and sufficient pure
lime-water added to render the solution alkaline. It should then be
boiled and filtered. The magnesia will, in this way, be completely
separated from the alkalies. The solution which has passed through the
filter is treated with ammonium carbonate in the manner first described,
and the process continued and completed as above mentioned.
If it be suspected that the whole of the alkalies have not been obtained
by the first fusion, the residue upon the filter can be rubbed up in a
mortar with an amount of ammonium chlorid equal to one-half the weight
of the mineral, mixed with fresh portions of calcium carbonate and
treated exactly as in the first instance. Any trace of alkali remaining
from the first fusion is thus recovered in the second one.
_Method of Heating the Crucible._—The apparatus used by Smith for
igniting the crucible is shown in Fig. 67. It consists of an iron
filter-stand HG, a clamp, ED, carrying the muffle NC, attached by the
supports AB, and heated by the lamp F. The muffle NC is a chimney of
sheet iron, eight to nine centimeters long, ten centimeters high, the
width at the bottom being about four centimeters on one side and three
centimeters on the other. It is made with the sides straight for about
four centimeters and then inclining toward the top so as to leave the
opening at the top about one centimeter in width. A piece is cut out of
the front of the chimney of the width of the diameter of the hole in the
iron support and about four centimeters in length, being semi-circular
at the top, fitting over the platinum crucible. Just above this part of
the chimney, is riveted a piece of sheet iron in the form of a flattened
hook, N, which holds the chimney in place by being slipped over the top
of the crucible support; it serves as a protection to the crucible
against the cooling effects of the currents of air.
[Illustration:
FIGURE 67.
SMITH’S MUFFLE FOR DECOMPOSITION OF SILICATES.
]
=356. International Method for Assimilable and Total Potash.=—In the
International Congress of Chemists held in Paris in 1889,[230] the
discrimination between the assimilable and total potash was declared to
be of prime importance. Unfortunately no method is known by which the
potash which is present in the soil in a state suited to the wants of
plants can be determined with approximate accuracy. In general, that
portion which is given up to weak acids may be assumed to be available.
In the treatment of soils with weak acid, as pointed out in the
Congress, it is demonstrable that with a 0.05 to 0.1 per cent nitric
acid solution, the quantity of potash which goes into solution increases
by continued stirring of the mixture with the time of action of the acid
up to a certain maximum which is reached in from three to four hours,
and after that, it is not changed even when the strength of the acid
mixture is increased to two per cent. From this time on, concentrated
acids withdraw from the soil which has already been exhausted by the
weak acid, a new quantity of potash. The soils which have been exhausted
by concentrated acids yield also an additional quantity of potash when
they are treated with hydrofluoric acid, or melted with barium or sodium
carbonate. Potash, therefore, appears to exist in the soil in various
forms.
First. In the form of indecomposable silicates which have,
agriculturally perhaps, very little interest.
Second. In the form of silicates which are more basic than those just
mentioned. These silicates are attacked by strong acids and give up
probably every year a portion of their potash to vegetation.
Third. In a form which is easily soluble in weak acids and consequently
directly assimilable by plants.
In view of the fact that it would be of interest to chemists and
agronomists to establish certain methods of investigation so as to be
able to obtain comparative results, it was decided to adopt the original
method recommended by Gasparin for the estimation of the potash
decomposable by concentrated acids. This method consists in the
treatment of the soil with boiling aqua regia until the sand which is
not decomposed, is white.
_Determination of the Fineness of the Earth which is Used for
Analysis._—For the estimation of potash, the soil should be divided as
finely as possible, and passed through a sieve of thirty meshes to the
centimeter. The decomposition is then completed in two hours, while if a
sieve of only ten perforations per centimeter is used, the acid must be
allowed to work for twelve hours.
The determination of the potash after solution, is accomplished by any
of the standard methods.
=357. Method of Tatlock as Used by Dyer.=—Attention was called in
paragraph =328= to the estimation of the total plant food in the soil by
extraction of the sample with citric acid. Dyer first determines the
total potash by Tatlock’s method which is as follows:
To determine potash ten grams of fine dry soil are treated with ten
cubic centimeters of hydrochloric acid and evaporated to dryness on the
water-bath, the residue taken up with another ten cubic centimeters of
acid, warmed, diluted with water, boiled, filtered, and washed. The
filtrate and washings are concentrated and gently incinerated to get rid
of organic matter, and the residue redissolved in hydrochloric acid, and
evaporated slowly with a considerable quantity of platinum chlorid. If
the evaporation be conducted slowly, the potassium platinochlorid
settles out well, despite the iron, aluminum, and calcium salts, and is
easily washed with some more platinum chlorid solution, followed by
alcohol. The application of this modification of the platinum chlorid
process to solutions containing comparatively minute quantities of
potash amid an overwhelming excess of iron, aluminum, and calcium salts
is probably new to many chemists. It works admirably, and obviates the
necessity for removing iron, aluminum, calcium, magnesium, etc., with
the necessary use of ammonia, and the tedious processes of concentration
and final volatilization of the ammonium salts; but, of course, the
process cannot be employed if soda also is to be determined.
The potash, soluble in hydrochloric acid, having been thus determined,
the undissolved siliceous matter is incinerated, weighed, and finely
ground in an agate mortar. A weighed portion of it is then, as in the
Smith method, mixed with a large bulk of pure calcium carbonate and a
little ammonium chlorid and heated, beginning with a low temperature,
rising slowly to bright redness. The mass is then boiled with water,
washed, incinerated, reground, mixed with some more ammonium chlorid,
and again heated, boiled, and washed. The process is repeated and the
filtrates from all the treatments concentrated, the calcium being
removed as carbonate, and the potash determined in the filtrate, after
evaporation and incineration at a low temperature, by means of platinum
chlorid.
Five hundred cubic centimeters of the citric acid solution of the soil,
made as described in paragraph =328=, corresponding to fifty grams of
soil, are evaporated to dryness in a platinum dish and ignited at a low
temperature. The residue is dissolved in hydrochloric acid filtered and
washed, and the filtrate again evaporated to dryness and treated again
as just described. The potash is then determined as above.
=358. Estimation of Total Alkalies and Alkaline Earths.=—To properly
determine the exact amount of these substances in a sample of soil it is
necessary first to remove the silica. This is accomplished in the
process of Berthelot and André[231] by intimately incorporating with the
sample, in a state of very fine powder, four or five times its weight of
ammonium fluorid. The mixture, in a platinum dish, is moistened with
strong sulfuric acid and allowed to stand for a few hours. It is then
gently heated until all fumes of hydrofluosilicic acid have disappeared,
but the mass is not raised to a red heat. If there is any doubt about
the complete decomposition of the silica the treatment is repeated.
At the end of the operation there remain only sulfates without excess of
sulfuric acid. The sulfates likely to be present are of potash, soda,
lime, magnesia, alumina, and iron. The separation of these bodies is
conducted in the ordinary manner.
Fusing the soil with potash does not give reliable results but it can be
used in certain cases for the rapid estimation of alumina and iron. In
this case after the separation of the silica in the ordinary way the
iron can be determined as ferric oxid.
The iron can also be directly determined by reducing to the ferrous
state and titrating with potassium permanganate.
_Comparison of Fluorin Method with Common Methods._—To establish the
difference in the data obtained by the old and new processes samples of
the same earth were treated by Berthelot and André by different methods
with the following results:
By the By the cold By the By
fluorin dilute concentrated incineration
method. hydrochloric hydrochloric and
acid method. acid method. subsequent
treatment
with boiling
hydrochloric
acid method.
Per cent. Per cent. Per cent. Per cent.
Potash 0.886 0.021 0.149 0.176
Soda 0.211 0.024 0.033 0.042
Magnesia 0.087 0.033 0.033 0.067
Lime 1.160 0.879 1.120 1.060
Alumina 3.950 0.102 1.009 2.631
Ferric oxid 2.150 0.296 1.401 1.678
The impossibility of getting all the alkalies and oxids into solution by
even the prolonged action of a boiling acid is clearly set forth in the
above table. Boiling sulfuric acid might do a little better but would
not give correct results. Lime alone of the elements in the soil can be
correctly determined by solution in boiling hydrochloric acid, a
circumstance due to the fact that lime is found chiefly as carbonate,
sulfate, and phosphate in the soil, and these compounds are easily
soluble in hot hydrochloric acid with the exception of the sulfate. Even
lime could not be thus determined in soils containing silicates rich in
lime. The other mineral elements cannot be determined by the wet method.
This is due to the forms in which they occur, being mostly silicates of
different composition, with excess of silica.
As to the silicates they may be divided into two groups. The first of
these are the hydrated silicates, resembling the zeolites, capable of
being completely decomposed by boiling acids. The first group of
silicates is doubtless of greater importance to vegetable life than the
second since it would, doubtless, give up its alkalies with greater
ease. This distinction is, however, arbitrary. It is, in fact,
impossible to place on one side the soluble and on the other the
insoluble silicates. This distinction represents only the unequal
degrees in the speed of decomposition of the different silicates
contained in the primitive rocks under the influence of atmospheric
agents, the soil being nothing more than the products of the
decomposition of these rocks with vegetable mold. The second group is
insoluble in acids.
That part of the silicates least decomposed at any given moment will be
attacked more easily by acids, while that portion whose decomposition
has been pushed furthest will be more slowly attacked. The action of the
acid will grow more feeble as the time of contact is prolonged, and
after a time a point is apparently reached where the results are nearly
constant. But it is evident that this distinction is purely conventional
and bears no necessary or even probable connection with the quantity of
alkali really assimilable by plants.
Vegetables, moreover, exert on a soil, for the extraction of its
alkalies and other matters, chemical reactions peculiar to themselves,
altogether distinct from the tardy action of atmospheric agents and
still more distinct from the rapid action of mineral acids.
It is well known with what energy, it ought to be said with what
admirable instinct, plants take from the soil the least traces of
phosphorus, of sulfur, of potash, of iron, and other substances
necessary to their sustenance.
These specific actions of vegetables on the soil merit, in the highest
degree, the attention of analysts and agronomists. Their intervention
plays a most important part in the restitution to the soil, by means of
complementary fertilizers, the mineral elements removed by vegetable
growth.
=359. Estimation of Lime by the French Method.=—The quantity of lime
contained in the soil varies within wide limits. Sometimes this base is
entirely absent to such a degree that it is even impossible to discover
feeble traces of it. Sometimes it composes almost the whole of the
earthy mass. Lime is found in the soil principally in the state of
carbonate. It is also found combined with organic matter under the form
of humates, with sulfuric acid, etc. It is customary to estimate the
lime as a whole, without distinguishing between the different states in
which it exists. The quantity of material which is used in the French
method[232] varies in proportion to the amount of calcareous matter
contained in it. For a soil which contains a large amount of lime, one
or two grams would be sufficient for the analysis. For a soil which is
poor in calcareous matter ten or even twenty grams must be taken. The
quantity of lime dissolved differs according to the strength of the
acids employed and length of contact of the acid with the soil. The
calcium carbonate, the sulfate, the nitrate, and the humate rapidly pass
into solution when treated with acid as above, but this is not the case
with calcium silicates which are attacked much more slowly. Sometimes
the silicates give only an insignificant increase in the amount of lime,
and in this case it is immaterial what process of solution is employed.
For simplicity it is best to adopt the method of solution in boiling
concentrated nitric acid, prolonging the boiling for a period of five
hours. This method of operation is sufficient to bring into solution at
one treatment, not only the lime, but also the potash and magnesia.
After having heated with acid for the necessary time there are added in
the capsule in which the solution took place ten cubic centimeters of
nitric acid and fifty cubic centimeters of water. The mixture is heated,
collected upon a filter and the residue washed. To the filtrate, the
volume of which should be from 400 to 500 cubic centimeters, a
sufficient quantity of ammonia is added to render it slightly alkaline.
There is formed a precipitate of alumina and of iron oxid containing
phosphoric acid and also sometimes a trace of the lime combined with the
same acid. In order to keep the whole of the lime in solution it is
necessary to add a little acetic acid, about ten cubic centimeters more
than is necessary to neutralize the ammonia which has been added in
excess. If the liquid is turbid on account of the presence of the iron
and aluminum phosphates it is necessary to filter it. There is
afterwards added a slight excess of ammonium oxalate in solution, and
the whole is left for twenty-four hours in order that the calcium
oxalate may deposit. Indeed, the complete precipitation is not always
immediate, and especially in the presence of magnesia it takes place
with slowness. The calcium oxalate is collected upon a filter and washed
with hot water. To determine the quantity of the lime the best procedure
consists in transforming the oxalate into carbonate by a careful
ignition, and afterwards heating in a Schloesing or Leclerc furnace for
four or five minutes. The oxalate for this purpose should be contained
in a covered platinum crucible. By this method the calcium carbonate is
transformed into calcium oxid, in which form it is weighed rapidly to
avoid absorption of moisture.
In laboratories which have no means of securing so high a temperature as
is mentioned before, the lime may be weighed as sulfate. For this
purpose the calcium oxalate is transformed into carbonate by ignition in
a platinum crucible. Afterwards it is treated with nitric acid until the
carbon dioxid is completely driven off. The platinum crucible is now
covered with a funnel which is afterwards washed in order to bring back
into the dish the small drops which have been projected in the process
of boiling. An excess of sulfuric acid is added and evaporated to
dryness on a sand-bath. Afterwards, in a muffle, the temperature is
carried to a feeble redness until the vapors of sulfuric acid are all
driven off. The lime is weighed in the form of sulfate, and the weight
multiplied by 0.412 gives the lime contained in the quantity of earth
analyzed.
In special researches in which it is desired to avoid attacking the
siliceous pebbles of the soil, the concentrated nitric acid is replaced
by dilute nitric acid in slight excess, and heated for a few moments
only. The calcium carbonate is then dissolved with the other calcareous
salts not combined with silica in the rock products. The analysis is
continued in other respects as just described.
=360. Estimation of the Actual Calcium Carbonate.=—The lime which is
found in the state of carbonate plays one of the most important rôles in
the chemical phenomena which take place in the soil. It is often of
great importance to determine it. The most certain process is to
estimate the carbon dioxid which is disengaged from the carbonate under
the influence of an acid and to receive this gas in a jar graduated to
measure it by volume. The flask recommended by the French Commission for
this purpose contains about 300 cubic centimeters. The neck of the flask
is connected with a condensing tube of about one centimeter interior
diameter, which is cooled by a current of water.
According to the presumed richness in calcium carbonate varying
quantities of earth are taken for analysis, from as little as half a
gram for soils which are rich in carbonate, up to five or even ten grams
for soils which are poor in carbonate. The apparatus is connected with a
mercury pump for the purpose of exhausting the air as completely as
possible therefrom. For this purpose the flask in which the carbonate is
disengaged is made in the shape of a tubulated retort. Through the
opening into the retort, a narrow tube is introduced and connected with
a small funnel by means of a rubber tube supplied with a pinch-cock.
When the retort has been connected with the mercury pump a slight vacuum
is produced and the pinch-cock is opened and forty cubic centimeters of
distilled water allowed to enter. The pinch-cock is closed soon enough
to retain a portion of the water in the funnel. The retort is then
heated and a vacuum partially produced by means of the pump. When the
flask is boiling, the steam drives out the air. A refrigerating jacket
is connected with the tube leading from the retort to the pump by means
of which the steam is condensed and falls back into the flask. After
some minutes of boiling, a vacuum is produced; the lamp is then taken
away and a cylinder, graduated at 100 cubic centimeters and filled with
mercury, is placed over the lower orifice of the pump, and there is
introduced into the apparatus, by the funnel above described, some
hydrochloric acid in small quantities, but sufficient only to saturate
the whole of the carbonate in the sample of soil taken. Usually three or
four cubic centimeters will be sufficient. The acid should be added in
such quantities as to prevent the production of any large amount of
foam. If frothing should be excessive a little oil can be added to the
flask. The whole of the carbon dioxid produced in the reaction is
withdrawn by means of the mercury pump and collected in the graduated
jar. Towards the end of the operation the flask is heated anew in order
to produce an ebullition which is continued for some time. The volume of
gas collected is measured after making the proper corrections for
pressure and temperature. Afterwards the carbon dioxid which has been
produced is absorbed by two or three cubic centimeters of a solution of
potash of 42° baumé. This potash is introduced into the graduated jar by
means of a pipette bent into the form of a =ᥩ= in the lower portion. If
the whole of the gas is not absorbed the volume which remains is read,
and this is subtracted from the original volume after having made the
proper corrections for pressure and temperature. The difference gives
the quantity of carbon dioxid contained in the amount of earth employed.
From this the actual weight of the calcium carbonate is computed. This
official French method does not appear to possess any advantage in
accuracy to the usual absorption method and is far more complicated.
=361. Estimation of the Active Calcareous Matter in Soils.=—Like other
soil elements, the calcium carbonate exists in different degrees of
fineness and availability in the soil. It must be admitted that the fine
particles play the most important rôle. The calcium carbonate, which
exists in large fragments, presents only a circumscribed surface and
remains almost inactive, although it is easily corroded by the rootlets
of plants. It is possible to estimate in a rapid way, the quantity of
fine carbonate in the soil, considering that in a time relatively short,
feeble acids act upon calcareous matter proportionally to the surface
which it presents, and that it attacks, therefore, especially the finest
particles. By measuring the amount of carbon dioxid set free under the
action of dilute acids it is possible to estimate the content of
available calcareous matter in the soil.
The apparatus of Mondesir is used for this purpose by the French
chemists. It is composed of a tubulated flask of about 600 cubic
centimeters capacity. The interior tubulature carries a manometer fixed
by means of a stopper. This is formed of a rubber tube, terminated by a
glass tube, whose extremity is united to a little rubber bag, very
flexible, placed in the interior of the flask.
_Graduation of the Apparatus._—If the apparatus is new it is necessary
to begin by graduating it. The rubber bag is filled with water, the air
being carefully excluded, in such a way that the level of the water
comes just a little above the bend in the tube. There are placed in the
flask 125 cubic centimeters of water and it is shaken for a few seconds.
The flask and the manometer are then unstoppered and the level of the
water in the manometer is made to equal the level of the water in the
flask. With a rubber ring the level of the water in the manometer tube
is marked. The manometer is then stoppered. There are then added to the
flask two-tenths gram of pure calcium carbonate. The flask is closed and
shaken for a minute. There are then added, enclosed in a little piece of
filter paper, six-tenths of a gram of pulverized tartaric acid and the
flask immediately closed and shaken several times. The manometer tube is
then uncorked and moved until the level of the water reaches the point
marked before. The difference in level after the height of the water
remains constant is then read. The depression in the level observed,
corresponds to two-tenths gram of pure calcium carbonate.
=362. Estimation of the Available Calcareous Matter in the Soil.=—There
is introduced into the flask of the apparatus a quantity of soil varying
in amount in accordance with the content of carbonate which it is
supposed to contain. There are added 125 cubic centimeters of water and
the flask is shaken for a minute. As in the test given before, the level
of the water in the manometer is then made to correspond to that of the
water in the flask. The level in the manometer is marked as before with
a rubber band, and the manometer is then closed. There are then added,
contained in a piece of filter paper, two grams of pulverized tartaric
acid and the operation is finished as described before. The amount of
tartaric acid added, in general, should be three times as much as the
amount of calcium carbonate supposed to be contained in the earth. The
pressure in the manometer being proportional to the quantity of carbon
dioxid disengaged, it is easy to calculate the quantity of calcium
carbonate in a state of fine division contained in the soil taken for
the test.
In order to fill the rubber bag it is necessary to put it in its proper
place in the apparatus. The flask is filled with water in order to
flatten the rubber bag and expel the air from it. It is then closed with
a cork. Afterwards, with the aid of a small funnel and with a copper
wire placed in the tube, the lower extremity of which descends just to
the elbow, the air in the tube is replaced by water. The operation is
finished by uncorking the flask and inclining it or shaking it after a
partial vacuum has been established. It is useless to attempt to drive
off the last particles of the air. The rubber bag should have a content
of about double the volume of the whole of the interior of the
manometric tube. In the washing which is necessary between two
successive operations, it is well to fill the flask entirely with water
in order to expel all the carbon dioxid which it may contain.
The same remark may be made of this method of determination as was made
of the last one. In the present case, however, the operation is not
quite so complicated. When the apparatus is once arranged, it will admit
of rapid determinations.
=363. Lime Method at the Riga Station.=—Ten grams of the non-ignited
sample of the fine earth are digested with 100 cubic centimeters of ten
per cent hydrochloric acid, in a 250 cubic centimeter erlenmeyer for
twenty-four hours on the steam-bath, with frequent shaking. The
filtrate, with washings after the addition of five cubic centimeters
strong hydrochloric acid, is evaporated to dryness in a porcelain dish
and the residue taken up with dilute hydrochloric acid. After filtering,
ammonia is added in excess, the excess removed by evaporation, and the
mass is again filtered. In the filtrate, the lime is thrown down with
ammonium oxalate, filtered, ignited, and weighed as calcium oxid.
The above method cannot give exact results chiefly because more or less
lime may be carried down with the phosphoric acid. Also if manganese be
present it will be thrown down with the lime. These errors are
compensatory, but only by chance could the compensation lead to
exactness. It would be better in all cases to remove the iron and
alumina in such a way as would avoid loss of time.
=364. Estimation of Assimilable Lime.=—In the determination of the total
lime in soils or even of that part present as carbonate, it is not to be
assumed that the quantity assimilable by plants is known; particles of
lime minerals in soils are corroded only superficially by the rootlets
of plants and any process which would attack only the superficies of the
lime particles would thus more nearly resemble the activity of the
solvent forces of plant growth. Oxalic acid is a reagent of this kind,
attacking only the surfaces of lime particles. Reverdin and de la Harpe
guided by this fact have based a method for determining the amount of
lime present in the soil in an available state on the solvent action of
oxalic acid.[233] After the total lime content has been determined,
twenty grams of the soil sample are covered with 200 cubic centimeters
of a solution containing in molecular proportion a known quantity of
sodium oxalate and carbonate. The mixture is digested on the water-bath
for one hour. By this treatment all lime minerals are converted
superficially into oxalate while particles containing magnesia are not
affected. After filtering and washing well, the filtrate and wash-waters
are acidulated with hydrochloric acid. If any precipitate of organic
matter be produced separate it by filtration. Treat the filtrate with a
slight excess of sodium acetate by which process the excess of
hydrochloric acid is replaced with acetic after which the oxalic acid
may be separated by treatment with calcium chlorid and subsequently
titrated with potassium permanganate in presence of excess of sulfuric
acid. The oxalic acid obtained, deducted from the quantity originally
present will give the amount consumed on the surfaces of the lime
particles and consequently the amount of lime corresponding thereto
which may be considered as available for plant growth.
=365. Method of the Halle Station for Lime.=[234]—a. _In Phosphates,
Limestones, etc._—Four grams of the prepared substance are heated with
fifty cubic centimeters of hydrochloric acid and five cubic centimeters
of nitric acid, in a porcelain dish on the water-bath to dryness, and
left for a few hours at 105° for the purpose of separating the silicic
acid. The dry residue is moistened with hot water and a few drops of
hydrochloric acid, and allowed to stand for some time with frequent
stirring. The contents of the dish are then washed into a half-liter
flask, filled up to the mark and the separated silicic acid removed by
filtration. If the silicic acid is not taken into account, the solution
can be made directly in a half-liter flask.
After filtration, an aliquot part of the filtrate is neutralized in a
500 or 250 cubic centimeter flask with ammonia, again acidified with a
few drops of hydrochloric acid and allowed to stand six hours at least,
in the cold, with ammonium acetate. For each four grams of the substance
fifty cubic centimeters of an ammonium acetate solution are used, made
by dissolving in one liter of water 100 grams of ammonium acetate. If
phosphoric acid is present in excess, iron and aluminum oxids are
precipitated completely as phosphates. If iron and aluminum oxids are in
excess, the excess must be precipitated by ammonia. If it is feared that
in the subsequent precipitation of the lime by ammonium oxalate there
may be still some phosphoric acid in solution, before precipitation with
ammonium acetate the proper amount of ferric chlorid is added and the
iron is afterwards precipitated with ammonia. It is certain that in the
presence of oxalic acid and phosphoric acid the lime is precipitated as
oxalate, but should it be feared that traces of calcium phosphate are
precipitated with the iron and aluminum phosphates the precipitate of
iron and aluminum phosphates may be dissolved in hydrochloric acid,
neutralized with ammonia, again acidified and a second time precipitated
with ammonium acetate and the filtrate added to that first obtained.
For the further estimation the filtrates are united and a quantity
corresponding to a given part of the original sample, and being in
volume from fifty to one hundred cubic centimeters is made slightly acid
with acetic acid and while hot precipitated with dilute ammonium
oxalate. The filtrate must contain acetic acid since calcium oxalate is
best precipitated from a slightly acetic acid solution. The filtering of
the calcium oxalate should not take place until from six to twelve hours
after precipitation, and during this time it should stand in a warm
place. Filter paper of the best quality should be used for the purpose.
The dried precipitate is brought into a platinum crucible together with
the filter; the filter is first incinerated over an ordinary bunsen and
the calcium oxalate converted into calcium oxid by ignition for fifteen
minutes over the blast. It is then cooled in a well-closed desiccator
and weighed as oxid. If in the precipitation of the iron and aluminum
phosphates sodium acetate be employed instead of ammonium acetate, the
precipitation must take place hot and filtration also be accomplished on
a hot filter.
b. _Estimation of Lime in Soils._—For the estimation of lime in soils
there may be used either the acid soil-extract, prepared as under the
direction for the estimation of potash, or twenty grams of the soil may
be treated with hydrochloric acid and a few drops of nitric acid, and
evaporated to dryness in a porcelain dish and the silicic acid separated
as described for the estimation of lime in phosphates and limestones. In
the case of soils, iron and aluminum oxids can be precipitated directly
with ammonia since the small quantity of phosphoric acid usually
contained in soils is not sufficient to influence in any way the
estimation of the lime. For example suppose there is 0.10 per cent of
phosphoric acid contained in a soil. In case the whole of this
phosphoric acid is taken down with the lime it would only amount to
about 0.10 per cent of calcium oxid precipitated as phosphate. This
case, however, is very improbable since it is much more likely that the
iron and aluminum phosphates will be precipitated and the whole of the
phosphoric acid be carried down with them instead of being precipitated
with the lime.
The precipitation of the lime and its subsequent treatment are to be
conducted as just described.
=366. Estimation of the Magnesia.=—Magnesia is a much more rare element
in the soil than lime. It is usually necessary to operate upon
considerable quantities of earth in order to determine the magnesia with
any degree of precision. From ten to twenty grams of the soil are taken.
The decomposition is accomplished as in the case of lime. A few drops of
barium nitrate are added for the purpose of precipitating any sulfuric
acid present. Some ammonia and ammonium carbonate are added to
precipitate the iron and aluminum oxids, the lime and the excess of
barium introduced, as well as the phosphoric acid. The operation is best
conducted on a dilute solution having a volume of from 400 to 500 cubic
centimeters. The solution from which the lime has been precipitated,
contains with the magnesia, large quantities of ammoniacal salts which
it is necessary to destroy. For this purpose the solution is
concentrated in a flask until its volume is about ten cubic centimeters.
About ten cubic centimeters of nitric acid are added and the whole
brought to the boiling point. Afterwards a few drops of hydrochloric
acid are added. Continuing the heating, hydrochloric acid is added in
small portions and, from time to time, some nitric acid until the
bubbles indicating the setting free of gaseous nitrogen, resulting from
the action of the nascent chlorin upon the ammonia, have completely
ceased to appear. The whole is then evaporated on a sand-bath in a
porcelain dish in order to separate the silica. The residue is taken up
by water containing a few drops of nitric acid. It is filtered and
evaporated to dryness in a covered porcelain dish. Upon the residue four
or five grams of oxalic acid, in a state of powder, are placed. A little
water is added in such a way that the moist mass covers entirely the
matter in the dish. In order to avoid all losses there is placed upon
the dish a funnel which serves as a cover. The dish is heated on a
sand-bath, but when the film which is formed begins to break there are
added from time to time, a little more oxalic acid and water until there
is no longer any disengagement of the vapor of nitric acid. Afterwards
it is evaporated to dryness and the heat raised to a low redness. The
magnesia is found in a free state or mixed with alkalies. It is washed
with a small quantity of water and collected upon a very small filter
paper. The filter paper is dried, burned, the ignition carried to
redness and afterwards cooled and weighed. In order to test the purity
of the magnesia it is transformed into sulfate by the addition of a few
drops of sulfuric acid. The excess of sulfuric acid is driven off by
heating moderately by means of a gas-burner moving it in a circular
manner round the bottom of the capsule and lifting the cover from time
to time in order to allow the vapors of sulfuric acid to escape. The
weight of the magnesium sulfate should correspond to that of the
magnesia from which it was formed.
Magnesia exists most often in the soil in the state of carbonate or
silicate. In this last state it is especially abundant in some soils,
such as those which are derived from mica schists, serpentines, etc. In
treating earth of this last quality with concentrated, nitric acid there
is dissolved also a notable part of the magnesia of the silicates. If,
however, it is treated for some minutes only with dilute hydrochloric
acid the amount of magnesia present as carbonate alone can be estimated
separately.
=367. Estimation of Magnesia in Soils.=—_Method of the Halle
Station._—For the estimation of magnesia the sample of soil or
fertilizer is brought into solution in the same way as is given for the
estimation of lime. After the separation of the silicic acid, the iron
and alumina are precipitated with sodium acetate. In the case of
phosphoric fertilizers, ferric chlorid should first be added in order
that the excess of phosphoric acid shall be in all cases certainly
combined with the iron. After this the lime is separated as usual with
ammonium oxalate. After the precipitation of the lime, the magnesia is
precipitated in an ammoniacal solution with sodium phosphate and the
ammonium magnesium phosphate estimated exactly as in the case with the
estimation of phosphoric acid, as magnesium pyrophosphate.
A simpler method for the estimation of magnesia consists in
precipitating it as ammonium magnesium phosphate in the presence of a
solution of ammonium citrate, the other bases remaining in solution. In
this case the operation is carried on in an inverse way as described
under the estimation of phosphoric acid, the proper quantity of the acid
solution being neutralized with ammonia and after the addition of sodium
phosphate, the required quantity of citrate solution added and a further
excess of ammonia supplied.
=368. Estimation of Manganese.=—The estimation of manganese in the
presence of Fe₂O₃, Al₂O₃, CaO, etc., presents peculiar difficulties. In
ordinary alluvial clays the quantity of manganese is proportionately
small and its estimation may be neglected. In volcanic clays the
quantity of manganese, in proportion to the lime and magnesia, is much
larger. The method used for estimating manganese is that of Carnot.[235]
The hydrochloric acid extract of the soil is evaporated to dryness and
heated with potassium bisulfate in order to destroy the organic
substance, the neutralized solution of the residue precipitated with
twenty cubic centimeters of hydrogen peroxid solution and thirty cubic
centimeters of ammonia. The colorless filtrate gives, with nitric acid
and bismuth peroxid, no trace of reaction for manganese. The
precipitate, washed by decantation, is carried into a carbon dioxid
apparatus and treated with oxalic acid and dilute sulfuric acid. From
the amount of carbon dioxid obtained, the quantity of manganese is
calculated on the supposition that the precipitate corresponds to the
formula Mn₆O₁₁.
=369. Estimation of the Manganese by the French Method.=—Manganese
exists in all plants and its presence in small quantities seems
necessary to vegetation. The method of estimation adopted by the French
Commission is the one proposed by Leclerc and is applicable even when
the base exists in small quantities.
Twenty grams of the soil are taken and the organic matter destroyed by
incineration. In a flask of 200 cubic centimeters capacity, are placed
thirty cubic centimeters of water and, little by little, some
hydrochloric acid for the purpose of decomposing the calcium carbonate.
When effervescence has ceased ten cubic centimeters of the same acid are
added and boiled for half an hour, filtered, washed, and the wash-water
and filtrate evaporated to dryness in a porcelain dish. Afterwards there
are added twenty cubic centimeters of nitric acid of one and two-tenths
density, and ten cubic centimeters of water. The liquor is boiled with
constant shaking. Afterwards there are thrown in, in two or three
portions, ten grams of lead dioxid. The boiling is stopped just at the
moment when all the lead oxid is introduced into the liquor and the
mixture is then shaken vigorously. The manganese is transformed by this
treatment into a highly oxygenized compound having a deep rose
coloration. It is transferred immediately afterwards to a graduated
cylinder of 100 cubic centimeters capacity, with the wash-waters the
volume is completed to 100 cubic centimeters and it is vigorously
stirred with a rod, flattened at its extremity, in order to obtain a
complete homogeneity of the liquid. The stirring rod is withdrawn and
the liquid left to settle. At the end of some minutes the principal part
of the liquid is clear, and it is decanted by means of a pipette
graduated at fifty cubic centimeters, and this quantity of the clear
liquid is poured into a small glass precipitating jar to which is added
immediately, with constant stirring, a solution of mercurous nitrate
from a graduated burette. The addition of the nitrate is arrested at the
moment when the rose color of the liquor disappears, and the volume of
the mercurous nitrate employed is read from the burette.
It is now necessary to determine the strength of the mercurous nitrate,
that is the quantity necessary to decolorize one milligram of manganese.
For this purpose dissolve by means of five cubic centimeters of
hydrochloric acid 150 milligrams of manganese dioxid, which is prepared
perfectly pure by means of precipitation. When the solution is complete
evaporate to dryness, add one cubic centimeter of sulfuric acid and heat
on a sand-bath until white fumes of sulfuric acid appear. Redissolve in
water and make the volume up to 100 cubic centimeters. Each cubic
centimeter of this solution should contain one milligram of manganese.
Take five cubic centimeters of this solution, equivalent to five
milligrams of manganese, treat in a capsule with twenty cubic
centimeters of nitric acid and ten cubic centimeters of water,
afterwards add ten grams of lead dioxid, carrying on the operation
exactly as described above. Fifty cubic centimeters, taken as before
described, are then decolorized by the solution of mercurous nitrate,
and thus it is easy to calculate the quantity of manganese which
corresponds to one cubic centimeter of the mercurous nitrate employed.
By a simple proportion the quantity of manganese contained in the twenty
grams of earth to be analyzed is calculated.
The mercurous nitrate is prepared by dissolving five grams of
crystallized mercurous nitrate in one liter of water; it is allowed to
repose for some time and is preserved in a well-stoppered flask.
=370. Estimation of Iron.=—Iron, in general, is quite abundant in the
soil where it is met with, principally in the state of anhydrous
sesquioxid or the hydrated sesquioxid of silicates. Some soils, however,
only contain iron in small proportions and it can happen that the
introduction of iron as a fertilizing element may be useful. Plants
assimilate iron only in small quantities, but it appears to be
indispensable to their development and to the proper functional activity
of their assimilating faculties. The method of estimation which is
recommended is based upon the decoloration of potassium permanganate by
iron in the ferrous state. The following description, based on the
method proposed by the French Commission, will illustrate the process to
be followed.
Ten grams of the soil are ignited in a porcelain capsule until all
organic matter is destroyed. The ignited mass is then introduced into a
flask of 100 cubic centimeters capacity with thirty cubic centimeters of
hydrochloric acid and fifteen cubic centimeters of water. It is boiled
for about half an hour. The iron oxid is dissolved and is found in
solution in the form of ferric chlorid. After filtering and washing, the
volume of the filtrate is reduced by evaporation to about twenty-five
cubic centimeters. The liquor is afterwards placed in a flask of from
100 to 150 cubic centimeters capacity, which is closed by a stopper
carrying a tube furnished with a valve destined to prevent the
re-entrance of the air. Ten cubic centimeters of dilute sulfuric acid
are added from a mixture containing twenty cubic centimeters of strong
acid and eighty cubic centimeters of water. Afterwards the iron is
reduced to the ferrous state by introducing into the flask, in
quantities of about five decigrams, metallic zinc and waiting after each
addition until the portion last added is dissolved before adding
another. This addition of zinc is continued until the iron is all
reduced. When this point is reached and the last portion of zinc added
is dissolved, the contents of the flask are transferred rapidly to a
precipitating glass of about one liter capacity, in which there has been
placed a little lately boiled but cold water. The flask is washed
several times with cold water, previously boiled, to remove from it all
traces of oxygen. The volume is made up to 500 cubic centimeters, and
afterwards, without any loss of time, by means of a graduated burette
and with constant stirring, a solution of potassium permanganate is
added which is stopped exactly at the moment when the liquor begins
taking on a light rose tint. The quantity of permanganate employed is
read from the burette and is proportional to the amount of iron
contained in the soil. A blank operation is made for the purpose of
detecting traces of iron which the zinc may contain. If, as often
happens, the soil contains a large amount of iron it is advisable to use
only one gram of it for this operation. The aspect of the earth will
indicate in general if it be very ferruginous.
_Preparation and Standardization of the Permanganate Liquor._—In one
liter of water are dissolved ten grams of crystallized potassium
permanganate and the quantity of iron which corresponds to one cubic
centimeter of this liquor is determined. It may be well enough to remark
that this liquor does not remain constant and it is necessary to titrate
it from time to time. For this purpose pure iron is taken. Piano wire
may be used, being almost pure iron. One-tenth of a gram of this wire is
dissolved in a flask in the manner recommended for treating the soil and
with the same quantities of acid and water. When the solution is
complete it is transferred to the flask to be estimated. It is made up
to one liter and permanganate added, just as in the case before
mentioned, until the rose color persists. There is thus determined the
quantity of iron which corresponds to each cubic centimeter of the
permanganate, and by a simple proportion, the quantity of iron contained
in the soil analyzed is determined.
The Italian agricultural chemists proceed essentially in the same manner
in determining the iron in soils, first igniting the sample and
afterwards extracting the iron in the ferric state with boiling
hydrochloric acid, reducing with hydrogen, and titrating with potassium
permanganate.
The following are the reactions which take place:
Fe₂O₃ + 6HCl = Fe₂Cl₆ + 3H₂O.
Fe₂Cl₆ + 2H = 2FeCl₂ + 2HCl.
10FeCl₂ + K₂Mn₂O₈ + 16HCl = 5Fe₂Cl₆ + 2KCl + 2MnCl₂ + 8H₂O.
Sulfuric may take the place of hydrochloric acid in the above reactions.
[Illustration:
FIGURE 68.
APPARATUS BY SACHSSE AND BECKER.
]
=371. Method of Sachsse and Becker.=[236]—Ferric oxid (not as silicate)
in soils can be estimated by reducing with hydrogen, and measuring the
hydrogen which is evolved by the action of the reduced iron on an acid.
The sample of soil is weighed in a platinum boat, the boat put into a
wide glass tube and heated in a stream of dry hydrogen. While this is
going on, water is boiled in the flask _A_ (see Fig. 68) from which the
stopper has been removed, to drive out the air. When the reduction of
the ferric oxid is complete, the boat is slipped out of the tube into
the flask without interrupting the hydrogen evolution. In order to
accomplish this without allowing the reduced iron to come in contact
with the air the flask is inclined, the end of the glass tube inserted
until it is covered with water and the boat is then dropped beneath the
water. The flask is closed with a cork provided with a funnel tube, _B_,
and a delivery tube _C_; the tap _a_ is opened, and tube _b_ connected
with a carbon dioxid apparatus from which the gas is passed into _A_
until all the air is displaced. This point is determined by filling the
burette _D_ with potash-lye by aspiration at _C_ and allowing the
escaping gas from _C_ to enter the burette as indicated in the figure.
Any residual gas in _D_ is removed by aspiration at _C_ and allowing the
potash-lye in _e_ to enter in its place. The end of the tube _C_ is now
placed under the measuring tube _D_, and the clamp _f_ opened and the
tap _a_ closed. The funnel is filled with dilute, boiled sulfuric acid,
the cork of _b_ replaced and connected with the carbon dioxid apparatus.
The burner under _A_ is lighted and acid let in. By continued boiling,
all the hydrogen is driven into _D_, the carbon dioxid being absorbed.
The measuring tube is then placed in a tall cylinder of water, the
volume of gas read and reduced to 0° and 760 millimeters barometric
pressure. To be certain that all carbon dioxid is absorbed, some fresh
potash-lye may be introduced into _D_ by carefully opening _d_. The iron
is then computed from the volume and weight of the hydrogen by the
formula (1) Fe + H₂SO₄ = 2H + FeSO₄.
If the substance analyzed contains iron silicates, these may be partly
decomposed with formation of ferrous sulfate, according to the reaction
(2) 2Fe + Fe₂(SO₄)₃ = 3FeSO₄. This will redissolve a part of the
metallic iron and yield ferrous oxid. In this case the contents of the
flask are cooled in an atmosphere of carbon dioxid, made up to 500 cubic
centimeters, of which 250 cubic centimeters are quickly filtered and
titrated with permanganate. In order to properly distribute the iron in
harmony with its previously existing states the following computations
may be made:
Represent the ferrous oxid corresponding to formula (1) by x
and that „ „ „ (2) by z
and that found by titration with permanganate by a.
We have then the equation x + z = a. Since seventy-two parts by weight
of ferrous oxid formed by formula (1) are equivalent to two parts by
weight of hydrogen, x parts of ferrous oxid would set free x/32 parts of
hydrogen; and this corresponds to the hydrogen found in _D_; _viz._, b.
If a = ¹⁄₃₆ then by solving the equations: Z = a − 36b and X = 36b. The
ferrous oxid arising according to formula (2), however, is derived in
such a way that only one-third of it corresponds to metallic iron. Then:
X + (⅓)z = (⅓)a + 24b. For computing the total ferric oxid reduced by
hydrogen there must, therefore, be added twenty-four parts by weight of
hydrogen for one-third of the ferrous oxid found by titration with
permanganate, and this quantity of ferrous calculated to ferric oxid.
Some silicates, such as the micas, give ferrous oxid with hot dilute
sulfuric acid. A correction for this is made by making one or more
determinations without previously reducing with hydrogen.
The method of procedure above described appears to be capable of giving
in an easily attainable manner some valuable indications of the state in
which iron exists in a soil. While plants do not use any notable
quantity of iron during their growth nevertheless its physiological
importance is unquestioned. The chief points of difficulty to be
considered are found in the changes which the iron may undergo even
while heating in a stream of hydrogen, and the practical difficulties of
obtaining carbon dioxid entirely free of air. The latter difficulty may
be overcome by making blank experiments with carbon dioxid alone and
estimating the volume of residual gas. The total volume of hydrogen
obtained is then to be diminished by the ascertained amount.
In regard to the second point it is known that both ferrous and ferric
oxids when ignited with hydrated silicates partly decompose and form new
silicates. Care should therefore be taken not to carry the temperature
too high during the process of ignition.
=372. Carnot’s Method for Estimating Phosphoric Acid in
Soils.=—Carnot[237] proposes the following procedure for the estimation
of phosphoric acid in soils. The principle of this method depends upon
the isolation of silica by the double precipitation of phosphomolybdate.
Ten grams of the sifted soil, dried at 100°, are charred if organic
matter be present. The charred mass is next moistened with water and
afterwards with nitric acid, until the carbonates are decomposed.
Afterwards the mass is digested with ten cubic centimeters of nitric
acid for two hours at about 100°, with frequent stirring and the
addition of fresh acid, from time to time, to replace that which has
been evaporated. After filtering and washing with hot water the filtrate
is evaporated to a volume of fifty cubic centimeters and treated with
five cubic centimeters of concentrated nitric acid and half a gram of
crystals of chromic acid. After covering the dish with a funnel to
return condensed vapors its contents are heated to the boiling point for
half an hour. At the end of this time five grams of ammonium nitrate are
added and afterwards fifty cubic centimeters of molybdate solution and
the mixture kept at a temperature of about 100° for an hour. The
precipitate obtained is washed twice by decantation with water
containing one-fifth of its volume of ammonium molybdate solution. It is
then dissolved in thirty cubic centimeters of ammonia diluted with an
equal bulk of warm water. The solution and the washings should measure
eighty cubic centimeters and the ammonia therein is neutralized with
nitric acid, keeping the temperature below 40°. When the yellow
precipitate formed ceases to redissolve on stirring, a mixture of three
cubic centimeters of pure nitric acid and five cubic centimeters of
water is added, together with the same quantity of molybdate solution.
The precipitate is brought upon a filter, washed first with water
containing one per cent of nitric acid and finally with a little pure
water, and dried at 100° and weighed. The weight of the precipitate
multiplied by the factor 0.0373 gives the quantity of phosphoric acid.
The object of the second precipitation is to relieve the process of the
necessity of rendering the silica insoluble, as the presence of silica
in the solution as above treated does not interfere with the complete
precipitation of the phosphate. This was proved by the author, by the
introduction of considerable quantities of sodium silicate and these
were found not to interfere with the accuracy of the operation.
The results are as accurate as those obtained by the methods of the
consulting committee of the agricultural stations. The coefficient
employed; _viz._, 0.0373, is not the same as that recommended by the
committee; _viz._, 0.043. The committee, however, itself has recognized
the inaccuracy of the latter number. The composition of the compound
obtained by double precipitation according to Carnot is
P₂O₅24MoO₃3(NH₄)₂O + 3H₂O.
=373. Method of the Halle Experiment Station.=—The available or easily
soluble phosphoric acid in soils is estimated by Maercker and Gerlach,
as follows:[238]
Sixty grams of the air-dried soil as prepared for analysis, are placed
in an erlenmeyer with 300 cubic centimeters of two per cent citric acid
solution and digested for twenty-four hours in the cold. It is necessary
in this time to shake the flask four or five times and to put the
stoppers in loosely in order to allow the escape of any evolved carbon
dioxid. Of this mixture 200 cubic centimeters are filtered and
evaporated in a 300 cubic centimeter dish to dryness. There remains, in
most cases, a sirupy-like mass from which even by strong heating the
silica is not completely separated. In order to reach this result the
residue is treated with twenty cubic centimeters of concentrated
sulfuric and five cubic centimeters of fuming nitric acid and heated
over a bunsen. As soon as the appearance of foam denotes the beginning
of the reaction the lamp must be removed. With strong foaming and the
evolution of red-brown vapors the citric acid is completely oxidized.
After the reaction is ended the contents of the dish are heated for
about fifteen minutes over a small flame so that a continuous, yet not
too violent evolution of sulfuric acid fumes takes place. After the
silicic acid and the greater part of the lime have been separated in
this way the contents of the dish are diluted with water, stirred with a
glass rod, washed into a 200 cubic centimeter flask, cooled, filled up
to the mark, and filtered. From the filtrate 100 cubic centimeters,
corresponding to twenty grams of the earth, are taken, made slightly
alkaline with ammonia, acidified by a few drops of hydrochloric acid,
and after cooling treated with fifty cubic centimeters of the citrate
solution and twenty-five cubic centimeters of the magnesia mixture. The
complete separation of the precipitate requires about forty-eight hours
and shaking of the precipitate is not necessary.
=374. Estimation of total Phosphoric Acid in Soils.=—In the method used
at the Halle Station[239] twenty-five grams of the soil sample are
boiled with twenty cubic centimeters of nitric acid and fifty cubic
centimeters of concentrated sulfuric acid for half an hour. With very
clayey soils only half the quantity of the sample mentioned above is
used in order to avoid the too great accumulation of soluble alumina.
The oxidation of the organic substances of the soils must be carried on
at a moderate heat to avoid foaming. During the boiling, the flask is to
be often shaken to prevent the soil constituents from accumulating too
firmly at the bottom. The total volume is finally made up to 500 cubic
centimeters.
For the estimation, 100 cubic centimeters of the solution, corresponding
to five (or two and a half) grams of the soil, are taken. In order to
nearly completely saturate the acid, the solution is treated with twenty
cubic centimeters of twenty-four per cent ammonia, care being taken that
the precipitate of iron and alumina which is formed is again completely
dissolved. The solution is cooled and treated with fifty cubic
centimeters of the citrate solution, and then with twenty cubic
centimeters of ammonia of above strength, and precipitated with the
magnesia mixture. The filtration of the precipitate should not be made
for at least forty-eight hours, during which time the flask should be
often shaken to prevent the attachment of ammonium magnesium phosphate
to its sides and bottom.
A detailed description of the citrate method for estimating phosphoric
acid will be found in the chapter devoted to this subject under
fertilizers.
=375. French Method for Phosphoric Acid.=—Phosphoric acid is found in
the soil principally in combination with alumina and iron oxid, with
organic matters, or with lime and magnesia. Whatever may be the state in
which it is found all the phosphoric acid, with the exception of that
which enters into the constitution of insoluble mineral particles, can
be brought into solution by acids and determined by some of the approved
methods. This method of solution, therefore, is capable of determining
very accurately the total proportion of phosphoric acid in the soil, but
it is incapable of rendering account to us of the state in which the
phosphorus is found and of its aptitude to be utilized by plants.
The estimation of soil phosphorus, as recommended by the French
Commission, is carried on in the following way:[240] Twenty grams of the
earth are submitted to ignition in a muffle heated to the temperature of
redness but not higher. This calcination eliminates the organic
materials, whose intervention in subsequent reactions might be able to
prevent the precipitation of a part of the phosphoric acid. The calcined
earth is placed in a capsule of about eleven centimeters diameter and
saturated with water. There is then added in small quantities, as long
as effervescence is produced, nitric acid of 36° baumé. When the
effervescence has ceased, after thorough shaking and the addition of a
new quantity of acid, it will be found that the whole of the calcium
carbonate in the soil has been decomposed. It is necessary then to
proceed to the solution of the phosphoric acid by adding twenty cubic
centimeters of nitric acid and heating on the steam-bath for five hours,
shaking from time to time, and avoiding complete desiccation. At the end
of this time the whole of the phosphoric acid has entered into solution.
It is taken up by warm water, filtered, and the insoluble residue washed
with small quantities of boiling water. But from the solution obtained,
which holds in addition to phosphoric acid, some oxid of iron, alumina,
lime, magnesia, etc., it is necessary to separate the silica which has
passed into solution. For this purpose the mass is evaporated to dryness
on a sand-bath, heating toward the end of the operation with precaution
and not allowing the temperature to pass beyond 110°–120°. In these
conditions there is obtained a magma which sometimes remains quite
sirupy when the earth is very highly impregnated with calcium carbonate,
but in which the silica is insoluble. It is indispensable that it be
eliminated wholly because it would introduce grave errors into the
results, as will be seen later on. If the temperature be carried too
high during the desiccation this silica would react upon the earthy
salts and alkaline earths forming silicates and it would be found
ultimately again in solution. The application of a too high temperature
would also render somewhat insoluble in nitric acid the iron and
aluminum oxids, and these would retain small quantities of phosphoric
acid. The desiccation, therefore, requires to be conducted with great
precaution. When it is accomplished there are placed in the capsule five
cubic centimeters of nitric acid and five cubic centimeters of water,
and the whole heated on the sand-bath until the entire amount of iron
oxid is dissolved, that is to say, until there is no ferruginous deposit
persisting in the liquid. The solution is then filtered and washed with
small quantities of boiling water in such a way that the total volume of
the filtrate will not exceed twenty-five to thirty-five cubic
centimeters. Afterwards there are added twenty cubic centimeters of
ammonium nitromolybdate and the whole is left at rest for twelve hours
at the ordinary temperature. At the end of this time the whole of the
phosphoric acid is precipitated in the form of ammonium
phosphomolybdate. In order to be certain that an excess of
nitromolybdate has been used in the precipitation, which is
indispensable to the total precipitation of the phosphoric acid, a few
cubic centimeters of the filtrate are removed by means of a pipette and
are mixed with their own volume of the ammonium nitromolybdate. If, at
the end of an hour or two, no precipitate is formed the operation can be
regarded as terminated.
In order to collect and weigh the ammonium phosphomolybdate some
precautions are necessary. Two smooth filters are used, one of which
serves as a counter-weight for the other on the balance. One of these
filters is placed within the other and the phosphomolybdate is collected
upon the inner filter. The part of the phosphomolybdate adhering to the
precipitating jar is detached by the aid of a stirring rod, one of the
ends of which is covered with a piece of rubber tubing. The washing is
accomplished with very small quantities of water containing five per
cent of its volume of nitric acid. When all of the precipitate is
collected upon the filter and the washing is terminated a few drops of
water are thrown upon the upper borders of the filters; this is done to
displace the acid liquor which has been used in washing. The filters are
then carried to the drying oven where they are dried at a temperature
not exceeding 90°. The application of a higher temperature would
decompose the ammonium phosphomolybdate and lead to results which would
be too low. After the drying is completed the two filters are separated
and placed upon the arms of the balance and the increase in weight
corresponds to the ammonium phosphomolybdate. This, multiplied by the
coefficient 0.043, (see page 404) gives the quantity of phosphoric acid
contained in the weight of the soil which has been employed. The
ammonium phosphomolybdate is pure if all the silica has been eliminated,
but if a part of that has remained in solution it would furnish an
ammonium silicomolybdate whose weight would be added to that of the
phosphomolybdate. The elimination of the silica, therefore, should be
made with the greatest care.
Different processes have been proposed in order to determine the forms
under which phosphoric acid should be regarded as most assimilable.
Deherain has proposed acetic acid as a solvent for this purpose. Other
scientists, oxalic or citric acid, and the ammonium oxalate or citrate.
The solubility of phosphoric acid in these different reagents gives some
information in regard to its state, but the relations which exist
between this solubility and the assimilability of the acid have not yet
been fixed.
_Preparation of the Ammonium Nitromolybdate._—One hundred grams of
molybdic acid are dissolved in 400 grams of ammonia with a density of
ninety-five. The mixture is filtered, and the filtered liquor is
received drop by drop in 1,500 grams of nitric acid of one and
two-tenths density, constantly stirring. This mixture is left standing
for some days in an unexposed locality, during which time a deposit is
formed. The clear part is decanted and used.
The above method of the French chemists unfortunately attempts to
determine the phosphorus content of the soil by weighing the yellow
precipitate and using an empirical factor for the calculation, a factor
which is probably too high.
Experience has shown that at this point it is far more accurate to
continue the process by dissolving the yellow precipitate, and
subsequently obtaining the phosphoric acid in combination with ammonia
and magnesia, or according to the process of Pemberton the content of
phosphoric acid in the yellow precipitate might be determined by
titration. In regard to the latter method which will be given in full
under fertilizers, it may be said that it has been found quite accurate
by several analysts, although it is difficult to see how a precipitate
which is so variable in its constitution as to be estimated with little
safety by weight may yet be capable of rather exact determination by
titration.
=376. Petermann’s Method for the Estimation of the Phosphoric Acid
Soluble in Alkaline Ammonium Citrate.=[241]—From twenty-five to fifty
grams of the sample of soil are triturated with 100 cubic centimeters of
alkaline ammonium citrate and placed in a flask of 250 cubic centimeters
capacity, and allowed to digest for one hour at a temperature of from
35°–40°. After cooling, make up to the mark, filter, take 200 cubic
centimeters of the filtrate, evaporate to dryness on a sand-bath in a
platinum dish, heat lightly at first, and afterwards to a higher
temperature. Take up the residue of the incineration with water and
about two cubic centimeters of nitric acid, heat a few minutes gently,
filter into a bohemian flask and precipitate with fifty cubic
centimeters of ammonium molybdate solution, and estimate the phosphoric
acid in the usual way.
=377. Method of Dyer for Total and Assimilable Phosphoric Acid.=—For the
determination of phosphoric acid, soluble in citric acid, secured as
described in paragraph =328=, 500 cubic centimeters of the filtrate
obtained, corresponding to fifty grams of the soil, are evaporated to
dryness in a platinum dish, gently ignited, extracted with hydrochloric
acid, again evaporated, ignited and extracted, and the phosphoric acid
determined as below in the method applied to the hydrochloric acid
extract of the soil itself.
The total phosphoric acid is determined in each case in ten grains of
the dried soil and also in twenty-five grams, the mean of the two
results being taken. The numbers obtained in each case are, however, all
but identical, the difference in the duplicate percentages being in most
cases only a small one in the third place of decimals.
The soil is incinerated and digested with hydrochloric acid, and
evaporated to dryness, redigested with acid, filtered, and washed. The
filtrate and washings are concentrated to a small bulk, and treated, in
the cold, with excess of a solution of ammonium molybdate in nitric
acid. After standing forty-eight hours, the liquor is decanted through a
filter, the precipitate washed several times by decantation, first with
dilute acid, then with pure water in very small doses, and finally
transferred to the filter and washed free from excess of acid. The
ammonium phosphomolybdate is then dissolved in ammonia, evaporated to
dryness in a platinum capsule, and dried to constant weight at 100°. The
residue contains three and one-half per cent of its weight of phosphoric
acid. This is the method of Hehner; and for determining small quantities
of phosphoric acid, such as occur in soils or in solutions of iron and
steel, is in the opinion of Dyer very much to be preferred to the
old-fashioned method of conversion into magnesium ammonium phosphate.
The solubility of the yellow precipitate in the small quantity of
wash-water used is in most cases negligible. As a matter of fact, the
quantity of wash-water used in these analyses was found capable of
dissolving only 0.005 gram of precipitate, of which only 0.00017 is
phosphoric acid, making an error of 0.0017 per cent on the soil if ten
grams be used, or of only 0.0006 if twenty-five grams be used. In the
citric acid experiments the solution from fifty grams of soil is used,
when the error due to solubility of precipitate shrinks to 0.0003 per
cent. The correction for this solubility is, however, made in each case.
It may be observed that the method of Hehner is not applicable if the
molybdic solution be added to a hot liquid, since, in that case, some
molybdic acid is sure to crystallize with the yellow precipitate.
Moderate and careful warming to about 35° hastens precipitation, but it
is preferable, when speed is not a special object, to precipitate cold,
and leave the beaker standing at the laboratory temperature over night,
or longer if the quantity to be determined is very minute.
=378. Methods of Berthelot and André.=—The phosphorus in the soil may be
found under three forms; _viz._,
1. Phosphoric acid in phosphates.
2. Phosphoric acid in ethers which alkalies decompose slowly and
oxidizing agents destroy with regeneration of phosphoric acid.
3. Organic compounds or mineral compounds of phosphorus which are
resolved by alkaline solutions with formation of phosphoric acid and
which are not reduced to this state by the reagents employed in the wet
way of decomposition except after a contact of indefinite length and
uncertainty.
It is, therefore, seen that the employment of oxidizing agents for the
valuation of phosphoric acid in soils and vegetables is not a very
reliable procedure. The same is true after incineration by which more or
less phosphorus may be lost or rendered insoluble in acids. The methods
used by Berthelot and André for the estimation of these forms of
phosphorus are as follows:[242]
_Total Phosphorus._—The sample is at first oxidized by a current of air
near a red heat and the vapors are conducted over a column of sodium or
potassium carbonate at the same temperature. The combustion is finished
in a current of pure oxygen. All phosphorus compounds, even those which
are volatile, are by this treatment converted into phosphoric acid. The
part of the acid held by the carbonate is to be determined with the
non-volatile portions.
A less certain method of oxidation consists in mixing the material with
potassium nitrate and carefully throwing it little by little into a red
hot platinum crucible.
_Estimation of the Phosphoric Acid Pre-existing as Phosphates._—The
sample is treated with a cold dilute acid incapable of exercising an
oxidizing or decomposing effect on the ethers. The dissolved acid is
precipitated and weighed in the usual way. The precipitate first
obtained should be ignited and the phosphoric acid taken up and
reprecipitated. This is necessary to remove any organic matter or silica
which the first precipitate may contain.
_Estimation of Ethereal Phosphoric Acid._—The sample is boiled for some
time with a non-oxidizing acid or with a concentrated solution of
potash. The phosphoric acid dissolved represents that which was present
as phosphates and as ethers. From this, deduct that portion pre-existing
as phosphates and the remainder represents the part derived from the
ethereal compounds.
_Estimation of Phosphorus in Organic Compounds and Special
Minerals._—From the total phosphoric acid deduct that found as
phosphates and ethers. The difference represents the quantity combined
as noted in the caption. Illustration.
A sample of soil contained:
Total phosphoric acid 0.292 per cent.
—————
Of this, pre-existing as phosphoric acid 0.109 „
As ethereal phosphoric acid 0.074 „
As organic phosphoric acid 0.109 „
—————
Sum 0.292 „
=379. Method Used at the Riga Station.=—In the method pursued at the
experimental station at Riga[243] the organic matter in the fine earth
is first destroyed by igniting twenty-five grams in a muffle. The
ignited residue is placed in a 250 cubic centimeter erlenmeyer and
digested with 150 cubic centimeters of ten per cent hydrochloric acid.
The digestion is continued for forty-eight hours with frequent shaking.
The filtrate is evaporated to dryness in a porcelain dish to separate
any dissolved silica. The residue is taken up with dilute hot nitric
acid, filtered, and the phosphoric acid precipitated with ammonium
molybdate. The final weighing is made as magnesium pyrophosphate,
following the usual procedure in respect of precipitation and washing.
Experiments show that approximately ninety-five per cent of the
phosphoric acid are obtained by one extraction and five per cent by a
second, conducted exactly as the first. Thoms draws the following
conclusions from a long series of determinations:
(1) For the simple purpose of determining the need of a soil for
phosphorus fertilizer a single extraction with ten per cent hydrochloric
acid is sufficient. The difference between the first and second
extraction; _viz._, five to six per cent is too small to be of any value
from a practical point of view.
(2) A soil which has been ignited until organic matter is destroyed
gives up to the hydrochloric acid solvent about fourteen per cent more
phosphoric acid than a non-ignited sample would.
(3) The mean temperature in the flask during extraction on a steam-bath
is 74°.
=380. Method of Hilgard.=[244]—The weighed quantity of soil (usually
from three to five grams) is ignited in a platinum crucible, care being
taken to avoid all loss. The loss of weight after full ignition gives
the amount of chemically combined water and volatile and combustible
matter.
The ignited soil is now removed to a porcelain or glass beaker, treated
with four or five times its bulk of strong nitric acid, digested for two
days, evaporated to dryness, first over the water-bath and then over the
sand-bath, moistened with nitric acid, heated and treated with water.
After standing a few hours on the water-bath it is filtered and the
filtrate is evaporated to a very small bulk (ten cubic centimeters) and
treated with about twice its bulk of the usual ammonium molybdate
solution, thus precipitating the phosphoric acid. After standing at
least twelve hours, first at a temperature of about 50°, it is filtered
and washed with a solution of ammonium nitrate acidified with nitric
acid. The washed precipitate is dissolved on the filter with dilute
ammonia water. After washing the filter carefully, the ammoniacal
solution is treated with magnesia mixture, by which the phosphoric acid
is precipitated. After allowing it to stand twenty-four hours it is
filtered, washed in the usual way, dried, ignited, and weighed as
magnesium pyrophosphate, from which the phosphoric acid is calculated.
When a gelatinous residue remains on the filter after dissolving the
phosphomolybdate with ammonia it may consist either of silica not
rendered fully insoluble in the first evaporation, or, more rarely, of
alumina containing phosphate. It should be treated with strong nitric
acid, and the filtrate with ammonium molybdate; any precipitate formed
is, of course, added to the main quantity before precipitating with
magnesia solution.
=381. Separation of Phosphoric Acid From Iron and Alumina.=—The
following methods are suggested by Wolff[245] for the complete
separation of the phosphoric acid from the iron and alumina in soil
analysis, where large quantities of these bases are found in solution:
1. After the separation of the greater part of the iron and alumina the
phosphoric acid is precipitated from the solution in nitric acid by
molybdic acid. The process is carried on as follows:
The acid extract is heated in a flask to boiling and the iron oxid
completely reduced by the gradual addition of small particles of sodium
sulfite. While still warm the free acid is neutralized with soda-lye,
and ammonia added until the ferrous hydroxid and the aluminum hydroxid
are completely separated. Acetic acid is now added in excess and until
about four-fifths of the whole precipitate have passed again into
solution. Then, after boiling for a moment, the whole is quickly
filtered through a large filter with a cover, and the contents of the
filter finally washed slightly. All the phosphoric acid is thus obtained
in combination with some alumina and a very little iron. Nearly the
whole of the iron and the larger part of the alumina, by this
precipitation, are found in the filtrate and therefore cannot disturb
the estimation of phosphoric acid in succeeding portions. The filter is
now filled with boiling water and a little nitric acid added. The
precipitate is dissolved and received in a beaker. The precipitation of
the phosphoric acid is then accomplished by ammonium molybdate in the
presence of nitric acid. After twenty-four hours all the phosphoric acid
is thus precipitated and the precipitate is free from iron.
2. By the method of Schulze[246] the iron is completely, and the
alumina, with the exception of a small quantity, separated, and the
precipitation of the phosphoric acid is accomplished either by the
addition of a small quantity of tartaric acid and afterwards magnesium
sulfate, or directly by means of ammonium molybdate. The principle of
the separation depends on the fact that when the hydrochloric acid is
nearly neutralized with soda or ammonia, and the solution boiled after
treatment with ammonium formate, the greater part of the alumina remains
in solution. The precipitate is quickly filtered, washed with hot water,
dried, taken from the filter and fused in a silver crucible with pure
caustic alkali, either soda or potash. On solution and boiling with
water, the iron is completely separated from the phosphoric acid, and
from the small quantity of the alumina present the precipitation of the
phosphoric acid can now be accomplished, either by saturation of the
alkaline solution with hydrochloric acid and the direct addition of the
magnesia solution after the addition of a little tartaric acid and
ammonia, or after the addition of nitric acid by ammonium molybdate.
=382. Estimation of Phosphoric Acid in Muck and Peat Soils.=—The amount
of phosphoric acid obtained by extraction with hydrochloric or sulfuric
acid is markedly less in these soils than that obtained after the
incineration of the sample, as pointed out by Schmoeger.[247] This is
due to the fact that the phosphoric acid is ordinarily combined in the
form of nuclein. Extraction of the soils with ether shows that it is not
present in the form of lecithin. The nuclein products, as is well known,
are decomposed by heating in presence of a liquid at a high temperature
for some time. The heating can either take place in an autoclave or in
sealed glass tubes. The method is as follows:
The sample of soil is thoroughly rubbed up in a mortar with water, and
then hydrochloric acid added until one gram of the water-free peat is
suspended in about ten cubic centimeters of twelve per cent hydrochloric
acid. The sample is placed in a glass or porcelain vessel in an
autoclave and heated to 140°–160° for ten hours. The phosphoric acid is
then determined in the extract in the usual way. The percentage of
phosphoric acid determined in this way is found to correspond to the
amount determined by the incineration of the substance.
The total phosphoric acid is determined in peats by the incineration of
the sample and the estimation of the phosphoric acid in the ash. The
phosphoric acid soluble in hydrochloric acid solution is determined by
extracting a sample of the soil with twelve per cent hydrochloric acid
in the usual way. The difference between this and the total is
calculated as phosphoric acid in organic compounds.
Or the total phosphoric acid is determined by treating the soil with
twelve per cent hydrochloric acid, in the proportion of one gram of soil
to ten cubic centimeters of the acid, and the solution is placed in an
autoclave and heated for ten hours to 140°–160°, as above described. The
phosphoric acid is then determined by the usual method. The difference
between the total phosphoric acid as thus determined and the phosphoric
acid soluble in hydrochloric acid is calculated as phosphoric acid in
organic compounds.
=383. Method of Goss.=—On account of the length of time required to
determine the phosphoric acid in soils by the usual methods, Goss[248]
has proposed the following modification which in his hands has given
satisfactory results:
Weigh ten grams of the air-dried soil, which has been sifted through a
one millimeter mesh sieve, and transfer to a pear-shaped, straight
necked, kjeldahl digestion flask, which has been marked to hold 250
cubic centimeters. Add approximately seven-tenths gram of yellow
mercuric oxid and twenty to thirty cubic centimeters of concentrated
sulfuric acid, as for the determination of nitrogen. Twenty cubic
centimeters of acid are nearly always sufficient, but in the case of
unusually finely divided clay soils containing little or no sand it is
necessary to use thirty cubic centimeters to prevent caking of contents
of flask. In doubtful cases twenty cubic centimeters of acid should
first be added and at the end of five or ten minutes, if contents show a
tendency to cake, ten cubic centimeters more should be introduced.
Thoroughly mix the contents of the flask by shaking, place on a suitable
support over a burner, boil for one hour, cool, add about 100 cubic
centimeters of water, five cubic centimeters of concentrated
hydrochloric acid, and two cubic centimeters of concentrated nitric
acid, boil gently for two minutes to oxidize iron, cool, make up to
volume, and filter through a dry folded paper until perfectly clear. In
order to secure a clear filtrate it will usually be found necessary to
pour the first portion of the filtrate back through the paper three or
four times. Transfer 100 cubic centimeters of the filtrate to an
ordinary flask of about 450 cubic centimeters capacity, add strong
ammonia until a permanent precipitate forms, then six or eight cubic
centimeters of nitric acid to dissolve the precipitate, and boil until
clear. In the case of many soils it is not absolutely necessary to
oxidize with hydrochloric and nitric acids, as a clear solution can be
secured at this point without further oxidation. In the case of some
soils, however, and especially in subsoils, the solution cannot be
cleared up even by prolonged boiling with nitric acid, but if the
solution have been previously oxidized, a clear solution can be secured
without any difficulty whatever. Remove the flask from the lamp and
after two minutes add seventy-five cubic centimeters of molybdate
solution, place the unstoppered flask in an open water-bath kept at a
temperature of 80° for fifteen minutes, shaking vigorously four or five
times while in the bath; then remove, let stand ten minutes to allow
precipitate to settle, filter through a nine centimeter filter avoiding
too strong a pressure at first, wash the flask and precipitate
thoroughly with ammonium nitrate solution, place the flask in which the
precipitation was made under the funnel, shut off pump and close all
valves to filtering jar to form an air-cushion and prevent too rapid
filtration, fill paper two-thirds full of hot water, add a few cubic
centimeters of strong ammonia, aid solution, if necessary, by stirring
precipitate with a small glass rod.
As pointed out by Hilgard, aluminum is sometimes carried down with the
phosphoric acid upon precipitating with molybdate solution, in which
case some of the phosphoric acid will not be dissolved in the treatment
with ammonia. This will be indicated, first, by the appearance of a
white precipitate upon dissolving the yellow precipitate in ammonia;
and, second, by the difficulty experienced afterward in washing. If such
a precipitate be present in any appreciable quantity, proceed as
follows:
After washing out all the ammoniacal solution in the usual manner, place
a small beaker under the funnel, close all valves, fill the filter
one-third full of hot water, add the same amount of concentrated
hydrochloric acid, proceed as if dissolving phosphomolybdate in ammonia,
and receive final solution and washings in flask used. As soon as the
yellow precipitate is dissolved open the valve to filtering jar but do
not turn on the pump; after the solution has all passed through rinse
the filter once with a small amount of hot water; after the last portion
has passed through remove the flask and place a No. 1 lipped beaker
under the funnel and heat the solution in the flask to boiling. If the
solutions have not been oxidized, a blue color is sometimes present upon
dissolving the yellow precipitate in ammonia. This can be discharged by
boiling the ammoniacal solution for a minute or two and shaking at the
same time. Again pour the solution through the filter, avoiding use of
pump at first, otherwise loss from spattering is likely to ensue, wash
out flask and filter with a small amount of hot water, (the total
filtrate should not exceed fifty cubic centimeters), add hydrochloric
acid to contents of beaker while hot, until yellow color appears, then
add a few drops of ammonia until solution clears, cool, add fifty cubic
centimeters of filtered magnesia mixture from a burette, a drop at a
time with constant stirring, let stand fifteen minutes, add twenty cubic
centimeters of strong ammonia specific gravity nine-tenths, let stand
over night, filter, wash precipitate with dilute ammonia, dry, ignite
intensely over blast-lamp for ten minutes, cool in desiccator and weigh
Mg₂P₂O₇ secured.
_Time of Digestion._—Experience has shown that very little phosphoric
acid is extracted from the sample by digestion with sulfuric acid after
the first thirty minutes.
_Time Required to Precipitate Phosphomolybdate._— When the yellow
precipitate is obtained according to the method of Goss practically the
whole of it will be thrown down in five minutes.
_Agreement with Standard Methods._—Comparative tests of the Goss method
against standard methods have shown that it gives almost identical
results with them. The variations were never more than 0.02 to 0.03 per
cent.
While this method has not been sufficiently tried to receive
unconditional recommendation it possesses merits which entitle it to the
attention of analysts.
=384. Estimation of the Sulfuric Acid.=—Sulfuric acid is generally
present in small proportions in soils. Since the plants have need of
sulfur it is proper to inquire into the presence of the compound which
is its principal source. It is in combination with lime that sulfuric
acid exists almost always. In addition to this there is also some sulfur
combined with the organic matter of the soil.
By digesting a soil for six hours with hot, concentrated nitric acid the
sulfates are dissolved, and there is transformed into sulfuric acid an
important part of the sulfur which is combined with the humic
substances. The quantity of soil to be operated upon should be about
fifty grams.
After filtering and washing with hot water the filtered liquor is
collected, in the French Commission method,[249] in a flask and carried
to boiling, and five cubic centimeters of a saturated solution of barium
chlorid or sufficient to be in slight excess are added. The boiling is
continued for some minutes and the flask is allowed to stand for
twenty-four hours. The filtrate is received upon a filter and washed
with boiling water. The filter is dried and incinerated, allowed to
cool, and as there may have been a slight reduction of the sulfate a few
drops of nitric acid are added and a drop of sulfuric acid. It is now
evaporated to dryness on a water-bath, heated to redness for a few
moments, cooled and weighed. The weight of the barium sulfate obtained
multiplied by 0.3433, gives the quantity of sulfuric acid obtained from
the fifty grams of soil.
If it is desired to estimate only the sulfur which exists in the form of
sulfate it is necessary to treat the soil with hydrochloric acid in a
very dilute state, heating for a few moments only and afterwards
precipitate by barium nitrate. If, on the other hand, it is desired to
estimate the total sulfur which is sometimes of great interest, it is
necessary to employ the process of Berthelot and André.
=385. Method of Berthelot and André.=—Sulfur may exist in the soil in
three forms; _viz._,
1. Mineral compounds, consisting generally of sulfates and sometimes of
sulfids.
2. Sulfur, existing in ethereal compounds or their analogues, as in
urine.
3. Organic compounds containing sulfur.
_Estimation of Total Sulfur._—The principle on which this operation, as
described by Berthelot and André, rests is that already described for
phosphorus; _viz._, oxidation in a current of oxygen and passing the
vapors over a column of alkaline carbonate at or near a red heat.[250]
The ordinary methods of oxidation in the wet way give generally inexact
results.
_Estimation of Sulfur Pre-existing as Sulfates._—The sample is treated
with cold, dilute hydrochloric acid. The filtrate is treated with barium
chlorid, the precipitate collected, dried, ignited, washed with a
mixture of sulfuric and hydrofluoric acids to remove silica, and
afterwards weighed as barium sulfate.
_Estimation of Sulfur as Sulfids._—The sample is distilled with dilute
hydrochloric acid, and the hydrogen sulfid produced is made to pass
through an acidulated solution of copper sulfate in such a way as to
transform the sulfur in the hydrogen sulfid into a sulfid, which is
afterwards collected and weighed in the usual way. The use of a titrated
solution of iodin is not advisable on account of the organic matter
which may be present.
_Estimation of Sulfur in Ethereal Compounds._—These compounds can be
decomposed by boiling with a solution of potash or concentrated
hydrochloric acid. The resulting sulfuric acid is precipitated with
barium chlorid. Subtract from the sulfates thus obtained those
pre-existing as sulfates; the difference represents the sulfur present
in ethers.
_Estimation of Sulfur in Other Organic Compounds._—This is estimated
indirectly by subtracting from the total sulfur that present as
sulfates, sulfids, and ethers.
=386. Method of Von Bemmelén.=—As Von Bemmelén[251] observes, the
estimation of sulfuric acid in soils presents a number of difficulties.
A small part of it can be present as sulfate insoluble in water. In
addition to this, there is always some sulfur in the organic bodies
present. If the soil is extracted with water then the sulfuric acid can
be estimated therein when only a trace of humus substance has gone into
solution. On the contrary, if there is much humus substance in solution,
and also iron oxid, as is the case when the extraction is made with
hydrochloric acid, then both of these must be removed, otherwise the
estimation is very inexact. By fusing the residue of the solution with
sodium carbonate and a little potassium nitrate the organic substance is
destroyed, and after treatment with water the iron oxid is separated. If
any sulfur has been dissolved in the organic substance present, this is
then oxidized to sulfuric acid. The estimation of the sulfuric acid and
of the sulfur, therefore, remains unsatisfactory.
In a sample of clay from Java, which was rich in calcium carbonate, but
which contained no basic iron sulfate, there was found the following
percentages of sulfuric acid:
Exhausted in the cold with very weak hydrochloric acid, 0.04 per cent;
the residue treated in the cold with concentrated hydrochloric acid, the
solution evaporated and fused with sodium carbonate and potassium
nitrate, 0.07 per cent; again, the residue treated with aqua regia to
oxidize the sulfur, the solution evaporated to dryness, fused with
sodium carbonate and potassium nitrate, 0.14 per cent; in all 0.25
percent. A sample of the same soil treated directly with aqua regia, and
then evaporated and fused as above, gave two-tenths per cent sulfuric
acid. A sample of the same soil ignited in a crucible with sodium
carbonate and potassium nitrate gave 0.16 per cent of sulfuric acid. The
difference between 0.04 and 0.07 per cent can be attributed to the
sulfur in the organic substance which was dissolved by the concentrated
hydrochloric acid; the quantity, however, is too small to draw any safe
conclusion. Possibly it might have been that the very dilute
hydrochloric acid did not dissolve all of the sulfate. The quantity of
sulfur combined in the organic substance in the above soil may be
derived from the following equation; _viz._, (0.2–0.07)/(80) × 32 = 0.05
per cent of sulfur.
The estimation of the sulfur in a sample of soil from Deli was carried
on with still greater exactness by three different methods.
The quantities of hydrochloric acid, nitric acid, and sodium carbonate
employed were measured or weighed, and the minute content of sulfuric
acid therein estimated and subtracted from the final results. The
methods employed were as follows:
(A) Extraction with water and afterwards with very dilute hydrochloric
acid.
(B) Extraction with cold hydrochloric acid, one part to three of water.
(C) Extraction with aqua regia.
(D) Ignition with sodium carbonate and potassium nitrate.
(F) Ignition in a combustion tube with sodium carbonate in a stream of
oxygen.
The percentages of sulfuric acid obtained by the different methods were
as follows:
(A) 0.058 per cent.
(B) 0.070 „ „
(C) 0.140 „ „
(D) 0.125 „ „
(F) 0.106 „ „
=387. Method of Wolff.=—In regard to the sulfuric acid Wolff calls
attention to the fact that in soils which have been ignited, a larger
quantity of this acid is found than in soils containing humus.[252]
This, doubtless, arises from the oxidation of the organic sulfur. The
following special method for determining the sulfuric acid is therefore
proposed:
Fifty grams of fresh air-dried soil are placed in a platinum dish with a
concentrated solution of pure sodium nitrate. After drying, the heat is
raised gradually to redness. In this way the complete ignition of the
humus present takes place. After cooling, the mass is diluted with
hydrochloric acid, with the addition of a little nitric acid, and
boiled. In the solution, the silicic acid is first separated and the
sulfuric acid estimated in the usual way with barium sulfate.
=388. Method of the Italian Chemists.=—The determination of the sulfuric
acid is conducted as follows by the Italian chemists:[253] The soil is
completely extracted by diluted hydrochloric acid and the sulfuric acid
precipitated in the solution with barium chlorid. If a soil is very rich
in calcium sulfate it should first be treated with a warm solution of
sodium carbonate to decompose the calcium sulfate, and the sulfuric acid
determined in the solution after having added hydrochloric acid.
=389. Estimation of the Chlorin.=—The estimation of the chlorin is of
great importance in certain cases. When this element is lacking in the
soil, which, however, is rare, certain plants appear to suffer from its
absence. The quality of the forage plants in particular is influenced by
it; but when the chlorids are too abundant, which is a frequent case,
they prevent or arrest completely the progress of vegetation. Salty
soils are, in general, completely sterile. In the proportion of one
pound in a thousand in the earth, sodium chlorid is to be regarded as
injurious. It is necessary, therefore, in analysis to take account of
two cases; _viz._, those of soils poor in chlorids and those of soils
rich in chlorids.
For soils poor in chlorids the French method directs that[254] 200 grams
of the earth are to be washed on a filter with boiling water. The liquor
is evaporated to dryness and gently heated to a temperature inferior to
redness in order to destroy the organic matter. The residue is taken up
by small quantities of water and to the filtered liquor the volume of
which should not exceed forty to fifty cubic centimeters are added ten
cubic centimeters of pure nitric acid and a sufficient quantity of
silver nitrate to produce a complete precipitation. The precipitate is
vigorously shaken and allowed to stand for a few hours in a darkened
locality. The precipitate is collected upon a double filter and the
silver chlorid, after proper desiccation, is weighed.
When the soil is rich in chlorids it is washed as has just been
described upon a filter. The wash-waters are made up to one liter and
fifty cubic centimeters, equivalent to ten grams of the soil, are taken
for analysis. This quantity is treated exactly as described above.
=390. Wolff’s Method of Estimating Chlorin in Soils.=[255]—Three hundred
grams of the soil are treated with 900 cubic centimeters of pure water
containing a little nitric acid for forty-eight hours with frequent
shaking. Four hundred and fifty cubic centimeters are then filtered and
the clear liquid evaporated to 200 cubic centimeters. The chlorin is
then precipitated with silver nitrate. The quantity obtained,
corresponds to that found in 150 grams of the air-dried soil.
A second method, Mohr’s, is as follows: Fifty grams of the soil are
placed in a platinum dish and moistened with a concentrated solution of
potassium nitrate, free from chlorin. The mass is evaporated to dryness
and gradually heated to a red heat. After cooling it is moistened with
water and washed into a beaker and the solid mass quickly separated. The
clear liquid is poured off and the residue again washed with water. The
clear liquid obtained is saturated with acetic acid, carefully
evaporated to dryness and after solution in water, filtration and the
addition of a little nitric acid, the chlorin therein is precipitated by
a silver nitrate solution, and the precipitate collected and weighed as
usual.
=391. Method of Petermann.=[256]—Chlorin in the soil is estimated at the
Gembloux station by digesting 1,000 grams of the sample with two liters
of distilled water with frequent shaking for thirty-six hours. After
allowing to stand for twelve hours with the addition of one gram of
powdered magnesium sulfate to facilitate the deposition of suspended
matter one liter of liquid is siphoned and evaporated in a platinum dish
with the addition of a few drops of a solution of potassium carbonate
free from chlorin and nitric acid. The concentrated solution is
filtered, washed, and made up to 250 cubic centimeters. Take 100 cubic
centimeters of the solution add some nitric acid and precipitate the
chlorin with silver nitrate. The rest of the solution is reserved for
the estimation of nitrate.
=392. Estimation of Silicic Acid.=—_Direct Estimation._—The sample of
soil in the method of Berthelot and André[257] is mixed with two or
three times its weight of pure sodium carbonate and fused in a silver
crucible until complete decomposition has taken place. The residue is
dissolved in water and dilute hydrochloric acid. The silicates are
decomposed by this treatment and the solution is evaporated to dryness
on the water-bath, and when dry slightly heated. The silicic acid
(silica) is by this treatment rendered insoluble. It is collected on a
filter, washed, ignited, and weighed. The resulting compound should be
mixed with ammonium fluorid and sulfuric acid, and after the
disappearance of the silica the residue should be dried and weighed. The
loss in weight represents the true silica. The loss in weight should be
corrected by calculating the sulfates of the alkalies back to oxids.
This correction can be neglected when the work has been carefully done,
and the washing of the original silica has been well performed.
_Indirect Estimation._—The total silica may be estimated indirectly by
subtracting from the total weight of the sample the sum of the weights
of the other constituents resulting from the separate estimation of each
of them after decomposing the sample with hydrofluoric acid.
=393. Simultaneous Estimation of Different Elements.=—The operations and
processes for the estimation of each of the elements have been
described, but it is often best to carry on an operation in such a way
as to gain time by making a single decomposition upon a quantity of soil
of some considerable magnitude, and using the results of the solution
for the determination of the different substances. From the operations
already described it will be easy to make a combination of methods by
which all or nearly all the important constituents in a soil may be
determined in a single sample. Of the various methods proposed, that of
the commission of the French agricultural chemists may be taken as a
type.[258] In the case of the estimation of lime, potash, magnesia, and
sulfuric acid, in which the operation is carried on in a soil which is
not incinerated, time may be saved by digesting a considerable quantity
of the soil with concentrated nitric acid for a period of five hours. It
is best to take 100 grams of the soil and increase proportionally the
nitric acid. The filtrate, after washing, is made up to one liter and
thoroughly shaken. From this amount of liquid, portions are taken
corresponding to the weights of soil upon which the operation for the
determination of each of the constituents would be conducted. For
example, for the estimation of lime in the case of a very calcareous
earth, ten cubic centimeters representing one gram of the original
sample, in the case of a soil poor in carbonates 100 cubic centimeters
representing ten grams, and for the estimation of potash, magnesia, and
sulfuric acid 200 cubic centimeters, representing twenty grams of the
soil, should be taken. This method avoids frequent weighings of the
earth and separate treatments thereof by the acid.
On the other hand, in the same portion of the solution, the different
elements can be estimated. For example, for the estimation of the potash
as has been indicated, in the place of precipitating as a whole the
sulfuric acid, lime, etc., and of afterwards separating the magnesia in
the sole aim of eliminating these bodies, they can be collected
separately and weighed, thus securing at a single operation several
determinations.
At the end, some barium chlorid is added and if the barium sulfate is
then collected and weighed, the estimation of the sulfuric acid is
effected. To the filtrate there are afterwards added some ammonia and
ammonium carbonate to precipitate, at once, the excess of barium and the
iron and aluminum oxids, the lime and the phosphoric acid. This
separation being effected the filtrate contains still the magnesia and
the alkalies. The first can be separated by carbonation by means of
oxalic acid, collected, and weighed. Finally the potash itself can be
estimated in the state of perchlorate. It has thus been possible in the
same suite of operations to estimate in a given quantity of the liquid,
the sulfuric acid, the magnesia, the lime, and the potash.
=394. Estimation of Kaolin in Soils.=—True kaolin is a hydrated aluminum
silicate, having the formula H₄Al₂Si₂O₉. This substance is, even in
concentrated hydrochloric acid, almost completely insoluble. It
contains, theoretically, 13.94 per cent of water of combination. The
following methods, due to Sachsse and Becker,[259] can be used for its
determination.
_Estimation of the Water of Combination._—Heat from one to two grams of
kaolin, dried at 100°, for half an hour in a covered platinum crucible
to a temperature which shows an incipient red heat when the crucible is
partly protected from the daylight with the hand. This treatment does
not quite give the whole of the water of combination but nearly all of
it. A kaolin is changed by this treatment into a substance which is
easily soluble in dilute hydrochloric acid.
_Estimation of the Kaolin in Impure Kaolins._—Mineral kaolin, or the
kaolin obtained by silt analysis, is dried at 100° to constant weight.
It is then heated with strong hydrochloric acid until all the matters
which will pass into solution have been dissolved. The residual kaolin
is then washed thoroughly with water and ignited for half an hour at a
low red heat. The residual mass is a second time extracted with
hydrochloric acid until silica no longer passes into solution. The
soluble silica is then estimated in the usual way and calculated to
kaolin. The result will give the pure kaolin in the sample examined.
The estimation may also be made as follows: Two samples of the impure
kaolin are taken and dried to constant weight at 100°. One is extracted
with hydrochloric acid in the manner described above and the amount of
silica determined. The second is treated directly by ignition to low
redness for half an hour, dissolved in hydrochloric acid and the amount
of silica determined. The difference in the two percentages of silica
corresponds to the silica equivalent to the pure kaolin.
_Statement of Results._—It is convenient to incorporate the data
obtained by the above methods with the complete mass analysis of the
silicate examined. In the sample given below the analysis was made on a
clay silt obtained with a velocity of two-tenths millimeter per second.
The mass analysis gave the following data:
Loss on ignition 10.04
SiO₂ 51.52
Al₂O₃ 17.93
Fe₂O₃ 7.42
CaO 1.57
MgO 6.27
K₂O 4.1
Na₂O 1.61
The loss on ignition was made up of the combined water and a trace of
humus. On gentle ignition only 7.52 per cent of water came off.
The examination of the non-ignited and the gently ignited silica by
means of dilute hydrochloric acid, gave the following data:
Non-ignited. Gently ignited. Difference.
Water 10.04 10.04
Insoluble residue 40. 34.54 –5.46
Al₂O₃ 9.04 10. +0.96
Fe₂O₃ 5.96 7.27 +1.31
SiO₂ 25.27 28. +2.73
Alkalies and alkaline earths 9.69 10.15 +0.46
By a comparison of these data with those obtained by the mass analysis,
the following representation of the distribution of the various
components in the clay is obtained:
23.52 per cent SiO₂ in the form of quartz and undecomposed silicates.
2.73 per cent SiO₂ in the form of kaolin.
25.27 per cent in the form of easily decomposable silicates and of the
hydrates of SiO₂.
7.93 per cent Al₂O₃ in the form of undecomposed silicates.
0.96 per cent Al₂O₃ in the form of kaolin.
9.04 per cent Al₂O₃ in the form of easily decomposed silicates and of
hydrates.
0.15 per cent Fe₂O₃ in the form of undecomposed silicates.
1.31 per cent Fe₂O₃ in the form of kaolin.
5.96 per cent Fe₂O₃ in the form of easily decomposable silicates and
hydrates.
3.40 per cent of alkalies and alkaline earths in the form of
undecomposed silicates.
9.69 per cent of alkalies and alkaline earths in the form of easily
decomposable silicates.
10.04 per cent of water, including a trace of humus.
Collecting these results the following statement is obtained.
The clay analyzed contained:
10.04 per cent of water, a trace of humus.
35.15 per cent of undecomposed silicates and quartz.
5.00 per cent of kaolin.
50.27 per cent of easily decomposable silicates, hydrates of SiO₂ and
hydroxids.
ESTIMATION OF NITROGEN IN SOILS.
=395. Introductory Considerations.=—The great economic and biologic
value of nitrogen as a plant food renders its estimation in soils of
especial importance. It is necessary, first of all, to remember that the
nitrogen present in soils may be found in three forms; _viz._, first, in
organic compounds, second, as ammonia, and third, as nitric or nitrous
acid. Further than this each of these classes of nitrogen may be
subdivided. The organic nitrogen may be in a form easily nitrified and
rendered available for plant food, or it may be inert and resistant to
nitrification, as in humus, or exist in an amid state. The ammoniacal
nitrogen may exist in small quantities as gaseous ammonia, or be
combined with mineral or organic acids. As nitric or nitrous acid the
nitrogen will be found combined with bases, or perhaps in minute
quantities as free acid, in passing under the influence of the
nitrifying ferment from the organic to the inorganic state. To the
latter state it must finally come before it is suited to absorption by
plants.
In general, far the largest part of soil nitrogen, excluding the
atmosphere diffused in the pores of the soil, is found in the organic
state and is derived from the débris of animal and vegetable life and
from added fertilizers. As ammonia, the nitrogen can only be regarded as
in a transition state, arising from the processes of decay, or
incomplete nitrification. As nitric acid, it is found as a completed
product of nitrification, or as the result of electrical action. The
processes of nitrification and the isolation and determination of the
nitrifying organisms will be considered in a special chapter of this
manual. By reason of the great solubility of the nitrates, and the
inability of the soil to retain them, there can never be a great
accumulation of nitric acid in the soil save in localities deficient in
rain-fall or in specially protected spots, such as caves. The nitric
acid, therefore, produced in the soil passes at once into growing
vegetation, or is found eventually in the drainage waters.
The formation of ammonia in soil containing much vegetable matter is
thought by Berthelot and André[260] to be due to the progressive
decomposition of amid principles under the influence of dilute acids or
alkalies, either in the cold or at an elevated temperature. Soils of the
above description, of themselves, contain neither free ammonia nor
ammoniacal salts, and the ammonia which is found in the analysis of
these soils comes from the reaction above indicated. The ammonia which
comes from these soils, in place of what is given off to the surrounding
atmosphere, comes from the same class of decompositions, and these
decompositions, in this case, are effected by the water itself, and by
the alkaline carbonates of the soil. The amid principles which are thus
decomposed belong either to the class of amids proper, derived by the
displacement of hydrogen in ammonia by acids, or to the class of
alkalamids derived from nitrogenous bases, both volatile and fixed.
Among these alkalamids some are soluble in water and some insoluble, and
the decomposition of these last by acids or by alkalies may furnish
bodies which themselves are either soluble or insoluble in water.
To determine the nature of the nitrogenous principles in a soil rather
rich in humus, Berthelot and André applied the following treatment:
A soil containing 19.1 grams of carbon and 1.67 grams of nitrogen per
kilogram was first subjected to treatment, at room temperature, with a
concentrated solution of potash. By this treatment 17.4 per cent of the
nitrogen content was set free under the form of ammonia. One-quarter of
this was obtained during the first three days; one-eighth during the
next three days. Afterward the action became much more feeble and was
continued during forty days longer, and the evolution of the gas was
diminished almost proportionately to the time. It appears from the above
observations that the amid principles of the soil, decomposable by
potash, belong to two distinct groups, which are broken up with very
unequal velocities. The soil, treated on the water-bath for three hours
at 100° with strong potash, showed the following behavior in respect of
its nitrogenous constituents: Nitrogen eliminated in the form of
ammonia, sixteen per cent; nitrogen remaining in the part soluble in
potash, ten per cent; nitrogen remaining in the part insoluble in
potash, seventy-four per cent.
_Treatment with Acid._—The nitrogenous compounds of the soil are also
decomposed by dilute acids, and often more rapidly than by the alkalies.
The method of treatment is substantially the same as that set forth
above. The decompositions effected either by alkalies or by acids tend
in general to lower the molecular weights of the resulting products. The
prolonged action of alkalies at the temperature of boiling water
rendered soluble, after twenty-four hours of treatment, 93.6 per cent of
the organic nitrogen found in the vegetable mould. By treating the earth
successively with alkalies and acids 95.5 per cent of the total nitrogen
were decomposed. These experiments show how the insoluble nitrogen in
humic compounds can be gradually rendered assimilable. The action of
vegetables is not assuredly identical with those which acids and
alkalies exercise. However, both present certain degrees of comparison
from the point of view of the mechanisms set in play by the earthy
carbonates and carbon dioxid, as well as by the acids formed by
vegetation. The reactions which take place naturally, while they are not
so violent as those produced in the laboratory, make up by their
duration what they lack in intensity.
For a more detailed study of the nature of the nitrogenous elements in
soil the following method of treatment, due to Berthelot and André, is
recommended:
_Treatment of the Soil with Alkalies._—1. Reaction with cold, dilute
solution of potash. Take fifty grams of the sample, dried at 110°, and
mix with a large excess of ten per cent potash solution and place under
a bell-jar containing standard sulfuric acid. The mixture is left for a
long time in order to secure as fully as possible the ammonia set free.
Example: Fifty grams of a soil contained 0.0814 gram of nitrogen.
Treated as above it gave the following quantities of nitrogen as
ammonia:
Nitrogen as Ammonia.
After 3 days 0.0034 gram.
„ 6 „ 0.0054 „
„ 11 „ 0.0065 „
„ 17 „ 0.0078 „
„ 25 „ 0.0093 „
„ 41 „ 0.0107 „
„ 46 „ 0.0141 „
It is seen that the action still continued after forty days. In the
space of forty days 17.4 per cent of the total nitrogen contained in the
soil had been converted into ammonia by dilute potash. According to the
above observations the amid principles transformed into ammonia under
the influence of dilute potash, exist in groups which are acted on with
very unequal rapidity.
2. Reaction with hot dilute solution of potash. Take 200 grams of the
soil sample, mix with one and one-half liters of dilute potash solution
containing fifty grams of potash. Place in a flask and heat on boiling
water-bath for six hours. The flask is furnished with a stopper and
tubes, and a current of pure hydrogen is made to pass through the
liquid, having the double object of preventing any oxidizing effect from
the air and of carrying away the ammonia which may be formed. The
escaping hydrogen and ammonia are passed into a bulb apparatus
containing titrated sulfuric acid.
The sample of soil employed contained in 200 grams, 0.3256 gram of
nitrogen. There was obtained at the end of six hours’ heating, 0.0366
gram of nitrogen. In other words, 11.24 per cent of the total nitrogen
in the sample appeared as ammonia.
_Examination of Residue._—After the separation of the ammonia as above
described, pour the residue in the flask on a filter, wash with hot
water, and determine nitrogen in filtrate and in solid matter on the
filter by combustion with soda-lime. The filtrate is, of course, first
evaporated to dryness after being neutralized with sulfuric acid.
The insoluble part contained 0.041 gram of nitrogen, _i. e._, 12.84 per
cent of the entire amount.
The soluble part contained 0.2411 gram of nitrogen, _i. e._, 74.05 per
cent of the whole.
_Summary of Data._—In the sample analyzed the following data were
obtained:
Of the whole.
Nitrogen as ammonia 11.24 per cent.
„ in insoluble part 12.84 „ „
„ „ soluble part 74.05 „ „
„ not determined 1.87 „ „
——————
Sum 100.00 „ „
The same experiment in which the heating on the water-bath was continued
for thirteen hours gave the following data:
Of the whole.
Nitrogen as ammonia 16.03 per cent.
„ in insoluble part 9.98 „ „
„ „ soluble part 74.01 „ „
——————
Sum 100.00 „ „
_Further Treatment of Matter Insoluble in Hot Dilute Potash._—A portion
of the insoluble portion from the last experiment was treated for
thirteen hours longer under the same conditions with dilute hot potash.
The soluble and insoluble portions were determined as already described.
Of the nitrogen insoluble after thirteen hours, 64.21 per cent remained
insoluble after the second thirteen hours. This fact shows that slow and
progressive decomposition of the alkalamids in the soil occurs under the
influence of hot dilute potash.
_Treatment of Matter Insoluble in Hot Dilute Potash with Hydrochloric
Acid._—A part of the material insoluble in hot potash after thirteen
hours is mixed with dilute hydrochloric acid, in such proportion as to
have one-fifth the weight of pure hydrochloric acid to the dry solid
matter. Heat in flask on a boiling water-bath for thirteen hours.
Determine the nitrogen in the insoluble residue.
_Example_: In the case given it was found that 54.91 per cent of the
nitrogen insoluble in dilute hot potash were dissolved by the hot
hydrochloric acid.
This fact shows that insoluble nitrogen compounds contained in the soil
are dissolved by dilute acids even more readily than by dilute alkalies
at the temperature of boiling water.
Several reactions appear to take place simultaneously when potash is
brought into contact with the nitrogenous principles of arable earth.
Some of these principles, during the first period of the action become
soluble and even form compounds which are not precipitable by acids.
When, however, the action of the potash is prolonged, the dissolved
bodies lose little by little a part of their nitrogen as ammonia or as
soluble alkalamids. They become thus changed either to compounds no
longer soluble in the potash, or to those insoluble in the solution when
acidified. These compounds, it is true, contain nitrogen, but are poorer
in this element and have a higher molecular weight, or, in other words,
are condensation products. These last principles are not absolutely
stable in the presence of potash, but are decomposed much more slowly
than the original principles from which they were derived.
In general, it may be said that under the influence of alkalies on the
nitrogenous principles of the soil there is a tendency to form two
classes of bodies, the one more soluble with a lower molecular weight,
the other less soluble with a higher molecular weight. The inverse
relation between solubility and condensation is in agreement with what
is observed in similar reactions with organic bodies in general. It
certainly plays an important rôle in the transformations which an arable
soil undergoes, either through the mild influences of the air and
natural waters, or the more energetic action of vegetables themselves.
The methods of estimating nitric nitrogen will be made the theme of a
special study in connection with the chapter on nitrification. There
will be considered first, therefore, the methods of determining organic
and ammoniacal nitrogen with only such incidental treatment of the
methods for nitric nitrogen as the processes applicable to the other
forms may contain.
=396. Provisional Methods of the Association of Official Agricultural
Chemists.=[261]—The nitrogen compounds in the soil are usually placed in
three classes.
1. The nitrogen combined with oxygen as nitrates, or nitrites, existing
as soluble salts in the soil.
2. The nitrogen combined with hydrogen as ammonia, or organic nitrogen
easily convertible into ammonia. The ammonia may exist as salts, or may
be occluded by hydrated ferric or aluminum oxids and organic matter in
the soil.
3. The inert nitrogen of the soil or the humus nitrogen.
_Active Soil Nitrogen._—The material proposed for reducing the nitrates
to ammonia, and at the same time to bring ammonia salts and organic
nitrogen into a condition for separation by distillation, is sodium
amalgam. Liquid sodium amalgam may be readily prepared by placing 100
cubic centimeters of mercury in a flask of half a liter capacity,
covering the warmed mercury with melted paraffin, and dropping into the
flask at short intervals pieces of metallic sodium, the size of a large
pea (taking care that the violence of the reaction does not project the
contents from the flask), till 6.75 grams of sodium have combined with
the mercury. The amalgam contains one-half per cent of sodium and may be
preserved indefinitely under the covering of paraffin. To estimate the
active soil nitrogen, weigh fifty grams of air-dried soil and place it
in a clean mortar. Take 200 cubic centimeters of ammonia-free distilled
water, rub up the soil with a part of the water to a smooth paste,
transfer this to a flask of one liter capacity, washing the last traces
of the soil into the flask with the rest of the water. Add twenty-five
cubic centimeters of the liquid sodium amalgam and shake the flask so as
to break the sodium amalgam into small globules distributed through the
soil. Insert a stopper with a valve and set aside in a cool place for
twenty-four hours. Pour into the flask fifty cubic centimeters of milk
of lime, and distill, on a sand-bath, 100 cubic centimeters into a flask
containing twenty cubic centimeters of decinormal sulfuric acid, and
titrate with decinormal soda solution, using dimethyl-orange as
indicator. Estimate the nitrogen of the ammonia found as active soil
nitrogen.
If the ammonia produced is too small in amount to be readily estimated
volumetrically, determine the ammonia by nesslerizing the distillate.
_Estimation of Nitrates in the Soil._—When it is desired to estimate
separately the nitrates in the soil the following method may be used:
Evaporate 100 cubic centimeters of the soil extract to dryness on the
water-bath, dissolve the soluble portion, of the residue in 100 cubic
centimeters of ammonia-free distilled water, filtering out any insoluble
residue, place the solution in a flask and add ten cubic centimeters of
liquid sodium amalgam, insert stopper with valve, set it aside to digest
in a cool place for twenty-four hours, add fifty cubic centimeters of
milk of lime, distill and titrate as above, and estimate the nitrogen as
N₂O₅.
Nesslerizing may be substituted for titration when the amount of
nitrates is small.
An approximate estimation of the amount of nitrates will be of value in
determining which method of estimation to use. This may be done by
evaporating a measured quantity of the soil extract, say five cubic
centimeters, on a porcelain crucible cover on a steam-bath or radiator,
having first dissolved a minute fragment of pure brucin sulfate in the
soil extract. When dry pour over the residue concentrated sulfuric acid,
free from nitrates, and observe the color reactions produced.
If the nitrate (reckoned as KNO₃) left upon evaporating the quantity of
water taken does not exceed the two-thousandth part of a milligram, only
a pink color will be developed by adding the sulfuric acid; with the
three-thousandth part of a milligram, a pink with faint reddish lines;
with the four-thousandth part, a reddish color; with the five-thousandth
part, a red color.
By increasing or diminishing the amount of soil extract evaporated to
secure a color reaction of a certain intensity, an approximate estimate
may be made of the amount of nitrates present.
Blank experiments to test the acid and the brucin sulfate will be
required before confidence can be placed in such estimations.
_Total Nitrogen of Soils._—The total nitrogen of soils may be determined
by the usual combustion with soda-lime, but this process is often
unsatisfactory because of the large amount of material required when the
organic matter or humus is small in amount.
A modification of the kjeldahl method is more easy to carry out and
gives results equally satisfactory. Place twenty grams of soil in a
kjeldahl flask, and add twenty cubic centimeters of sulfuric acid (free
from ammonia) holding in solution one gram of salicylic acid. If the
soil contain much lime or magnesia in the form of carbonate, enough more
sulfuric acid must be added to secure a strongly acid condition of the
contents of the flask. Add gradually two grams of zinc dust, shaking the
contents of the flask to secure intimate mixture. Place the flask in a
sand-bath and heat till the acid boils, and maintain the boiling for ten
minutes. Add one gram of mercury and continue the boiling for one hour,
adding ten cubic centimeters of sulfuric acid if the contents of the
flask are likely to become solid. Cool the flask and wash out the
soluble materials with 200 cubic centimeters of pure water, leaving the
heavy earthy materials. Rinse the residue with 100 cubic centimeters of
water, and add this to the first washings. Place this soluble acid
extract in a liter digestion flask, add thirty-five cubic centimeters of
a solution of potassium sulfid, and shake the flask to secure intimate
mixture of the contents. Introduce a few fragments of granulated zinc,
pour in seventy-five cubic centimeters of a saturated solution of
caustic soda, connect the flask with a condenser and distill 150 cubic
centimeters into a flask containing twenty cubic centimeters of acid,
using the same acid and alkali for titration used in the kjeldahl method
under fertilizers.
Enter the nitrogen found in this operation as total soil nitrogen.
The difference between the total soil nitrogen and the active soil
nitrogen will express the inert nitrogen of the soil.
=397. Hilgard’s Method.=[262]—The humus determination will, in the case
of virgin soils, usually indicate approximately the store of nitrogen in
the soil, which must be gradually made available by nitrification.
Ordinarily (outside of the arid regions) the determination of ammonia
and nitrates present in the soil is of little interest for general
purposes, since these factors will vary with the season and from day to
day. Kedzie proposes to estimate the active soil nitrogen (ammonia plus
nitrates and nitrites) by treatment of the whole soil with sodium
amalgam and distillation with lime. The objection to this process is
that the formation of ammonia by the reaction of the alkali and lime
upon the humus amids would greatly exaggerate the active nitrogen and
lead to a serious overestimate of the soil’s immediate resources.
The usual content of nitrogen in black soil-humus is from six to eight
per cent in the regions of summer rains. From late determinations it
would seem that in the arid regions the usually small amount of humus
(often less than two-tenths per cent) is materially compensated by a
higher nitrogen percentage. It thus becomes necessary to determine the
humus nitrogen directly; and this is easily done by substituting in the
grandeau process of humus extraction potash or soda-lye for ammonia
water, and determining the nitrogen by the kjeldahl method in the
filtrate.
The lye used should have the strength of four per cent in the case of
potassium hydroxid, three per cent in that of sodium hydroxid. The black
humus filtrate is carefully neutralized with sulfuric acid, evaporated
to a small bulk in a beaker or evaporating basin, and the reduced liquid
finally evaporated to dryness in the kjeldahl flask itself by means of a
current of air. The beaker or basin is washed either with some of the
alkaline lye, or, after evaporation, with warm concentrated sulfuric
acid, which is then used in the nitrogen determination in the usual way.
For the determination of nitrates in the soil it is, of course, usually
necessary to use large amounts of material, say not less than 100 grams,
and, according to circumstances, five or more times that amount. In the
evaporated solution the nitric acid is best determined by the reduction
method, as ammonia.
Usually the soil filtrate is clear and contains no appreciable amount of
organic matter that would interfere with the determination; yet in the
case of alkaline soils (impregnated with sodium carbonate) a very dark
colored solution may be obtained. In that case the soil may
advantageously be mixed with a few per cent of powdered gypsum before
leaching; or the gypsum may be used in the filtrate to discolor it by
the decomposition of sodium carbonate and the precipitation of calcium
humate. The evaporated filtrate can then be used for the nitrate
determination by either the kjeldahl, griess, or the nessler process,
which will, of course, include such portions of the ammoniacal salts as
may have been leached out.
For the separate determination of these and of the occluded ammonia,
when desired, it is probably best to mix the wetted soil intimately with
about ten per cent of magnesium oxid and distill into titrated
hydrochloric acid. For general purposes, however, this determination is
usually of little interest.
=398. Müller’s Modified Kjeldahl Method.=—Numerous difficulties, as
stated by Müller,[263] have attended the attempts to apply the kjeldahl
method for the estimation of nitrogen to samples of soil, and he has
modified the method to some extent and made comparisons of the quantity
of nitrogen by this modified method and by the soda-lime method.
The principal difficulty encountered by him has been in the regular
heating of the mixture of fuming sulfuric acid and soil. The particles
of soil are deposited at the bottom of the flask and the result is that
the bottom layers become overheated, and, being poor conductors of heat,
fail to transmit a sufficient quantity of heat to penetrate to the upper
layers of the liquid to complete the reaction. In order to avoid this
difficulty Müller heats his flask in a small stove formed with a
straight vertical cylinder of iron or copper, the upper end of which is
covered with a sheet of iron pierced with a hole which allows the neck
of the flask to pass through, while the lower end is closed with a piece
of sheet iron furnished on its upper surface with a layer of asbestos.
This cylinder of metal is surrounded with a second one, concentric with
the first through which passes a current of heated gases furnished by an
ordinary bunsen. By heating the flask in this stove or furnace an even
distribution of the heat is secured to all parts of the mixture, but the
little drops of sulfuric acid, which are condensed on the cold part of
the neck, sometimes lead to the fracture of the glass as they run down
the sides of the flask to the hot portions. To prevent the reflux of
this condensed acid, which only needs to be done near the end of the
reaction, when it is necessary to heat to a very high temperature, the
neck of the flask is bent at the point immediately above its emergence
at the upper surface of the furnace, and carried into a flask of about
seventy-five cubic centimeters capacity, which will receive the drops of
sulfuric acid condensed during the operation. The furnace has the
following dimensions; height, twelve centimeters; diameter of interior
cylinder, five and one-half centimeters; diameter of exterior cylinder,
seven and one-half centimeters.
It is supported on a triangle of large iron wire and is heated by an
ordinary bunsen, or by a concentric bunsen, according to the temperature
which it is necessary to obtain. The proportions which should be
observed between the amount of earth employed and the sulfuric acid are
about as follows:
Of the dry earth, fifteen grams; of the fuming sulfuric acid, thirty
cubic centimeters. There should also be added to the mixture about
three-tenths of a gram of pure stearic acid, or better, benzoic acid.
When the soil to be analyzed does not contain carbonate, the sulfuric
acid should be added in two portions. At first add about twenty cubic
centimeters of the acid, and after shaking it, the other ten cubic
centimeters, running it in from a burette or a pipette in such a manner
as to wash thoroughly the neck and sides of the flask. If the earth
contain carbonate, however, it is necessary to add the fuming acid in
small portions of about five cubic centimeters at a time, waiting each
time until the disengagement of the gas caused by the previous addition
has ceased. A soil which contains from thirty to forty per cent of
calcium carbonate should be carefully treated in a porcelain capsule
with a slight excess of sulfuric acid, pure and dilute. The mixture is
afterward to be evaporated to dryness upon a sand-bath and the residue
heated in a drying oven to 110°. The mass is then pulverized, introduced
into the flask, treated with three-tenths of a gram of benzoic acid and
thirty cubic centimeters of fuming sulfuric acid, and treated as
indicated above. In all cases it is necessary to continue the heating
until the contents of the flask are colorless.
With soils containing considerable quantities of iron, however, a slight
red color will probably be observed which will not interfere with the
accuracy of the tests.
The heating should at first be gentle and the temperature afterward
elevated little by little, and finally the heat should be sufficiently
great to distill about one and one-half cubic centimeters of sulfuric
acid. The operation lasts from twelve to thirteen hours. As the reaction
is terminated the cooled mass is taken up with water absolutely free
from ammonia. It is filtered into a flask, and washed upon the filter
until the volume of the filtered liquid is about 350 cubic centimeters.
Afterward an excess of soda-lye, at 50° baumé is added, then a few
pieces of quartz to facilitate boiling. The flask is then connected with
a condenser, the liquid distilled and received in a conical flask closed
by a cork having two holes, of which one permits the entrance of the end
of the condenser, and the other a glass tube which is connected with a
small flask containing water, the neck of the receiving flask being
inclined toward the condenser to avoid the entrainment of any of the
alkaline liquid which may be distilled. The receiving flask rests upon
two or three pieces of sheet iron and is heated with an ordinary burner,
and ebullition is perfectly regular. From 170 to 180 cubic centimeters
of the liquid are distilled in from three and one-half to four hours.
The distilled liquid, treated with a few drops of litmus, is titrated by
a solution of sulfuric or hydrochloric acid, of which one cubic
centimeter corresponds to 0.001 cubic centimeter of nitrogen.
=399. Modification of the Kjeldahl Method by Arnold and
Wedemeyer.=[264]—For the oxidizing liquid a mixture of three grams of
benzoic acid with forty cubic centimeters of H₂SO₄ is employed. After
placing in the digestion flask with the nitrogenous body the whole is
gently shaken for a few minutes to prevent clotting. The temperature is
then raised until acid vapors begin to come off, when one gram of copper
sulfate and one gram of mercuric oxid are added; and after ten to
fifteen minutes, to avoid foaming, ten to twenty grams of potassium
sulfate. The sublimate noticed on the walls of the flask is benzoic acid
and does not interfere with the accuracy of the determination.
This method has given good results with the alkaline nitrates, the
nitrates of barium, mercury, silver, lead, and with strychnia, ammonia,
pyridin, azobenzol, dinitrobenzol, and picric acid.
=400. Prevention of Bumping During Distillation.=—Daffert has employed
the modified kjeldahl method, but found considerable difficulty in using
the same owing to the violent bumping of the liquid in the distillation.
This was especially the case where the sample contained a large
proportion of sand. To overcome this annoyance and danger he devised the
following process:[265]
Fit into the mouth of a large-mouthed distillation flask a stopper
having two perforations. Through one of the perforations pass the usual
distillation tube, through the other a similar tube connected with a
supply of steam. Bring the contents to a brisk boil, after which a small
current of steam is turned on, allowing the same to pass in a small
stream throughout the distillation. By this means, not only is all
danger from bumping avoided, but the time required for the distillation
shortened. By the old method it usually requires from fifteen to twenty
minutes, whereas the former requires from six to ten minutes.
It is advisable to filter all samples of soils having a large proportion
of sand.
=401. Determination of Organic Nitrogen by the Soda-Lime Method.=—In the
description of the method following, the directions of the French
Commission of Agricultural Chemists have been taken as the basis of the
analytical process.[266] This method is, in this country, almost
superseded by the moist combustion process with sulfuric acid. By reason
of its long use, however, and because it is still regarded as the best
method by the agricultural chemists of France, Italy, and England, it
merits a full description. It is recommended also by Berthelot and
André,[267] by the International Congress of Chemists, held in Paris in
1889, by the Italian chemists, and by the official Belgian method,[268]
in all cases where nitrates are not present in notable quantities.
The nitrogen which is found in soils in the organic state is transformed
into ammonia when it is heated with soda-lime. This reaction is the base
of the process of analysis which has so long been used for this class of
bodies. The analytical process is conducted as follows:
A well-cleaned glass combustion tube, closed at one end, is used. The
length of the tube is from thirty-five to forty centimeters. It is
filled first to a depth of two centimeters with calcium oxalate;
afterwards to a depth of five centimeters with soda-lime in small
fragments; afterwards with the mixture to be analyzed; _viz._, of ten
grams of the sample of soil, or twenty grams if poor in nitrogen and
organic matters, with soda-lime reduced to a coarse powder. This mixture
should occupy a length of about twenty centimeters in the tube. The soil
and soda-lime are mixed in a mortar. Afterwards the mortar is rubbed
with small quantities of soda-lime, and this, together with the copper
boat which has been used in introducing the mixture, is thoroughly
washed with the soda-lime, which is poured into the tube until it is
filled to within four centimeters of its open extremity. The open end of
the tube is then closed with a wad of asbestos packed sufficiently tight
to prevent the carrying off of the soda-lime by the gas which may be
generated during the combustion. The combustion should be commenced by
heating the tube near the open extremity until it is red and carrying
the heat progressively towards the part containing the soil mixed with
the soda-lime. An ordinary gas combustion furnace should be used and the
heat graduated in such a way that the bubbles of gas pass off regularly
and not too rapidly. The gas is conducted into a bulb tube containing a
decinormal standard sulfuric acid colored with litmus. The combustion is
continued until the whole of the organic material is decomposed, care
being taken not to raise the combustion tube above a low redness in
order to avoid its softening. At the end, however, the temperature of
the combustion tube should be raised to a bright red, and the part
containing the calcium oxalate should be heated little by little for the
purpose of evolving hydrogen, which is used to drive out the last traces
of ammonia. After the combustion is completed, and the last traces of
ammonia driven out, the standard acid which has received the evolved
ammonia is removed, the tube leading to it washed, the wash-water
collected with the rest of the liquid and titrated with a standard
solution of lime-water, the strength of which has previously been
determined against standard sulfuric acid.
=402. Preparation of the Standard Sulfuric Acid.=—The sulfuric acid to
be used in making the standard solutions should be previously boiled for
half an hour in a platinum dish and allowed to cool in a desiccator. It
should contain 61.25 grams of sulfuric acid in one liter. It is
recommended that the flask which holds the sulfuric acid should be one
which has been used for a long time for holding concentrated sulfuric
acid, in order to avoid any action of the alkali in the glass upon the
acid after its strength has been determined. The solution before
described is of such strength as to have each cubic centimeter
equivalent to one milligram of nitrogen.
For the estimation of the nitrogen in the soil a tenth normal solution
should be used, which is prepared by taking 100 cubic centimeters of the
normal solution, described above, and diluting to one liter.
_Preparation of the Lime-Water._—From 200 to 300 grams of slaked lime
are placed in a closed flask of about five liters capacity. This is
filled with water and shaken frequently, and left to deposit the matter
in suspension. The water which contains the saline particles which may
have been present in the lime is then poured off. Fresh water is then
poured on and the flask shaken from time to time. To use this lime-water
the clear part of it is decanted into a flask, avoiding, as much as
possible, access to the air. The flask is closed with a cork carrying
two tubes drawn out and bent at a right angle. One of these serves for
pouring off the water and the other serves for the entrance of the air.
These two tubes are themselves closed by means of a rubber tube carrying
a pinch-cock. The strength of the lime-water is fixed by titration with
the decinormal standard sulfuric acid.
_Preparation of the Soda-Lime._—Six hundred grams of slaked lime in fine
powder are saturated with 300 grams of caustic soda dissolved in 300
cubic centimeters of water. The whole is rubbed into a paste and
introduced into a crucible which is heated to redness. The contents of
the crucible, still hot, are poured out, and rapidly reduced to
fragments in a copper mortar in such a manner as to have the pieces
about the size of a pea, and without having too much finely powdered
soda-lime mixed with it. While the matter is still hot it is placed in a
flask and well-stoppered. In order that this reagent should contain no
nitrogen it is indispensable to use in its preparation materials which
contain no trace of nitrates.
_Preparation of the Calcium Oxalate._—In a small copper vessel place 100
grams of oxalic acid and add gradually, bringing it to boiling, enough
water to dissolve it. Afterwards place in the solution small portions of
slaked lime in a state of powder, constantly testing it until turmeric
paper indicates that there is a little lime in excess. It is then
evaporated, stirring vigorously on the open fire, and the evaporation is
finally finished on a steam-bath. The dried material is placed in a
flask and well-stoppered. The oxalic acid which is used in this
preparation should be free from every trace of nitrogen.
_Preparation of the Litmus Solution._—Five grams of litmus are placed in
a flask with a flat bottom. Afterwards a few cubic centimeters of
ammonia are added, twenty five grams of crystallized sodium carbonate,
and ten cubic centimeters of water. This mixture is left to digest for
sometime, with frequent stirring, at a temperature of from 60°–80°. The
digestion is finished in about four or five days, during which time, at
intervals, a few drops of ammonia are added, sufficient to maintain
always the ammoniacal odor. At the end of this time 200 cubic
centimeters of water are added and the digestion allowed to continue
several days more, still maintaining the solution alkaline with ammonia.
A slight excess of hydrochloric acid is added, and the matter which is
precipitated is received upon a filter where it is washed several times
with cold water and allowed to dry at a low temperature.
For use, from one to two grams of this dry precipitate are dissolved in
100 cubic centimeters of alcohol, and there is thus obtained a litmus
solution of extreme sensibility.
=403. Treatment of Soil Containing Nitrates.=—Nitrates exist in small
quantities in all arable soils. When treated for nitrogen by the
soda-lime method above described, a part of the nitric nitrogen is
changed to the state of ammonia, while another part escapes estimation
altogether, causing an error which it is important to point out. When
the soils contain only small quantities of nitrates this error is
insignificant and does not affect sensibly the results, but in the case
of earths rich in nitrates it is necessary first to eliminate them
before the determination of the nitrogen by the soda-lime method. The
operation is carried on as follows:
Twenty grams of the soil are washed on a small funnel, furnished with a
plug of asbestos, with small quantities of pure water, in such a way as
to cause thirty to forty cubic centimeters of water to pass through. The
whole of the nitrate is thus removed. The soil is now dried and
submitted to analysis by the soda-lime method as just described. There
are removed with the nitrate only small traces of organic nitrogen, too
small to influence the results of the analysis. If, however, it is
desired to remove altogether this slight cause of error, evaporate the
wash-waters, above described, to two or three cubic centimeters; add a
few drops of a concentrated solution of ferrous chlorid and as much
hydrochloric acid, and boil some minutes in order to drive off, in the
state of nitrogen dioxid, all the nitric acid. The residue is evaporated
to dryness and contains the traces of organic nitrogen. This is added to
the soil which is to be treated by the soda-lime method.
=404. Müller’s Method.=—The determination of nitrogen in the soil by
soda-lime is carried on as follows by Müller:[269]
Fifteen grams of fine earth, dried and mixed with a little sugar, are
mixed with thirty grams of soda-lime in powder. The bottom of the
combustion tube contains a little moist soda-lime, which is heated at
the end of the operation at the same time that a current of pure
hydrogen is made to pass through it, and the temperature of the tube is
raised, little by little, to a distinct redness. The contents of the
receiving bulbs are distilled, after the addition of water and soda, in
the same apparatus which served in the estimation of nitrogen, by the
kjeldahl method; the determinations and titrations are made also under
the same conditions.
Blank determinations are also made under the same conditions to
determine the amount of correction to be made by the two methods.
Soda-lime, heated with pure sugar, gave 0.0002 gram of nitrogen for a
total weight of fifty-five grams of the soda-lime contained in the tube.
The fuming sulfuric acid gave 0.0011 cubic centimeters of ammoniacal
nitrogen for the volume of thirty cubic centimeters.
The numbers obtained by the kjeldahl method in general, are lower than
those obtained by the soda-lime method when no stearic or benzoic acid
is used. The numbers obtained when stearic acid alone was used were
sometimes inferior to those obtained by the soda-lime method. The
numbers obtained when benzoic acid is used are, in general, about the
same as those obtained by the soda-lime method.
It would seem that the double distillation, outlined above, for the
kjeldahl method, would not be necessary if due care were exercised in
the first distillation. This variation, therefore, seems to be
unnecessary.
In the soda-lime method, time would be saved by the reception of the
ammonia in standard acid, and its titration in the usual way, unless a
further purification of the nitrogenous products of the combustion by
the final distillation be desired.
=405. Volumetric Determination of the Nitrogen.=—Instead of separating
the nitrates, the total nitrogen in the soil can be determined directly
by the classic method of Dumas, which consists in bringing the whole of
the nitrogen into a gaseous state and afterwards measuring its volume.
The following method illustrates the general principles of the
determination:
A glass combustion tube closed at one end, about one meter in length, is
selected. In the bottom of this tube is placed some potassium
bicarbonate in a crystalline form, in small pieces, filling the tube to
a distance of about twenty centimeters. Afterwards copper oxid is placed
to the depth of ten centimeters and finally a mixture of from twenty to
thirty grams of the earth with thirty to forty grams of copper oxid in a
fine state of subdivision, and about ten grams of metallic copper
obtained by reducing the copper oxid by hydrogen. Next the tube is
filled with copper oxid to a depth of from twenty to twenty-five
centimeters, and afterwards with reduced copper to the depth of at least
twenty-five centimeters, and after this another layer of copper oxid of
about five centimeters, and finally a plug of asbestos. The combustion
tube is closed with a stopper carrying a glass tube of about ninety
centimeters in length, of which the extremity, bent into the form of a
=ᥩ=, extends to a mercury trough. The glass combustion tube is
surrounded with brass gauze, except that part which contains the
potassium bicarbonate. The beginning of the operation consists in
heating the tube to decompose a part of the potassium bicarbonate, until
the whole of the apparatus is filled with carbon dioxid. In order to
determine that the whole of the air has been expelled and that the
apparatus is entirely filled with carbon dioxid, a part of the gas which
is disengaged, is received into a jar filled with mercury, in which a
little potash-lye has been placed. If the gas is entirely absorbed by
the potash, so that there remain only unappreciable particles, the tube
can be regarded as completely free of air. When assurance is given that
the air is all out of the apparatus, a jar of about 300 cubic
centimeters capacity, filled with mercury and containing from thirty to
forty cubic centimeters of a solution of potash of a density of 42°
baumé, is placed over the outlet tube. The combustion is commenced by
heating the anterior part of the tube, avoiding the heating of the part
containing the earth. When the first part of the tube has reached the
red stage the part containing the earth is gradually heated in order to
obtain a gentle evolution of gas. The temperature of the tube is carried
to redness and the heating gradually carried back toward the closed
extremity, but avoiding raising the temperature of the part containing
the potassium bicarbonate. The red heat is continued as long as bubbles
of gas are discharged into the reservoir. When the evolution of gas has
ceased the apparatus is again filled with carbon dioxid for the purpose
of driving out the last traces of nitrogen, by heating again the part of
the tube containing the potassium bicarbonate. The evolution of the
carbon dioxid should be maintained for about fifteen minutes. At the end
of this time all the nitrogen will be found in the receiving jar.
Sometimes a small quantity of nitrogen dioxid is formed incidentally in
the operation. After waiting for a quarter of an hour, in order to
permit all the carbon dioxid which may have escaped into the reservoir
to be completely absorbed, the receiving jar is carried to a water-basin
and the mercury allowed gradually to escape; its place being taken by
the water. The gas is then transferred into an azotometer where its
volume and temperature are read in the usual way.
In order to absorb any nitrogen dioxid which may be admixed with the
nitrogen itself, a little crystal of ferrous sulfate is introduced. The
reservoir containing the nitrogen is carried to the mercury trough, and
the water which it contains is nearly all run out in such a way as to be
replaced with mercury, great care being exercised to avoid any escape of
gas. Afterwards there is introduced over the mercury a crystal of
ferrous sulfate and the azotometer is shaken until this crystal is
dissolved by the water which it still contains. It is then allowed to
remain for twenty hours. At the end of this time the nitrogen dioxid is
absorbed and the volume of the gas is again read as before. One-half
only of the total loss should be subtracted, since the volume of the
nitrogen dioxid is twice the volume of the nitrogen itself. For the
practice of this method, in connection with the use of a mercury pump,
the directions which will be given under fertilizers may be consulted.
=406. Estimation of Ammonia.=—Ammonia exists ordinarily only in very
small quantities in the soil, since it is incessantly transformed into
nitrate or diffused in the air. Nevertheless, it is sometimes
interesting to determine its quantity.
The method of determining the ammonia in soils is one of extreme
delicacy on account of the small proportion therein, and the difficulty
of expelling it without at the same time converting some of the organic
nitrogen into ammoniacal compounds. The various methods employed for
this purpose may be classified as follows:
1. Treatment of the soil with soda-lye in the cold, and the absorption
of the ammonia given off by standard sulfuric acid.
2. The method of Boussingault, which consists in replacing the soda-lye
with magnesia and distilling the ammonia at a boiling temperature,
absorbing the distillate in a standard acid.
3. A modification of the above method, due to Schloesing, which consists
first in extracting the ammonia by hydrochloric acid and subjecting the
extract to distillation with magnesia.
4. The method of Knop consists in treating the soil in a closed cylinder
with soda-lye containing bromin. The ammonia set free by the lye is
decomposed in the presence of bromin into free nitrogen and hydrochloric
acid. The nitrogen is collected and measured in an azotometer. The
brom-soda-lye is prepared by dissolving 100 grams of sodium hydroxid in
1,200 cubic centimeters of water and adding twenty-five cubic
centimeters of bromin.
5. The process described under 4, as shown by Baumann,[270] does not
give accurate results and it has been modified by him as follows: Two
hundred grams of soil are treated with 100 cubic centimeters of dilute
hydrochloric acid (one part acid and four of water) free of ammonia; 300
cubic centimeters of ammonia-free distilled water are added and the
whole digested for two hours with frequent stirring. If a soil contain
much calcium carbonate larger quantities of acid must be used. Two
hundred cubic centimeters of the filtrate are placed in an evolution
flask, connected with an azotometer, with five grams of freshly burned
magnesia. The mixture is then oxidized as follows: Ozone is generated by
adding three parts by weight of sulfuric acid to one part of dry and
powdered potassium permanganate. A stream of air is drawn through the
ozone generator by an aspirator, and the ozone is conducted into a flask
containing the hydrochloric acid extract of the soil and magnesia. The
oxidation is completed in about ten minutes. The mixture is then brought
into the evolution flask of the azotometer and the nitrogen set free and
measured in the usual way.
It has been shown that if asparagin or glutamin be present in the soil
they are decomposed by the soda-lye and the results obtained are too
high. It has been further proved that soils which contain a notable
quantity of humus give, with soda-lye in the cold, a practically
continuous evolution of ammonia. Moreover, soils which are rich in humus
and which have been treated by distillation with magnesia give, on
subsequent treatment with soda-lye, considerable additional quantities
of ammonia.
_Comparison of Methods of Estimating Ammonia._—Baumann has determined
the ammonia-nitrogen in various soils by the soda-lime method;
distillation of the hydrochloric acid extract with magnesia, and the
azotometric method modified as indicated above. These methods will be
designated as 1, 2, 3, respectively in the following table.
METHOD.
1. 2. 3.
Ammonia-nitrogen in one
kilogram of soil.
—————— —————— ——————
No. of sample. Gram. Gram. Gram.
1 0.0448 0.02227 0.02781
2 0.0168 0.01105 0.01326
3 0.0336 0.01771 0.02214
4 0.0056 0.00443 0.00443
5 0.0280 0.02337 0.02894
6 0.0196 0.01243 0.01672
From the above figures it is seen that the method usually attributed to
Schloesing gives uniformly higher numbers than either of the other
processes, while the third gives slightly higher values than the second.
=407. The Magnesia Distillation Process.=—If a sample of soil be
distilled directly with magnesia and water, there is danger on the one
side of not extracting all the ammonia, by reason of the absorbing power
of these bodies, and on the other, of transforming into ammonia the
nitrogen of the organic matters. It is therefore preferable to separate
the ammonia from the soil in the form of chlorid, and to subject this
extract to distillation.
In fifty grams of the soil the humidity is determined by drying at 100°
until there is no further loss of weight. The quantity of moisture being
known, 200 grams of soil are taken and moistened with water, and then
there is added, in small portions, some dilute hydrochloric acid,
shaking frequently until the whole of the calcium carbonate present is
decomposed. The liquor should remain acid at the end of the operation,
but without containing a notable excess of acidity. Knowing beforehand
the quantity of moisture contained in the 200 grams, water is added
until the total quantity shall be equal to 500 cubic centimeters. The
whole is then shaken and allowed to repose, and filtered rapidly,
covering the funnel with a glass vessel and receiving the liquid which
runs through in a flask with a narrow opening. Two hundred and fifty
cubic centimeters of this liquor, or mixture, represent 100 grams of
earth of known humidity. This quantity is introduced into a flask for
determining the ammonia and five grams of calcined magnesia added.
Before commencing the distillation, assurance should be had that the
magnesia has completely saturated the acid in excess, and that the
liquor is alkaline.
If, by chance, the liquor should be still acid it would be necessary to
add sufficient magnesia in order that the reaction should be manifestly
alkaline. Afterwards the distillation is begun and the ammonia is
received in an appropriate vessel containing one-tenth normal sulfuric
acid and titrated in the usual way, or nesslerized.
Inasmuch as the quantities of ammonia contained in the earth are
generally very small it is necessary to be very particular in order to
avoid errors. The distilled water which is employed should be deprived
of all traces of ammonia by prolonged ebullition, and the hydrochloric
acid should be distilled in the presence of a little sulfuric acid. The
treatment with hydrochloric acid is for the purpose of destroying the
absorbing properties of the soil for ammonia, and to permit this last to
enter into solution as chlorid. When there is need of very great
precision it is convenient to make a blank operation with the
hydrochloric acid and water which are employed, in order to make a
correction for the traces of ammonia which these reagents may contain.
=408. Estimation of Ammoniacal and Amid Nitrogen by the Method of
Berthelot and André.=[271]—Heat one hundred grams of earth for thirty
hours on a steam-bath with about .500 cubic centimeters of dilute
hydrochloric acid (fifteen grams of hydrochloric acid to 500 cubic
centimeters of water). At the end of this time throw the contents of the
flask on a filter and wash with hot water until acid reaction has
ceased. Determine both the ammoniacal and amid nitrogen in the soluble,
and the total nitrogen in the insoluble portion, the ammoniacal by
distillation with magnesia, and the amid and total with soda-lime.
_Example._ A soil contained 0.1669 per cent total nitrogen. Of this
there were obtained:
As ammoniacal nitrogen 13.7 per cent
In the soluble part as amid nitrogen 56.2 „ „
In the insoluble part, total nitrogen 29.7 „ „
————
Sum 99.6 „ „
_Treatment of the Insoluble Portion._—Treat the part insoluble in
hydrochloric acid with a three per cent solution of potash on a
steam-bath for thirty hours. Estimate the nitrogen remaining insoluble,
from which the part dissolved can be determined by difference. The
potash will dissolve usually about two-thirds of the remaining nitrogen.
About ninety per cent of the total nitrogen present in an arable soil
will be rendered soluble by successive treatment with acid and alkali.
The reverse treatment will give practically the same result. It is
therefore immaterial, from an analytical standpoint, whether the acid or
alkali be used first.
=409. Estimation of Volatile Nitrogenous Compounds Emitted by Arable
Soil.=—The following method, due to Berthelot and André,[272] may be
practiced:
Porcelain pots, containing one kilogram of soil, are placed under
bell-jars of fifty liters capacity adjusted to glass dishes designed to
receive the waters of condensation.
During the first period the pots are to be sprinkled from time to time,
during the duration of the experiment, through the upper tubulature, so
as to prevent the soil from becoming dry. The water is partly condensed
on the sides of the bell-jar. It is removed each week through the
inferior tubulature, treated with a little dilute sulfuric acid, and
preserved for further study. A small vessel containing dilute sulfuric
acid is placed near the porcelain pot for the purpose of collecting, as
far as possible, the evolved ammonia.
During the second period the pots are not sprinkled, the soil becomes
dry and there is no longer any condensation of water on the walls of the
bell-jar. The two periods should include about five months, from May to
October.
At the end of the second period the following determinations are to be
made:
1. The ammonia absorbed by the dilute sulfuric acid.
2. The ammonia set free by distillation with magnesia, such as may have
accumulated in the condensed water.
3. The organic nitrogen contained in the latter after elimination of the
ammonia. This is determined by adding a slight excess of acid,
evaporation to dryness, and combustion with soda-lime, or by moist
combustion with sulfuric acid.
Example:
_Earth Employed._—One kilogram of sandy clay containing total nitrogen,
0.09 gram. Nitrogen in sprinkling water, 0.000048 gram.
_Nitrogen in Exhaled Products._—
FIRST PERIOD. SPRINKLING.
Ammoniacal nitrogen collected in the dilute sulfuric 0.00012 gram.
acid
Ammoniacal nitrogen collected in the condensation waters 0.00012 „
Organic nitrogen in condensation waters 0.00220 „
————————
Sum 0.00244 „
SECOND PERIOD. NO SPRINKLING.
Ammoniacal nitrogen in dilute sulfuric acid 0.000007 gram.
„ „ „ condensed water 0.000007 „
Organic „ „ „ „ 0.000040 „
————————
Sum 0.000054 „
_Conclusions._—The exhalation of nitrogenous compounds takes place with
a certain relative activity, about two milligrams in two months and a
half, as long as the soil is kept moist by sprinkling.
In the second period, without sprinkling, the exhalation is reduced to a
mere trace.
The vessel containing the dilute sulfuric acid placed near the porcelain
pot absorbs only about one-half of the ammoniacal nitrogen set free. The
nitrogen emitted under other forms than ammonia is, in every instance,
greatly superior in quantity, and this is the most important of the
observed phenomena. This is true at least with the kind of soil with
which the experiment was made. With arable soil containing twenty times
as much nitrogen as the soil described above this order is
reversed,[273] the ammoniacal prevailing over the non-ammoniacal
nitrogen volatilized.
These phenomena are doubtless greatly influenced in soil under culture
by microbes, and the lowest orders of vegetation to which are doubtless
due the traces of non-ammoniacal volatile nitrogenous compounds, a sort
of vegetable ptomaines.
=410. General Conclusions.=—In the light of our present knowledge
concerning the methods of nitrogen determination in the soil in the form
of organic compounds and ammonia, moist combustion with sulfuric acid is
to be preferred to the older soda-lime process. For the nitrogen
combined as ammonia, the extraction of the sample with hydrochloric acid
and subsequent distillation with an excess of freshly calcined magnesia,
are recommended. For the study of the progressive decomposition of the
nitrogenous compounds, the various processes devised by Berthelot and
André are the best.
The origin of the nitric acid in the soil, the methods of studying the
various nitrifying organisms, and of estimating the nitric acid
produced, will form the subject of the next part.
NOTE.—At the Eleventh Annual Convention of the Association of Official
Agricultural Chemists, held in Washington, August 23, 24 and 25, 1894,
the following process of soil extraction was adopted as the official
method:
_Preparation of the Sample._—500 grams or more, of the air-dried soil,
which may be either the original soil or that which has been passed
through a sieve of coarser mesh, are sifted upon a sieve with circular
openings one-half millimeter in diameter, rubbing, if necessary, with a
rubber pestle in a mortar, until the fine earth has been separated as
completely as possible from the particles that are too coarse to pass
through the sieve. The fine earths thoroughly mixed and preserved in a
tightly stoppered bottle from which the portions for analysis are
weighed out.
The coarse part is weighed and may be subjected to further examination,
(as in _Bulletin 38, Div. of Chem._, pp. 65, 75 and 200.) It may
sometimes be necessary to wash the soil through the one-half millimeter
sieve with water, in which case proceed as directed on pp. 65 and 75 of
the above _Bulletin_. The use of water is to be avoided whenever
possible.
_Determination of Moisture._—Heat two to five grams of the air-dried
soil in a flat-bottomed, tared platinum dish; heat for five hours in a
water-oven kept briskly boiling; cover the dish, cool in a desiccator,
and weigh.
Repeat the heating, cooling, and weighing at intervals of two hours till
constant weight is found, and estimate the moisture by the loss of
weight. Weigh rapidly to avoid absorption of moisture from the air. An
air-bath must not be used in this determination.
_Determination of Volatile Matter._—The platinum dish and soil used to
determine moisture are used also to determine volatile matter. Heat the
dish and dried soil to full redness until all organic matter is burned
away. If the soil contain appreciable quantities of carbonates, the
contents of the dish, after cooling, are to be moistened with a few
drops of a saturated solution of ammonium carbonate, dried and heated to
dull redness to expel ammonium salts, cooled in the desiccator and
weighed.
The loss in weight represents the organic matter, water of combination,
ammonium salts, etc.
_Extraction of Acid-Soluble Materials._—In the following scheme for soil
analysis it is intended to use the air-dried soil from the sample bottle
for each separate investigation. The determination of moisture, made
once for all on a separate portion of air-dried soil, will afford the
datum for calculating the results of analysis upon the soil dried at the
temperature of boiling water. It is not desirable to ignite the soil
before analysis or to heat it so as to change its chemical properties.
The acid digestion is to be performed in a flask so arranged that the
evaporation of acid shall be reduced to a minimum, but to take place
under atmospheric pressure and at the temperature of boiling water. Any
flask resistant to acids is suitable, but it is not necessary to use a
condenser, as a simple bohemian glass tube eighteen inches in length
will answer the purpose of preventing loss of acid. Where it is not
desired to determine sulfur trioxid, an erlenmeyer fitted with a rubber
stopper and hard glass tube will answer. The flask must be immersed in
the water-bath up to the neck or at least to the level of the acid and
the water must be kept boiling continuously during the digestion.
In the following scheme, ten grams of soil are taken, this being a
convenient quantity in most soils, in which the insoluble matter is
about eighty per cent. If desired, a larger quantity of such soil may be
taken, using a proportionately larger quantity of acid and making up the
soil solution to a proportionately larger volume. In very sandy soils,
where the proportion of insoluble matter is ninety per cent or more,
twenty grams of soil are to be digested with 100 cubic centimeters of
acid and the solution made up to 500 cubic centimeters or a larger
quantity may be used, preserving the same proportions. It is very
important that the analyst assure himself of the purity of all the
reagents to be used in the analysis of soils before beginning the work.
_Acid Digestion of the Soil._—Place ten grams of the air-dried soil in a
150 to 200 cubic centimeter bohemian flask, add 100 cubic centimeters of
pure hydrochloric acid of specific gravity 1.115, insert the stopper
with condensing tube, place in a water or steam-bath and digest for ten
hours continuously at the temperature of boiling water, shaking once
each hour. Pour the clear liquid from the flask into a small beaker,
wash the residue out of the flask with distilled water on a filter
adding the washings to the contents of the beaker. The residue after
washing until free of acid, is to be dried and ignited as directed
below. Add one or two cubic centimeters of nitric acid to the filtrate,
and evaporate to dryness on the water-bath, finishing on a sand or
air-bath to complete dryness; take up with hot water and a few cubic
centimeters of hydrochloric acid, and again evaporate to complete
dryness. Take up as before, filter and wash thoroughly with cold water
or with hot water slightly acidified at first with hydrochloric acid.
Cool and make up to 500 cubic centimeters. This is solution “A.” The
residue is to be added to the main residue and the whole ignited and
weighed, giving the insoluble matter.
The determination of the various components of the solution remains
essentially as described in the provisional methods of the Association
which have already been given.
It is directed that all results of soil analysis be calculated on the
basis of the sample dried to constant weight at the temperature of
boiling water.
AUTHORITIES CITED IN PART SIXTH.
Footnote 189:
Annales de Chimie et de Physique, sixiéme serie, Tome 25, pp. 292, et
seq.
Footnote 190:
L’Analyse du Sol, p. 14.
3. Die Landwirtschaftlichen Versuchs-Stationen, Band 38, S. 311.
Footnote 191:
Bulletin 38, Chemical Division United States Department of
Agriculture, p. 201.
Footnote 192:
Untersuchung Landwirtschaftlich und Gewerblich Wichtiger Stoffe, S.
14.
Footnote 193:
Vid. op. cit. supra.
Footnote 194:
Vid. op. cit. 1.
Footnote 195:
Vid. op. cit. 3, Band 37, S. 279.
Footnote 196:
Journal of the Chemical Society, September, 1880, p. 617.
Footnote 197:
Wanklyn, Philosophical Magazine, Series 5, Vol. 5, p. 466.
Footnote 198:
Vid. op. cit. 1.
Footnote 199:
Traité d’Analyse des Matiéres Agricoles, p. 148.
Footnote 200:
Bulletin 38, Division of Chemistry, United States Department of
Agriculture, pp. 84, et seq.
Footnote 201:
Encyclopedie Chimique, Tome 4, p. 182.
Footnote 202:
Die Landwirtschaftlichen Versuchs-Stationen, Band 37, S. 280.
Footnote 203:
Vid. op. cit. supra, Band 28, S. 229.
Footnote 204:
Comptes rendus, 1890, pp. 290, et seq.
Footnote 205:
Annales Agronomiques, 1890, p. 558.
Footnote 206:
Bulletin 13, Division of Chemistry, p. 590.
Footnote 207:
Bulletin de la Société Chimique, Serie 3, Tome 2, pp. 483, et seq.
Footnote 208:
Zeitschrift für analytische Chemie, Band 3, S. 165.
Footnote 209:
Petermann, L’Analyse du Sol, p. 20.
Footnote 210:
Zeitschrift für analytische Chemie, Band 3, S. 92.
Footnote 211:
Journal of the Chemical Society, March, 1894, p. 141.
Footnote 212:
Agricultural Science, January, 1894, p. 2.
Footnote 213:
American Journal of Science, Vol. 7, 1874, p. 20.
Footnote 214:
Geological and Agricultural Report of Kentucky, Vol. 3.
Footnote 215:
Bulletin 38, Division of Chemistry, p. 77.
Footnote 216:
Op. cit. supra, p. 83.
Footnote 217:
Die Landwirtschaftlichen Versuchs-Stationen, Band 37, S. 311.
Footnote 218:
Zeitschrift für analytische Chemie, Band 3, S. 92.
Footnote 219:
Traité d’Analyse des Matiéres Agricoles, p. 144.
Footnote 220:
Op. cit. 28, pp. 202, et seq.
Footnote 221:
Op. cit. 28, pp. 77, et seq.
Footnote 222:
Op. cit. 22, p. 21.
Footnote 223:
Manuscript communication to author.
Footnote 224:
Annales de la Science Agronomique, Huitiéme Année, Tome 1, p. 278.
Footnote 225:
Die Landwirtschaftlichen Versuchs-Stationen, Band 37, S. 311.
Footnote 226:
Comptes rendus, 1890, p. 289.
Footnote 227:
Thoms, Zur Werthschätzung der Ackererde, S. 120.
Footnote 228:
Le Stazioni Sperimentali Agrarie Italiane, Vol. 16, p. 679.
Footnote 229:
Crookes’ Select Methods in Chemical Analysis.
Footnote 230:
Chemiker Zeitung, Band 13, S. 1391.
Footnote 231:
Annales de Chimie et de Physique, serie sixiéme, Tome 15, p. 309.
Footnote 232:
Vid. op. cit. 37, pp. 270, et seq.
Footnote 233:
Chemiker Zeitung, Band 13, S. 726.
Footnote 234:
Die Agrikultur Chemische Versuchs-Station, Halle, a/S., S. 80.
Footnote 235:
Comptes rendus, Tome 107, pp. 999 and 1150.
Footnote 236:
Die Landwirtschaftlichen Versuchs-Stationen, Band 41, S. 453.
Footnote 237:
Bulletin de la Société Chimique de Paris, 1893, p. 343.
Footnote 238:
Die Agrikultur Versuchs-Station, Halle, a/S., S. 70.
Footnote 239:
Op. cit. supra., S. 68.
Footnote 240:
Op. cit. 37, p. 267.
Footnote 241:
L’Analyse du Sol, p. 20.
Footnote 242:
Op. cit. 1, pp. 303, et seq.
Footnote 243:
Op. cit. 40, S. 114.
Footnote 244:
Bulletin 38, Division of Chemistry, p. 80.
Footnote 245:
Zeitschrift für analytische Chemie, Band 3, S. 92.
Footnote 246:
Chemisches Centralblatt, 1861, p. 3.
Footnote 247:
Berichte der deutschen chemischen Gesellschaft, Band 26, S. 386.
Footnote 248:
Manuscript communication to author.
Footnote 249:
Op. cit. 37, p. 285.
Footnote 250:
Op. cit. 44, pp. 305, et seq.
Footnote 251:
Die Landwirtschaftlichen Versuchs-Stationen, Band 37, S. 284.
Footnote 252:
Op. et loc. cit. 58.
Footnote 253:
Op. et loc. cit. 41.
Footnote 254:
Op. et loc. cit. 37.
Footnote 255:
Op. et loc. cit. 58.
Footnote 256:
L’Analyse du Sol, p. 17.
Footnote 257:
Op. cit. 44, p. 308.
Footnote 258:
Op. cit. 37.
Footnote 259:
Op. cit. 64, Band 40, S. 251.
Footnote 260:
Bulletin de la Société Chimique, May, 1891, pp. 643, et seq.
Footnote 261:
Bulletin 38, Division of Chemistry, p. 204.
Footnote 262:
Bulletin 38, Division of Chemistry, p. 81.
Footnote 263:
Op. cit. 44, March, 1891, pp. 393, et seq.
Footnote 264:
Op. cit. 58, Band 31, S. 525.
Footnote 265:
Relatorio Annual do Instituo Agronomico do Estado de San Paulo
(Brazil), 1892, p. 107.
Footnote 266:
Op. cit. 37, pp. 253, et. seq.
Footnote 267:
Op. cit. 44, Tome, 25, pp. 299, et seq.
Footnote 268:
Petermann, L’Analyse du Sol, p. 17.
Footnote 269:
Op. et. loc. cit. 76.
Footnote 270:
Die Landwirtschaftlichen Versuchs-Stationen, Band 33, Ss. 247, et seq.
Footnote 271:
Op. cit. 44, Tome 25, pp. 327, et seq.
Footnote 272:
Op. cit. 44, Tome 25, pp. 330, et seq.
Footnote 273:
Op. et loc. cit. supra.
PART SEVENTH.
THE ORIGIN AND ESTIMATION OF OXIDIZED NITROGEN IN SOILS, RAIN AND
DRAINAGE WATERS.
=411. Introductory Considerations.=—The estimation of oxidized nitrogen
in the soil would properly find a place in the preceding part; but on
account of the late progress in our knowledge of the source of this
indispensable and costly plant food it has become necessary to give it
especial attention. The present part will, therefore, be devoted to a
brief statement of our present knowledge in respect of the origin of
oxidized nitrogen, a description of the nitrifying ferments and methods
for their isolation and determination and finally the most approved
methods of estimating the ammonia, nitrous, and nitric acids formed
thereby both in the soil and the waters pertaining thereto or proceeding
therefrom. It is scarcely necessary to caution the reader not to
consider this part in any sense a treatise on the bacteria active in
soil chemistry. Its object is rather to place in the hands of the soil
analyst data which will enable him to intelligently study the soil
phenomena depending on these organisms and to determine the extent and
character of their biological and chemical functions. These are matters
which, up to the present time, have found no place in manuals dedicated
to agricultural analysis.
=412. Organic Nitrogen in the Soil.=—With the exception of the small
quantities of nitric acid added to the soil directly by rain water, the
whole of the supply of this substance is derived from the products of
the oxidation of nitrogenous bodies. These products are either stored as
the results of past nitrification or are formed synchronously with their
consumption by the growing plant. Nitrogenous compounds are present as
organic vegetable or animal remains and as humus. All vegetable and
animal material deposited in or on the soil contains more or less of
these proteid or nitrogenous matters while the amount of nitric acid
supplied in this way is probably represented entirely by the quantity in
the organism of the plant or animal and unabsorbed at the time of its
death. In other words it is not demonstrated that nitrates or nitrites
are in any sense a special product of plant growth save in the case of
nitrifying organisms themselves which are supposed to be of a vegetable
nature. Animal organisms do not in any sense assimilate nitric nitrogen.
With most plants, the quantity of proteid nitrogen which they can
deliver to the soil is in no case greater than the sum of organic and
nitric nitrogen supplied in their food and they can therefore be
regarded only as the carriers and conservers of this substance. On the
other hand there are some plants notably those belonging to the
leguminous family which permit of the development on their rootlets of
colonies of bacteria which have the faculty of rendering atmospheric
nitrogen available for plant growth. Whether or not there exist plants
other than the micro-organisms mentioned which are capable of directly
oxidizing and fixing atmospheric nitrogen is still an unanswered
question. It is not probable, however, that the difficult task of
oxidizing atmospheric or free nitrogen would be accomplished in nature
in only one way. In fact it has already been established that organisms
do exist which are capable of oxidizing free nitrogen in a manner wholly
independent of other plant life and to produce weighable quantities of
nitric acid when developed in media of mineral matters and pure
carbohydrates to which free nitrogen has access. It is, therefore, fair
to assume that the fixation of free nitrogen is a function of chemical
activity quite independent of ordinary plant life and that the
leguminous plants take no further part in this process than that of
providing in their radical development a favorable nidus for the growth
of the nitrifying organism.
By the action of denitrifying organisms a portion of the nitrogen of
nitric acid is constantly restored to a free state, a far larger
portion, perhaps, than is fixed in the atmosphere itself by the action
of electricity. Were it not, therefore, for the activity of the
nitrifying ferments the stores of nitrogen available for growing plants
would constantly become less. Instead of this being the case, however,
it is probable that the contrary is true and that, by a wise system of
agriculture, the total nitrogen at the disposal of plants may become
greater and greater in quantity.
=413. Development of Nitric and Nitrous Acids in Soils.=—Owing to the
solubility of nitrates there can be but little accumulation of them in
soils in those countries where there is any considerable amount of
rain-fall. On the other hand in arid regions there may be found
extensive deposits of nitrates. The occurrence of a certain quantity of
nitrates in the soil, however, is essential to the growth of plants.
Until within a few years little was known of the origin of nitric acid
in the soil. The presence of nitrates in drainage waters was well
established, likewise the consumption of nitric acid by the growing
plant, but the method of its supply was unknown. In a general way it was
said that the nitric acid came from electrical action and the oxidation
of the albuminous bodies in the soil, but without specifying the manner
in which this change takes place. The researches of Schloesing and
Müntz, of Springer, Winogradsky, Frankland, Warington and others have
demonstrated the fact that this oxidation is caused by means of bacteria
and that the nitrates formed can be consumed and destroyed by other
species of this organism. In the one case the process has been called
nitrification and in the other denitrification.[274]
The influence of these low organisms both in producing fertility in a
soil and maintaining it in a state of fertility is of the highest
importance.
=414. Conditions Necessary for Nitrification.=—In order to properly
understand the reasons for many of the steps in investigating a soil for
nitrifying organisms, it will be useful to state the general conditions
on which nitrification depends.
The nitrifying organism, like every other one, first of all feels the
necessity for food. In general, food which is given to microbes of all
kinds consists of some organic matter together with the addition of
mineral substances necessary to growth. These substances in general are
phosphoric acid, potash, and lime. Of these articles of bacterial food
phosphoric acid seems to be the most important. With the nitrifying
organisms, however, it has been found that the organic matter can be
omitted. In fact, as will be seen further on, the omission of organic
matter supplies the best condition for the proper isolation of the
organisms. In other words some forms of the nitrifying organisms have
the property of subsisting wholly on mineral substances, _i. e._, are
true vegetables.
The presence of oxygen is also necessary to the growth of the common
nitro-organisms. In an atmosphere deprived of oxygen or in which the
oxygen is reduced to a very low percentage, the process of nitrification
is retarded or stopped as the oxygen diminishes or disappears.
The presence of a base with which the nitrous or nitric acid formed may
unite is also essential to the proper conduct of the process. For this
reason the nitrification should take place in a solution which is feebly
alkaline or in the presence of a base which can be easily decomposed so
that no acidity can take place. Calcium carbonate is a base well suited
to favor the nitrifying process and its presence in a soil favors the
rapid oxidation of proteid matter. The mistake must not be made,
however, of supposing that an excess of alkali would favor
nitrification. The contrary is true. A slight excess of alkali may
prevent nitrification altogether when it is due to the common organisms
present in an arable soil. It may be that in soils charged with alkali a
different organism exists which is capable of exercising its functions
when the alkali is in excess.
The temperature to which the nitrifying body is subjected is also a
matter of importance. The nitrifying organisms have the property of
remaining active at lower temperatures than most bodies of their class.
On the contrary their action is retarded and destroyed by high
temperatures. The most favorable temperature for nitrification is about
that of blood heat; _viz._, 37°. At 50° the organism shows very little
activity and at 55° its activity ceases altogether. Nitrification,
however, according to Warington, cannot be started in a solution if the
initial temperature is 40°.
Desiccation has the same retarding influence on nitrification that a
high temperature has. Even thoroughly air-drying a soil may destroy its
nitrifying qualities.
Darkness is also necessary to the proper progress of nitrification. In a
strong light, the activity of the organism is very much diminished or
destroyed altogether. A bright light like sunshine may even stop
nitrification which has set in.
=415. Effect of Potassium Salts on Rate of Nitrification.=—Dumont
and Crochetelle have described some experiments to determine the
effect of potassium salts alone and in combination with lime on
nitrification.[275]
Soil rich in vegetable mold (18.5 per cent of humus and 0.29 per cent of
lime) was treated with varying amounts of potassium sulfate and
carbonate and kept for twenty days at 25°. In the untreated soil the
amount of nitric acid produced was twenty-five parts per million. When
potassium carbonate was applied in quantities of from one-tenth to six
per cent the amount of nitric acid increased from forty-seven parts per
million to 438 parts when four and one-half per cent of the potassium
salt were used. Larger quantities caused a decrease in the amount of
nitric acid produced. Very little effect, on the contrary, was produced
by the action of potassium sulfate. When one-half per cent was employed
the quantity of nitric acid formed rose to fifty parts per million,
while with quantities as high as five per cent it fell below the normal;
_viz._, twenty-five parts per million.
When calcium carbonate was added to the soil in conjunction with
potassium sulfate there was a marked increase in the amount of nitrogen
oxidized. The activity of potassium sulfate in promoting nitrification
is therefore increased by the presence of the calcium salt, potassium
carbonate and calcium sulfate being formed.
=416. Production of Nitrous and Nitric Acids.=—In the following pages
the study of the methods of isolating the nitrous and nitric ferments
will be considered as one process, the final isolation of the two
classes of bodies being the result of their synchronous cultivation in
appropriate media. The special process of the production of ammonia by
oxidation is not so well-known, and will therefore be described in
brief.
It is now generally conceded that the action of the nitrous organism is
precedent to that of the nitric, but the two processes go on so nearly
together as to prevent the accumulation of any large quantities of the
lower salt in the soil.
Whether or not the formation of ammonia precedes that of nitrous acid is
still a subject for experimental demonstration. Chemically, both nitrous
acid and ammonia may be produced by the reduction of nitric acid. In
nature, the reverse of this process may be the customary method.
=417. Production of Ammonia in the Soil by the Action of Microbes.=—It
is highly probable that organic nitrogen in the soil in passing into the
form of nitric acid exists at some period of the process in the form of
ammonia.
Marchal has isolated and studied some of these ammonia-making
bacteria.[276] Bacillus mycoides is the most active of these organisms.
It occurs constantly in surface soils and is present in the air and in
natural waters. In decomposing albumen it produces a strongly alkaline
solution due to ammonium carbonate. Organic carbon, during this process,
is converted chiefly into carbon dioxid, but small quantities of formic,
propionic, and butyric acids are also produced. Any organic sulfur which
is present is converted into acid. No hydrogen or nitrogen is eliminated
in a free state. While slight alkalinity is favorable to the development
of this bacterium, yet it may be propagated in a feeble sulfuric acid
solution when the acid is less than one per cent.
The greatest activity of this organism is manifested at 30°. Below 5°
and above 42° no ammonia is produced. The bacillus will not develop in
an atmosphere of hydrogen or carbon dioxid, except in solutions of
organic matter and nitrate. In addition to its action on egg albumen it
decomposes other proteid bodies as well as leucin, tyrosin, creatin, and
asparagin. It, however, does not oxidize urea, nor does it develop in
solutions of ammonium salts and nitrates, except as mentioned above.
When soluble carbohydrates are present, acids are formed. It is
concluded from these experiments that the final oxidation of organic
nitrogenous matter is preceded by its conversion into ammonium
carbonate.
=418. Summary of Statements.=—All nitrogenous matters which would be
naturally present in the soil may become subject to nitrification when
the proper conditions are supplied. Munro has also succeeded in
nitrifying ethylamin, thiocyanates, and gelatin, urea, asparagin, and
the albuminoids of milk and rapeseed.
The products of nitrification are ammonia, nitrous or nitric acid,
carbon dioxid, and water. The ammonia and nitrous acid may not appear in
soils as the final products of nitrification, as the nitric organism
attacks the latter at once and converts it into nitric acid. Nitrous
acid and ammonia may also be produced in soils as one of the retrograde
steps in denitrification.
To summarize the conditions necessary for nitrification it may be said
that first, the proper material must be supplied; _viz._, an organic or
inorganic nitrogenous compound capable of oxidation. In the second
place, the medium must be faintly alkaline, the temperature must not be
too high, the nitrifying organisms must have abundant food, and the
process must take place in the dark.
=419. Order of Oxidation.=—It is quite definitely determined that
activity of the ammoniacal and nitrous organisms is the first step in
the process, since the nitric organism appears to have no power whatever
to oxidize proteid compounds; while, on the other hand, the nitrous
organism can not, in any case, complete the conversion of nitrous into
nitric acid.
The conditions which permit certain organisms to oxidize free nitrogen
have not been definitely determined. The presence of such bodies in the
tubercles attached to the rootlets of certain leguminous plants has been
established. Lately, Winogradsky has isolated from the soil a nitrifying
organism which is capable of converting free nitrogen into forms suited
to nourish plant growth. This organism is cultivated in dextrose with
careful exclusion of all nitrogen, save that which exists in the air
carefully freed of every trace of ammonia or oxidized nitrogen.
Under the influence of the growth of this organism the sugar undergoes a
butyric fermentation, and nitrogen in an oxidized form is assimilated in
an amount apparently equal to about one five-hundredth of the sugar
consumed.
This result leads Warington[277] to remark that it is a fact of
extraordinary interest, both to the physiologist and chemist, that a
vegetable organism should be able to acquire from the air all the
nitrogen it needs.
=420. The Nitrification of Ammonia.=—The same organism which converts
organic nitrogen into nitrous acid acts also on ammonia and its
compounds with a similar result. In fact, the formation of ammonia may
be regarded as one of the stages on the road from albuminoid to nitric
nitrogen.
Data have been collected by Schloesing on the nitrification of ammonia
taking place in arable soil, tending to show that this phenomenon is
accomplished without appreciable loss of nitrogen in the gaseous
state.[278] This, however, does not hold good when the quantity of
ammonium carbonate introduced into the earth is largely increased. In
two experiments, conducted by Schloesing, with a larger quantity of
ammonium carbonate, the loss of nitrogen was very notable. In certain
conditions the production of nitrous acid may take place, and it is
interesting to know whether the appearance of nitrites has any influence
on the disengagement of free nitrogen. In order to determine this
question a solution of calcium nitrite was prepared by decomposing
silver nitrite with calcium chlorid. From the results of the experiments
made it was seen that the nitrites were only the results of a retarded
and partially incomplete nitrification. They are, moreover, thus an
obstacle to the normal work of the nitrifying organisms. It is also
established that when they are present a disengagement of gaseous
nitrogen takes place, whether the nitrites are formed during the
progress of the experiment, or whether they were originally present.
However, it is not best to say that the nitrites themselves have been
the cause of the disengagement of the nitrogen. It may happen that the
disengagement of the nitrogen and the presence of nitrites are simply
simultaneous and due to one and the same cause. The destruction of
nitrates in the midst of reducing agents furnishes, according to the
nature of these bodies and the circumstances, nitrous acid, nitrogen
dioxid, nitrogen protoxid, free nitrogen, and even ammonia.
This destruction of nitrates and the appearance of oxids of nitrogen and
of free nitrogen are more likely to be due to the presence of a separate
denitrifying ferment as pointed out by Springer than to have arisen in
the manner mentioned above by Schloesing. In the present state of our
knowledge, moreover, we can hardly regard the presence of nitrites as an
obstacle to complete nitrification. On the other hand, it seems to be
well established that the production of nitrites or ammonia is a
necessary step between organic nitrogen and nitric acid.
=421. Occurrence of Nitrifying Organisms.=—According to the observations
of Schloesing and Müntz the nitrifying organisms are widely
distributed.[279] Arable soil containing considerable humus seems to be
the medium in which they grow most freely and in which they accomplish
their most important functions. Sewage waters are also rich in
nitrifying ferments, and, in fact, all waters containing organic matter.
They are also found in running waters but not in great numbers. They
affect chiefly the surface of bodies, and especially are found on the
bottom of culture-flasks.
These authors have not found the nitrifying organisms in normal air.
They could not seed sterilized flasks by admitting air freely. The
absence of these ferments from the air is explained by reason of their
sensitiveness to desiccation.
The method used by Schloesing and Müntz for the separation of the
organism consisted in the preparation of original and subcultures in
sterilized solutions containing nitrifiable matters. The proof of
isolation was assumed when a given subculture contained only one kind of
organism as seen with the microscope. The appearance of this organism,
as described by the authors, was that of the later isolations by
Warington and Winogradsky, but the method used could hardly now be
regarded as decisive.
=422. Determination of Nitrifying Power of Soils.=—In studying the
distribution of the nitrifying organisms in a soil the general method of
procedure is based on the production of nitrification in a convenient
solution by the organisms present in a given sample of soil. If the
solution seeded with the given portion of soil remain unaffected, it
will show that there were no nitrifying organisms present in the seed
used. On the other hand, the vigor of the nitrifying process when once
it is started, may be taken as an evidence of the number and activity of
the organisms in the soil, a sample of which was used for seed.
=423. Composition of the Culture Medium.=—The solution recommended by
Warington for the culture and isolation of the nitrifying ferments has
the following composition:
Ammonium chlorid 80 milligrams.
Sodium potassium tartrate 80 „
Potassium phosphate 40 „
Magnesium sulfate 20 „
calcium carbonate about 200 „
Pure bacteria-free water to make one liter.
=424. Apparatus and Manipulation.=—The experiments are conducted in
short, wide-mouthed bottles. The initial volume of the solution in each
bottle is 100 cubic centimeters, and the bottle should be of such size
as to give a depth of liquid of from three to five centimeters.
The neck of the bottle is closed with a plug of cotton and this is
protected from dust by tying over it a cap of filter paper. Arranged in
this way, filtered air has free access to the solution. The bottle with
the solution thus protected is placed in a water-oven and kept near the
temperature of boiling water for six to eight hours to destroy any
organisms present. When cool, the solution is ready for use.
The calcium carbonate used should be prepared by precipitation and added
in a moist state. The calcium carbonate solution should be added after
the sterilization of the liquid, the precipitated carbonate being boiled
just before it is added.
_Preparation of Seed._—The seed employed to start the nitrification
should be a small quantity of fresh soil, usually about one-tenth of a
gram. If a previously nitrified solution be used for seed it should be
thoroughly shaken and about one cubic centimeter of the solution removed
for seeding the new bottle.
In introducing the nitrifying liquor into the bottle the plug should be
lifted slightly and a small pipette inserted by means of which the
liquor is added. The operation should be carried on in a room perfectly
free from dust and to which no one but the operator has access. The
greatest care should be exercised to prevent any particles of matter
entering the solution except that which is purposely added. In
withdrawing the liquor from the nitrifying solution cotton wool should
be pressed around the top of the pipette so that the entering air may be
filtered before admission to the interior of the bottle. The pipette
which is used should be kept in boiling water until it is required for
use. After use it should be washed and replaced in boiling water until
again required.
After seeding, the bottles should be placed in a dark cupboard and
exposed to the ordinary temperature of the laboratory. If a higher or
stated temperature be desired, the bottle should be placed in a metal
box the temperature of which can be regulated to any degree.
_Test of the Commencement of Nitrification._—The beginning of the
nitrification can be determined in a solution by testing it with
diphenylamin. One cubic centimeter of the solution withdrawn as above
indicated, is placed in a small beaker, a drop of solution of
diphenylamin sulfate in sulfuric acid added, and then two cubic
centimeters of concentrated sulfuric acid and the contents of the beaker
well shaken. The development of a violet-blue color shows the presence
of nitric or nitrous acid. This test will detect one part of nitric
nitrogen in twenty million of water.
_Determining the Progress of Nitrification._—The progress of
nitrification is determined by repeated examinations for ammonia by
nesslerizing, and for nitrous acid with metaphenylenediamin. Each
experiment is made with five cubic centimeters of the solution withdrawn
as above indicated and placed in test-tubes, always of the same size.
The reaction with the nessler solution is then made by adding it in the
usual way. The colorations are recorded as, trace, small, moderate,
considerable, large, and abundant.
If the change produced by the organism consisted in the formation of
nitrites only, the ammonia in the original solution would fall from
_large_ to _trace_, while the nitrous acid would increase from _trace_
to _large_. If the nitrification consisted in the production of nitrates
only, the ammonia would diminish without any corresponding production of
nitrous acid. In mother solutions which contain ammonium carbonate
instead of sulfate, it should not be forgotten that the ammonia might
gradually disappear owing to the volatilization of the carbonate without
any corresponding production of free nitrites or nitrates. The complete
disappearance of the ammonia in the above experiments shows the
completion of the process.
=425. To Determine the Distribution of the Nitrifying Organism in the
Soil.=—The principle on which the determination of the distribution of
the nitrifying organism in the soil depends, rests upon seeding the
growth solutions with samples of soil taken at different depths and
carefully protected from the time of sampling until the time of seeding
from any admixture of accidental organisms.
The method of Warington is the simplest and best to follow.[280] The
samples of soil are taken by digging a pit of convenient depth usually
from eight to ten feet. A fresh surface is then cut on one of the sides
of the pit at the spot selected for sampling. This surface is scraped
with a freshly ignited platinum spatula. The spatula should then be
washed, re-ignited, and cooled, and a small portion of the soil, at the
depth required, detached with the spatula and transferred at once into
one of the growth bottles already described.
The growth solution best suited for the purpose contains four cubic
centimeters of urine per liter. Each bottle should also contain some
freshly precipitated calcium carbonate. In sterilizing urine solutions
the calcium carbonate should be added before the heating instead of
afterwards. The quantity of soil taken for each seeding should be about
one-tenth of a gram.
Inasmuch as the cotton stopper has to be lifted to introduce the soil,
opportunity is given for the entrance of any organisms floating in the
air. Experience, however, has shown that air free from soil dust very
seldom contains nitrifying organisms. The seeded bottles are placed in a
dark cupboard of moderate temperature as already described.
=426. Sterilized Urine Solution.=—The sterilized urine solution used for
the determination of the distribution of the nitrifying organisms in the
soil, is made by taking four cubic centimeters of healthy urine,
diluting to one liter, adding some freshly precipitated calcium
carbonate, stoppering with cotton wool and heating for several hours at
the boiling temperature of water.
As a result of Warington’s experiments it was shown that the nitrifying
organism in the soil did not exist, at least in portions of one-tenth of
a gram, to a greater depth than eighteen inches. In only one case was
nitrification produced from a sample of soil taken at a greater depth
and this may have been due to the accidental introduction of organisms
from other sources. It may be assumed that any long delay in the
commencement of nitrification under favorable conditions, implies the
presence of a very limited quantity of organisms in the solution. Thus a
comparative study of the period of incubation and the progress of
nitrification in solutions seeded with soils taken at different depths
or at different places, becomes a fair index of the number and vitality
of the nitrifying organisms contained therein.
=427. Depth to Which Micro-Organisms are Found.=—Koch states that at the
depth of about one meter, the soil is nearly free from every kind of
bacteria.[281] These observations have been corroborated by Pumpelly and
Smyth who find that no infection of a bacterial nature is produced in a
sterilized solution from samples of clay taken at the depth of nine feet
below the surface.[282]
It is evident from the nature of the experiments above described that
the nitrifying processes go on almost exclusively in those portions of
the soil which are subject to cultivation, while in the subsoil and
below the processes of nitrification are either retarded or arrested.
Any stores of nitrogenous matter, therefore, in an insoluble state,
resting in the subsoil, are preserved from oxidation and consequent
waste until such time as they may be removed to near the surface.
=428. Isolation of the Nitrous and Nitric Organisms in the Soil.=—The
action of the organisms which produce nitrification either in form of
nitrites or nitrates, having been thoroughly established, and the method
of testing the soil therefor given, it remains to describe a method by
means of which these organisms in the soil may be isolated and obtained
in a state of purity. The difficulties attending this process are
extremely great on account of the similarity of the two organisms. All
earlier attempts to make pure cultures of the two separate organisms
were attended with but little success.
According to Winogradsky the method of culture on gelatin so long
practiced is not to be relied upon.[283] It is very difficult to
eliminate by this process the organisms which grow rapidly in gelatin
and which mature their colonies in two or three days, but where they
require eight or ten days to produce a colony the method is successful.
In fact, by the gelatin process as it was at first practiced, a good
deal was owing to chance, but sometimes by a happy accident a pure
nitro-bacterium might be isolated.
Formerly it was considered that a liquid could be regarded as sterile if
it gave no growth upon gelatin. It has, however, now been demonstrated
that a liquid may contain large numbers of nitro-bacteria and still
produce no growth upon gelatin. However, for the organisms which
accompany the nitro-bacteria in soils, it is regarded as certain that if
no growth on gelatin is produced by them they are absent. Therefore in
the case of a solution which has been seeded with a soil, if it can be
brought to such a state as to produce no growth on gelatin, it may be
safely assumed that it contains no bacterial organisms save those which
are capable of producing nitrites or nitrates. Therefore if such a
solution produce nitrification and at the same time no growth upon
gelatin, it may be considered as a proof of the isolation of the
nitro-organisms from all others.
This method was also worked out independently by Mr. and Mrs.
Frankland.[284]
Winogradsky says further he confesses that he has advanced these views
only provisionally and without being convinced of their infallibility.
Strictly speaking, the proof of seeding gelatin is not sufficient alone
because the absence of growth can not be regarded as the exclusive
privilege of the nitro-bacteria. Such might be the case sometimes for an
accidental mixture of microbes, introduced with any given sample of soil
into the cultures, but the criterion is not absolute. Microbes, for
example, of a sulfurous or ferruginous nature may be cited, for which
the gelatin layer is not only unfavorable but even fatal. It may thus
happen that there may be eliminated from the solution all that will grow
upon gelatin without freeing it from some special kinds of cultures,
refractory like the nitro-bacteria, but which might reappear if they
should be resown in some favorable nutritive solution. On account of
this fault in the process, Winogradsky has been impressed with the
necessity of bringing out a better method.
In using the gelatin media it is necessary to find the one that is
suited to nourish these organisms, which would evidently be the way
promising the greatest success. This having been found, and those
organisms which produce colonies being easily recognizable, a great step
towards the solution of the problem will have been made and the more so
as the medium would be at the same time absolutely unfavorable to other
forms of microbes. On account of the slow degree of development of the
nitro-organisms, all others would probably have opportunity to grow and
strengthen to their exclusion, unless these interfering organisms could
be completely removed.
=429. The Culture Solution.=—The culture-solution, first proposed by
Winogradsky, had the following composition:
To ten grams of gelatin or one part of agar-agar in 100 cubic
centimeters of water add potassium phosphate, one-tenth of a gram;
magnesium sulfate, five-hundredths of a gram; calcium chlorid, trace;
and sodium carbonate, half a gram. The solution being sterilized in the
usual way by heating, there are added to it a few cubic centimeters of a
sterilized solution containing two-tenths per cent of ammonium sulfate.
Such a solution has been proved to be very favorable to nitro-organisms.
Nevertheless the experiments with such solutions gave no definite
results and they were abandoned.
The non-success of this method led Winogradsky to adopt a nitrifying
solution which absolutely excluded all organic substances. Instead of
using an animal or vegetable gelatinous substance he used one of a
mineral nature, first proposed by Graham and Kühne.[285] Two of these
gelatinous mineral substances were considered; _viz._, the aluminum
hydroxid and the hydrate of silica. The latter was chosen.
=430. Preparation of the Mineral Gelatinous Solution.=—The soluble glass
which is found in commerce is generally of a thick, sirupy consistence.
It is first diluted with three times its volume of water. One hundred
cubic centimeters of this liquid are poured with constant stirring into
fifty cubic centimeters of dilute hydrochloric acid and the mixture
placed in a dialyzer. It is useless to employ a standard solution of
silica. All that is necessary is to submit to dialysis a liquid with an
excess of acid and sufficiently dilute not to be exposed to the danger
of being spontaneously gelatinized in the dialyzer. The dialyzer is left
for one day in running water and two days in distilled water, often
renewed. The solution is then ready for use. This is the case when it is
no longer rendered turbid on the addition of silver nitrate, showing
that the hydrochloric acid has been entirely extracted. The solution is
then to be sterilized by boiling, and preserved in a glass flask closed
with a plug of cotton.
More recent instructions by Winogradsky for preparing the gelatinous
silica recommend dialyzing the soluble glass after treatment with
hydrochloric acid in a parchment tube.[286] The proportions of silicate
and acid are 100 cubic centimeters of the silicate solution (1.06
specific gravity) and 100 cubic centimeters of hydrochloric acid (1.1
specific gravity). With a dialyzing tube placed two days in running
water and one day in distilled water frequently changed it will be found
that the acid is completely removed. One hundred cubic centimeters of
the residual liquor giving no reaction for hydrochloric acid are
concentrated to twenty cubic centimeters. When cold there is added one
cubic centimeter each of a solution of ammonium sulfate and of sodium
carbonate, together with corresponding quantities of the other nutrient
salts commonly employed. The ammonium sulfate should never exceed two to
two and a half, and the sodium carbonate four parts per thousand. To the
flask containing the above substances is added one drop of the
seed-liquor, which may be a soil water or a drop from some previous
culture. The flask is shaken and the mixture poured into a low circular
glass dish which is covered by one slightly larger in diameter (Petri
double dish). To the liquid in the dish is added a drop of a cold
saturated solution of common salt, and it is then stirred with a
platinum spatula. The addition of the salt greatly favors the setting of
the jelly. The jelly may set in from two to three hours, but a longer
time secures better results in the end.
In employing these preparations as seed, after the organisms have grown,
it is absolutely necessary to use the isolated cellules and not the
aggregated masses (zoöglœæ). The latter are rarely free of foreign germs
which adhere to their gelatinous envelope. Since the zoöglœæ can not be
broken up by any artificial means it is necessary to await their
spontaneous disintegration in order to separate the mobile monads. The
opalescence of the culture-liquid is a sure index of this separation.
The particles of mineral gelatin to be used as seed for nitrifying are
best taken as follows:
A glass tube is drawn out immediately preceding the operation, until the
end is as fine as a hair. The surface of the mineral gelatin is
magnified by means of a dissecting microscope magnifying 80 to 100, to
the proper degree and the preparation table is so arranged as to give a
perfect support to the right hand which should hold the filament of
glass. The smallest colony is then pricked with the needle and the end
of the glass is broken and dropped into the flask which is to be seeded.
The seed is thus selected in as small a particle as may be desired, only
a few cells, but it can always be ascertained with certainty that some
of the particles have been obtained by this operation.
The method of cultivation on mineral jelly is considered by Winogradsky
an important resource in the study of the nitrifying organisms. It
removes the chief difficulties heretofore existing in discovering and
characterizing these organisms among the innumerable micro-organisms of
the soil. The long series of cultures necessary to separate the
organisms are rendered nugatory. By directly introducing a little of the
earth into the silicic jelly the active organisms in nitrification can
be at once discovered. It is preferable, however, as indicated below, to
previously produce a nitrification in an aqueous solution by a trace of
earth and to take from it the seed for impregnating the solid medium. In
order to show at once a proof of its nitrifying character, it is only
necessary to take a small bit of the mineral jelly, the size of a grain
of rye, and to throw it into a little sulfuric acid which has been
treated with diphenylamin. There is at once formed a blue spot equal in
intensity to a saturated solution of anilin blue.
In regard to the growths which nitro-organisms make in a medium of the
kind described, they are far from being so marked as are those produced
by ordinary micro-organisms.
A nitro-bacterium is not capable of the energy of growth which is
recognized for the greater number of microbes. The colonies contained in
the gelatin always remain small. The largest among them are just visible
to the naked eye like white points. Along the striae, on the contrary,
there is formed quite a thick white crust. To the naked eye, in general,
there is nothing very characteristic in the formation of colonies in a
medium of this nature. But this impression changes altogether when the
placques are examined with a low magnifying power. The colonies,
especially those of the interior surface, reveal then an aspect so
curious as to be well remembered when once seen.
This mineral gelatin, as has already been noticed, is very unfavorable
to the growth of microbes other than nitro-bacteria and becomes altered
only under the action of the air. If the placques be carefully preserved
from desiccation the culture of these organisms can be continued for
several weeks. Although they do not seem to increase, the colonies, as
well as the jelly, are still in a good condition at the end of that
time. Nevertheless the expectation that this medium would prevent the
formation of any foreign organism has not been realized. Some of the
organisms which accompany the nitro-bacteria in soil, also grow upon the
silicic jelly; but they do not form colonies, properly so-called, and
their growth is extremely slow. They generally make their appearance
before the nitro-bacteria and spread exclusively upon the surface in
form of white spots, so transparent that without careful examination
they would not be discovered. Having reached a certain size the spots do
not change during entire weeks. This circumstance renders the operations
of isolation somewhat delicate, but does not prevent them.
=431. Preparation and Treatment of the Solution to be Nitrified.=—The
organisms having been grown on the siliceous gelatin in the manner
described they are tested for their nitrifying power as follows:
The mineral solution which is to be nitrified with the above preparation
is composed of ammonium sulfate, four-tenths gram; magnesium sulfate,
half a gram; potassium phosphate, one-tenth gram; calcium chlorid,
trace; sodium carbonate, six-tenths to nine-tenths gram; and distilled
water, 100 cubic centimeters. The sulfates with the calcium chlorid on
the one hand, and the phosphate and carbonate on the other, are
dissolved separately and the two solutions sterilized separately and
mixed after cooling. The seeding is then done as described above.
=432. Isolation of the Nitrous and Nitric Organisms.=—Instead of
proceeding immediately to the isolation of special organisms in the
soil, the preliminary period of purification is prolonged by Winogradsky
by allowing the free growth to take place of all the organisms which can
be maintained in the ordinary medium.[287]
The composition of the culture solution employed is as follows:
Distilled water, 1,000 parts; potassium phosphate, one part; magnesium
sulfate, half a part; calcium chlorid, trace. Each flask receives
besides this some magnesium carbonate, freshly washed with boiling water
and added in slight excess.
The flasks thus charged are sterilized, and after sterilization there
are added two cubic centimeters of a solution of two per cent of
ammonium sulfate, which, when added to fifteen or twenty cubic
centimeters of liquid give from two to two and a half parts per
thousand.
They are then seeded with soil. The reasons for this preliminary
treatment are as follows: First, all the observations upon the
enfeeblement of the oxidizing power of these organisms have been made
upon cultures seeded simply by the fresh soil, and in cultures derived
therefrom. In the second place, the existence of the two forms, one
nitrous and the other nitric, prevents at once the isolation of a single
organism.
Samples of soil from Europe, Africa, Asia, Australia, and America, were
used for seed for the experiments. First, the cultures were made by
seeding with a small quantity of each of these samples of soil, and each
one of these cultures served as a point of departure for a series of
subcultures. The temperature of the cultures should be kept constantly
at 30°.
The method of following the nitrification adopted by Winogradsky is
essentially that of Warington, the percentage of ammonia remaining at
any time being determined by nesslerizing. To detect the presence of
nitric acid the nitrous acid is decomposed by boiling with ammonium
chlorid in excess, or with urea, and then diphenylamin is used as a
reagent. By treatment with ammonium chlorid and boiling, the ammonium
nitrite is resolved into free nitrogen and water as indicated by the
equation NH₄NO₂ = N₂ + 2H₂O. Or the total oxidized nitrogen may be
estimated by the Schloesing method or by any of the standard methods
hereafter given. The nitrous acid is then determined by potassium
permanganate and the nitric acid by difference.
A great difference is to be noted between freshly taken earth and that
which has been kept for a long while, especially when sealed. With fresh
earth taken near the surface a mere trace is sufficient to produce
nitrification. With samples of earth which have been kept for a long
while and thoroughly dried, several grams must be added in order to
secure perfect nitrification. The period of incubation with the samples
of earth ranges from three to twenty days. The beginning of the
phenomenon is revealed by the appearance of nitrous acid, of which the
quantity is increased very rapidly, but in the end it disappears and is
transformed into nitric acid.
=433. Statement of the Results.=—The method of stating the results of
examination of soils for nitrifying organisms is illustrated by the
following example:
Soil from Zurich. The culture was seeded on the 11th of October, one
gram of soil being taken. On the 20th of October the nitrous acid had
reached its maximum of intensity and there was no ammonia left. On the
29th of October the nitrous acid remained almost stationary and there
was hardly any nitric acid present. On the 1st of November the reaction
for nitrous acid began to decrease. On the 5th of November the reaction
for nitric acid was very intense. On the 11th of November the nitrous
acid had all disappeared except a mere trace.
The above order of phenomena was observed with all the samples of soil
tried, from which it is concluded with certainty that nitrifying
organisms transplanted directly from their natural medium in the soil
into a liquid easily nitrifiable produce at once nitrous acid in
abundance. The phenomenon of nitrification is divided into two periods
therefore, of which the first is devoted to the production of nitrites,
and the second consists in the oxidation of the nitrites, and this does
not commence until the total disappearance of the ammonia. Occasionally
the formation and oxidation of the nitrites practically go on together,
but never equally, the oxidation of the nitrites being always sensibly
behind their formation.
=434. Method for Subcultures.=—From the mother cultures described above,
Winogradsky makes subcultures as follows:
The solution to be nitrified is prepared as in the mother cultures. The
seeding is accomplished by adding a small quantity of the liquor of the
mother culture after shaking. Subcultures can be made in this way to the
seventh generation.
In respect of the oxidation of the nitrites the results may be entered
as negative if they have not disappeared at the end of two months.
To determine whether the process of oxidizing the nitrites is in
progress or not the total nitrous acid is estimated, and the process
repeated at the end of eight or ten days. Should there be no diminution
of the nitrous acid within this time it may be considered that the
further oxidizing action is not taking place.
=435. Use of a Solid Medium.=—It may be justly claimed that the action
of nitrifying organisms in a liquid is not to be compared with their
action in a solid medium, such as a soil which is their natural habitat.
It might be, therefore, that the inability of the nitrous organism to
produce nitrates is due to the nature of the medium in which it is
cultivated. Winogradsky in order to determine this question cultivated
the organism in a solid medium of two kinds, first a silicate gelatin
impregnated with an ammonium salt and second in sterilized earth. The
silicate jelly is prepared as follows:
Mix a jelly of silica containing some ammonium sulfate with sterilized
soil. The seeding is done with one of the subcultures which no longer
has the power of producing nitrates.
In the case of the jelly the seeding is accomplished as follows:
A minute drop of a culture liquid is taken with a capillary glass tube
and applied in striae to different parts of the solid jelly; or a minute
drop of the culture liquid may be mixed with the jelly before
solidification. The Petri dishes in which these cultures are made can be
preserved in a moist atmosphere and thus the desiccation be easily
prevented for a long time. From time to time small pieces of the jelly
as large as a pea can be taken and tested for the progress of
nitrification.
_Results._—The nitrous reaction, both in the prepared jelly and in
sterilized soil, will appear in a few days. At the end of from seven to
twelve days it will have attained its maximum intensity and will then
remain stationary indefinitely. Sterilized soil has no power to generate
the nitric from the nitrous ferment. The two organisms are, therefore,
of different species.
After a few generations the power of producing nitrates seems to be lost
although the nitrous ferment may still be active. This suppression of
the power to oxidize the nitrites is not due to any pernicious influence
of the culture-medium but to the condition of the successive solutions
at the time of taking the seeding samples.
=436. Microscopic Examination.=—A small particle of the deposit in
the culture-liquid is spread on a glass slide and dried. There is
then added a drop of very dilute perfectly transparent malachite
green solution. Malachite green is Bittermandelölgrün, or
tetramethyldiamidotriphenylcarbinol. Use the zinc chlorid double
salt or oxalate. In about half a minute it is washed and colored by
a very dilute solution of gentian violet which is left to act for
some time. The cells then appear distinctly colored on a colorless
background.
In examining in this way nitrous cultures under a moderate enlargement
there are seen particles of material covered with scattered groups and
massive zoöglœæ composed of cells which are, doubtless, identical.
By their round or roundish forms, by their relative size and especially
by their numbers and uniformity they are at once distinguished from the
other vegetations which are generally of a purely bacillus shape.
With the exception of some shreds of mycelium coming from some oidium in
the soil the microscope reveals nothing but the organisms described. The
microscopic appearance[288] of the nitrous ferment is shown in Fig. 69.
[Illustration:
Figure 69. (Upper figure.) Nitrous ferment prepared by Winogradsky
from soil from Cito.
Figure 70. (Lower figure.) Nitric ferment prepared by Winogradsky from
soil from Cito.
]
The general conclusions of Winogradsky are:
1. Each soil possesses but one organism capable of oxidizing ammonia.
2. Soils from one locality have always the same kind of nitrifying
ferment.
3. Soils from different and distant countries contain nitrifying
organisms which differ from one another in some respects so much so that
it may be necessary to distinguish a few species or even genera in these
bodies.
=437. Isolation of the Nitric Ferment in Soils.=—The principle of the
separation of this ferment as described by Winogradsky rests upon the
fact that in culture solutions of a mineral nature free from ammonia the
nitrous ferment will not grow, whereas if nitrite or nitrous acid be
present the nitric ferment will grow.[289] In a few generations,
therefore, the nitrous ferment will be entirely eliminated.
Solution employed:
Distilled water 1,000 grams.
Potassium phosphate 1 gram.
Magnesium sulfate 0.5 „
Calcium chlorid trace.
Potassium nitrite 0.22 gram.
To culture-flasks containing 100 cubic centimeters of the above mixture
after sterilization about one-tenth gram of fresh soil is added. In
favorable conditions the nitrous acid will disappear in about fifteen
days.
Subcultures are made by seeding fresh portions of the sterilized
solution with one or two cubic centimeters of the mother culture. The
operation is continued until the nitrous ferment is eliminated.
The organisms in the deposit in the culture-flasks are then subjected to
microscopic examination in the manner already described for the nitrous
ferment; or proceed as follows:
=438. Culture on Solid Media.=—Take a liquid which has been employed in
the culture of a nitrous ferment and evaporate to one third of its bulk.
Gelatinize the residue by adding double its volume of the silicic acid
solution prepared as already directed.
The jelly is placed in the glass vessels usually employed. The seeding
may be done with a few drops of a culture-liquid containing the nitric
ferment as obtained above. The first reaction will appear in from eight
to ten days. In about forty-five days the nitrous acid in the jelly will
have entirely disappeared. Two classes of colonies are noticed under the
microscope. The first to appear are small colonies which never extend
beneath the surface of the jelly. In cultures seeded with these colonies
there is no oxidation of nitrous acid. The second class of colonies
extends into the interior of the jelly. They are much larger than the
first, of a yellowish-gray color and not spherical but rather lenticular
in shape. Cultures seeded with these colonies will lose their nitrous
acid in about ten days or two weeks.
The growth of these organisms in a liquid scarcely merit the name of
cultures. The naked eye can usually distinguish no form of vegetation.
The liquid remains clear, the surface is free from any film, no flocks
are deposited. Colored and examined in the microscope the organisms
found are so puny as to make doubtful their oxidizing power. There is an
apparent contradiction between the powerful chemical action that these
organisms can produce and their apparent deficiency in physical
properties.
These organisms are best found by cultivating them in a very limpid
solution. The bottoms of the culture boxes will be found covered with an
extremely tenuous gelatinous deposit communicating to the glass a feeble
grayish-blue tint. The culture bottle is inclined and the bottom
scratched with a recently drawn-out capillary tube. The colonies rise in
the tube together with a little of the liquid. The colonies are dried,
mounted, and colored as already described and when examined with the
microscope are found to be composed exclusively of masses of an organism
of extreme minuteness.
The organism remains attached so firmly to the bottom of the culture
bottle that it can be washed several times with pure water without
danger of detachment and thus rendered more pure.
In old cultures which are sustained by new additions of nitrite an
extremely transparent pellicle on the bottom of the flask can be
distinguished. By shaking the liquid some fragments may be detached and
made to float through the fluid. With a little care and patience these
flocks can be captured, mounted, and colored. Since they show the nitric
organism in its natural state their preparations are of the greatest
interest.
The best preparations are made by coloring with malachite green and
gentian violet and then coloring again hot with magenta. Afterwards the
preparation is washed with warm water at 50°–60° which takes almost the
whole of the color from the gelatinous matter. The cells are then
clearly presented colored a reddish violet on a rose background. These
organisms[290] are shown in figure 70.
The figure shows the cells united by a gelatinous membrane and grouped
in small dense masses composed often of a single layer of organisms. The
cells are generally elongated, rarely regularly spherical or oval. Their
mean length does not exceed half a micromillimeter and their thickness
is from two to three times less.
The difference in form of the nitrous and nitric ferments is very marked
and leaves no doubt of the existence of these two forms which are as
distinct as could be desired in microbic discrimination.
=439. Dilution Method of Warington.=—The method pursued by Warington in
preparing pure cultures of the nitrifying ferment is based on the
well-known principle of dilution which may be expressed as follows:[291]
In a liquid containing bacterial ferments dilution may be practiced
until a drop of the liquid may be taken which will contain no more than
a single organism of any one kind. If now proper solutions be seeded
with single drops of this solution, some of them may give colonies of
pure cultures of any given organism. The solution to be nitrified
employed by Warington had the following composition:
Water 1000 parts.
Ammonium carbonate 0.25 „
Ammonium chlorid 0.50 „
Potassium phosphate 0.04 „
Magnesium sulphate 0.02 „
Calcium sulphate 0.02 „
The ammonium chlorid is added to prevent the precipitation of magnesium
and calcium phosphates. The solution is kept in wide-mouthed, stoppered
bottles to prevent the loss of ammonium carbonate, the bottles being
only half full. About 100 cubic centimeters are taken for each
experiment. These bottles are sterilized and seeded with fresh soil in
the ordinary way. They are then covered with paper caps and placed in a
dark cupboard at a constant temperature of 22°.
_Special Media._—A quantity of arable soil is exhausted of nitrates by
washing with cold water under pressure. The soil is then boiled with
water and filtered. The clear amber-colored solution obtained may be
used instead of water in the above formula.
_Solid Media._—(1) Ordinary ten per cent gelatin made with beef broth
and peptone. (P)
(2) A ten per cent urine solution solidified with six per cent of
gelatin. (U)
(3) A solution of one gram of asparagin, one-half gram sodium acetate,
one-half gram potassium phosphate, two-tenths gram magnesium sulfate,
two-tenths gram calcium sulfate, and one liter of water solidified with
six per cent gelatin. (As)
Other solid media may also be employed for the purpose of favoring, as
much as possible, the growth of the nitrifying organisms.
The first culture in the ammonium carbonate solution given above, is
always made by seeding with a little unmanured arable soil. Subcultures
are seeded from this mother culture by seeding new solutions with a few
drops of the original. In all cases tried by Warington the subcultures
produced only nitrous fermentation while the original cultures produced
the nitric fermentation.
=440. Microscopic Examination.=—The microscopic examination of the
organisms formed is conducted as follows:
The cover glasses for microscopic objects are placed at the bottom of
the culture-flask, the cover glasses being previously sterilized. At the
end of the nitrification the liquid is removed with a pipette and the
flask containing the cover glasses dried at 35°. The cover glasses are
then removed and stained. The microscopic appearance of the organisms
obtained by the previous cultures showed masses of corpuscles usually of
oval shape and having a length generally exceeding one micromillimeter.
An immersion objective giving a magnification of 800 diameters is
suitable for this work.
Other forms of organisms are also met, the whole series being
characterized as follows:
(1) The corpuscles already mentioned. Larger ones are frequently rough
in outline resembling masses of siliceous sea-sand. The smaller oval
corpuscles are regular in form.
(2) Some very small circular organisms often appearing as mere points
and staining much more plainly than the preceding.
(3) A few slender bacilli, staining faintly.
All the cultures obtained by the above method give abundant growth on
gelatin.
=441. Trials with the Dilution Method.=—One part of the third subculture
in the ammonium carbonate solution described above, is mixed with 500
parts of thoroughly boiled water and one drop from a sterilized
capillary tube is added to each of five bottles containing the
sterilized ammonium carbonate solution. In Warington’s experiments one
of the five bottles was found to have nitrified after forty-one days.
After ninety-one days two more were nitrified. Two bottles did not
nitrify at all. All three solutions which nitrified gave growths on
gelatin. The growths took place more speedily on gelatin U and As than
on P.
The organisms obtained on gelatin were seeded in appropriate liquid
media but no nitrification was obtained.
A subculture from solution No. 2 of the first dilution mentioned above,
was diluted to one one-thousandth, one ten-thousandth, one
one-hundred-thousandth, and one one-millionth. Each of these dilutions
was used for seeding with five sterilized solutions of ammonium
carbonate, using the method of seeding above described. At the end of
190 days not one of these solutions had nitrified.
Warington supposed that the cause of failure in the method just
mentioned might be due to the alkalinity of the ammonium carbonate.
While this solution could be seeded in the ordinary way with fresh earth
it might be that the faint alkalinity which it presented might prevent
it altogether from action when the nitrifying agent was reduced to a few
organisms.
He therefore changed the solution to one of the following composition:
Water 1,000 parts.
Ammonium chlorid 0.02 part.
Potassium phosphate 0.06 „
Magnesium sulfate 0.03 „
Calcium sulfate 0.03 „
The solution was divided in twenty stoppered bottles which were half
filled. The bottles were divided into four series, A, B, C, D, each one
consisting of five bottles, and these were respectively seeded with one
drop from dilutions to one one-thousandth, one ten-thousandth, one
one-hundred-thousandth, and one one-millionth of a second subculture of
No. 3 in the first dilution series.
After 115 days, nitrification had occurred in ten of the bottles. The
other ten did not nitrify at all. Each of the nitrifying solutions was
spread on gelatin, P and U being employed. Growth took place far more
easily on gelatin U than on gelatin P. Of the ten nitrified solutions
there were three which gave no growth on gelatin U, either when spread
on the surface or introduced into the substance of the jelly. There were
therefore secured nitrifying solutions which did not contain organisms
capable of growing on gelatin. The supposition is therefore fair that
they were pure nitrifying organisms. These fresh, pure organisms had the
faculty of converting ammonia into nitrous acid only and not into nitric
acid.
With the organisms thus prepared a number of solutions of potassium
nitrite containing phosphates and other mineral ingredients were seeded.
In no case was any loss of nitrite found, which is proof that the
solution contained no organisms capable of oxidizing nitrous acid. The
organisms prepared as above, have the power of nitrifying organic
substances containing nitrogenous bodies.
The organism isolated as described and examined under the microscope is
seen to contain two forms. The first one is nearly spherical in shape,
the corpuscles varying in size from mere points to a diameter of one
micromillimeter. The form is very striking and easily stained. The
second form is oval-shaped and attains a length distinctly exceeding one
micromillimeter. Sometimes it is a regular oval and sometimes it is
egg-shaped. This form is stained less easily than the preceding or
spherical form.
=442. Method of Staining.=—The method of staining employed is as
follows:
A drop of the culture-liquid is placed on a glass slide and mixed with
the filtered stain by means of a wire. A cover glass is placed on the
drop and allowed to stand for half an hour. It is then pressed down on
the slide and the liquid which exudes wiped off and hollis glue run
around the cover glass. In this way the organism is stained in its own
culture-fluid and can be seen in its true form without any possibility
of the destruction of its shape by drying. The plate is bright and clear
though colored.
If the preparation is to be mounted in balsam a drop of the culture is
dried in the center of a cover glass. It is then placed for some minutes
in dilute acetic acid to remove matter which would cause turbidity. The
cover glass with its contents is then washed, dried, and stained for
some hours in methyl violet.
=443. Classification of Nitrifying Organisms.=—The names proposed by
Winogradsky for the various organisms are the following:
For the general group of microbes transforming ammonia into nitric acid,
_Nitro-bacteria_.
For the nitrous ferments of the Old World
Genus, _Nitrosomonas_:
Species, _Nitrosomonas europaea_.
_Nitrosomonas javanensis._
For the nitrous microbes of the New World:
Genus, _Nitrosococcus_.
Species, not determined.
For the nitric ferment:
Genus, _Nitrobacter_.
=444. Nitrification in Sterilized Soil.=—The process of nitrification in
sterilized soil, when seeded with pure cultures, is determined as
follows:
_Preparation of Sample._—The fresh sample of arable soil is freed from
pebbles and vegetable débris and reduced to as fine a state of
subdivision as is possible in the fresh state. It is placed in
quantities of about 800 grams in large crystallizing dishes.
One dish is set aside for use in the natural state, and the other,
hermetically closed, is placed in a sterilizing apparatus and subjected
to the action of steam for two and a half hours. This treatment is
repeated three times on as many successive days.
_Seeding of Sample._—Each of the two dishes is moistened with fifty
cubic centimeters of pure water containing 500 milligrams of ammonium
sulfate. The sterilized portion is then seeded with a preparation of the
pure nitrous ferment, produced as before described. The seed is prepared
by filtering a few cubic centimeters of the nitrous culture liquid
through asbestos. The asbestos is well washed and then thrown into a
flask containing a few cubic centimeters of sterilized water and well
shaken. The water carrying the filaments of asbestos is poured drop by
drop on the surface of the soil in as many places as possible. The two
dishes of soil are kept at an even temperature of 20° in a dark place.
Winogradsky found that, treated in this way, the unsterilized soil
produced only nitrates, while the sterilized portions produced only
nitrites.[292]
=445. Variation of the Determinations.=—To vary the conditions of the
experiment Winogradsky uses twelve flasks of the erlenmeyer shape, four
having bottoms twelve centimeters in diameter, and eight of them five
centimeters in diameter. In each of the four large flasks are placed 100
grams of fresh soil, and in each of the eight small flasks twenty-five
grams. The eight small flasks are designated a, b, c, d, and a′, b′, c′,
d′, and the four large flasks A, B, C, D.
The flasks a, b, c, d, and a′, b′, c′, d′, are placed in a stove at 30°
for several days before use, while A, B, C, and D, are kept at 22°–23°
for the same length of time. The soil in the small flasks is, therefore,
somewhat drier than that in the large ones.
The flasks are treated as follows:
a, a′, A, contain the soil as prepared above for control.
b, b′, B, are sterilized at 135° and seeded with a drop of the pure
nitrous culture.
c, c′, C, sterilized as above and seeded with a little of the
unsterilized earth.
d, d′, D, sterilized as above and seeded with pure nitrous and pure
nitric cultures.
After sterilization there was added to the small flasks two cubic
centimeters of a twenty per cent sterilized ammonium sulfate solution,
and to the large ones six cubic centimeters. At the end of a month or
six weeks the contents of the flasks are thrown on a filter and washed
with cold water until a drop of the filtrate gives no blue color with
diphenylamin. The respective quantities of nitrite and nitrate are then
determined in the filtrates by the usual processes, which will be fully
described further along.
=446. Sterilization.=—One of the chief requisites for success in the
bacteriological investigation of soils is found in the thoroughness of
the sterilizing processes. The value of cultures depends chiefly on the
care with which the introduction of foreign germs is prevented. In the
following description a mere outline of the method of sterilization is
presented, while those who wish to study more carefully the details of
the process are referred to the standard works on bacteriology.
=447. Sterilization of the Hands.=—It is important that the hands of the
operator handling apparatus and materials for bacteriological work
should be sterilized. The sterilization may be accomplished in the
following way:
The nails should be cut short and thoroughly cleaned with soap and
brush. The hands are thoroughly washed in hot water with soap. After
washing in hot water the hands should be washed in alcohol and ether.
They are then dipped in the sterilizing solution.
This liquid may consist of a three per cent solution of carbolic acid,
which is the one most commonly employed. A solution of corrosive
sublimate, however, is perhaps the best disinfectant. It should contain
from one to two parts of the crystallized salt to 1,000 parts of water.
It has lately been advised to use the sublimate in an acid solution.
Acetic acid or citric acid may be employed, but hydrochloric acid is
recommended as the best, in a preparation of one-half part per 1,000.
For stronger solutions of sublimate containing more than a half per
cent, equal quantities of common salt should be added. The solution
should be made with sterilized water.
After dipping the hands in the sterilizing solution they should be dried
with a napkin taken directly from a sterilizing oven, where it has been
kept for some time at the temperature of boiling water. Where only
ordinary work in bacteriology is contemplated this sterilization of the
hands is not necessary. It is practiced chiefly in antiseptic surgery.
=448. Sterilizing Apparatus.=—With platinum instruments the most
effective and easiest way for sterilizing is to hold them in the flame
of a bunsen until they are red hot. Steel and copper instruments can not
be treated in this way without injury. They are best sterilized by
submitting them to dry heat in a drying oven at a temperature of
150°–160° for two hours. Glass and porcelain apparatus can be sterilized
best in the same way.
All apparatus and materials employed should be used in a space as free
as possible from dust, so that any germs which might be carried in the
dust can be excluded from the apparatus in transferring it from one
place to another.
=449. Methods of Applying Heat.=—Sterilization by means of heat may take
place in several ways.
_First. Submitting the Materials to Dry Heat Without Pressure._—The
temperature in sterilization of this kind may vary from the temperature
of boiling water at sea-level to 160° obtained by an oil-bath or by an
air-oven.
_Second. Sterilization in a Liquid Under Pressure._—This form of
sterilization may be effected by sealing the liquid in a strong vessel
and submitting it to the required temperature. If the temperature
required be greater than that of boiling water the vessel can be
immersed in a solution of some mineral salt which will raise the
boiling-point.
_Third. Sterilization in Steam Under Pressure._—This method of
sterilization consists in placing the body in a proper receptacle in
vessels to which the steam can have access and then admitting steam from
a boiler at any required pressure. In the case of small apparatus, such
as the autoclave, the steam can be generated in the apparatus itself.
The variety of apparatus used in the above method of sterilization is
very great, but all the forms of apparatus employed depend upon the
principles indicated.
=450. The Sterilizing Oven.=—The apparatus for sterilization by means of
hot, dry air usually consists of a double-walled vessel made of
sheet-iron, usually with a copper bottom. The apparatus is shown in Fig.
71.
The temperature is controlled by means of a thermometer, T, and the
gas-regulator, _R_. This is one of the ordinary gas-regulators by means
of which the amount of gas supplied to the lamp is increased if the
temperature should fall, and diminished if it should rise above the
required degree. The best form of the sterilizing ovens is provided with
a means for circulating the hot air so that the temperature may be made
uniform throughout the mass. This can be accomplished by introducing a
mechanical stirrer, or by the movement of the air itself.
[Illustration:
FIGURE 71. STERILIZING OVEN.
]
Between the walls of the vessel may be placed water, provided the
temperature of sterilization be that of boiling water. If it should
require a higher temperature than boiling water, a solution of salt can
be added until the required temperature is reached, or the space between
the two walls may be left vacant and hot air made to circulate around
the oven.
The exterior of the oven, except at the bottom where the lamp strikes
the copper surface, should be protected by thick layers of asbestos or
other non-conducting material. To avoid danger of flying filaments, this
covering should be coated with some smooth paint which will leave an
even surface not easily abraded.
=451. Sterilization with Steam at High Pressure.=—The apparatus used for
this is commonly called an autoclave and is shown in Fig. 72.
The top is movable and held in place by the clamp, _a_, which is fixed
by the screw worked by the lever, _b_. The vessel itself is
double-jacketed and the pressure is obtained from water in the vessel
heated by means of the lamp, _c_. The actual steam pressure is indicated
by the index _d_. The safety-valve, _e_, allows any excess of steam to
escape above the amount required for the maintenance of the pressure.
This, however, is best regulated by the lamp. The outer jacket permits
the heat from the lamp to circulate around the inner pressure vessel,
and the holes near the top, _oo_, are for the escape of the heated
gases. Enough water is placed in the bottom of the inner pressure vessel
to supply all the aqueous vapor necessary to produce the required
pressure and still leave some water in excess.
[Illustration:
FIGURE 72. AUTOCLAVE STERILIZER.
]
The materials to be sterilized are held on the shelves of the stand and
the vessels may be of various kinds according to the nature of the
material to be sterilized. The vessels containing the material being
covered, the steam does not come in actual contact with it. At the end
of the operation the safety-valve must not be opened to allow the escape
of the steam, otherwise the remaining water would be rapidly converted
into vapor and would be projected over the materials on the shelves. The
lamp should be extinguished and the apparatus allowed to cool. The
autoclave is not only useful for sterilizing purposes but can be made of
general use in the laboratory where heat under pressure, as in the
estimation of starch, etc., is required.
These two forms of apparatus are sufficient to illustrate the general
principles of sterilization by hot air and steam. There are, however,
many variations of these forms designed for special use in certain kinds
of work. For full descriptions of these, reference is made to catalogues
of bacteriological apparatus.
=452. Arnold’s Sterilizing Apparatus.=—A very simple and cheap steam
sterilizer has been devised by Arnold.
[Illustration:
FIGURE 73. ARNOLD’S STERILIZER.
]
Water is poured into the pan or reservoir, B, Fig. 73, whence it passes
through three small apertures into the shallow copper vessel, A. It is
there converted into steam by being heated with any convenient lamp, and
rises through the funnel in the center to the sterilizing chamber. Here
it accumulates under moderate pressure at a temperature of 100°. The
excess of steam escapes about the cover, becomes imprisoned under the
hood, E, and serves to form a steam-jacket between the wall of the
sterilizing chamber and the hood. As the steam is forced down from above
and meets the air it condenses and drips back into the reservoir. Such
an apparatus as this is better suited to commercial purposes, as the
sterilizing of milk, than for scientific uses.
=453. Thermostats for Culture Apparatus.=—It is important in the culture
of micro-organisms that the temperature should be kept constant during
the entire time of growth. Inasmuch as some operations continue for as
much as three months it is necessary to have special forms of apparatus
by means of which a given temperature, during the time specified, can be
maintained. This is secured by means of an oven with an automatic
temperature regulator, practically built on the principle of the hot air
sterilizing oven already described.
The essential principles of construction are, however, that the
regulator for the temperature should be delicate and that the
non-conducting medium surrounding the apparatus should be as perfect as
possible, so that the variations in temperature from changes in the
exterior temperature, are reduced to a minimum. This delicacy is secured
by introducing a drop of chloroform-ether into a confined space over the
mercury of the regulating apparatus. The doors of the chamber are
double, the interior one being of glass so that the exterior door can be
opened for inspection of the progress of the bacterial growth without
materially interfering with the interior temperature. A convenient form
is shown in Fig. 74.
[Illustration:
FIGURE 74. LAUTENSCHLÄGER’S THERMOSTAT.
]
The apparatus figured, is oval in shape, although circular or other
forms are equally as effective. The arrangement of the lamp, _a_,
thermometers, _t t t_, and gas-regulator, _g_, and the double doors,
_d d_, is shown in the engraving and does not require further
description.
The usual temperatures for cultures range from 22° to 35°, and the
apparatus once set at any temperature will remain fixed with extremely
minute variations for an indefinite time. The apparatus possesses a heat
zone which, by the arrangement of the regulator, is kept absolutely
constant. The space between the walls of the apparatus being filled with
water, the temperature is maintained even in every part. The apparatus,
as constructed, is independent not only of the surrounding temperature
within ordinary variations, but also of the pressure of the barometer.
Three thermometers are employed to determine the temperature of the
heating zone, the water space and the inner space. The arrangement of
the gas-regulator is of an especial kind, as mentioned above, by means
of which the consumption of gas is reduced to a minimum. This apparatus
can be regulated to suit the character of the work.
=454. Microscopic Apparatus Required.=—Any good microscope, capable of
accurate observation, of high power, may be used for the bacteriological
observations necessary to soil analysis.
Preference should be given to the patterns adapted to receive any
additional accessories which may be subsequently required for advanced
work. The stage, in addition to being fitted with a sliding bar, should
have a large circular or horseshoe opening to facilitate focusing
operations. A mechanical stage is a desirable acquisition if really well
made, but a plain stage is preferable for many purposes. A rackwork,
centering sub-stage is essential for advanced work, and in the absence
of the more complete form, there should at least be a fitting beneath
the stage to take the diaphragm and condenser. An iris diaphragm will be
found more useful than any other kind in practice, since the size of the
opening can be increased very gradually at will.
One of the best lamps is known as the paraffin lamp and is fitted with a
half-inch wick. This will give even more light than is actually
required, and a steady flame, perfectly under control, may be obtained.
For the minute details to be observed in high-grade microscopic work,
such as is required in the bacteriological examination of soils,
reference must be had to the standard works on bacteriology and
microscopy.
=455. General Conclusions.=—The nitrogenous food of plants is provided
in several ways; _viz._, (1) By the nitrogen brought to soil in rain and
snow. This nitrogen is chiefly in the form of ammonia and nitric acid.
The nitrogen gas in solution in rain water has no significance as a
plant food. (2) By the action of certain anaerobic organisms herding in
the rootlets of leguminous plants, free nitrogen may be oxidized and put
into form for assimilation. (3) By the action of certain organisms on
nitrogenous compounds pre-existing in the soil, ammonia, nitrous acid,
and finally, nitric acid, are produced. It is believed that the plant
organism, unaided by the activity of a micro-organism, is unable to
assimilate nitrogen unless it be fully oxidized to nitric acid. (4)
There exist micro-organisms capable of acting directly on free nitrogen
independent of other plant growth, but the significance of this possible
source of plant food is, at the present time, unknown. (5) The
micro-organisms of importance to agriculture may be isolated and
developed to the exclusion of other organisms of a similar character.
This isolation is best accomplished in culture-media consisting
essentially of a mineral gelatin to which is added only pure
carbohydrates and the necessary mineral nourishment. (6) The nitrifying
ferments consist probably of several species, of different geographic
distribution. Different types of soils probably have nitrifying
organisms of different properties. This is illustrated by the fact that
nitrification is accomplished in dry alkaline soils under conditions in
which the ordinary nitrifying organisms would fail to develop. (7) The
study of typical soils in respect of the kind, activity, and vigor of
their nitrifying organisms has become as important a factor in soil
analysis as the usual determination of physical and chemical
composition.
DETERMINATION OF NITRIC AND NITROUS ACIDS IN SOILS.
=456. Classification of Methods.=—The minute quantities in which highly
oxidized nitrogen exists in soils render the operations of its
quantitative estimation extremely delicate. On the other hand, the easy
solubility of these forms of combination and the absence of absorptive
powers therefor, in the soil, render the separation of them from the
soil a matter of great ease. It is possible, therefore, to secure all
the nitrates and nitrites present in a large quantity of earth in a
solution which can be concentrated under proper precautions to a volume
convenient for manipulation. The method of this extraction is the same
for all the processes of determination. The methods of analysis suited
to soil extracts, as a rule, may also be used in the determination of
the same compounds in rain, drainage, and sewage waters, and for the
qualitative and quantitative control of the progress of nitrification.
The various processes employed may be classified as follows:
1. The conversion of the nitrogen into the gaseous state and the
determination of its volume directly. This is accomplished by combustion
with copper oxid and metallic copper.
2. The conversion of the nitrogen into nitric oxid and the volumetric
determination thereof. The decomposition of a nitrate with ferrous
chlorid in the presence of free hydrochloric acid is an instance of this
type of methods.
3. The oxidation of colored organic solutions and the consequent
disappearance of the characteristic color, or its change into a
different tint. The indigo and indigotin processes are examples of this
method.
4. The production of color, in a colorless or practically colorless
solution, by the treatment thereof with the nitrate in presence of an
acid which decomposes it with the liberation of oxidizing compounds. The
depth of color produced is compared with that formed by a known quantity
of a pure nitrate solution until the two colorations are alike. The
methods depending on the use of carbazol or acid phenol sulfate
illustrate this class of reactions.
5. The conversion of the nitrogen into ammonia by moist combustion with
sulfuric acid in the presence of certain organic compounds, _e. g._,
salicylic acid, and the collection of the ammonia in standard acid, the
excess of which, is determined by titration.
6. The reduction of nitrates to ammonia by nascent hydrogen and the
recovery of the ammonia produced by distillation and collection in
standard acid.
7. The reduction of nitrates by electrolytic action and the collection
of the ammonia as above.
8. The decomposition of nitrates with the quantitative evolution of a
different element, and the direct or indirect measurement of the evolved
substance. The quantitative evolution of chlorin on treating a nitrate
with hydrochloric acid, the collection of the chlorin in potassium
iodid, and the determination of the iodin set free, form a process
belonging here.
=457. Relative Merit of Methods.=—The processes mentioned in the
classifications embraced under numbers (1) and (5) of the preceding
schedule are sufficiently described in the paragraphs devoted thereto,
under soil and fertilizers. In practice at the present time it is rare
to determine the nitrogen in nitrates by the copper oxid method. The
more rapid and equally exact processes of colorimetric comparison or
reduction by nascent hydrogen are in all respects to be preferred.
The indigo methods among the colorimetric processes are not so much in
use now as those which depend on the development of a color. Lawes and
Gilbert considered them far inferior to the Schloesing method. The
developed color methods are especially delicate and are to be preferred
in all cases where the detection of the merest traces of nitrates is
desired. Where nitrates are present in considerable quantities the
reduction method with nascent hydrogen is to be preferred over all
others. In all these cases the judgment of the analyst must be
exercised. The particular method to be employed in any given case can
not be determined save by the intelligent discrimination of the
operator.
=458. The Extraction of Nitric Acid from the Soil.=—The easy solubility
of nitric acid and of nitrates in water is taken advantage of in the
separation of these bodies from the soil. A convenient quantity, usually
about 1,000 grams of the fine soil, is taken for the extraction. Instead
of freeing the soil entirely from water, it is better to determine the
amount of water in the air-dried or prepared sample, and base the
calculation on 1,000 grams of the water-free soil.
All samples of soil, when taken for the purpose of examining for
nitrates, should be rapidly dried to prevent the process of
nitrification from continuing after the sample is taken. For this
purpose the soil should be placed in a thin layer in a warm place,
50°–60°, until air-dried. It still contains in this case a little
moisture but not enough to permit nitrification to go on.
One thousand grams of soil prepared as above are treated with 2,000
cubic centimeters of distilled water, free of nitric acid, and allowed
to stand for forty-eight hours with frequent shaking. One thousand cubic
centimeters of the extract are then filtered, corresponding to 500 grams
of the dry soil. A small quantity of pure sodium carbonate should be
added to the filtrate which is then evaporated on a water-bath to a
volume of about 100 cubic centimeters. Should a precipitate be formed
during evaporation it should be separated by filtration, the filter
washed thoroughly, and the filtrate again evaporated to a volume of 100
cubic centimeters.
In taking a soil for the determination of nitrates, it is well to extend
the sampling to a considerable depth. If the sample be taken only to the
depth of nine inches, it should be in dry weather when the nitrates are
near the surface.
The temperature at which a soil is dried has also an influence on the
quality of nitric nitrogen remaining after desiccation.
If a wet soil be dried at 100°, the nitrates present will suffer partial
decomposition. This is probably due to deoxidation by organic matter
present. On the other hand, ordinary air-drying affords opportunity for
continued nitrification, thus increasing the residuum of oxidized
nitrogen. The above method is essentially that followed by Warington at
Rothamstead.
The method of drying practiced at Rothamstead, in order to secure
results as nearly accurate as possible is the following:[293]
The soil is broken up directly after it is taken from the field, and
spread on trays in layers one inch deep. The trays are then placed in a
room at 55°. The drying is completed in twenty-four hours. After drying,
stones and roots are removed, and the soil is finely powdered and placed
in bottles.
For extracting the nitrates, a funnel is prepared by cutting off the
bottom from a bottle four and a half inches in diameter. A nicely
fitting disk of copper gauze is placed in the bottom of this funnel, and
this is covered with two filter papers, the upper one having a slightly
greater diameter than the lower. The paper is first moistened, and then
from 200 to 500 grams of the dry powdered soil introduced. The funnel is
connected with the receiving flask of a filter pump, and pure water
poured on the soil until it is thoroughly saturated. The water is then
added in small quantities. When the filtrate amounts to 100 cubic
centimeters the process may be discontinued, since all the nitrates in
the soil will be found in this part of the filtrate.
The extract obtained above is evaporated to convenient bulk for the
determination of nitric nitrogen.
THE NITRIC OXID PROCESS.
=459. Method of Schloesing.=—The processes for estimating nitrogen by
combustion with copper oxid and by moist combustion with sulfuric acid
have both been used for the determination of the quantity of nitrogen
existing in a highly oxidized state. These processes will be fully
discussed under the head of fertilizers. In the case of soil extracts,
drainage waters, etc., it will be sufficient to discuss, for the
present, only those processes adapted especially to a quick and accurate
estimation of oxidized nitrogen.
The principle of the method of Schloesing depends on the decomposition
of nitrates in the presence of a ferrous salt and a strong mineral
acid.[294] The nitrogen in the process appears as nitric oxid, the
volume of which may be directly measured, or it may be converted into
nitric acid and titrated by an alkali.
The typical reactions which take place are represented in the following
equation:
6FeCl₂ + 2KNO₃ + 8HCl = 3Fe₂Cl₆ + 2KCl + 4H₂O + 2NO.
=460. Schloesing’s Modified Method.=—The Schloesing method as now
practiced by the French chemists is conducted in the apparatus shown in
Fig. 75.[295] The carbon dioxid is generated by the action of the
hydrochloric acid in F on the fragments of marble in A. After passing
the wash-bottle the gas enters the small tubulated retort, C, which
contains the nitrate in solution. For ordinary soils 100 grams are
placed in an extraction flask, plugged with cotton, and a layer of the
same material is placed over the soil for the purpose of securing an
even distribution of the extracting liquid. This liquid is distilled
water containing in each liter one gram of calcium chlorid. The purpose
of using the calcium chlorid is to prevent the soil from becoming
compacted which would render the extraction of the nitrate difficult.
The extracting liquid is allowed to fall drop by drop from a mariotte
bottle until the filtrate amounts to 500 cubic centimeters. This volume
is concentrated on a sand-bath until it is reduced to ten or fifteen
cubic centimeters when it is transferred to a flat-bottomed dish and the
evaporation finished over steam, care being taken not to allow the
temperature to exceed 100°.
[Illustration:
FIGURE 75. SCHLOESING’S APPARATUS FOR NITRIC ACID.
]
Another and more rapid method for dissolving the nitrate, may also be
practiced. In a flask holding about one liter, place 220 grams of the
soil and 660 cubic centimeters of distilled water and shake vigorously,
or enough water to make 660 cubic centimeters together with the moisture
remaining in the air-dried sample taken. All the nitrates pass into
solution. Throw the contents of the flask into a filter and take 600
cubic centimeters of the filtrate which will contain all the nitrates in
200 grams of the sample taken. This filtrate is evaporated as described
above.
In the flat dish containing the dried nitrates, pour three or four cubic
centimeters of ferrous chlorid solution and stir with a small glass rod
until complete solution of the nitrate takes place. By means of a small
funnel the solution is poured into C, and the capsule and funnel are
well rinsed with two cubic centimeters of hydrochloric acid. The washing
is repeated three times as above described, and once with one cubic
centimeter of water, which is added cautiously so as to form a layer
over the surface of the heavier liquid. The tubulated flask is then
connected with the carbon dioxid apparatus, previously freed from air,
and the gas allowed to flow evenly until the whole of the apparatus is
completely air-free. The other details of the method are essentially the
same as those adopted by the Commission of French Agricultural Chemists
which will be given below.
=461. The French Agricultural Method.=—The Schloesing method as
practiced by the French agricultural chemists is very slightly different
from the procedure just described.[296] The process with soils is
carried on as follows:
Five hundred grams of the soil are taken and introduced into a flask of
about two liters capacity and shaken thoroughly with a liter of
distilled water. The whole of the nitrates of the soil is thus passed
into solution. The solution is filtered and 400 cubic centimeters of the
filtrate are taken, which correspond to 200 grams of the soil. This
liquid is evaporated in a flask, adding a fragment of paraffin to
prevent foaming, until its volume is reduced to fifteen or twenty cubic
centimeters. It is afterwards transferred through a filter into a
capsule with a flat bottom in which the evaporation is finished on a
steam-bath, taking care that the temperature does not exceed 100°. An
important precaution is, not to allow the contact of the water with the
soil to be too prolonged, to avoid the reduction of the nitrates which
could take place under the influence of the denitrifying organisms which
are developed with so great a rapidity in moist earth. The apparatus in
which the transformation of the nitrates into nitric oxid takes place is
essentially that already described (Fig. 75). The carbon dioxid
generator is connected by means of a rubber tube and a small wash-bottle
to the small retort in which the reaction takes place, and from which
the exit tube leads to a mercury trough. The gas which is disengaged is
received under a jar drawn out to a fine point in its upper part, which
carries about fifteen cubic centimeters of potash solution containing
two parts of water to one of potash.
The operation is conducted as follows:
Into the small capsule which contains the dried matter, three or four
cubic centimeters of ferrous chlorid are poured. By means of a stirring
rod the residue sticking to the sides of the capsule is detached with
care and all the matter is thus collected in the bottom. By means of a
small funnel the contents of the capsule are introduced into the retort.
About two cubic centimeters of hydrochloric acid are used for washing
out the materials and this acid is also introduced into the retort. The
washing with hydrochloric acid is repeated three or four times, and
finally the apparatus is washed with one cubic centimeter of water,
which is also poured in by the small funnel with great care, so that
this water may form a layer over the surface of the liquid. The
apparatus is now connected and filled completely with carbon dioxid.
Since it is necessary that this gas should be completely free of air,
the flask, which generates it, is first filled with the acidulated water
from the acid flask, and the air is thus almost totally displaced by the
liquid. The evolution of carbon dioxid gas which follows, completely
frees the apparatus from air. When this is accomplished the retort is
connected with the rest of the apparatus and the gas allowed to pass for
about two minutes until the air is completely driven out of all the
connections. The current is arrested for a moment by pinching the rubber
tube which conducts the carbon dioxid into the retort, and the vessel
which is to receive the gas is then placed over the delivery-tube, this
vessel being filled with mercury and a strong solution of potash. The
communication between the retort and the carbon dioxid flask is broken
and the flask is heated slightly by means of a small lamp. The first
bubbles of gas evolved should be entirely absorbed by the potash. This
will be an indication of the complete absence of the air. When the
liquid is in a state of ebullition the nitrogen dioxid is set free. The
boiling is regulated in such a way that the evolution is regular and the
liquid of the retort may not, by a too violent boiling, pass into the
receiver. The boiling is continued until the larger part of the liquid
is distilled and only three or four cubic centimeters remain in the
retort. At this time a few bubbles of carbon dioxid are allowed to flow
through in order to cause to pass into the receiver the last traces of
nitric oxid. The gas received is left for some minutes in contact with
the potash.
Afterward in a small flask, G, the neck of which is drawn out to a fine
point, and carrying a bulb-tube, H, and a piece of rubber tubing, there
are boiled twenty-five or thirty cubic centimeters of water for five or
six minutes in order to drive all the air out of the flask, and while
the boiling is continued the rubber tubing is fastened to the drawn-out
part of the jar containing the nitric oxid. Within the rubber tubing the
drawn-out point is broken and the vapor of water is forced into the jar
and drives before it the solution of potash which has filled the
capillary part of the drawn-out tube. As soon as the point is broken,
the boiling of the flask is stopped and by its cooling the nitric oxid
passes into it. It is necessary to press the rubber tubing with the
fingers in order that the passage of the gas into the flask be not too
rapid. As the solution of potash rises in the bell-jar which contains
the nitric oxid near to the point where the rubber tubing covers its
drawn-out portion, the fingers are removed and a clamp put in their
place. There still remains a little nitric oxid in the flask and to
drive this out it is necessary to introduce five or six cubic
centimeters of pure hydrogen, which are allowed to pass over into the
receiving flask, by releasing the clamp in the same way as the nitric
oxid. The hydrogen being introduced in successive portions, finally
carries all the nitric oxid into the flask without allowing any of the
potash to enter.
The flask containing the nitric oxid is now connected with a reservoir
of oxygen. The oxygen is allowed to enter, bubble by bubble, by means of
cooling the flask by immersion in water. The transformation of nitric
oxid into nitric acid is not entirely complete for twenty-four hours. It
is necessary, therefore, to wait that long after the introduction of the
oxygen before determining the amount of nitric acid produced.
The contents of the flask are placed in a titration-jar, the flask being
washed two or three times and a few drops of tincture of litmus being
added. The nitric acid is then determined by a standard solution of
calcium hydroxid or some other standard alkali. From the titration the
content of nitric acid is calculated.
The French Committee further suggests that this method may be modified
in the way of making it more rapid by collecting the nitric acid in a
graduated tube filled with mercury and containing some potash. The
volume of the gas is determined and the pressure of the barometer and
the temperature observed, and the usual calculations made to reduce the
volume to zero and to a pressure of 760 millimeters of mercury. Each
cubic centimeter of nitric oxid thus measured corresponds to 2.417
milligrams of nitric acid. The presence of organic matter does not
interfere with the determination of nitric acid by either of the methods
given above.
=462. Modification of Warington.=—The method of procedure and
description of apparatus used, as employed by Warington, are as follows:
The vessel in which the reaction takes place is a small tubulated
receiver, A (Fig. 76), about four centimeters in diameter, mounted and
connected as shown in the illustration. The delivery-tube dips into a
jar of mercury in a trough containing the same liquid. The long supply
funnel-tube _a_ is of small bore, holding in all only one-half cubic
centimeter. The connecting tube F, carrying a clamp, is also of small
diameter and serves to connect the apparatus with a supply of carbon
dioxid.
[Illustration:
FIGURE 76. WARINGTON’S APPARATUS FOR NITRIC ACID.
]
In practice, the supply-tube _a_ is first filled with strong
hydrochloric acid and carbon dioxid passed through the apparatus until
the air is all expelled. This is indicated when a portion of the gas
collected over the mercury, is entirely absorbed by caustic alkali.
At this point the current of carbon dioxid is stopped by the clamp C,
and a bath of calcium chlorid, B, heated to 140° is brought under the
bulb A, until the latter is half immersed therein. The temperature of
the bath is maintained by a lamp. By allowing a few drops of
hydrochloric acid to enter the receiver, the carbon dioxid is almost
wholly expelled. The end of the delivery-tube is then connected with the
tube, T, filled with mercury, and the apparatus is ready for use.
The nitrate, in which the nitric acid is to be determined, in a dry
state, is dissolved in two cubic centimeters of the ferrous chlorid
solution (one gram of iron in ten cubic centimeters), one cubic
centimeter of strong hydrochloric acid is added, and the whole is then
introduced into the receiver through the supply-tube, being followed by
successive rinsings with hydrochloric acid, each rinsing not exceeding
one-half cubic centimeter. The contents of the receiver are, in a few
moments, boiled to dryness; a little carbon dioxid is admitted before
dryness is reached, and again afterwards to drive over all remains of
nitric oxid. In the recovered gas the carbon dioxid is first absorbed by
caustic potash, and afterwards the nitric oxid by ferrous chlorid. All
measurements of the gas are made in Frankland’s modification of
Regnault’s apparatus. The carbon dioxid should be as free as possible
from oxygen. The carbon dioxid generator is formed of two vessels, the
lower one consisting of a bottle with a tubule in the side near the
bottom; this bottle is supported in an inverted position and contains
the marble from which the gas is generated. The upper vessel consists of
a similar bottle standing upright and containing the hydrochloric acid
required to act on the marble. The two vessels are connected by a glass
tube passing from the side tubule of the upper vessel to the inverted
mouth of the lower vessel. The acid from the upper vessel thus enters
below the marble. Carbon dioxid is generated and removed at pleasure by
opening a stop-cock attached to the side tubule of the lower vessel thus
allowing hydrochloric acid to descend and come in contact with the
marble. A good Kipp’s generator of any approved form may also be used
instead of the simple apparatus, above described.
The fragments of marble used are previously boiled in water in a strong
flask. After boiling has proceeded for some time, a rubber stopper is
fixed in the neck of the flask and the flame removed. Boiling will then
continue for some time in a partial vacuum.
The hydrochloric acid is also well boiled and has dissolved in it a
moderate quantity of cuprous chlorid. As soon as the acid has been
placed in the upper reservoir, it is covered by a layer of oil. The
apparatus being thus charged is at once set in active work by opening
the stop-cock of the marble reservoir; the acid descends, enters the
marble reservoir, and the carbon dioxid produced drives out the air. As
the acid reservoir is kept on a higher level than the marble reservoir,
the latter is always under internal pressure, and leakage of air from
without, into the apparatus, cannot occur.
The presence of the cuprous chlorid in the hydrochloric acid not only
insures the removal of dissolved oxygen, but affords an indication to
the eye of the maintenance of this condition. While the acid remains of
an olive tint, oxygen is absent; but should the color change to a
blue-green, more cuprous chlorid must be added. All the reagents
employed should be previously boiled.
In order to secure absolute freedom from air, the following
modifications on the above process have been adopted by Warington: The
apparatus having been mounted as described, the funnel-tube attached to
the bulb retort is filled with water, and the apparatus connected with
the carbon dioxid generator. Carbon dioxid is then passed through the
apparatus until a moderate stream of bubbles rises in the mercury
trough. The stop-cock is left in this position, and the admission of gas
is controlled by the pinch-cock. The bath of calcium chlorid is so
adjusted as to cause the bulb retort to be almost entirely submerged,
and the temperature of the bath is kept at 130° to 140°. Small
quantities of water are next admitted into the bulb and expelled as
steam in the current of carbon dioxid, the supply of this gas being shut
off before the evaporation is entirely completed, so as to leave as
little carbon dioxid as possible in the apparatus. Previous to very
delicate experiments it is advisable to introduce through the
funnel-tube a small quantity of potassium nitrate, ferrous chlorid, and
hydrochloric acid, rinsing the tube with the latter reagent. Any trace
of oxygen remaining in the apparatus is then consumed by the nitric oxid
formed; and after boiling to dryness and driving out the nitric acid
with carbon dioxid, the apparatus is in a perfect condition for a
quantitative experiment.
=463. Preparation of the Materials to be Analyzed.=—According to
Warington, soil extracts may be used without other preparation than
concentration.
Vegetable juices which coagulate when heated, require to be boiled and
filtered or else evaporated to a thin sirup, treated with alcohol, and
filtered. A clear solution being thus obtained, it is concentrated over
a water-bath to a minimum volume in a beaker of small size. As soon as
cool, it is mixed with one cubic centimeter of a cold saturated solution
of ferrous chlorid and one cubic centimeter of hydrochloric acid, both
reagents having been boiled and cooled immediately before use.
In mixing with the reagents, care must be taken that bubbles of air are
not entangled, which is apt to occur with viscid extracts.
The quantity of ferrous chlorid mentioned is amply sufficient for most
soil extracts, but it is well to use two cubic centimeters in the first
experiment, the presence of a considerable excess of ferrous chlorid in
the retort being thus insured. With bulky vegetable extracts more
ferrous chlorid should be employed. To the sirup from twenty grams of
mangel-wurzel sap, five cubic centimeters of ferrous chlorid and two
cubic centimeters of hydrochloric acid are usually added.
=464. Measurement of the Gas.=—The measurement of the gas was for some
time conducted by the use of concentrated potash for absorbing the
carbon dioxid, and ferrous chlorid for absorbing the nitric oxid. The
use of the ferrous chlorid, however, was found to introduce a source of
error. The treatment of the gas with oxygen and pyrogallol over potash
has therefore been substituted by Warington for its absorption by
ferrous chlorid.
The chief source of error attending the oxygen process lies in the small
quantity of carbon monoxid produced during the absorption with
pyrogallol; this error becomes negligible if the oxygen be only used in
small excess. The amount of oxygen employed can be regulated by the use
of Bischof’s gas delivery-tube. This may be made of a test-tube having a
small perforation half an inch from the mouth. The tube is partly filled
with oxygen over mercury, and its mouth is then closed by a finely
perforated stopper made from a piece of wide tube and fitted tightly
into the test-tube by means of a covering of rubber. When this tube is
inclined, the side perforation being downwards, the oxygen is discharged
in small bubbles from the perforated stopper, while mercury enters
through the opening. Using this tube, the supply of oxygen is perfectly
under control and can be stopped as soon as a fresh bubble ceases to
produce a red tinge on entering. Warington concludes his description by
stating that in the reaction proposed by Schloesing the analyst has a
means of determining a very small quantity of nitric acid with
considerable accuracy, even in the presence of organic matter; but to
accomplish this, the various simplifications consisting in the omission
of the stream of carbon dioxid, and the collection of the gas over
caustic soda must be abandoned, and special precautions must be taken to
exclude all traces of oxygen from the apparatus.
=465. Spiegel’s Modification.=—Spiegel noticed inaccuracies in the
results of the ferrous chlorid method of estimating nitric acid when
carbon dioxid is used, which sometimes amounted to three per cent of the
nitric acid present in the sample. The following suggestions are made by
him for the improvement of the process:[297]
As regards the use of carbon dioxid in the operation, the first
difficulty consists in obtaining it entirely free from air. By the use
of small pieces of marble, which, before being placed in the Kipp
apparatus, are kept for a long while in boiling water, a product is
obtained which, after thirty minutes of moderate evolution, leaves only
a trace of unabsorbed gas in contact with potash-lye. The apparatus used
is illustrated in Fig. 77.
[Illustration:
FIGURE 77. SPIEGEL’S APPARATUS FOR NITRIC ACID.
]
A is a round flask of about 150 cubic centimeters capacity, furnished
with a well-fitting rubber stopper provided with two holes, one for the
entrance of the funnel-tube B and the other for the delivery-tube C. The
tube B ends about two centimeters above the bottom of A and carries a
bulb-shaped funnel at its top capable of holding about fifty cubic
centimeters. The gas-tube D is ground into the bulb of B as shown in the
figure.
After the flask had been filled with the solution to be examined, carbon
dioxid is conducted through D and the flask is heated to boiling until
the gas which escapes through C no longer contains any air. The
measuring tube is brought over the end of the delivery-tube C, in the
usual manner, but not shown in the figure. In the funnel of B are placed
twenty cubic centimeters of previously prepared and boiled ferrous
chlorid solution and this liquid is allowed to flow partly into A by
lifting slightly the gas-tube, D. About forty cubic centimeters of
concentrated, boiled hydrochloric acid are afterwards added to it in the
same way. As soon as the liquid in the flask A is again boiling, the
stream of carbon dioxid is shut off and allowed to flow again only
towards the end of the operation, when the contents of the flask are
reduced almost to dryness. As will be seen from the above directions no
unboiled liquids of any kind are to be used as reagents in the apparatus
described. If the flask A were made much smaller the efficiency of this
apparatus would be increased. It appears to have few, if any, advantages
over Warington’s process.
=466. Schulze-Tiemann Method.=—The modification of Schulze-Tiemann in
the ferrous salt method consists chiefly in the omission of the use of
carbon dioxid, and in the simplified form of apparatus, which permits
rapid work and gives, also, according to some authorities, very exact
and reliable results.[298] The extract, representing 500 grams of the
fine soil, is reduced by evaporation to 100 cubic centimeters and placed
in a glass flask, _A_ (Fig. 78), of 500 cubic centimeters capacity. The
flask is closed with a rubber stopper, carrying two bent glass tubes
which pass through it. The tube _a b c_ is drawn out into a point at _a_
and reaches about two centimeters below the surface of the rubber
stopper. The tube _e f g_ passes just to the lower surface of the rubber
stopper. The two tubes mentioned are connected, by means of rubber tubes
and pinch-cocks, with the tubes _d_ and _h_. The pinch-cocks at _c_ and
_g_ must be capable of closing the tubes air-tight. The end of the tube
_g h_ passes into a crystallizing dish, _B_, and is bent upward to a
point passing two to three centimeters into the measuring tube _C_. The
point within the tube is covered with a piece of rubber tubing. The
measuring tube _C_ is divided into tenths of a cubic centimeter, and
together with the crystallizing dish _B_, is filled with a ten per cent
solution of boiled soda-lye, which is obtained by dissolving 12.9 parts
of sodium hydroxid in 100 parts of water.
[Illustration:
FIGURE 78. SCHULZE-TIEMANN’S NITRIC ACID APPARATUS.
]
The liquid which is to be examined for nitric acid, the pinch-cocks
being opened and the tube _g h_ not dipping into the crystallizing dish,
is boiled for one hour in order to drive the air out of the flask _A_.
The end of the tube _e f g h_ is then brought into the crystallizing
dish containing the sodium hydroxid solution so that the steam escaping
from the flask _A_, escapes partly through the tube _b c d_, and partly
through the tube _f g h_, not allowing, however, the bubbles to enter
the measuring tube _C_. To determine whether the air is all expelled,
the pinch-cock at _g_ is closed and the soda-lye will thereupon rise to
_g_ in case no air interferes. It is best to close the tube at _g_ first
with the thumb and finger and then the rise of the soda-lye to that
point can be determined by the impulse felt. The tube is then firmly
closed by means of the pinch-cock _g_. The rest of the steam is allowed
to escape through the tube _a b c d_, and the evaporation is continued
until the contents of the flask are evaporated to about ten cubic
centimeters. The flask into which the tube _c d_ dips, is filled with
freshly boiled water. The lamp is removed from the flask _A_, the
pinch-cock is closed, whereupon the tube _c d_ becomes filled with the
freshly boiled water. The measuring tube _C_, filled with freshly boiled
soda-lye is closed with the thumb and brought into the dish _B_, care
being taken that no bubble of air enters. It is placed over the end of
the tube _g h_.
The pressure of the external air will now flatten the rubber tubes at
_c_ and _g_. The flask at the end of _c d_ holding freshly boiled water
is then replaced with one filled with a nearly saturated solution of
ferrous chlorid containing some hydrochloric acid. The flask containing
the ferrous chlorid solution should be graduated so that the amount
which is sucked into the flask _A_ can be determined. The pinch-cock _c_
is opened and from fifteen to twenty cubic centimeters of the ferrous
chlorid solution allowed to flow into _A_. The end of the tube _c d_ is
then placed in another flask containing strong hydrochloric acid, and
the latter allowed to flow into the tube in small quantities at a time
until all the ferrous chlorid is washed out of the tube _b c d_ into
_A_. At the point _b_ there is sometimes formed a little bubble of
hydrochloric acid in the state of gas, which by heating the flask _A_
completely disappears.
The flask _A_ is next warmed gently until the rubber tubes at the
pinch-cocks begin to assume their normal condition. The pinch-cock at
_g_ is now replaced by the thumb and finger, and as soon as the pressure
within the flask _A_ is somewhat stronger, caused by the nitric oxid gas
evolved from the mixture, it is allowed to pass through the tube
_e f g h_ and escape into the measuring cylinder _C_. By a manipulation
of the finger and thumb at _g_, it is possible to prevent regurgitation
of the sodium hydroxid into _A_, and at the same time to relieve the
pressure of the nitric oxid in _A_, which would be difficult to do by
means of the pinch-cock alone.
The boiling of the liquid is continued until there is no longer any
increase of the volume of gas in the measuring cylinder _C_. After the
end of the operation the tube _g h_ is removed from the dish _B_ and the
measuring tube _C_ is closed by means of the thumb while its end is
still beneath the surface of the soda-lye, and it is shaken until all
traces of any hydrochloric acid, which may have escaped absorption, are
removed. It is then placed in a large glass cylinder filled with water
at the temperature at which the volume of gas is to be read. After being
kept at this constant temperature for about half an hour the volume of
the nitric oxid can be read. For this purpose the measuring cylinder _C_
is sunk into the water of the large cylinder until the level of the
liquids within and without the tube is the same. The usual correction
for pressure of the atmosphere, as determined by the barometer, and for
the tension of the aqueous vapor at the temperature at which the reading
is made, is applied. The correction is made by means of the following
formula:
V′ = (V × 273 × (B − f)) ÷ ((273 + t) × 760)
In this formula V′ denotes the volume of the gas at the temperature of
zero, and at 760 millimeters barometric pressure; V the volume of the
gas as read at the barometric pressure observed, B, and the temperature
observed, _t_, while _f_ denotes the tension of the aqueous vapor in
millimeters of mercury pressure at the observed temperature _t_. The
tension of the aqueous vapor at temperatures from zero to 26°, expressed
in millimeters of mercury, is given in the following table:
─────┬─────────────
Temp.│ Tension in
° │mm. mercury.
─────┼─────────────
0│ 4.6
1│ 4.9
2│ 5.3
3│ 5.7
4│ 6.1
5│ 6.5
6│ 6.9
7│ 7.4
8│ 8.0
9│ 8.5
10│ 9.1
11│ 9.7
12│ 10.4
13│ 11.1
14│ 11.9
15│ 12.7
16│ 13.5
17│ 14.4
18│ 15.3
19│ 16.3
20│ 17.4
21│ 18.5
22│ 19.6
23│ 20.9
24│ 22.2
25│ 23.5
26│ 25.0
─────┴─────────────
From the gas volume reduced by the above formula the nitric acid is
calculated as follows:
One cubic centimeter of nitric oxid weighs at 0° and 760 millimeters
barometric pressure 1.343 milligrams.
Since two molecules of NO (molecular weight sixty) correspond to one
molecule of N₂O₅ (108) we have the following equation: 60 : 108 = 1.343
: x. Whence x = 2.417 milligrams, the weight of nitric acid
corresponding to one cubic centimeter of nitric oxid.
[Illustration:
FIGURE 79. DE KONICK’S APPARATUS.
]
[Illustration:
FIGURE 80. END OF DELIVERY-TUBE.
]
=467. DeKonick’s Modification of Schloesing’s Method.=—This modification
consists in an arrangement of the gas delivery-tube, whereby the
regurgitation of the water in the measuring burette into the evolution
flask is prevented by a device for sealing the delivery-tube with
mercury.[299] The apparatus is arranged as shown in Fig. 79. The flask
in which the decomposition takes place is provided with a long neck,
into which a side tube is sealed and bent upwards, carrying a small
funnel attached to it by rubber tubing. The piece of rubber tubing
carries a pinch-cock, by means of which the solution containing the
nitrate and hydrochloric acid can be introduced into the flask. The
small gas delivery-tube is arranged as shown in the figure, and carries
at the end next the burette a device shown in Fig. 80. The cork
represented in this device has radial notches cut in it, so as to permit
of a free communication between the water in the burette and in the
pneumatic trough. The open end of the burette, when the apparatus is
mounted ready for use, rests on the notched surface of the cork, and the
end of the delivery-tube is placed in the crystallizing dish resting on
the bottom of the pneumatic trough.
The end of the delivery-tube, as indicated, has fused onto it a vertical
tube open at both ends and six to seven centimeters in length, and
carrying the notched cork already described. The crystallizing dish in
the bottom of the pneumatic trough is filled with mercury until the
point of union of the delivery-tube with the vertical end is sealed to
the depth of a few millimeters. As the gas is evolved it bubbles up
through the mercury into the measuring tube and the displaced water
passes out through the notches in the cork. Should any back pressure
supervene the mercury at once rises in the delivery-tube which is of
such a length as to prevent its entrance into the flask. The operation
can then be carried on with absolute safety.
To make an estimation there are placed in the flask about forty cubic
centimeters of ferrous chlorid solution containing about 200 grams of
iron to the liter, and also an equal volume of hydrochloric acid of one
and one-tenth specific gravity. The side tube is also filled up to the
funnel with the acid. The contents of the flask are boiled until all air
is expelled, which can be determined by holding a test-tube filled with
water over the end of the delivery-tube. The solution containing the
nitrate is next placed in the funnel, the pinch-cock opened and the
liquid allowed to run into the flask by means of the partial vacuum
produced by stopping the boiling and allowing the mercury to rise in the
delivery-tube. All the solution is washed into the flask by successive
rinsings of the funnel with hydrochloric acid, being careful to allow no
bubble of air to enter. The contents of the flask are again raised to
the boiling-point and the nitric oxid evolved collected in the
nitrometer. The solution examined should contain enough nitrate to
afford from sixty to eighty cubic centimeters of gas. Without refilling
the flask, from eight to nine determinations can be made by regenerating
the ferrous chlorid by treatment with zinc chlorid. Care must be
exercised not to add the zinc chlorid in excess, otherwise ammonia and
not nitric oxid will be produced. The side tube and funnel must also be
carefully freed from zinc chlorid by washing with hydrochloric acid.
=468. Schmidt’s Process.=—In the case of a water, or the aqueous extract
of a soil, according to the content of nitric acid, from fifty to one
hundred cubic centimeters are evaporated to thirty cubic centimeters,
and the residue sucked into the generating flask of the apparatus, Fig.
81, and, with the rinsings with distilled water, evaporated again to
from twenty to thirty cubic centimeters, and the flask then connected,
as shown in the figure, to a Schliff measuring apparatus, B.[300] This
apparatus is previously filled to _i_ with mercury, and the bulb _g_
connected with _k_ by a rubber tube.
[Illustration:
FIGURE 81. SCHMIDT’S APPARATUS.
]
The apparatus is then filled with a twenty per cent, previously boiled
and still warm, caustic soda solution until the bulb _g_ is partially
filled when raised a little above the cock _h_. Then _h_ is closed and
_g_ held, by an appropriate support, on about the same level with _h_.
The cock at _b_ is then closed and _e_ opened. Meanwhile the ebullition
in the flask is continued, and the air bubbles rising in the Schliff
apparatus are removed, from time to time, by carefully opening _h_ and
raising _g_. When bubbles no longer come over, the cock at _e_ is closed
and at _b_ opened, and the steam issuing at _a_ is conducted through a
mixture of ferrous chlorid and strong hydrochloric acid to free it, as
far as possible, from air. When the contents of the flask have been
evaporated to about five cubic centimeters, _b_ is closed and the lamp
at once removed.
By carefully opening _b_ about ten cubic centimeters of a mixture of
ferrous chlorid and hydrochloric acid are allowed to enter the flask,
when _b_ is closed and the flask slowly heated until the positive
pressure is restored. The pinch-cock _e_ is then opened and the contents
of the flask evaporated nearly to dryness. The cock _e_ is again closed
and the flame removed. Another quantity (fifteen cubic centimeters) of
ferrous chlorid and hydrochloric acid solution is sucked into the flask
and the process of distillation repeated, whereby the whole of the
nitric oxid is collected in _h_. The nitric oxid evolved is measured in
the usual way and calculated to nitric acid, one cubic centimeter of
nitrogen dioxid being equal to 2.417 milligrams of nitric acid.
=469. Merits of the Ferrous Chlorid Process.=—The possibility of an
accurate determination of nitrates; by decomposition with a ferrous salt
in presence of an excess of acid, has been established by many years of
experience and by the testimony of many analysts. The method is
applicable especially where the quantity of nitrate is not too small and
when organic matter is present. In the case of minute quantities of
nitrate, however, the process is inapplicable and must give way to some
of the colorimetric methods to be hereafter described.
In respect of the apparatus modern practice has led to the preference of
that form which does not require the use of carbon dioxid for displacing
the air. Steam appears to be quite as effective as carbon dioxid and is
much more easily employed. That form of apparatus should be used which
is the simplest in construction and has the least cubical content.
The measurement of the evolved gas is most simply made by collecting
over lye in an azotometer, reading the volume, noting the reading of the
barometer and thermometer and then reducing to standard conditions of
pressure and temperature by the customary calculations. Where a very
strong lye is used the tension of the aqueous vapor may be neglected.
While every analyst should have a thorough knowledge of the ferrous
chlorid method and the principles on which it is based it can not be
compared in simplicity to the later methods with pure nitrates which are
based on the conversion of the nitric acid into ammonia by the action of
nascent hydrogen. In accuracy, moreover, it does not appear to have any
marked advantage over the reduction methods.
=470. Mercury and Sulfuric Acid Method.=—This simple and accurate method
of determining nitric acid in the absence of organic matter is known as
the Crum-Frankland process.[301]
The method rests on the principle of converting nitric acid into nitric
oxid by the action of mercury in the presence of sulfuric acid. The
operation as at first described is conducted in a glass jar eight inches
long by one and a half inches in diameter filled with mercury and
inverted in a trough containing the same liquid. The nitrate to be
examined, in a solid form, is passed into the tube together with three
cubic centimeters of water and five of sulfuric acid. With occasional
shaking, two hours are allowed for the disengagement of the gas, which
is then measured.
=471. Warington’s Modification.=—A graduated shaking tube is employed
which allows the nitrate solution and oil of vitriol to be brought to a
definite volume. The nitrate solution, with rinsings, is always two
cubic centimeters and enough sulfuric acid is added to increase the
volume to five cubic centimeters. The sulfuric acid should give no gas
when shaken with distilled water. Any gas given off in the apparatus
before shaking, is not expelled but is included in the final result. The
persistent froth sometimes noticed where some kinds of organic matter
are present, is reduced by the addition of a few drops of hot water
through the stop-cock of the apparatus. The nitric oxid is finally
measured in Frankland’s modification of Regnault’s apparatus.
This method, accurate for pure nitrates, unfortunately fails in the
presence of any considerable amount of organic matter.
According to Warington’s observations the presence of chlorids is no
hindrance to the accurate determination of both nitric and nitrous acids
by the mercury method. This simplifies the operation as carried on by
Frankland who directs that any chlorin present, be removed before the
determination of the nitric acid is commenced.
=472. Noyes’ Method.=—In the analyses made by Noyes for the National
Board of Health, the Crum-Frankland method was employed.[302] The
apparatus used was essentially that which is now known as Lunge’s
nitrometer and it will be described in the next paragraph. No correction
is made by Noyes for the tension of aqueous vapor in the measurement of
the nitric oxid because of the moderate dilution of the sulfuric acid by
the liquid holding the nitric compounds in solution. The chlorin was not
removed from the dry residue of the evaporated water as its presence in
moderate quantity does not interfere with the accuracy of the process.
In order to obtain the amount of nitrogen in the form of nitrates, the
total volume of nitric oxid must be diminished by that due to nitrites
present, which must be determined in a separate analysis. The method of
manipulation is given in the following paragraph.
[Illustration:
FIGURE 82. LUNGE’S NITROMETER.
]
=473. Lunge’s Nitrometer.=—The apparatus employed by Noyes, in a
somewhat more elaborate form, is known as Lunge’s nitrometer.[303] This
apparatus is shown in Fig. 82. It consists of a burette, _a_, divided
into one-fifth cubic centimeters. At its upper end it is expanded into a
cup-shaped funnel attached by a three-way glass stop-cock. Below, the
burette is joined to a plain tube, _b_, of similar size, by means of
rubber tubing. The apparatus is first filled with mercury through the
tube _b_, the stop-cock being so adjusted as to allow the mercury to
fill the cup at the top of _a_. The cock is then turned until the
mercury in the cup flows out through the side tube carrying the rubber
tube and clamp. The three-way cock is closed, and the solution
containing the nitrate placed in the cup. By lowering the tube _b_ and
opening the cock the liquid is carefully passed into _a_, being careful
to close the cock before all the liquid has passed out of the cup. By
repeated rinsings with pure concentrated sulfuric acid, every particle
of the nitric compound is finally introduced into _a_, together with a
large excess of sulfuric acid. The total volume of the introduced liquid
should not exceed ten cubic centimeters. The mixture of the mercury,
nitric compound, and sulfuric acid is effected by detaching _a_ from its
support, compressing the rubber connection between _a_ and _b_, placing
_a_ nearly in a horizontal position, and quickly bringing it into a
vertical position with vigorous shaking.
After about five minutes the reaction is complete, and the level of the
liquids in the two tubes is so adjusted as to compensate for the
difference in specific gravity between the acid mixture in _a_ and the
mercury in _b_; in other words, the mercury column in _b_ should stand
above the mercury column in _a_ one-seventh of the length of the acid
mixture in _a_. This secures atmospheric pressure on the nitric oxid
which has been collected in _a_. The measured volume of nitric oxid
should be reduced to 0° and 760 millimeters barometric pressure. Each
cubic centimeter of nitric oxid thus obtained corresponds to 1.343
milligrams NO; 2.417 milligrams N₂O₅; 4.521 milligrams KNO₃; 1.701
milligrams N₂O₃; 2.820 milligrams HNO₃; and 3.805 milligrams NaNO₃.
=474. Lunge’s Improved Apparatus.=—Lunge has lately improved his
apparatus for generating and measuring gases and extended its
applicability.[304] The part of it designed to measure the volume of a
gas is the same in all cases. For generating the gas, the apparatus
varies according to the character of the substance under examination.
The measuring apparatus is shown in Fig. 83. It is composed essentially
of three tubes, conveniently mounted on a wooden holder with a box base
for securing any spilled mercury. The support is not shown in the
illustration.
The tubes A, B, C, are mutually connected by means of a three-way tube
and rubber tubing with very thick walls to safely hold the mercury
without expansion. In the middle of the measuring tube A, is found a
bulb of seventy cubic centimeters capacity. Above and below the bulb the
tube is divided into tenths of a cubic centimeter, and its diameter is
such, _viz._, 11.3 millimeters, that each cubic centimeter occupies a
length of one centimeter. The upper end of A is closed with a glass cock
with two oblique perforations, by means of which communication can be
established at will, either through _e_ with the apparatus for
generating the gas, or through _d_ with the absorption apparatus, or the
opening be completely closed.
[Illustration:
FIGURE. 83.
LUNGE’S IMPROVED APPARATUS.
]
The volume of air under the observed conditions which would measure
exactly 100 cubic centimeters at 0° and 760 millimeters pressure of
mercury, is calculated by the formula
V = (100(273 + _t_)760)/(273(_b_ − _f_));
where _t_ equals observed temperature, _b_ the barometric pressure less
the correction noted above and _f_ the tension of the vapor of water
under existing conditions. For example:
Let the temperature be 18°
Barometric reading 755
Correction for _t_ 2
Corrected barometer 753
Vapor of water tension 16
Then V = (100(273 + 18)760)/(273(753 − 16)) = 109.9.
This indicates that 109.9 cubic centimeters of air would occupy a volume
of 100 cubic centimeters when subjected to standard conditions.
The tubes A, B, and C are filled with mercury of which about two and a
half kilograms will be required. By means of the leveling tube B, the
stopper in C being opened, the mercury in C is brought exactly to 109.9
cubic centimeters. The stopper in C is then closed, mercury poured into
D, which is then closed with a rubber stopper, carrying a small glass
tube as indicated in the figure.
The leveling tube B serves to regulate the pressure on the gas in A and
this is secured by depressing or elevating it as the case may require.
The tube for reducing the volume to standard conditions of temperature
and pressure, _viz._, 0° and 760 millimeters of mercury, is shown in C.
In its narrow part which has the same internal diameter as A it is
graduated into tenths of a cubic centimeter. The upper end of C is
furnished with a heavy glass neck D surmounted by a glass cup. In the
neck is placed a ground-glass stopper, carrying a groove below, which
corresponds to a similar groove above in the side of the neck whereby
communication can be established at will between the interior of C and
the exterior. The joint is also sealed by pouring mercury into D as is
shown in the figure. When the stopper is well ground and greased the
reduction tube may be raised or lowered as much as may be necessary
without any danger of escape or entrance of gas. To determine the
position of the reduction tube C the reading of the barometer and
thermometer at room temperature is taken. From the reading of the
barometer subtract one millimeter if the temperature be below 12°, two
millimeters at a temperature from 12° to 19°, three from 20° to 25°, and
four above 25°.
When a gas has been introduced into the measuring tube A it is brought
to the volume which it would assume under standard conditions by
adjusting the tube C in such a way as to bring the level of mercury in C
and A to the same point and the level of the mercury in C is exactly at
100 cubic centimeters. The gas in A is then at the volume which it would
occupy under standard conditions and this volume can be directly read.
This adjustment is secured by moving the tubes B and C up or down. If
gases are to be measured wet, a drop of water should be put on the side
of the upper part of C, and, if dry, of sulfuric acid, before the
adjustment for temperature and pressure.
=475. Method of Manipulation.=—By the action of mercury in the presence
of sulfuric acid, the nitrogen in nitrates, nitrites, nitrosulfates,
nitroses, nitrocellulose, nitroglycerol, and the greater number of
explosives, may be obtained and measured as nitric oxid. The nitrogen
compounds are decomposed in the apparatus shown in Fig. 84.
To make an analysis, the apparatus is filled with mercury, through F,
until the two openings in the cock and _i_ are entirely occupied with
that liquid. The cock _h_ is then closed, and the nitrogen compound, in
solution, introduced through _g_, care being taken that no air enters
_g_ when F is depressed and _h_ opened to admit the sample. The funnel
_g_ is washed several times with a few drops of sulfuric acid, which are
successively introduced into G. The total liquid introduced should not
exceed ten to fifteen cubic centimeters, of which the greater part
should be sulfuric acid. The rubber tube connecting G and F is carefully
closed with a clamp and G violently shaken for a few minutes until no
further evolution of nitric oxid takes place. In shaking, the apparatus
should be so held as to prevent the escape of the mercury from the small
tube _i_ by keeping it closed with the finger or drawing over it a
rubber cap.
[Illustration:
FIGURE 84.
LUNGE’S ANALYTIC APPARATUS.
]
After the evolution of the gas has ceased, the tube _e_, Fig. 83, is
brought into contact with _i_, Fig. 84, and the two are joined by a
tight-fitting piece of rubber tubing in such a way as to exclude any
particle of air. The tube F, Fig. 84, is lifted and B and C, Fig. 83,
depressed. On carefully opening the cocks _h_ and _b_ and bringing _i_
and _e_ into union, the gas is passed from G into A. When all the gas
has entered A and the acid mixture from G has reached _b_ the latter is
closed, and also _h_. The apparatus G is disconnected and removed. The
gas in A is then reduced to normal conditions by manipulating the
reduction tube C in the manner already described.
The gas in A is measured dry by reason of having been generated in
presence of rather strong sulfuric acid. Consequently, for this
operation the adjustment of the volume of gas in C should be made in
contact with a drop of strong sulfuric acid. In order to make the
readings, a quantity of material must be taken which will give less than
thirty or from 100 to 140 cubic centimeters of nitric oxid.
The quantities of the different compounds of nitric acid corresponding
to the number of cubic centimeters of nitric oxid, measured under
standard conditions, are shown in the following table:
CORRESPONDING TO
———————————— ———————————— ————————————
Cubic Weight in N₂O₃ in HNO₃ in NaNO₃ in
centimeters milligrams. milligrams. milligrams. milligrams.
of NO.
1 1.343 1.701 2.820 3.805
2 2.682 3.402 5.640 7.610
3 4.029 5.103 8.460 11.415
4 5.372 6.804 11.280 15.220
5 6.715 8.506 14.100 19.025
6 8.058 10.206 16.920 22.830
7 9.401 11.907 19.740 26.635
8 10.744 13.608 22.560 30.440
9 12.087 15.309 25.380 34.245
=476. Utility of the Method.=—Where it is desirable that the nitric oxid
method be used, and at the same time heating be avoided, the
decomposition of a nitrate by means of metallic mercury and sulfuric
acid affords a convenient and accurate procedure. But, as a rule, there
is no objection to the application of the lamp, and in such cases the
mercury method appears to have no advantage over the ferrous chlorid
process. Nevertheless, in the hands of a skilled worker the results are
reliable, and the process is a quicker one, on the whole, than by
distillation with ferrous chlorid and hydrochloric acid. This method,
however, can not be recommended as in any way superior to the reduction
methods to be hereinafter described.
ESTIMATION OF NITRIC ACID BY OXIDATION OF A COLORED SOLUTION.
=477. Method of Boussingault.=—The process for the estimation of nitric
acid by the decoloration of a solution of indigo is due originally to
Boussingault.[305] In this method the extract, obtained by washing
slowly 200 grams of soil until the filtrate amounts to 300 cubic
centimeters, is evaporated until its volume is no greater than two or
three cubic centimeters, and it is transferred to a test-tube, with
washings, and again evaporated in the tube until the volume is not
greater than that last mentioned. A few drops of solution of indigo are
added, and then two cubic centimeters of pure hydrochloric acid; the
whole is then heated. As the color of the indigo disappears more is
added. When the color ceases to fade, the liquid in the test-tube is
concentrated by boiling. If concentration fail to destroy the blue or
green color, another one-half cubic centimeter of hydrochloric acid is
introduced. The reaction is completed when neither concentration nor
fresh addition of hydrochloric acid destroys the excess of indigo
present. The color produced by a small excess of indigo is a bright
sap-green; this tint is the final reaction sought. The small excess of
indigo necessary to produce a green color is deducted in every
experiment.
When more than mere traces of organic matter are present, Boussingault
advises that the nitric acid be first separated by distillation and then
reduced by the indigo solution. For this purpose the concentrated
solution of the nitrate, two or three cubic centimeters, is placed in a
small tubulated retort with two grams of manganese dioxid in fine
powder. The retort is next half filled with fragments of broken glass,
over which is poured one cubic centimeter of concentrated sulfuric acid.
The retort is heated carefully by means of a small flame, which is kept
in motion so as to successively come in contact with all parts of the
bottom of the retort. The distillate is received in a graduated
test-tube which is kept cool. The distillation is continued until the
vapors of sulfuric acid begin to appear. The apparatus is allowed to
cool, the stopper of the retort removed, two cubic centimeters of water
introduced, and the distillation again made until fumes of sulfuric acid
are again seen. The distillation with water is made twice in order to
remove every trace of nitric acid from the retort. The distillate is
neutralized with a solution of potassium hydroxid and concentrated to
two cubic centimeters, and the nitric acid estimated in the manner
already described. The manganese dioxid used should be previously well
washed and the sulfuric must be free of nitric acid.
_Preparation of the Indigo Solution._—Fifty grams of indigo in fine
powder are digested for twenty-four hours, at 40°, in a liter of
distilled water. The water is then poured off and replaced with a fresh
supply. After the second decantation the residue is treated with 750
cubic centimeters of equal parts of water and pure concentrated
hydrochloric acid and boiled for an hour. After cooling, the undissolved
portion is collected on a filter and washed at first with hot, and
afterwards with cold water, until the filtrate is no longer colored and
is free of acid. The dried residue is treated with ether under a
bell-jar, or in a continuous extraction apparatus, until the ether is
only of a faint blue tint. The fifty grams of indigo at first taken will
give about twenty-five grams of the purified article, which, however,
will still leave a little ash on combustion.
_Solution in Sulfuric Acid._—Five grams of the purified indigo are
placed in a flask having a ground-glass stopper, treated with
twenty-five grams of fuming sulfuric acid, and allowed to digest two or
three days at a temperature of from 50° to 60°. From seventy to 200
drops of the solution thus made are placed in 100 cubic centimeters of
water for use in the process.
_Standardization of the Indigo Solution._—The solution as prepared above
is standardized by a solution of one gram of pure potassium nitrate in
1,000 cubic centimeters of distilled water. The oxidation of the indigo
solution is accomplished as described above. For this strength of
standard nitrate solution two cubic centimeters are taken corresponding
to two milligrams of potassium nitrate. The indigo solution for this
strength should have only twenty drops of the sulfuric acid solution of
indigo to 100 cubic centimeters of water. If twenty grams of potassium
nitrate are taken for 1,000 cubic centimeters of the standard solution
then 200 drops of the sulfindigotic acid should be used to 100 cubic
centimeters of water.
=478. Method of Marx.=—As usually practiced, the indigo method is
conducted according to the variation described by Marx.[306] There are
required for the process the following reagents and apparatus:
_a._ A solution of pure potassium nitrate containing 1.8724 grams per
liter. One cubic centimeter of the solution is equivalent to one
milligram of nitric anhydrid (N₂O₅).
_b._ A solution of the best indigo carmine in water which should be
approximately standardized by solution in the manner described
hereafter, and then diluted so that six to eight cubic centimeters equal
one milligram of nitric acid.
_c._ Chemically pure sulfuric acid of specific gravity 1.842, perfectly
free from sulfurous and arsenious acids and nitrogen oxids.
_d._ Several thin flasks of about 200 cubic centimeters capacity.
_e._ A small cylindrical measure holding fifty cubic centimeters and
divided into cubic centimeters.
_f._ A Mohr’s burette divided into tenths of a cubic centimeter.
_g._ A twenty-five cubic centimeter pipette or another burette.
_h._ A five cubic centimeter pipette divided into cubic centimeters or
half cubic centimeters.
_i._ A measuring flask of 250 cubic centimeters capacity.
_Preliminary Trial._—Twenty-five cubic centimeters of the sample are
transferred to a flask; the fifty cubic centimeter measure is filled
with sulfuric acid and the burette with indigo solution. The sulfuric
acid is added to the sample all at once, shaken for a moment, and the
indigo run in as quickly as possible with shaking until a permanent
greenish tint is produced. If the sample do not require more than twenty
cubic centimeters of indigo solution of the above strength, it can be
titrated directly, otherwise it must be diluted with a proper quantity
of pure water, and subjected again to the preliminary trial.
_The Actual Titration._—(1) Twenty-five cubic centimeters of the sample
properly diluted if necessary, are measured and poured into a flask, and
as much indigo as was used in the preliminary trial, is added; a
quantity of sulfuric acid, equal in volume to the liquid in the flask,
is added all at once, the mixture shaken, and indigo solution run in
quickly out of the burette until the liquid remains permanently of a
greenish tint.
(2) The last experiment is repeated as often as may be necessary adding
to the water at first half a cubic centimeter less indigo than the total
quantity used previously, afterwards proceeding as in (1) until the
final test shows too little indigo used.
(3) From the rough titration of the indigo, calculate the amount of
potassium nitrate solution corresponding with the indigo solution used
in (2), multiply the result by ten, transfer this quantity of the
standard nitrate solution to a 250 cubic centimeter flask, fill with
pure water to the mark, and titrate twenty-five cubic centimeters of
this fluid with indigo as in (2). If the quantity of indigo solution
used is nearly the same as that required in (2), its exact value may be
calculated, but if it is not, another nitrate solution may be made up in
the 250 cubic centimeter flask, more closely resembling the sample in
strength, and the titration with the indigo solution must be repeated.
(4) If the water contain any considerable amount of organic matter, it
must first be destroyed by potassium permanganate. In this case, the
estimation of the organic matter and nitric acid may be conveniently
combined.
The use of permanganate in the above case is likely to introduce an
error as has been shown by Warington. The method therefore can not be
recommended in the presence of organic matter.
=479. Method of Warington.=—The modification of the indigo method as
used by Warington, applicable only in absence of organic matter, is the
one chiefly employed in England.[307]
Instead of the ordinary indigo of commerce, indigotin is used. The
normal solution of the coloring matter is made of such a strength as to
be equivalent to a solution of potassium nitrate containing 0.14 gram of
nitrogen per liter. Where large quantities of the coloring matter are to
be used it is advisable to prepare it about four times the strength
given above and then dilute it as required. Four grams of sublimed
indigotin will furnish more than two liters of the color solution.
The solution is prepared as follows:
Four grams of indigotin are digested for a few hours with five times
that weight of Nordhausen sulfuric acid, diluted with water, filtered,
and made up to a volume of two liters. The strength of the indigotin
solution is determined with a solution of potassium nitrate of the
strength mentioned above. The process is performed as follows:
From ten to twenty cubic centimeters of the standard nitrate solution
are placed in a wide-mouthed flask of about 150 cubic centimeters
capacity. A portion of the indigotin solution is next added, such as
will be deemed sufficient for the process, and the whole is well mixed.
Strong sulfuric acid is next measured out from a burette into a
test-tube, in volume equal to the united volumes of the nitrate solution
and indigotin. The whole of the sulfuric acid is then poured as quickly
as possible, into the solution in the flask and rapidly mixed, and the
flask transferred to a calcium chlorid bath, the temperature of which
should be maintained at 140°. It is essential to the success of the
operation that the sulfuric acid should be mixed with the greatest
rapidity. It should be poured in at once and the whole well shaken
without waiting for the test-tube containing the acid, to drain. The
flask should be covered by a watch-glass while it is held in the bath.
As soon as the larger part of the indigotin is oxidized the flask in the
bath should be gently rotated. With very weak solutions of nitrate it
may be necessary sometimes to keep the flask in the bath for five
minutes. When the indigo color is quickly discharged it shows the
presence of nitric acid in considerable excess and a considerably larger
quantity of indigo must be taken in the next experiment. The experiments
are continued until just the quantity of indigo necessary to consume the
nitric acid is taken, the amount of indigo being in very slight excess,
not exceeding one-tenth cubic centimeter of the indigo solution used.
The tint produced by the small excess of indigo remaining is best seen
by filling the flask with water. On substances of approximately known
strength about four experiments are usually necessary to determine the
amount of indigo to be taken, but with unknown substances a larger
number may be necessary.
Usually in determinations of this kind it is directed to use double the
volume of sulfuric acid mentioned above. In this case not only is the
quantity of indigo oxidized much greater than with a smaller portion of
acid, but the prejudicial effect of organic matter is also greater than
when the smaller quantity of acid is employed.
An indigo solution standardized as above is strictly to be used for a
solution of nitrate of the strength employed during the standardization.
The quantity of indigo oxidized in proportion to the nitric acid present
diminishes as the nitrate solution becomes more dilute. Instead of
determining this during each series of experiments it may be estimated
once for all and a table of corrections used.
The following table is based upon experimental determinations:
Strength of Indigo Difference Nitrogen Difference Difference
niter required, between corresponding between the in the
solution cubic amounts of to one cubic nitrogen nitrogen
used. centi- indigo, centimeter of values, values for a
meters. cubic indigo, gram. gram. difference
centi- of one cubic
meters. centi- meter
in the
amount of
indigo,
gram.
⁸⁄₆₄ Normal 10.00 0.000035000
⁷⁄₆₄ „ 8.71 1.29 0.000035161 0.000000161 0.000000125
⁶⁄₆₄ „ 7.43 1.28 0.000035330 0.000000169 0.000000132
⁵⁄₆₄ „ 6.14 1.29 0.000035627 0.000000298 0.000000231
⁴⁄₆₄ „ 4.86 1.28 0.000036008 0.000000381 0.000000298
³⁄₆₄ „ 3.57 1.29 0.000036763 0.000000756 0.000000586
²⁄₆₄ „ 2.29 1.28 0.000038209 0.000001445 0.000001129
¹⁄₆₄ „ 1.00 1.29 0.000043750 0.000005541 0.000004295
The table is used as follows:
Suppose that twenty cubic centimeters of water under examination have
required 5.36 cubic centimeters of indigo solution for the oxidation of
the nitric acid contained therein. By inspection of the table it is seen
that this number is five-tenths cubic centimeter above the nearest
quantity given; _viz._, 4.86 cubic centimeters. From the last column in
the table it is found that the correction for five-tenths cubic
centimeter of indigo solution is 0.000000149 cubic centimeter, being
half that for the one cubic centimeter given in the table. This is to be
subtracted from the unit value in nitrogen given in the first “gram”
column of the table; _viz._, 0.000036008. It is thus seen that the 5.86
cubic centimeters of indigo solution are equivalent to 0.000035859 gram
of nitrogen per cubic centimeter. The water under examination,
therefore, contains nine and six-tenths parts of nitrogen as nitric acid
per million.
Attention must also be paid in standardizing indigo solutions to the
initial temperature of the solutions. A rise in the initial temperature
will be attended by a diminution in the quantity of indigo oxidized.
Experiments with a room temperature of 10° and a room temperature of
20°, being the initial temperatures of the experiments, showed that at
the higher temperature the amount of indigo consumed was about five per
cent less when the strong solutions of nitrate were employed. The indigo
solution should, therefore, be standardized at the same temperature at
which the determinations are made.
If twenty cubic centimeters of the standard nitrate solution employed be
used in setting the indigo solution, this standard will enable the
operator to determine nitric acid up to 17.5 parts of nitrogen per
million in water or soil extracts.
The presence of an abundance of chlorids in the water under examination
tends to diminish the content of nitric acid found, and also tends to
introduce an error, which is sometimes of a plus and sometimes of a
minus quantity, according to the strength of the nitric acid present.
The reaction is shortened in weak solutions by the presence of chlorids,
and the quantity of indigo consumed is consequently increased. The error
introduced by chlorids is usually of an insignificant nature.
On account of the interference of organic matters with the reaction of
indigo it is not of much use in the examination of nitrates washed out
of soils, although in some cases the results may be quite accurate. This
method must, therefore, be considered as applicable, in general, to
waters or soil extracts which contain little or no organic matter.
In analytical work pertaining particularly to agriculture, the use of
the indigo method for determining nitric acid has been largely employed,
both in the analyses of soil extracts and drainage and irrigation
waters. The method, however, can hardly survive as an important one in
such work in competition with more modern and speedy processes of
analysis.
DETERMINATION OF NITRIC NITROGEN BY REDUCTION TO AMMONIA.
=480. Classification of Methods.=—When nitrogen is present in a highly
oxidized state, _e. g._, as nitric acid, it may be quickly and
accurately estimated by reduction to ammonia. This action is effected by
the reducing power of nascent hydrogen, and this substance may be
secured in the active state by the action of an acid or alkali on a
metal, or by means of an electric current. The processes depending on
the use of a finely divided metal in the presence of an acid or alkali
have come into general use within a few years, and are now employed
generally instead of the more elaborate estimations depending on the use
of copper oxid or indigo.
The typical reaction which takes place in all cases is represented by
the following equation:
2HNO₃ + 8H₂ = 2NH₃ + 6H₂O.
The method will be considered under three heads; _viz._, 1. Reduction in
an alkaline solution; 2. Reduction in an acid solution; 3. Reduction by
means of an electric current.
In the first class of processes the reduction and distillation may go on
together. In the second class the reduction is accomplished first and
the distillation effected afterwards, with the addition of an alkali. In
the third class of operations the reduction is accomplished by means of
an electric current and the ammonia subsequently obtained by
distillation, or determined by nesslerizing. These processes may be
applied to rain and drainage waters, and to soil extracts. On account of
the ease with which the analyses are accomplished, the short time
required and the accuracy of the results, the reduction methods for
nitrates have already commended themselves to analysts, and are quite
likely to supersede all others for practical use where weighable
quantities of nitrates are present. For the minute traces of nitrates
found in rain and drainage waters, and in some soil extracts, the
reduction method may also be applied, but in these cases the ammonia
which is formed must be determined by colorimetry (nesslerizing) and not
by distillation. The processes about to be described are especially
applicable to the examination of soils and waters rich in nitrates.
REDUCTION IN ALKALINE SOLUTIONS.
=481. Provisional Method of the Association of Official Agricultural
Chemists.=[308]—
_Extraction of the Nitrates._—Place one kilogram of the dried soil,
calculated to water-free substance, on a percolator of glass or tin.
Moisten the soil thoroughly with pure distilled water, and allow to
stand for half an hour. Add fresh portions of pure distilled water until
the filtrate secured amounts to one liter. If the first filtrate be
cloudy before use it may be refiltered.
_Qualitative Test for Nitrates._—Evaporate five cubic centimeters of the
soil extract in a porcelain crucible, having first dissolved a small
quantity of pure brucin sulfate therein. When dry, add to the residue a
drop of concentrated sulfuric acid free of nitrates. If the nitrate
calculated as potassium nitrate does not exceed the two-thousandth part
of a milligram only a pink color will be developed; with the
three-thousandth part of a milligram a pink color with reddish lines;
with the four-thousandth part of a milligram a reddish color; with the
five-thousandth part of a milligram a distinct red color.
_Estimation of the Nitrates._—Evaporate 100 cubic centimeters of the
soil extract to dryness on a steam-bath. Dissolve the soluble portions
of the residue in 100 cubic centimeters of ammonia-free distilled water,
filtering out any insoluble residue. Place the solution in a flask, add
ten cubic centimeters of sodium amalgam, stopper the flask with a valve
which will permit the escape of hydrogen, and allow to stand in a cool
room for twenty-four hours. Add fifty cubic centimeters of milk of lime
and titrate the ammonia produced by distillation, with standard acid and
estimate as nitrogen pentoxid. Where the amount of ammonia is small,
nesslerizing may be substituted for titration.
_Preparation of Sodium Amalgam._—Place 100 cubic centimeters of mercury
in a flask of half a liter capacity; warm until paraffin will remain
melted over the surface; drop successively in the paraffin-covered
mercury, pieces of metallic sodium of the size of a pea until 6.75 grams
have united with the mercury. The amalgam contains then 0.5 per cent of
metallic sodium and may be preserved indefinitely under the covering of
paraffin.
=482. Method of the Experiment Station at Möckern.=[309]—The principle
of this reaction is based on the reducing action exercised by nascent
hydrogen on a nitrate, the hydrogen being generated by the action of
soda-lye on a mixture of zinc dust and finely divided iron.
Ten grams of nitrate are dissolved in 500 cubic centimeters of water. Of
this solution twenty-five cubic centimeters, corresponding to one-half
gram, are placed in a distillation flask of about 400 cubic centimeters
capacity, 120 cubic centimeters of water added, and about five grams of
well-washed and dried zinc dust and an equal weight of reduced iron. To
the solution are added eighty cubic centimeters of soda-lye of 32° B.
The flask is then connected with the condensing apparatus and the
distillation carried on synchronously with the reduction, the ammonia
being collected in twenty cubic centimeters of titrated sulfuric acid.
The distillation is continued from one to two hours, or until 100 cubic
centimeters have been distilled, and the remaining sulfuric acid is
titrated in the usual way. Soil extracts and sewage waters should be
concentrated until they have approximately the proportion of nitrates
given above.
=483. Method of Devarda.=—The inconvenience due to slow action and other
causes, arising from the use of pure metals in the reduction of nitrates
to ammonia, has been overcome, to some extent, by Devarda, by use of an
alloy, in a state of fine powder, consisting of aluminum, copper, and
zinc.[310] The alloy consists of forty-five per cent of aluminum, fifty
per cent of copper, and five per cent of zinc. In dissolving, the copper
is left in a finely divided state, which is a great help in distillation
in preventing bumping.
The analytical process is carried out as follows: The solution
containing the nitrate, in quantity equivalent to about one-half gram of
potassium nitrate, is placed in a flask having a capacity of about one
liter, and diluted with sixty cubic centimeters of water and five cubic
centimeters of alcohol, and then forty cubic centimeters of caustic
potash solution added of specific gravity one and three-tenths. From two
to two and one-half grams of the alloy, described above, are introduced,
and the flask attached to a condenser with a receiver containing
standard acid. The connection between the flask and the condenser is
made by means of a tube having on the limb next the flask a bulb filled
with glass beads to prevent the contents of the flask splashing over
into the receiver, and on the other limb another bulb to prevent the
acid in the receiver finding its way into the distillation flask, should
regurgitation occur. When the flask has been thus connected with the
condenser it is gently heated for half an hour, at the end of which time
the evolution of hydrogen will have slackened or ceased, and then the
distillation is begun, at first cautiously, until the zinc of the alloy
has completely dissolved, and then more vigorously, the time necessary
being about twenty minutes from the time when the contents of the flask
begin to boil. The distillate is caught in standard acid and the ammonia
determined by backward titration in the ordinary way. It is to be noted
that the strength of the alkali used is of importance, as if it be too
strong, the action on the alloy is unduly vigorous at the beginning of
the operation, and if too weak, the contents of the flask have to be
heated overmuch, the result in both cases being the formation of a fine
spray of caustic solution, which is very difficult to stop, even with
complicated washing attachments to the distilling flask. The test
analyses on pure nitrates are satisfactory. This method has been used
with satisfaction in the laboratory of the Department of Agriculture,
but does not appear to have any special advantage over the process of
Ulsch, to be described further on.
=484. Variation of Stoklassa.=—Stoklassa has subjected the method of
Devarda to a comparative test with the following methods:[311]
1. Wagner’s Schloesing-Grandeau method.
2. Lunge’s nitrometer method.
3. Stutzer’s method.
[Illustration:
FIGURE 85. STOKLASSA’S NITRIC ACID APPARATUS.
]
The reduction takes place in a copper erlenmeyer, A, Fig. 85, in which,
in addition to the solution containing the nitrate, are placed 200 cubic
centimeters of water, forty cubic centimeters of potassium hydroxid
solution of 33° B., five cubic centimeters of alcohol, and finally two
and one-half grams of the finely powdered Devarda alloy. The distillate
passes through a tube, B, filled with glass pearls and into the
condenser D, through the bulbs, C C′.
After the flask is connected with the distilling apparatus, it is gently
warmed and the reduction is ended in about twenty minutes. The ammonia
which is formed is then distilled into E, containing the standard acid,
S, requiring about twenty minutes more. The comparative results given,
show that the Devarda method is equally as accurate as any of the other
methods mentioned, giving practically theoretical results.
In so far, however, as speed of an analysis, is concerned the first
place is awarded to the Lunge nitrometer method, with which a complete
analysis can be made in from thirty to forty minutes. In the second
rank, so far as speed is concerned, the Devarda method is recommended.
All the methods give accurate results.
=485. Method of Sievert.=[312]—Two grams of potassium or sodium nitrate
are dissolved and made up to 1,000 cubic centimeters. Fifty cubic
centimeters of the solution are placed in a 600 cubic centimeter flask
and diluted with fifty cubic centimeters of water, and from eighteen to
twenty grams of caustic alkali added. After the alkali is dissolved,
seventy-five cubic centimeters of ninety-six per cent alcohol are added
and a few pieces of bone-black to prevent foaming. From ten to fifteen
grams of zinc or iron dust are then added to the flask which is closed
and connected with a =ᥩ= tube holding about 200 cubic centimeters, which
contains about ten cubic centimeters of normal sulfuric acid. This =ᥩ=
tube is kept cool by being immersed in water. The whole mixture is now
allowed to stand for three or four hours and then the alcohol is
distilled slowly and the ammonia formed by the reduction of the nitrates
is carried over with it. The distillation lasts for about two hours. The
contents of the =ᥩ= tube are carefully rinsed into a dish and the excess
of sulfuric acid titrated with one-fourth normal soda-lye.
For soil extracts and substances containing unknown quantities of nitric
acid, a preliminary test will indicate approximately the amount thereof,
and this will be an indication for the quantity to be used in the
analysis.
The method of Stutzer differs from the foregoing in the substitution of
aluminum dust instead of iron or zinc.[313] The reducing power of
aluminum, however, varies greatly according to the method in which the
metal has been prepared. Pure aluminum prepared by the electric method,
reduces the nitric acid much less vigorously than the metal prepared by
the older methods of fusion with sodium. For this reason the method of
Stutzer is not to be preferred to that of Sievert.
REDUCTION IN AN ACID SOLUTION.
=486. Variation of the Sodium Amalgam Process.=—This method is described
by Monnier and Auriol.[314]
[Illustration:
FIGURE 86. VARIATION OF THE SODIUM AMALGAM PROCESS.
]
The principle of the operation depends on the reduction of the dissolved
nitrate by titrated sodium amalgam in presence of an acid, and the
estimation of the quantity of nitric acid present from the deficit in
the volume of hydrogen. The apparatus employed is conveniently mounted
as shown in Fig. 86. The brass vessel A is movable by means of the cord
on the pulley B, in such a way as to be fixed at any required altitude.
It is filled with water and connected by a rubber tube to the cooling
tube D. Within the cooling tube there is a graduated cylinder open at
its lower end. Its upper end is connected directly with the apparatus C.
The cooling tube D has a small side tube, _c_, near its upper end, by
means of which the air can enter or escape when the position of A is
changed. The apparatus C, in which the reaction takes place, is a glass
cylinder. Its upper end is continuous with the =⟙= tube provided with
the stop-cocks _a_ and _b_. One arm of the =⟙= permits connection with
the graduated measuring tube by means of a rubber union. The lower end
of C is closed with a large hollow ground-glass stopper, carrying a
small receptacle within, so that it forms two separate water-tight
compartments, open at the top.
The sodium amalgam is prepared as follows:
In a clay crucible are heated 400 grams of mercury, and, little by
little, with constant stirring, four grams of dry sodium are added. When
cold, the amalgam is placed in a burette, having a ground-glass stopper,
and covered with petroleum. The strength of the amalgam is established
in the following manner. A small glass thimble, ground even at the top,
is filled with the amalgam and struck off even with a ground-glass
straight edge. In this way the same quantity of amalgam is taken for
each test. This measured portion of the amalgam is placed in the inner
vessel of the glass stopper to C. Ten cubic centimeters of water,
containing sixty centigrams of tartaric acid, are placed on the outer
ring of the glass stopper, which is then inserted, well oiled, in C,
closing it air- and water-tight. The tartaric acid solution also carries
a piece of litmus paper, so that its constant acidity may be insured.
The vessel A is then fixed in a position which brings the water in the
graduated burette and tube D exactly to the 0 mark. The cock _a_ is next
closed, _b_ opened, and C is inverted until all the amalgam is poured
into the solution of tartaric acid. The evolved hydrogen mixed with the
air contained in the apparatus, is passed into the graduated burette.
After fifteen minutes, the reaction is ended. The water level within and
without the graduated tube is restored and the volume of gas evolved
noted and reduced by the usual tables to 0° and 760 millimeters pressure
of the barometer.
An amalgam prepared as above will give about three cubic centimeters of
hydrogen for each gram. The thimble should hold from twelve to fifteen
grams.
The estimation of nitric acid should be made in a solution containing
about one-tenth per cent of nitrate. Ten cubic centimeters are taken, to
which six-tenths gram of tartaric acid is added, and placed in the outer
part of the glass stopper. The rest of the process is conducted exactly
as described above. The deficit in hydrogen is calculated to nitrogen
pentoxid.
The reduction by sodium amalgam is not so convenient a form of
estimating nitric acid as many of the other forms of using nascent
hydrogen. As practiced by calculating from the deficit of hydrogen,
however, it has some advantages by reason of the fact that no heating is
required. The presence of organic neutral bodies, or even those of an
acid nature, like humus, does not, therefore, interfere with the work.
Likewise, mineral bodies in solution, which are not reduced by nascent
hydrogen, do not interfere with the accuracy of the reaction.
=487. Method of Schmitt.=—In the method of Schmitt forty cubic
centimeters of glacial acetic acid are placed in a flask of 600 cubic
centimeters content, and fifteen grams of a mixture of zinc and iron
dust added.[315] To this a quantity of the solution containing the
nitrate, representing about half a gram of the pure nitrate, is added
with constant shaking, in portions which do not evolve hydrogen too
rapidly. After about fifteen minutes when the evolution of nitrogen has
somewhat diminished, an additional fifteen grams of the metal dust are
added. If the contents of the flask should become thick they can be
diluted with thirty cubic centimeters of water. The reduction is
complete in from thirty to forty minutes. The contents of the flask are
now saturated with enough soda-lye not only to neutralize the excess of
acetic acid, but to keep the zinc hydroxid also in solution. For this
purpose about 200 cubic centimeters of soda-lye of 1.25 specific gravity
are necessary. The ammonia is obtained by distillation into standard
acid in the usual way.
=488. Method of Ulsch.=—In practice the method of Ulsch has come into
general use.[316]
For the determination of nitrogen by this method half a gram of
saltpeter or four-tenths gram of sodium nitrate is taken and dissolved
in twenty five cubic centimeters of water, in a flask with a content of
about 600 cubic centimeters. Five grams of iron reduced by hydrogen, and
ten cubic centimeters of sulfuric acid diluted with two volumes of water
are then added to the flask. To avoid mechanical losses during the
evolution of hydrogen a pear-shaped glass stopper is hung in the neck of
the flask. After the first violent evolution of hydrogen has passed, the
flask is slowly heated until in about four minutes it is brought to a
gentle boil. The boiling is continued for about six minutes when the
reduction is complete. About fifty cubic centimeters of water are then
added; also an excess of soda-lye and a few particles of zinc and the
ammonia is distilled and collected in standard acid in the usual way.
The method of Ulsch can also be applied, according to Fricke, to the
analysis of nitrates contained in drinking and drainage waters, and it
is regarded by him as one of the best methods to be employed in such
investigations.[317]
The method of Ulsch in this laboratory has given entirely satisfactory
results, and is generally used in preference to other methods in cases
where a considerable quantity of nitrates is present. It is based on the
following reactions:
2KNO₃ + H₂SO₄ = K₂SO₄ + 2HNO₃
2HNO₃ + 8H₂ = 2NH₃ + 6H₂O
2NH₃ + H₂SO₄ = (NH₄)₂SO₄.
REDUCTION BY THE ELECTRIC CURRENT.
=489. Method of Williams-Warington.=—From the losses which naturally
occur during the evaporation of water, even with all the precautions
noted, Warington was led to try some method for the determination of
nitrates and nitrites in waters without previous concentration.[318] The
reduction of these bodies by the copper-zinc couple formed the basis of
these experiments, and they resulted in the following method of
manipulation, which is based on a process devised by Williams.[319]
The method consists in boiling rapidly one liter of the rain water in a
retort, with a little magnesia previously raised to a low red heat and
then washed, until 250 cubic centimeters have distilled over. The
residue is then made up to 800 cubic centimeters, transferred to a
wide-mouthed, stoppered bottle supplied with strips of copper and zinc
forming electric couples, and set aside, at a constant temperature of
from 21°–24°, for three days. A measured portion of the solution is then
distilled, and the ammonia determined in the distillate by nesslerizing.
This plan has two advantages: First, the ammonia, as well as the
nitrogen as nitrates and nitrites, can be determined in the course of
the same operation and in the same sample of water. For this purpose it
is only necessary to fit the retort to an efficient condenser and to
remove all ammonia from the apparatus by boiling distilled water in the
retort before introducing the rain water. The distillate of 250 cubic
centimeters from the rain water, as described above, is well mixed and
the ammonia determined, in from twenty-five to one hundred cubic
centimeters thereof, diluted to 150 cubic centimeters with ammonia-free
water. Second, the nitrogen, as nitrates and nitrites, is determined
directly and alone; the error of the determination is as small as
nesslerizing admits of, since it is possible, if necessary, to distill
600 cubic centimeters of the boiled rain water corresponding to 750
cubic centimeters of the original, and thus obtain a full amount of
ammonia for determination, even when the rain water has been poor in
nitrates.
The determination of nitric nitrogen, in a given sample, by the above
method gave a mean quantity of product of 0.162 part per million, while
the determination, in the same lot of samples, by the modified
Schloesing method gave 0.125 part per million. This result confirms the
supposition that in the complete evaporation necessary to the
manipulation of the Schloesing method there is a loss of nitrogen. The
amount of nitrogen as nitrates and nitrites in the rain water at
Rothamstead, for the twelve months ending April 1, 1888, was found, by
the Schloesing method, to be 0.614 pound per acre, the total rain-fall
being 21.96 inches. For the year ending April 1, 1889, by the
copper-zinc method, it amounted to 0.917 pound per acre, the total
rain-fall being 29.27 inches.
The amounts found in other localities are quite different from the
above, as for instance, the mean of seven stations in Germany for
thirteen years, beginning in 1864, showed 10.18 pounds of nitrogen per
acre. The average amount for ten years at the observatory of Mont
Sauris, near Paris, showed 12.36 pounds of nitrogen per acre. The
average for three years at Lincoln, as determined by Professor G. Gray,
shows one and six-tenths pounds of nitrogen per acre per annum. At
Tokio, in Japan, Kellner found, for one year, 1.02 pounds per acre.
=490. Determination of the Ammonia.=—The method used at Rothamstead is
to make one determination of ammonia in the whole of the distillate
obtained, the strength of which is regulated by varying the amount
introduced into the retort, so that it shall be equal to about two cubic
centimeters of the standard ammonia solution. A 150 cubic centimeter
cylinder is first filled with the rain water, and fifty cubic
centimeters of nessler reagent added. The depth of tint indicates what
quantity of rain water will be required for distillation. This having
been determined, the appropriate volume of the rain water, provided it
do not exceed 600 cubic centimeters, is placed in the retort described
above, and the distillation continued until the 150 cubic centimeter
cylinder is filled. The titration is made in the usual way.
=491. Preparation of the Copper-Zinc Couple.=—For 800 cubic centimeters
of boiled rain water, prepared as described, six strips of zinc foil,
four inches long by one and a quarter inches wide, are taken and bent at
right angles along their center to obtain stiffness. The couple is
cleansed and coated by washing in a series of five beakers containing,
respectively, dilute solution of sodium hydroxid, very dilute sulfuric
acid, a three per cent solution of copper sulfate, ordinary distilled
water, and distilled water free from ammonia. Through these five beakers
the zinc foil is successively passed. It is rinsed both after the alkali
and the acid. But after the copper has been deposited, the strips are
simply drained and carefully placed in the distilled water, it being
difficult to rinse without removing the copper. The couples should be
entirely submerged when placed in the rain water. The strips should
remain in the copper sulfate solution long enough to be well covered
with copper.
=492. Substitution of an Aluminum-Mercury Couple for
Copper-Zinc.=—Ormandy and Cohen have proposed to use an aluminum-mercury
couple for the copper-zinc in the process described above.[320]
This couple acts more quickly than the copper-zinc, and the results are
equally as accurate. Nitrites are reduced in about one hour by this
apparatus, while the zinc-copper couple of Gladstone and Tribe requires
about six times as long. Aluminum foil, free of grease, should be used.
The foil should be heated over a bunsen just before amalgamation. The
clean, very thin foil is coated with mercury by shaking with a
concentrated solution of mercuric chlorid. It should be prepared
immediately before use.
The amalgamated foil is introduced into the sample of water to be
analyzed, and left until all the aluminum is converted into oxid. The
presence of the oxid favors the prevention of bumping during the
subsequent distillation. The distilled ammonia, collected in dilute
acid, is determined by nesslerizing, the free ammonia in the sample
having been previously determined. The increase in ammonia is due to
nitrates or nitrites reduced by the couple.
IODOMETRIC ESTIMATION OF NITRIC ACID IN NITRATES.
=493. Method of De Koninck and Nihoul.=—This process is applicable only
in the absence of organic bodies and other reducing agents.
The principle on which it rests, as applied by McGowan, is as
follows:[321]
When a fairly concentrated solution of a nitrate is warmed with an
excess of pure, strong hydrochloric acid, the nitrate is completely
decomposed, and the production of nitrosyl chlorid and chlorin is
quantitative. The reaction, as shown by Tilden, is represented by the
following equation:[322]
HNO₃ + 3HCl = NOCl + Cl₂ + 2H₂O.
One molecule of nitric acid thus yields two atoms of chlorin and one
molecule of nitrosyl chlorid capable of setting free three atoms of
iodin. The iodin can be estimated in the usual manner by titration with
sodium thiosulfate. The nitrosyl chlorid is decomposed by the potassium
iodid, nitric oxid escaping.
The apparatus employed is shown in Fig. 87.
A is a small, round-bottomed flask, into the neck of which a glass
stopper, _x_, is accurately ground (with fine emery and oil). The
capacity of the bulb is about forty-six cubic centimeters, and the
length of the neck, from _x_ to _y_, ninety millimeters. The first
condenser is a simple tube, slightly enlarged at the foot into two small
bulbs.[323] The length from _a_ to _b_ is 300 millimeters, from _b_ to
_c_ 180 millimeters, and from _e_ to _f_ thirty millimeters. The
capacity of the bulb B is twenty-five cubic centimeters, and the total
capacity of the two bulbs and tube, up to the top of C, forty-one cubic
centimeters. This condenser is immersed, up to the level of _c_, in a
beaker full of water. D is a geissler bulb apparatus, E is a calcium
chlorid tube, filled with broken glass, which acts as a tower and _g_ is
a small funnel, attached by rubber and clip to the branch =⟙= tube _h_.
Between the =⟙= tube _i_ and the wash-bottle for the carbon dioxid is
placed a short piece of glass tubing, _s_, containing a strip of filter
paper, slightly moistened with iodid of starch solution. This tube _s_
is really hardly necessary, as no chlorin escapes backwards if a
moderate current of carbon dioxid is kept passing, but it serves as a
check. A glance at the joints _o_, _p_, and _q_, which are of narrow
india-rubber tubing, is sufficient to show that, by using this
arrangement, practically no rubber is exposed to the action of the
chlorin. The tiny piece of rubber tubing at the joint _o_ may be done
away with, the narrower tube there being accurately ground into the
wider one; this makes the condensing apparatus practically perfect.
[Illustration:
FIGURE 87. MCGOWAN’S APPARATUS FOR THE IODOMETRIC ESTIMATION OF NITRIC
ACID.
]
The actual operation is performed in the following manner:
The evolution flask is washed and thoroughly dried, and the nitrate
(say, about 0.25 gram of potassium nitrate) is tapped into it from the
weighing tube. Two cubic centimeters of water are now added, and the
bulb is gently warmed, so as to bring the nitrate into solution, after
which the stopper of the flask is firmly inserted. About fifteen cubic
centimeters of a solution of potassium iodid (one in four) are run into
the first condensing tube, any iodid adhering to the upper portion of
the tube being washed down with a little water, and five cubic
centimeters of the same solution, mixed with eight to ten cubic
centimeters of water, are sucked into the geissler bulbs whilst the
glass in the tower E is also thoroughly moistened with the iodid. The
geissler bulbs should be so arranged that gas only bubbles through the
last of them, the liquid in the others remaining quiescent.
All the joints having been made tight the carbon dioxid is turned on
briskly and passed through the apparatus until a small tubeful collected
at _l_, over caustic potash solution, shows that no appreciable amount
of air is left in it. The small outlet tube _l_, is now replaced by a
calcium chlorid tube, filled with broken glass which has been moistened
with the above-mentioned iodid solution, and closed by a cork through
which an outlet tube passes, the object of this trap tube being to
prevent any air getting back into the apparatus. The brisk current of
carbon dioxid is continued for a minute or two longer, so as to
practically expel all the air from this last tube. The stream of gas is
now stopped for an instant, and about fifteen cubic centimeters of pure
concentrated hydrochloric acid, free from chlorin, run into A through
the funnel _g_ (into the tube of which it is well to have run a few
drops of water before beginning to expel the air from the apparatus),
and A is shaken so as to mix its contents thoroughly. A slow current of
carbon dioxid is now again turned on (one to two bubbles through the
wash-bottle per second), and A is gently warmed over a burner. It is a
distinct advantage that the reaction does not begin until the mixed
solutions are warmed, when the liquid becomes orange-colored, the color
again disappearing after the nitrosyl chlorid and chlorin have been
expelled. The warming should be very gentle at first in order to make
sure of the conversion of all the nitric acid, and also because the
first escaping vapors are relatively very rich in chlorin; afterwards
the liquid in A is briskly boiled. A very little practice enables the
operator to judge as to the proper rate of warming. When the volume of
liquid in A has been reduced to about seven cubic centimeters (by which
time it is again colorless) the stream of carbon dioxid is slightly
quickened and the apparatus allowed to cool a little. The burner is now
set aside for a few minutes, and two cubic centimeters more of
hydrochloric acid, previously warmed in a test-tube, run in gently
through _g_; there is no fear either of the iodid solution running back,
or of any bubbles of air escaping through _y_ if this is done carefully.
This is a precautionary measure, in case a trace of the liberated
chlorin might have lodged in the comparatively cool liquid in the tube
_h_. The carbon dioxid is once more turned on slowly and the liquid in A
is boiled again until it is reduced to about five cubic centimeters. It
is now only necessary to allow the apparatus to cool, passing carbon
dioxid all the time, after which the contents of the condensers are
transferred to a flask and titrated with thiosulfate. At the end of a
properly conducted experiment, the glass in the upper part of tower E
should be quite colorless and there should be only a mere trace of iodin
showing in the lower part of the tower, while the liquid in the last
bulb of the geissler apparatus ought to be pale yellow. During the
operation, the stopper of A and the various joints can be tested from
time to time by means of a piece of iodid of starch paper, and before
disjointing it is well to test the escaping gas (say at _m_) in the same
way, to make sure that all nitric oxid has been thoroughly expelled.
The method is capable of giving accurate results, but it can not be
preferred to the reduction or colorimetric processes.
=494. Method of Gooch and Gruener.=—The principle on which this method
rests depends on the decomposition of a nitrate in presence of a hot
saturated solution of manganous chlorid and hydrochloric acid in an
atmosphere of carbon dioxid.[324] The products of decomposition are
passed into a solution of potassium iodid and the liberated iodin is
titrated with standard sodium thiosulfate. The products of the reaction
are chlorin, nitric oxid, and possibly nitrosyl chlorid, and under
proper precautions the iodin set free is quantitively proportional to
the weight of nitrate decomposed. The manganous mixture is acted on
slowly at ordinary temperatures, but on heating, the nitrate is
decomposed with the formation of a higher manganese chlorid and nitric
oxid. When the heat is continued a sufficient length of time the chlorin
from the higher chlorids is evolved and only manganous chlorid remains.
During the heating the color of the solution passes from green to black
and at the end the green color is restored. The apparatus employed is
shown in Fig. 88.
[Illustration:
FIGURE 88. APPARATUS OF GOOCH AND GRUENER.
]
A plain pipette bent as is shown in the figure serves as the generating
flask and for the attachment on the one hand to the carbon dioxid
apparatus and on the other to the system of absorption bulbs for
containing the potassium iodid. The latter should be glass, sealed to
the evolution bulb of the pipette to prevent the action of the evolved
gases on organic materials. The point of the potassium iodid apparatus
is drawn out so as to be pushed well into the second receiver, being
held in place by a piece of rubber tubing. The third tube acts simply as
a trap to exclude the air from the absorption apparatus. The first
receiver contains in solution three grams, the second two, and the third
one gram of potassium iodid. During the reaction the first receiver is
kept cool by immersion in water. Before connecting the apparatus with
the carbon dioxid generator the solution of manganous chlorid and
afterwards the nitrate solution are drawn into the bulb of the pipette
by gentle suction. After connecting the apparatus the current of carbon
dioxid is started and kept up until all the air is expelled. Heat is
then applied to the bulb of the pipette and the distillation continued
until all the liquid has passed over. At the end of the reaction the
contents of the receivers are united by disconnecting the apparatus from
the carbon dioxid generator and passing water through the pipette. The
introduction of the manganous chlorid into the mixture does not
interfere with the titration of the iodin. This is accomplished in the
usual way with sodium thiosulfate using starch as an indicator. The
quantity of material used should contain about the amount of nitric acid
that is found in two-tenths of a gram of potassium nitrate. This method,
so similar to the preceding, is somewhat less complex, and, to that
extent, preferable to it.
ESTIMATION OF NITRIC ACID BY COLORIMETRIC COMPARISON.
=495. Delicacy of the Method.=—The remarkable delicacy of those methods
of chemical analysis, which depend on the production of a pronounced
color, which can be compared with that produced by a known quantity of a
given substance, has been long illustrated by the nesslerizing process
for the estimation of ammonia. By such methods minute qualities of
substances can be quantitively determined with great accuracy, when they
would escape all effort for their estimation by gravimetric methods.
Processes based on this principle are, therefore, peculiarly applicable
to the detection and estimation of oxidized nitrogen in waters and soil
extracts, whether they be present as nitric, nitrous, or ammoniacal
compounds.
In the following paragraphs will be given with sufficient detail for the
needs of the analyst, the principles and practice of the colorimetric
comparison methods which have been approved as best by the experience of
analysts. These methods are applicable especially to cases in which only
minute quantities of the substances looked for are present, and where
celerity of determination is especially desirable. They are, therefore,
of especial value in the analysis of rain, drainage, and irrigation
waters, and of soil extracts poor in oxidized nitrogen.
=496. Hooker’s Method.=—The quantitive action depends upon the deep
green coloration given by nitric acid, when dissolved in sulfuric acid
and carbazol.[325] Other oxidizing bodies, such as iron, chlorin,
bromin, chromic acid, etc., give the same reaction, but not in such a
prominent manner. Such bodies with the exception of chlorin and iron,
are not often found in waters. In the application of the process, iron,
if present in quantities greater than one-tenth part per one hundred
thousand, must be removed. Chlorids also, even when present in very
small quantities, interfere with the delicacy of the reaction and must
be removed. Easily destructible organic matter tends to lower the
result, but not materially, unless present in large excess. Calcium
carbonate and sulfate, soda, and other alkalies, in the quantities in
which they are usually present in water, do not affect the result. The
following reagents are required:
1. Concentrated sulfuric acid.
2. An acetic acid solution of carbazol; diphenylimid, (C₆H₄\/C₆H₄/
NH.)
3. A sulfuric acid solution of carbazol.
4. Standard solutions of potassium nitrate.
5. A solution of aluminum sulfate.
6. A solution of silver sulfate.
1. The sulfuric acid, used for all purposes in the process, should be
entirely free from nitrogen oxids. It may be readily tested by
dissolving in it a small quantity of carbazol. If the solution be at
first golden-yellow or brown, the acid is sufficiently pure; if it be
green or greenish, another and better sample must be taken. It is
essential also that the specific gravity of the acid be fully 1.84, and
it is well to ascertain that this is really the case.
2. The acetic acid solution of carbazol is prepared by dissolving
six-tenths gram in about ninety cubic centimeters of strongest acetic
acid, by the aid of gentle heat. It is allowed to cool, and is then made
up to 100 cubic centimeters by the further addition of acetic acid. The
exact strength of this solution, is of no material importance to the
success of the process, and the above proportions have been selected
principally because they are convenient. The solution will remain
unchanged for several months. The use of this solution merely
facilitates the preparation of that next described, which will not keep,
and has, consequently, to be freshly prepared for each series of
determinations.
3. The sulfuric acid solution of carbazol is easily made in a few
seconds, but it is advisable to allow it to stand from one and one-half
to two hours before using. It is prepared by rapidly adding fifteen
cubic centimeters of sulfuric acid, to one cubic centimeter of the
above-described acetic acid solution. This quantity usually suffices for
from two to three nitrate estimations. When freshly prepared it is
golden-yellow or brown; it changes gradually, however, and in the course
of one and one-half or two hours it becomes olive-green. This change is
probably due to traces of oxidizing agents, which occur in the sulfuric
and acetic acids, and which, although not present in sufficient quantity
to act immediately, gradually bring about the reaction described. The
greenish color does not interfere with the process, as might at first be
supposed; on the contrary, the solution is not sensitive to small
quantities of nitric acid until it has undergone the change to
olive-green, and it is for this reason, that it should be prepared about
two hours before required for use. This solution may be thoroughly
depended on for six hours after preparation. The intensities of color
produced by the more concentrated solutions of nitrates after this time,
gradually approach each other and become ultimately the same.
4. The standard solutions of potassium nitrate are very readily
prepared. The solutions which are to be compared directly with the
waters examined, may be prepared as required, but if many determinations
are to be made with a variety of waters, it will be found best to
prepare a complete series, differing from each other by 0.02 part
nitrogen in 100,000. This series may include solutions containing
quantities of nitrogen in 100,000 parts, represented by all the odd
numbers from 0.03 up to 0.39. It will be found convenient to prepare
them in quantities of 100 cubic centimeters at a time, from a stock
solution of potassium nitrate which contains 0.00001 gram nitrogen, or
0.000045 nitric acid in one cubic centimeter. Each cubic centimeter of
this solution, when diluted to 100 cubic centimeters, represents 0.01
nitrogen in 100,000, and consequently if it is desired to make a
solution containing 0.35 part nitrogen in 100,000, thirty-five cubic
centimeters are taken and made up to 100 cubic centimeters, and so on.
The solution of potassium nitrate (b) is best prepared from a stronger
one (a) containing 0.0001 gram nitrogen to the cubic centimeter, or
0.7214 gram potassium nitrate to the liter; 100 cubic centimeters of (a)
made up to one liter give the solution (b). It is obvious that the
series of solutions, above described, could be made directly from (a),
but by first making (b), greater accuracy is secured.
5. For purposes which will be presently described, a solution of
aluminum sulfate is required, containing five grams to the liter. The
salt used must be free from chlorin and iron; and the solution should
give no reaction when tested with carbazol.
6. The solution of silver sulfate is required for the removal of chlorin
from the water or soil extract to be examined. It is prepared by
dissolving 4.3943 grams of the salt in pure distilled water and making
up to one liter. The sulfate is preferably obtained by dissolving
metallic silver in pure sulfuric acid. The solution should be tested
with carbazol in the same way as will be presently described for water;
if perfectly pure, no reaction will be obtained. As silver sulfate is
often prepared by precipitation from the nitrate, it is very apt to
contain nitric acid, and consequently, if the source of the salt be
unknown, this test should on no account be omitted.
The analytical process is carried on as follows:
Two cubic centimeters of the water are carefully delivered by means of a
pipette into the bottom of a test-tube; four cubic centimeters of
sulfuric acid are added, and the solution thoroughly mixed by the help
of a glass rod. The test-tube is then immersed in cold water, and when
well cooled, one cubic centimeter of the sulfuric acid solution of
carbazol is added, and the whole again mixed as before. The intensity of
the color is now observed, and a little experience enables a fairly good
opinion to be formed of the quantity of nitric acid present. Suppose
that the water be roughly estimated to contain about 0.15 part nitrogen
per 100,000; in such a case solutions of potassium nitrate containing
0.11, 0.15, 0.19 part nitrogen are selected from the series. Two cubic
centimeters are taken from each, and treated, side by side, with a fresh
quantity of the water, precisely as described for the preliminary
experiment, the various operations being performed as nearly
simultaneously as possible with each of the samples, and under precisely
similar conditions. Two or three minutes after the carbazol has been
added, the intensity of the color of each is observed. If that given by
the water is matched by any of the standard solutions, the estimation is
at an end. Similarly, if it falls between two of these, the mean may be
taken as representing the nitrogen present in cases in which great
accuracy is not required. If this be done, the maximum error will be
0.02 part nitrogen, or 0.09 part nitric acid per 100,000. If greater
exactness be required, or it be found that the color given by the water
is either darker or lighter than that given by all the standard
solutions, a new trial must be made. In such a case the water must be
again tested simultaneously with the solutions with which it is to be
compared. This is rendered necessary principally for the reason that the
shade of the solutions to which the carbazol has been added is apt to
change on standing. Hence it is desirable that the water, and the
standard potassium nitrate with which it is to be compared, should have
the carbazol added at as nearly the same time as possible. When finally
the color falls between that given by any two consecutive members of the
standard potassium nitrate series, the estimation may be considered at
an end, and the mean of these solutions taken as representing the
nitrogen present.
The greatest neatness should be observed in all steps of the analysis.
The quantity of water operated upon is so small that if the greatest
care be not exercised throughout, sources of error may be readily
introduced. The test-tubes should be rinsed out with nitrate-free water
before being used and then dried. The tint should be determined by
looking through the tube and not through the length of the column of
liquid.
_Influence of Nitrites._—If the quantity of nitrous acid in the water is
known a correction can be applied for nitrates by deducting one-fifth of
the number found for nitrites when estimated as nitrates.
_Influence of Iron._—Although ferrous salts give no reaction with
carbazol, nitrates are apt to be overestimated in their presence. Oh the
other hand, ferric compounds, like other oxidizing agents, may give a
characteristic green color with carbazol. In all cases when iron is
present in any considerable quantity it is best to remove it by
rendering the water slightly alkaline, evaporating to dryness, and
redissolving the soluble residue until the solution reaches the original
volume.
_Influence of Chlorids._—The presence of chlorids furnishes by far the
most serious source of error in the process by intensifying the action
of the nitric acid. If, however, nitrates be absent chlorids give no
reaction with carbazol. The chlorids are removed by a standard silver
sulfate solution, the quantity of chlorids present having been first
determined by a standard silver nitrate solution. For this purpose an
ordinary sugar flask can be employed marked at 100 and 110 cubic
centimeters. This flask is filled to the 100 cubic centimeter mark with
the water to be examined; the necessary quantity of silver sulfate is
added and then two cubic centimeters of the solution of aluminum
sulfate, previously described, and the contents of the flask brought up
to 110 cubic centimeters by the addition of pure distilled water. The
whole is shaken up and filtered, the first portion of the filtrate being
rejected. The aluminum sulfate by reacting with the carbonates usually
present in the water and producing the precipitation of alumina,
facilitates the removal of the precipitated silver chlorid.
The above-described method on account of its delicacy is not well suited
to aqueous solutions of soils except where the quantity of nitric
nitrogen present is extremely minute.
Hooker also first suggested the use of diphenylamin for detecting the
presence of nitrates,[326] a method afterwards worked out by
Spiegel.[327]
In the variation of the method as practiced by Rideal the standard
potassium nitrate and the pure sulfuric acid mentioned below are
required, and in addition, the following reagents:[328]
(a) Silver sulfate solution containing 4.3945 grams per liter.
(b) Aluminum sulfate solution free from chlorids and iron, five grams
per liter.
(c) Carbazol solution; six-tenths gram carbazol dissolved in glacial
acetic acid and made up to 100 cubic centimeters with the glacial acid.
For use, one cubic centimeter of this solution is withdrawn by a pipette
and mixed with fifteen cubic centimeters of pure redistilled sulfuric
acid.
The process is carried out as follows: To 100 cubic centimeters of water
the amount of chlorin which has been previously ascertained is removed
by the silver sulfate solution. Two cubic centimeters of the aluminum
sulfate are added and the whole made up to a convenient volume, say
about 110 cubic centimeters. The liquid is filtered and two cubic
centimeters of the filtrate taken for an estimation of nitrates. To the
two cubic centimeters are added four cubic centimeters of concentrated
sulfuric acid and the mixture cooled.
One cubic centimeter of the carbazol solution in sulfuric acid is added
and a bright green color appears in a few moments, if nitrates are
present. Comparison is made with solutions of standard potassium
nitrate.
=497. Phenylsulfuric Acid Method.=—Rideal also proposes a variation of
the method described by Hooker, which consists in the substitution of
phenylsulfuric acid for carbazol.[329]
The solutions required are:
(a) A standard solution of potassium nitrate containing 0.7215 gram of
the pure crystallized salt in a liter of water.
(b) Phenylsulfuric acid, (acid phenyl sulfate,) prepared by dissolving
fifteen grams of pure crystallized phenol in 92.5 cubic centimeters of
pure, redistilled sulfuric acid free from nitrates and diluted with
seven and one-half cubic centimeters of water.
The process is conducted as follows:
A known volume of water, from twenty-five to one hundred cubic
centimeters, according to its richness in nitrates, is evaporated to
dryness in a porcelain dish, one cubic centimeter of phenylsulfuric acid
added then one cubic centimeter of pure water and three drops of strong
sulfuric acid and the mixture gently warmed. A yellow color shows the
presence of nitrates. Dilute to about twenty-five cubic centimeters with
water and add ammonia in slight excess. Pour into a narrow nessler tube
and add the washings and make up to 100 cubic centimeters. Imitate the
color of the solution with the standard potassium nitrate treated with
the same reagents.
The phenylsulfuric acid should be prepared some time before use, as the
fresh solution imparts a greenish tint to the yellow of the ammonium
picrate formed.
=498. Variation of Leffmann and Beam.=—The phenyl sulfate process, as
described by Leffmann and Beam, is conducted as follows:[330]
_Solutions Required._—_Acid phenyl sulfate_: 18.5 cubic centimeters of
strong sulfuric acid are added to one and one-half cubic centimeters of
water and three grams of pure phenol. Preserve in a tightly-stoppered
bottle.
_Standard potassium nitrate_: 0.722 gram of potassium nitrate,
previously heated to a temperature just sufficient to fuse it, is
dissolved in water, and the solution made up to 1000 cubic centimeters.
One cubic centimeter of this solution will contain 0.0001 gram of
nitrogen.
_Analytical Process._—A measured volume of the water is evaporated just
to dryness in a platinum or porcelain basin. One cubic centimeter of the
acid phenyl sulfate is added and thoroughly mixed with the residue by
means of a glass rod. One cubic centimeter of water, and three drops of
strong sulfuric acid are added, and the dish gently warmed. The liquid
is then diluted with about twenty-five cubic centimeters of water,
ammonium hydroxid added in excess, and the solution made up to 100 cubic
centimeters.
The reactions are:
Acid phenyl sulfate. Trinitrophenol (picric acid).
HC₆H₅SO₄ + 3HNO₃ = HC₆H₂(NO₂)₃O + H₂SO₄ + 2H₂O.
Ammonium picrate.
HC₆H₂(NO₂)₃O + NH₄HO = NH₄C₆H₂(NO₂)₃O + H₂O.
The ammonium picrate imparts to the solution a yellow color, the
intensity of which is proportional to the amount present.
Five cubic centimeters of the standard solution of potassium nitrate are
similarly evaporated in a platinum dish, treated as above, and made up
to 100 cubic centimeters. The color produced is compared to that given
by the water, and one or the other of the solutions diluted until the
tints of the two agree. The comparative volumes of the liquids furnish
the necessary data for determining the amount of nitrate present, as the
following example will show:
Five cubic centimeters of standard nitrate are treated as above, and
made up to 100 cubic centimeters, representing 0.0005 gram nitrogen.
Suppose 100 cubic centimeters of water similarly treated are found to
require dilution to 150 cubic centimeters before the tint will match
that of the standard; then
100 : 150 :: 0.005 : 0.0075
_i. e._, the water contains seven and one-half milligrams of nitrogen as
nitrate per liter.
The ammonium picrate solution keeps very well, especially in the dark. A
good plan, therefore, is to make up a standard solution equivalent to,
say, ten milligrams of nitrogen as nitrate per liter, to which the color
obtained from the water may be directly compared.
The results obtained by this method are quite accurate. Care should be
taken that the same quantity of acid phenyl sulfate be used for the
water and for the comparison liquid, otherwise different tints instead
of depths of tints are produced.
With subsoil and other waters probably containing much nitrates, ten
cubic centimeters of the sample will be sufficient for the test, but
with river and spring waters, twenty-five to one hundred cubic
centimeters may be used. When the organic matter is sufficient to color
the residue, it will be well to purify the water by addition of alum and
subsequent filtration, before evaporating. The method may also be used
to determine small quantities of nitrates in aqueous extracts of soils
when the quantity is too small for estimation by the ferrous chlorid or
reduction processes.
=499. Variation of Johnson.=—The ammonium picrate method has given very
satisfactory results as practiced by Johnson, who varies the process as
described below.[331]
The standard solution of potassium nitrate is prepared by dissolving
0.7215 gram of the pure salt in a liter of distilled water. Dilute 100
cubic centimeters of this solution to one liter with distilled water.
Ten cubic centimeters of this dilute solution contain nitrogen
equivalent to one part as nitrates in 100,000.
_The Solution of Acid Phenyl Sulfate._—This is prepared by pouring two
parts by measure of pure crystallized phenol liquefied by hot water into
five parts by measure of pure concentrated sulfuric acid and digesting
the whole in the water-bath for eight hours. After cooling, add one and
one-half volumes of distilled water and one-half volume strong
hydrochloric acid to each volume of the above mixture.
The analytical processes are carried on as follows: Ten cubic
centimeters of the water under examination and ten cubic centimeters of
the standard potassium nitrate are placed in small beakers and put near
the edge of a hot plate. When nearly evaporated they are put on the top
of the water-bath and left there until completely dry. The residue, in
each case, is then treated with one cubic centimeter of the acid phenyl
sulfate and the beakers placed on the top of the water-bath. In good
water, a red color ought not to appear for about ten minutes.
After standing about fifteen minutes, the beakers are removed, the
contents of each washed successively into 100 cubic centimeter flasks,
about twenty cubic centimeters of 0.96 per cent. ammonia added, and the
100 cubic centimeters made up by the addition of water and the yellow
liquid transferred to the nessler glass and the tints appropriately
compared.
=500. Estimation of Nitric in Presence of Nitrous Acid.=—The detection
of nitrous in presence of nitric acid can be accomplished by the method
proposed by Griess, as described further on, through the development of
azocolors, with metaphenylenediamin and other bodies, which are not
produced under similar conditions by nitric acid. The detection and
estimation of nitric in the presence of nitrous acid, however, is not so
easy. Lunge and Lwoff propose brucin for this purpose, which, contrary
to most authorities, does not give the red-yellow color with nitrous
acid.[332] The reagent is prepared by dissolving two-tenths gram of
brucin in 100 cubic centimeters of sulfuric acid, pure and concentrated.
It is almost impossible to prepare a sulfuric acid which does not give a
trace of color with brucin; but with the purest acids this trace may be
neglected.
A solution of nitrate is also prepared containing 0.01 milligram of
nitrogen as nitric acid in one cubic centimeter. It is made by
dissolving 0.0721 gram of pure potassium nitrate in 100 cubic
centimeters of distilled water, and diluting ten cubic centimeters
thereof with pure concentrated sulfuric acid to 100 cubic centimeters.
Both solutions are conveniently preserved in burettes with glass
stop-cocks. The liquid to be tested for nitric acid should be mixed with
sulfuric acid in such a way that the mixture will have a specific
gravity of one and seven-tenths. If the liquid to be tested is water,
this concentration is reached by adding three times its volume of the
strong acid. For the comparison of colors, cylinders of colorless glass
are employed, marked at fifty cubic centimeters. They should be about
twenty-four centimeters high and extend about ten centimeters above the
mark. There is placed in the cylinder one cubic centimeter of the
solution of nitrate in sulfuric acid, and the same quantity of the
brucin mixture, and it is filled to the mark with pure sulfuric acid.
The contents of the cylinder are poured into a flask and warmed at from
70°–80°, until the final yellow tint is secured, and then poured into
the cylinder again. The liquid to be tested is treated in exactly the
same way. The tints are then equalized by pouring out a part of the
contents of the deeper colored cylinder, taking account of the volume,
and filling up with pure concentrated sulfuric acid.
In this manner the content of nitric acid in the liquid under
examination can be compared directly with the solution of potassium
nitrate of known strength. The coloration is distinctly produced with
0.01 milligram in fifty cubic centimeters of liquid, at least
three-fourths of which must be sulfuric acid.
=501. Piccini Process.=—The method proposed by Piccini may also be
used.[333]
About five cubic centimeters of the nitrite solution are placed in a
small beaker, some pure urea dissolved therein and a few drops of
sulfuric acid, and then held in boiling water for three minutes. The
nitrous acid is thus completely destroyed. Ammonium chlorid may be
substituted for urea. The reaction is given on page 478. The nitric acid
present is then determined by diphenylamin or other suitable reagents.
Diphenylamin reacts both with nitrous and nitric acids, producing a
violet tint. Warington calls attention to a slight difference, however,
in its deportment with these two acids. When the solution of the reagent
is not too strong a drop of it produces but little turbidity when added
to water or to a solution containing nitric acid. When, however, nitrous
acid is present, a cream-colored turbidity is produced. The violet color
also appears at once on adding sulfuric acid when a nitrite is present,
while in the case of nitrates, more sulfuric acid is required, except
when the solution is very strong. In this connection, it must not be
forgotten that in heating nitrites with urea or ammonium chlorid in the
presence of a slight excess of sulfuric acid a trace of nitric acid may
be formed.
ESTIMATION OF NITROUS ACID BY COLORIMETRIC COMPARISON.
=502. Application of the Method.=—The most minute traces of nitrous acid
may be detected by colorimetric methods and the determination of the
quantity present may be approximated with great exactness by comparison
with a solution of a nitrite of known strength. Especially in following
the progress of nitrification is this method, in some of its forms, of
essential importance. In delicacy and celerity it has the same
advantages as the colorimetric methods for the determination of nitric
acid.
=503. Metaphenylenediamin Method.=—This process depends upon the
development of a yellow color in water containing nitrous acid on the
addition of a reagent containing metaphenylenediamin; m-C₆H₄(NH₂)₂. This
variety of the phenylenediamins is readily obtained from common
dinitrobenzene. It melts at 63° and boils at 287°. In order to preserve
the reagent in shape for use it should be prepared in the following
manner:
Dissolve two grams of the chlorid in ten cubic centimeters of ammonia,
and place the solution in a glass-stoppered flask. To this solution are
added five grams of powdered animal-black, and the whole vigorously
shaken. After allowing to settle, the shaking is repeated at intervals
of an hour, three or four times, and the flask then allowed to remain at
rest for twenty-four hours.
The supernatant liquid is generally sufficiently decolorized by this
treatment. If not, the shaking and subsidence must be repeated until a
completely colorless liquid is obtained. The solution can be kept,
indefinitely, in contact with the animal-black. Aqueous and alcoholic
solutions of the salt can not be kept.
The test is applied by mixing five drops of the reagent with five cubic
centimeters of sulfuric acid. The mixture must be colorless. To the
mixture add 100 cubic centimeters of the water to be tested, and heat on
the water-bath for five minutes. A yellow coloration indicates the
presence of nitrous acid.
The metaphenylenediamin test is fairly satisfactory in perfectly
colorless waters and aqueous extracts. Many waters and soil extracts,
however, have a yellowish tint, and this interferes in a marked way with
a proper judgment of the yellow triaminazobenzol developed in the
application of the above test.
The decoloration of such waters by means of sodium carbonate or hydroxid
and alum, is a matter of some difficulty and not wholly without action
on the nitrites which may be present. The method, therefore, is inferior
to the one next described.
=504. Sulfanilic Acid and Naphthylamin Test for Nitrous Acid.=—A very
delicate test for the presence of nitrous acid, first described by
Griess, is the coloration produced thereby in an acid solution of
sulfanilic acid and naphthylamin.[334]
Sulfuric or acetic acid may be used as the acidifying agent, preferably
the latter. The solutions are prepared as follows:
(1) Dissolve one-half gram of sulfanilic acid in 150 cubic centimeters
of dilute acetic acid.
(2) Boil one-tenth gram of naphthylamin with twenty cubic centimeters of
water, decant the colorless solution from the residue and acidify it
with 150 cubic centimeters of dilute acetic acid.
The two solutions may at once be mixed and preserved in a well-stoppered
flask. The action of light on the mixture is not hurtful, but air should
be carefully excluded because of the traces of nitrous acid which it may
contain. Whenever the mixed solutions show a red tint it is an
indication that they have absorbed some nitrous acid. The red color may
be discharged and the solution again fitted for use by the introduction
of a little zinc dust, and shaking.
The water, or aqueous solution of a soil, to be tested for nitrites, in
portions of about twenty cubic centimeters, is treated with a few cubic
centimeters of the mixed reagent and warmed to 70°–80°. If nitrous acid,
in the proportion of one part to one million be present, the red color
will appear in a few minutes. If the content of nitrous acid be greater,
_e. g._, one part in one thousand, only a yellow color will be produced,
unless a greater quantity of the reagent be used.
Leffmann and Beam recommend the following method of conducting the
determinations.[335]
Solutions required:
_Naphthylammonium Chlorid._—Saturated solution in water free from
nitrites. It should be colorless; a small quantity of animal charcoal
allowed to remain in the bottle will keep it in this condition.
_Paraamidobenzene Sulfonic Acid (Sulfanilic Acid)._—Saturated solution
in water, free from nitrites.
_Hydrochloric Acid._—Twenty-five cubic centimeters of concentrated pure
hydrochloric acid added to seventy-five cubic centimeters of water, free
from nitrites.
_Standard Sodium Nitrite._—0.275 gram pure silver nitrite is dissolved
in pure water, and a dilute solution of pure sodium chlorid added until
the precipitate ceases to form. It is then diluted with pure water to
250 cubic centimeters and allowed to stand until clear. For use, ten
cubic centimeters of this solution are diluted to 100. It is to be kept
in the dark.
One cubic centimeter of the dilute solution is equivalent to 0.00001
gram of nitrogen.
The silver nitrite is prepared in the following manner: A hot
concentrated solution of silver nitrate is added to a concentrated
solution of the purest sodium or potassium nitrite available, filtered
while hot and allowed to cool. The silver nitrite will separate in fine
needle-like crystals, which are freed from the mother-liquor by
filtration with the aid of a filter pump. The crystals are dissolved in
the smallest possible quantity of hot water, allowed to cool and
crystallize, and again separated by means of the pump. They are then
thoroughly dried in the water-bath, and preserved in a tightly-stoppered
bottle away from the light. Their purity may be tested by heating a
weighed quantity to redness in a tared, porcelain crucible and noting
the weight of the metallic silver. One hundred and fifty-four parts of
silver nitrite leave a residue of 108 parts of silver.
_Analytical Process._—One hundred cubic centimeters of the water are
placed in one of the color-comparison cylinders, the measuring vessel
and cylinder having previously been rinsed with the water to be tested.
By means of a pipette, one cubic centimeter each of the solutions of
sulfanilic acid, dilute hydrochloric acid, and naphthylammonium chlorid
is dropped into the water in the order named. It is convenient to have
three pipettes for this test, and to use them for no other purpose. In
any case the pipette must be rinsed out thoroughly with nitrite-free
water each time before using, as nitrites, in quantity sufficient to
give a distinct reaction, may be taken up from the air.
One cubic centimeter of the standard nitrite solution is placed in
another clean cylinder, made up with nitrite-free water to 100 cubic
centimeters and treated with the reagents, as above.
In the presence of nitrites a pink color is produced, which, in dilute
solutions, may require half an hour for complete development. At the end
of this time the two solutions are compared, the colors equalized by
diluting the darker, and the calculation made as explained under the
estimation of nitrates.
The following are the reactions:
Paraamidobenzene Nitrous acid. Paradiazobenzene sulfonic acid.
sulfonic acid.
C₆H₄NH₂HSO₃ + HNO₂ = C₆H₄N₂SO₃ + 2H₂O.
Naphthylammonium Azoalphaamidonaphthalene parazobenzene
chlorid. sulfonic acid.
C₆H₄N₂SO₃ + C₁₀H₇NH₃Cl = C₁₀H₆(NH₂)NNC₆H₄HSO₃ + HCl.
The last named body gives the color to the liquid.
The method pursued by Tanner, in the preparation of the reagent, is as
follows:
Sulfanilic acid is prepared by mixing thirty grams of anilin slowly,
with sixty grams of fuming sulfuric acid, in a porcelain dish. The
brown, sirupy liquid formed is carefully heated until quite dark in
color, and until the evolution of sulfurous fumes is noticed. After
cooling, the thick, semi-fluid mass is poured into half a liter of cold
water and allowed to stand for some hours. The liquid portion is then
decanted from the nearly black undissolved crystalline mass. To the
residue half a liter of hot water is added and allowed to stand until
cold, and the liquid again decanted. The undissolved portion is then
treated with one liter of hot water and filtered. The filtrate is
treated with animal charcoal to decolorize it, and allowed to stand for
twenty-four hours and again filtered, the filtrate diluted to 1,500
cubic centimeters and used as required. This solution tends to turn pink
on keeping, and thus its color interferes with the delicacy of the test,
and a small amount of animal-char is kept in a small bottle containing
the portion for immediate use, and this bottle is filled, from time to
time, from the larger one.
The solution of naphthylamin hydrochlorate is made with one gram of the
salt dissolved in 100 cubic centimeters of water. The solution is to be
occasionally filtered, and not more than 100 cubic centimeters should be
prepared at a time.
The analytical operations are carried on as follows:
A standard solution of pure potassium nitrite, made from the silver salt
in distilled water perfectly free from nitrites, is placed in a
color-glass, similar to those used in the nessler reaction, together
with a second glass containing the water to be tested. These glasses
should be marked to hold 100 cubic centimeters at the same depth. To
each of the tubes a few drops of pure hydrochloric acid are added and
two cubic centimeters of the sulfanilic solution. Afterwards, to each
tube are added two cubic centimeters of the solution of naphthylamin
hydrochlorate, and it is allowed to stand for twenty minutes, at the end
of which time the color should be fully developed. Each tube is covered
by a piece of glass in order to prevent access of air. It is unnecessary
to add that the standard solutions of nitrite of different strength
should be employed until the one is found which resembles, as nearly as
possible, the color developed in the sample of water under examination.
=505. Lunge and Lwoff’s Process for Nitrous Acid.=—The reaction of
nitrous acid with α naphthylamin, first described by Griess, may be made
reliable, quantitatively, by proceeding as below:[336]
Boil 0.100 gram of pure white α naphthylamin for fifteen minutes with
100 cubic centimeters of water, add five cubic centimeters of glacial
acetic acid, or its equivalent of dilute acid, and afterwards one gram
of sulfanilic acid dissolved in 100 cubic centimeters of hot water. The
mixture is kept in a well-closed flask. A slight red tint in the mixture
is of no significance, inasmuch as this completely disappears when one
part of it is mixed with fifty parts of the liquid to be examined. If
the coloration be very strong it can be removed by adding a little zinc
dust. One cubic centimeter of this reagent will give a distinct
coloration with 0.001 milligram of nitrous nitrogen in 100 cubic
centimeters of water.
The analysis is conducted in cylinders of white glass marked at fifty
cubic centimeters. One cubic centimeter of the above reagent is placed
in each of two cylinders with forty cubic centimeters of water and five
grams of solid sodium acetate. In one of the cylinders is placed one
cubic centimeter of a normal solution of a nitrite prepared by
dissolving 0.0493 gram of pure sodium nitrite corresponding to ten
milligrams of nitrogen in 100 cubic centimeters of water, and adding ten
cubic centimeters of this solution to ninety cubic centimeters of pure
sulfuric acid. This secures a normal solution of nitrosylsulfuric acid,
of which each cubic centimeter corresponds to 0.01 milligram of
nitrogen.
In the other cylinder is placed one cubic centimeter of the solution to
be examined, and the contents of both cylinders are well mixed so that
the nitrous acid in a nascent state may act on the reagent. The colors
are compared after any convenient period, but, as a rule, after five
minutes.
The chief improvement made by Lunge and Lwoff on the method of Griess is
in keeping the reagent in a mixed state ready for use, by means of which
any nitrous impurities in the components thereof are surely indicated.
Its advantage over the method of Ilosvay[337] consists in using the
comparative normal nitrite solution as nitrosylsulfuric acid, in which
state it is much more stable.
=506. Estimation of Nitrous Acid with Starch as Indicator.=—The method
of procedure, depending on the blue color produced in a solution of
starch in presence of a nitrite and zinc iodid when treated with
sulfuric acid, is not of wide application on account of the interference
produced by organic matter. The soil extract or water is treated in a
test-tube, with a few drops of starch solution and some zinc iodid, to
which is added some sulfuric acid. The decomposition of the nitrite is
attended with the setting free of an equivalent amount of iodin which
gives a blue coloration to the starch solution. The depth of the tint is
imitated by treating a standard solution of nitrite in a similar way
until the proper quantity is found, which gives at once the proportion
of nitrite in the sample examined. This process, however, is scarcely
more than a qualitative one.
=507. Estimation of Nitrites by the Method of Chabrier.=—In order to
make the estimation of the evolved nitrous acid more definite by the
iodin method, Chabrier has elaborated a plan for titrating it with a
reducing agent.[338]
The substance chosen for this purpose is sodium hyposulfite. In point of
fact, it is not the nitrous acid which is attacked by the hyposulfite,
but the equivalent amount of free iodin representing it. In the case of
a soil where the quantity of nitrites is usually very small, it is well
to take as much as one kilogram. The extraction should be made rapidly,
with water, free of nitrites, in order to avoid any reducing action on
the nitrates which may be present. In the case of water, from five to
ten liters should be evaporated to a small volume. The concentration
should take place in a large flask, rather than in an open dish, in
order to avoid any possibility of the absorption of nitrites produced by
combustion. When the volume has been reduced to about 100 cubic
centimeters it is transferred to a small flask and the concentration
continued until only ten or fifteen cubic centimeters are left. The
residue is filtered into a woulff bottle, F, Fig. 89, of about 100 cubic
centimeters capacity.
One of the side tubulures carries a burette, B, containing five per cent
sulfuric acid, the other one filled with a hyposulfite solution of known
strength. The middle tubule serves to introduce a glass tube through
which carbon dioxid or illuminating gas passes for the purpose of
driving out the air from the solution and the flask. If carbon dioxid be
used it should be generated by the action of sulfuric acid on marble.
The cork holding this is furnished with a slot or valve to permit the
exit of the air and the excess of the gas.
Before inserting the middle stopper, a few cubic centimeters of
potassium iodid solution and a few drops of thin starch paste are added,
the potassium salt being always used in excess of the nitrite supposed
to be present.
[Illustration:
FIGURE 89. METHOD OF CHABRIER.
]
After the air has all been expelled from the flask the analytical
process is commenced, the carbon dioxid current being slowly continued.
At first, a few drops of the dilute sulfuric acid are allowed to flow
into the flask. As soon as the liquid is colored blue a sufficient
quantity of the thiosulfate solution is added to discharge the color.
The successive addition of acid and thiosulfate is continued until
another portion of the acid fails to develop the blue color, thus
indicating that all the nitrite has been decomposed. From the volume of
thiosulfate used the quantity of nitrite is calculated.
_The Thiosulfate Solution._—The thiosulfate solution is conveniently
prepared, when a large number of analyses is to be made, by dissolving
twenty-five grams of pure crystallized sodium thiosulfate in 100 cubic
centimeters of water and diluting any convenient part thereof to 100 or
1,000 cubic centimeters, according to the supposed strength of nitrite
solution under examination.
For fixing the strength of the solution dissolve 3 348 grams of pure
iodin in a solution of potassium iodid and make the volume up to one
liter. Each cubic centimeter of this solution corresponds to one
milligram of nitrous acid. A given volume of the iodin solution is
titrated against the thiosulfate, but it is best not to add the starch
paste until the greater part of the iodin has been removed. The starch
paste is then added and the titration continued until the blue color has
been discharged. Ten cubic centimeters of the iodin solution is a
convenient quantity for the titration and the thiosulfate should be
diluted by adding to ten cubic centimeters of the solution mentioned
above, 990 cubic centimeters of water. Each liter of this dilute
solution contains two and a half grams of the sodium thiosulphate.
_Example._—Let us suppose that it has required 21.3 cubic centimeters of
thiosulfate to absorb ten cubic centimeters of the iodin solution;
further that ten liters of water have been evaporated and titrated as
described above, and that the volume of thiosulfate employed was 13.8
cubic centimeters. From this is derived the following formula: (13.8 ×
10)/(21.3) = 6.48 milligrams of nitrous acid; or 0.648 milligram per
liter.
=508. Estimation of Nitrous Acid By Coloration of Solution of Ferrous
Salt.=—This method, due to Picini is based on the production of the
well-known brown color formed by the action of nitric oxid on a ferrous
salt.[339] The nitrite is decomposed by heating with acetic acid while
nitrates thus treated do not develop the reaction. The tint produced is
imitated as above by testing against a standard solution of nitrite.
Ferrous chlorid is to be preferred to other ferrous salts for the above
purpose. The process should be carried on in solutions free of air.
=509. Estimation of Nitrous Acid By Decomposition with Potassium
Ferrocyanid.=—The method of Schaeffer was first described in 1851, but
little attention has been paid to it since. The method has lately been
brought into notice again by Deventer.[340]
The reaction depends upon the decomposition of nitrous acid by potassium
ferrocyanid in the presence of acetic acid with the formation of
potassium ferricyanid and acetate, and nitric oxid. The reaction is
expressed by the following equation:
2K₄FeCy₆ + 2HNO₂ + 2C₂H₄O₂
= K₆Fe₂Cy₁₂ + 2KC₂H₃O₂ + 2NO + 2H₂O.
[Illustration:
FIGURE 90.
SCHAEFFER’S NITROUS ACID METHOD.
]
A eudiometer with a glass stop-cock is arranged as shown in Fig. 90. The
lower part of the eudiometer is closed with a rubber stopper carrying a
glass tube which ends in the pan _f_ as shown at _e_. The eudiometer is
filled to the stop-cock with a solution of potassium ferrocyanid of
about fourteen per cent strength. The dish _f_ is also filled up to the
height indicated in the figure with the same solution. The solution of
nitrite is used in such quantities that the nitric oxid evolved will
occupy a space of about twenty cubic centimeters. The whole eudiometer
should contain about fifty-seven cubic centimeters. The nitrite solution
is added to the eudiometer by means of a funnel, _a_. The vessel
containing it is washed out with a little water and then with acetic
acid and finally with a few cubic centimeters of strong potassium
ferrocyanid solution. The last fluid flows through the solution of
nitrite and acetic acid and thus mixes it with the solution already in
the eudiometer. The liquids reacting on each other float together on the
strong ferrocyanid solution and each one of them is at once pressed
downward by the gases which are evolved. When the evolution of gas
becomes slower the apparatus should be shaken for about twenty minutes,
moving it back and forth without taking the bottom of it out of the
dish. When there is no longer any evolution of gas, water is added
through a slowly, until the heavy potassium ferrocyanid solution is
almost completely driven out of the eudiometer. The opening of the tube
at _e_ is then closed with the thumb, the apparatus is taken out of the
dish, shaken for some time in a vertical direction and again placed in
the dish. Water of any required temperature is now allowed to flow
through the jacket, _g_, _h_, until the temperature is constant, when
the volume of nitric oxid is read. The whole experiment can be performed
in less than an hour. Operating in this way, at the end there is in the
eudiometer a liquid which is not very different from water and one whose
coefficient of solubility for nitric oxid is practically the same as
that of water. The gas volume read is to be corrected for temperature,
pressure, tension of the aqueous vapor, height of the water column in
the eudiometer, and, after the end of the calculation, five per cent of
the volume of water remaining in the eudiometer is to be added to the
volume of gas obtained. This is to compensate for the volume of the gas
absorbed by the water. The method gives good quantitive results.
=510. Method of Collecting Samples of Rain Water for
Analysis.=—Warington collects rain water in a large leaden gauge having
an area of 0.001 of an acre.[341] Of the daily collection of rain, dew,
and snow water, an aliquot part amounting to a gallon for each inch of
precipitation is placed in a carboy; at the end of each month the
contents of the carboy are mixed, and a sample taken for analysis. In
the carboy receiving the rain for nitric acid estimation a little
mercuric chlorid is placed each month with the view of preventing any
change of ammonia into nitric acid. It may be doubted, however, if this
precaution is necessary, as the rain water thus collected always
contains a very appreciable amount of lead; and experiments have shown
that on the whole, rain water more frequently gains than loses ammonia
by keeping.
_Preparation of the Sample._—The method first employed by Warington was
to concentrate ten pounds of the rain water in a retort, a little
magnesia being used to decompose any ammonium nitrite or nitrate
present. Concentration by evaporation in the open air, and especially
over gas, results in a distinct addition to nitrites present. When
concentrated to a small bulk, the water is filtered and evaporated to
dryness in a very small beaker. The nitrogen as nitrates and nitrites is
then determined by means of the methods already described.
DETERMINATION OF FREE AND ALBUMINOID AMMONIA IN RAIN AND DRAINAGE WATERS
AND SOIL EXTRACTS.
=511. Nessler Process.=—The quantities of free ammonia in rain and most
drainage waters are minute, but may reach considerable magnitude in some
sewages. By reason of these minute proportions, gravi- and volumetric
methods are not suitable for its quantitive determination. Recourse is
therefore had to the delicate colorimetric reaction first proposed by
Nessler. This reaction is based on the yellowish-brown coloration
produced by ammonia in a solution of mercuric iodid in potassium iodid.
The coloration is due to the formation of oxydimercuric ammonium iodid,
NH₂Hg₂OI, and takes place between the molecule of free ammonia and the
mercuric iodid dissolved in the alkaline potassium iodid as represented
by the following equation:
Hg—O—Hg—I Hg
/ / \
O + 2H₂N = 2O NH₂I + H₂O
\ \ /
Hg—O—Hg—I HG
_Nessler Reagent._—Dissolve thirty-five grams of potassium iodid in 100
cubic centimeters of water. Add to this solution gradually a solution of
seventeen grams of mercuric chlorid in 300 cubic centimeters of water
until a permanent precipitate of mercuric iodid is formed. Add now
enough of a twenty per cent solution of sodium hydroxid to make 1000
cubic centimeters.
The mixed solutions, at room temperature, are treated with additional
mercuric chlorid until the precipitate formed, after thorough stirring,
remains undissolved. This precipitate is then allowed to subside, and
when the supernatant liquid is perfectly clear, it is decanted or
filtered through asbestos and kept in a well-stoppered bottle in a dark
place. The part in use should be transferred to a smaller bottle as
required. The solution should be made for a few days before using, since
its delicacy is increased by keeping. The nessler reagent should show a
faint yellow tint. If colorless it is not delicate, and shows the
addition of an insufficient quantity of mercuric chlorid. When properly
prepared, two cubic centimeters of the reagent poured into fifty cubic
centimeters of water containing 0.05 milligram of ammonia will at once
develop a yellowish-brown tint.
_Preparation of Ammonia-Free Water._—To pure distilled water add pure,
recently-ignited sodium carbonate, from one to two grams per one liter,
and distill. When one-fourth of the whole has passed over, the
distillate may be regarded as free from ammonia; fifty cubic centimeters
of the following distillate should give no reaction with the nessler
reagent. The distillation should be continued until the residual volume
in the retort is about one-fourth of the original, and the distillate
free of ammonia is carefully preserved in close glass-stoppered bottles
previously washed with ammonia-free water. Pure water, free of ammonia
may also be obtained by distilling with sulfuric acid.
_Comparative Solution of Ammonium Chlorid containing 0.00001 gram
Ammonia in one cubic centimeter._—Dissolve 3.15 grams H₄NCl in
ammonia-free water and make the volume up to one liter. Take ten cubic
centimeters of the above solution and dilute to 1000 with water, free
from ammonia.
_Solution containing 0.00001 gram Nitrogen in one cubic
centimeter._—Dissolve 3.82 grams H₄NCl in water, free from ammonia and
dilute with same to 1000 cubic centimeters. Dilute ten cubic centimeters
of the above solution to 1000.
_The Distillation._—Any kind of suitable retort or flask connected with
a good condenser may be used. The capacity of the retort should be from
700 to 1,000 cubic centimeters. The retort and condenser preferred by
Leffmann and Beam are shown in Fig. 91. Any good lamp may be used in
which the flame is under complete control. The gauze burner shown in the
figure is easily controlled and distributes the heat evenly over the
surface of the retort thus diminishing the danger of fracture. The
apparatus having been previously rinsed with distilled water receives
500 cubic centimeters of the liquid to be tested for ammonia, together
with a few pieces of recently ignited pumice stone to prevent bumping
and five cubic centimeters of the twenty percent sodium carbonate
solution to render its contents alkaline. The water is raised to the
boiling-point and with gentle ebullition fifty cubic centimeters of
distillate collected. The distillate is conveniently collected in a
color-comparison cylinder of thin white glass and flat bottom, about two
and a half centimeters in diameter, and marked at fifty and one hundred
cubic centimeters. Two cubic centimeters of the nessler reagent are
added and if ammonia be present a yellowish-brown color will be
developed, the intensity of which is matched by taking portions of the
ammonium chlorid solution, diluting to fifty cubic centimeters with pure
water and treating with the same quantity of the nessler reagent. The
process is repeated until a distillate is obtained which gives no
reaction for ammonia. The sum of the quantities obtained in the several
distillates gives the total amount of ammonia in the 500 cubic
centimeters of the water taken. In most cases practically all the
ammonia is obtained in three or four portions of the distillate.
[Illustration:
FIGURE 91. RETORT FOR DISTILLING AMMONIA.
]
_Albuminoid Ammonia._—The residue from the process just described is
employed for the purpose of determining the albuminoid ammonia. Two
hundred grams of potassium hydroxid and eight grams of potassium
permanganate are dissolved in 1,000 parts of distilled water. Fifty
cubic centimeters of the solution are placed in a porcelain dish with
100 cubic centimeters of distilled water and evaporated to fifty cubic
centimeters. This liquid is placed in the retort and the distillation
resumed and continued until an ammonia-free distillate is obtained. The
total albuminoid ammonia is determined by taking the sum of the
quantities in the several distillates.
=512. Nessler Reagent of Ilosvay.=—To secure greater delicacy in
nesslerizing, Ilosvay uses a reagent prepared as follows:[342]
Dissolve two grams of potassium iodid in five cubic centimeters of
water, heat the solution gently, and add three grams of mercuric iodid.
After the solution is cooled, add an additional portion of three grams
of the mercury salt, and then twenty cubic centimeters of water, and
wait until the precipitation is complete. After filtering, there are
added to the filtrate from twenty to thirty cubic centimeters of a
twenty per cent solution of potassium hydroxid. Only the limpid
supernatant liquid is used in the analytical work. With this reagent,
Ilosvay has been able to detect 0.02 milligram of ammonia in 110 cubic
centimeters of water.
AUTHORITIES CITED IN PART SEVENTH.
Footnote 274:
Comptes rendus, Tome 84, pp. 301, et seq. Journal of the Chemical
Society, (Transactions), 1878, p. 44; 1879, p. 429; 1884, p. 637.
American Chemical Journal, Vol. 4, p. 452. Proceedings of the American
Association for the Advancement of Science, Vol. 41, p. 105. Annales
de l’Institut Pasteur, Tome 4, pp. 218, 257, 760; Tome 5, p. 92.
Footnote 275:
Comptes rendus, Tome 118, p. 604.
Footnote 276:
Bulletin de la Academie royale de Belgique, [3], Tome 25, p. 727.
Journal of the Chemical Society, (Abstracts), June, 1894, p. 248.
Footnote 277:
Chemical News, Oct. 13, 1893, p. 176.
Footnote 278:
Comptes rendus, Tome 109, p. 883.
Footnote 279:
Op. cit. supra, Tome 89, pp. 891, et seq.
Footnote 280:
Journal of the Chemical Society, (Transactions), Vol. 45, pp. 645, et
seq.
Footnote 281:
Jahresbericht der Agricultur Chemie, 1881, S. 43.
Footnote 282:
Annual Report of the British Board of Health, 1883.
Footnote 283:
Annales de l’Institut Pasteur, 1891, S. 93.
Footnote 284:
Philosophical Transactions of the Royal Society of London, Vol. 181,
(1890).
Footnote 285:
Zeitschrift für Biologie, Band 9, S. 172.
Footnote 286:
Archives de Science Biologique à St. Petersbourgh, Tome 1, p. 1331.
Footnote 287:
Annales de l’Institut Pasteur, 1891, pp. 581, et seq.
Footnote 288:
Op. cit. supra, 1891, Plate 18, Fig. 2.
Footnote 289:
Op. cit. supra, 1891, pp. 595, et seq.
Footnote 290:
Op. cit. supra, 1891, Plate 18, Fig. 1.
Footnote 291:
Journal of the Chemical Society, (Transactions), 1891, pp. 498, et
seq.
Footnote 292:
Annales de l’Institut Pasteur, 1891, pp. 605, et seq.
Footnote 293:
Journal of the Chemical Society, (Transactions), 1882, p. 357.
Footnote 294:
Annales de Chimie et de Physique, 1854, Tome 40, p. 479. Zeitschrift
für analytische Chemie, 1870, S. 24; 1877, S. 291. Die
Landwirtschaftlichen Versuchs-Stationen, Band 12, S. 164. Journal of
the Chemical Society, (Transactions), 1880, p. 468; 1882, p. 345;
1889, p. 537.
Footnote 295:
Encyclopedie Chimique, Tome 4, p. 151.
Footnote 296:
Annales de la Science Agronomique, 1891, pp. 263, et seq.
Footnote 297:
Berichte der deutschen chemischen Gesellschaft, Band 23, S. 1361.
Footnote 298:
Zeitschrift für analytische Chemie, Band 9, S. 24, 401. Die
Landwirtschaftlichen Versuchs-Stationen, Band 9, S. 9. Berichte der
deutschen chemischen Gesellschaft, Band 6, S. 1038.
Footnote 299:
Zeitschrift für analytische Chemie, Band 33, S. 200.
Footnote 300:
Apotheker Zeitung, 1891, Band 5, S. 287.
Footnote 301:
Sutton’s Volumetric Analysis, 3d edition, p. 316. Warington, Journal
of the Chemical Society, (Transactions), 1879, p. 376.
Footnote 302:
Report of the National Board of Health, 1882, p. 281.
Footnote 303:
Berichte der deutschen chemischen Gesellschaft, Band 11, S. 432.
Footnote 304:
Bulletin de la Société Chimique, [3], Tomes 11–12, p. 625.
Footnote 305:
Encyclopedie Chimique, Tome 4, p. 154.
Footnote 306:
Zeitschrift für analytische Chemie, Band 7, S. 412. Fresenius,
Quantitative Analysis, Grove’s translation, special part, p. 118.
Footnote 307:
Journal of the Chemical Society, (Transactions), 1879, pp. 578, et
seq.
Footnote 308:
Bulletin 38, Department of Agriculture, Division of Chemistry, p. 204.
Footnote 309:
Die Landwirtschaftlichen Versuchs-Stationen, Band 41, S. 165.
Footnote 310:
Chemiker Zeitung, 1892, Band 16, S. 1952.
Footnote 311:
Zeitschrift für angewandte Chemie, 1893, S. 161.
Footnote 312:
Chemiker Zeitung, 1889, No. 15.
Footnote 313:
Vid. op. cit. 38, 1890, S. 695.
Footnote 314:
Archives de la Société Physique de Genève, Tome 31, p. 352.
Footnote 315:
Chemiker Zeitung, 1890, S. 1410.
Footnote 316:
Chemisches Centralblatt, 1890, Band 2, S. 926.
Footnote 317:
Vid. op. cit. 38, 1891, S. 241.
Footnote 318:
Vid. op. cit. 34, 1889, p. 538.
Footnote 319:
Op. cit. supra, 1881, p. 100.
Footnote 320:
Op. cit. supra, Vol. 57, p. 811.
Footnote 321:
Op. cit. supra, 1891, pp. 530, et seq.
Footnote 322:
Op. cit. supra, 1874, p. 630, and 1885, p. 86.
Footnote 323:
Sutton’s Volumetric Analysis, 4th edition, p. 103.
Footnote 324:
American Journal of Science, Vol. 44, p. 117.
Footnote 325:
American Chemical Journal, Vol. 11, p. 249.
Footnote 326:
Journal of the Franklin Institute, Vol. 127, p. 61.
Footnote 327:
Zeitschrift für Hygiene, Band 2, S. 163.
Footnote 328:
Chemical News, 1889, Nov. 29, 261.
Footnote 329:
Vid. op. cit. supra, p. 51.
Footnote 330:
Examination of Water for Sanitary and Technical Purposes, p. 28.
Footnote 331:
Chemical News, 1890, Jan. 10, p. 15.
Footnote 332:
Zeitschrift für angewandte Chemie, 1894, Heft 12, S. 347.
Footnote 333:
Journal of the Chemical Society, (Abstracts), 1891, p. 489.
Footnote 334:
Zeitschrift für analytische Chemie, Band 18, S. 597. Zeitschrift für
angewandte Chemie, 1889, S. 666. Bulletin de la Société Chimique, [3],
Tome 2, p. 347.
Footnote 335:
Op. cit. 57, p. 30.
Footnote 336:
Zeitschrift für angewandte Chemie, 1894, S. 349.
Footnote 337:
Bulletin de la Société Chimique, [3], Tomes 11–12, p. 218.
Footnote 338:
Encyclopedie Chimique, Tome 4, p. 262.
Footnote 339:
Peligot, Traité de Chimie Analytique appliqueè à Agriculture, p. 261.
Footnote 340:
Berichte der deutschen chemischen Gesellschaft, 1893, S. 589.
Footnote 341:
Journal of the Chemical Society, 1889, p. 537.
Footnote 342:
Op. cit. 64, p. 216.
NOTE.—On page 158, paragraph 172, third line, insert, “and determining
matters dissolved therein,” after “flow.”
PART EIGHTH.
SPECIAL EXAMINATION OF WATERS, VEGETABLE SOILS, AND UNUSUAL SOIL
CONSTITUENTS.
=513. Further Examination of Waters.=—Having described in the preceding
part the approved methods of determining the oxidized nitrogen in waters
and soil extracts there remains to be considered the examination of
waters for other substances of importance to agriculture. Rain waters
add practically nothing to the soil but nitric acid and ammonia, and,
therefore, demand no further discussion here. In drainage and sewage
waters, in addition to the oxidized nitrogen, there may be sufficient
quantities of phosphoric acid and potash to make their further analysis
of interest. But by far the most practical point to be considered is in
the case of waters used for irrigation purposes where the continued
addition to the soil of mineral matters may eventually convert fertile
fields into barren wastes. In irrigated lands there is practically no
drainage and the whole of the water is removed by superficial
evaporation. It is easily seen how these mineral matters tend to
accumulate in that part of the soil in which the rootlets of plants seek
their nourishment.
=514. Estimation of Total Solid Matter.=—The total solid contents of a
sample of water are determined by evaporating a known volume or weight
to dryness and weighing the residue. For comparative purposes a given
volume of water may be taken if the solid contents do not exceed four
grams in a United States gallon. The water should be measured at a
temperature of about 15°.5. Where the content of mineral matter is
greater it is best to weigh the water and calculate the solid contents
to parts per one hundred thousand. For practical purposes in the United
States it is customary to state the content of solid matter in grains
per gallon. Since, however, the gallon has so many different values it
is always necessary to indicate what particular measure is meant.
In ordinary spring and well waters the volume to be used is conveniently
taken at 100 cubic centimeters. To avoid calculation a volume in cubic
centimeters corresponding to some decimal part of a gallon in grains may
be taken and the weight in milligrams will then be equivalent to the
grains per gallon. Thus in the imperial gallon which contains 70,000
grains of distilled water at 15°.5, seventy cubic centimeters may be
taken. If the residue weigh twenty-five milligrams the water contains
twenty-five grains of solid matter per gallon. The United States gallon
at 15°.5 contains 58,304 grains of distilled water. In this case 58.3
cubic centimeters should be used, or double this amount and the weight
in milligrams be divided by two.
The evaporation may be made in a platinum, porcelain, or aluminum dish,
preferably with a flat bottom; The dish does not need to hold the whole
volume at once, but the water may be added from time to time as the
evaporation continues. The dish, however, should, as a rule, hold not
less than 100 cubic centimeters. The evaporation is best conducted over
a steam-bath, and after the complete disappearance of the liquid the
heating should be continued until the residue is perfectly dry.
In the case of mineral waters highly impregnated with inorganic salts, a
smaller volume or weight may be taken, and greater care must be
exercised in drying the residue. For the purpose of qualitively
determining the percentage of special ingredients, quantities of the
water should be taken inversely corresponding to the content of the
ingredient desired. In general, it will not be necessary to evaporate
the sample to complete dryness, but only to concentrate it to a volume
convenient for the application of the analytical process. Where a
complete quantitive analysis of the solid residue is desired, a
sufficient quantity of the water is evaporated to give a weighable
amount of the least abundant ingredient. The total solid content of the
water having been previously determined, the actual weight or volume of
the water taken to obtain the above residue is of no importance.
=515. Estimation of the Chlorin.=—The chlorin in the solid residue from
a sample of water may be determined directly by dissolving the soluble
salts in distilled water, to which enough nitric acid is added to
preserve the solution slightly acid. After filtering and washing, silver
chlorid is added, little by little, with constant shaking until a
further addition of the reagent produces no further precipitate. The
beaker or flask should be placed in a dark place, on a shaking apparatus
which is kept in motion until the precipitate has entirely settled in a
granular state. The silver chlorid is then collected on a gooch, washed
free of all soluble matter, dried at 150° and weighed. If the
precipitate be ignited to incipient fusion, a porcelain gooch should be
used.
A more convenient method is to determine the chlorin directly in the
water, or, where the quantity is too minute, after proper concentration,
volumetrically by means of a titrated solution of silver nitrate, using
potassium chromate as indicator. As soon as the chlorin has all united
with the silver, any additional quantity of the silver nitrate will form
red silver chromate, the permanent appearance of which indicates the end
of the reaction. This process is especially applicable to water, which
in a neutral state contains no other acids capable of precipitating
silver. The chromate indicator does not work well in an acid solution.
=516. Solutions Employed.=—A quantity of pure silver nitrate, about five
grams, is dissolved in pure water and made up to a volume of one liter.
For determining the actual strength of the solution, 0.824 gram of pure
sodium chlorid is dissolved in water and the volume made up to half a
liter. Twenty-five cubic centimeters of this solution are placed in a
porcelain dish, and a few drops of the solution of potassium chromate
added. The silver nitrate solution is allowed to flow into the porcelain
dish from a burette graduated to tenths of a cubic centimeter. The red
color produced as each drop falls, disappears on stirring as long as
there is any undecomposed chlorid. Finally a point is reached when the
red color becomes permanent, a single drop in excess of the silver
nitrate being sufficient to impart a faint red tint to the contents of
the dish.
The solution of potassium chromate is prepared by dissolving five grams
of the salt in 100 cubic centimeters of water. Silver nitrate solution
is added until a permanent red precipitate is produced, which is removed
by filtration, and the filtrate is employed as the indicator as above
described. Water with any considerable quantity of chlorin can be
treated directly with the reagents; when the percentage of chlorin is
low, previous concentration to a convenient volume is advisable.
In waters containing bromids and iodids these halogens would be included
with the chlorin estimated as above. For agricultural purposes such
waters have little importance. In the case of soluble carbonates capable
of precipitating silver this action can be prevented by acidifying the
water with nitric acid and afterwards removing the excess of acid with
precipitated calcium carbonate. In this reaction McElroy recommends the
use of Congo paper, which is not affected by the carbon dioxid but is
turned blue as soon as an excess of nitric acid is added. After the
addition of the calcium carbonate the mixture should be boiled to expel
carbon dioxid.[343]
Irrigation waters from natural sources or derived from sewage rarely
contain enough chlorin to make their use objectionable. On the other
hand, when water is obtained for this purpose from artesian wells it may
often contain a quantity of chlorin which will eventually do more harm
to the arable soil than the water will do good.
=517. Carbon Dioxid.=—Free carbon dioxid in water has no significance in
respect of its use for irrigation purposes. Such waters, however, are
usually of a highly mineral nature and thus are justly open to suspicion
when used for farm animals and on the field. The presence of free carbon
dioxid as has already been pointed out in paragraph =42=, gives to
water, one of its chief sources of power as an agent for dissolving
rocks and ultimately forming soil. The estimation of the total free
carbon dioxid in a sample of water issuing from a spring or well is a
matter of some delicacy by reason of the tendency of this gas to escape
as soon as the water reaches the open air and is relieved from the
natural pressure to which it has been subjected. The actual quantity of
the gas remaining in solution at any given time is determined as
follows: 100 cubic centimeters of the water are placed in a flask with
three cubic centimeters of a saturated solution of calcium and two of
ammonium chlorid. To this mixture is added forty-five cubic centimeters
of a titrated solution of calcium hydroxid. The flask is stoppered, well
shaken, and set aside for twelve hours to allow the complete separation
of the calcium carbonate formed.
When the supernatant liquid is perfectly clear an aliquot part thereof,
from fifty to one hundred cubic centimeters, is removed and titrated
with decinormal acid with phenacetolin or lacmoid as an indicator. From
the quantity of calcium hydroxid remaining unprecipitated the amount
which has been converted into carbonate can be determined by difference.
The difference between the quantity of calcium hydroxid originally
present in the solution and that remaining after the above treatment
multiplied by the factor 0.0022 will give the weight of carbon dioxid
present in the water in a free state or in excess of that present as
normal carbonates.
UNUSUAL CONSTITUENTS OF SOIL.
=518. Boric Acid.=—Boron, while not regarded as an essential plant food,
is yet found quite uniformly in the ashes of a large number of plants.
It may, therefore, be of some interest to the agricultural analyst to
determine the amount of it which may be present in a soil extract or
mineral water. For this purpose the following method due to Gooch may be
employed.[344] To one liter of the water supposed to contain boric acid
add enough sodium carbonate to produce distinct alkalinity. After
evaporation to dryness acidify the residue with hydrochloric acid, apply
a piece of turmeric paper and dry at a moderate heat. The usual
brown-red tint will reveal the presence of boric acid.
The quantitive estimation of the acid is accomplished as follows: One or
more liters of the water rendered alkaline as above are evaporated to
dryness. With the aid of as small a quantity as possible of acetic acid
the dry residue is transferred to a distillation flask and condenser
arranged as shown in Fig. 92. About one gram of recently ignited pure
lime, cooled in a desiccator and weighed accurately, is introduced into
the flask at the bottom of the condenser and slaked by a few cubic
centimeters of water. When the flask is attached, the terminal tube of
the condensing apparatus should dip into the lime-water in the flask.
The heating-bath is partly filled with paraffin at a temperature of
about 120°. The paraffin-bath is raised so that the entire bulb of the
flask is immersed therein and the distillation continued until all the
liquid has been distilled. The bath is removed and after cooling
somewhat, ten cubic centimeters of methyl alcohol are introduced by
means of the stoppered funnel-tube and the process of distillation
repeated. This operation with methyl alcohol is repeated five times. The
boric acid passes off with the distillate and is found in the flask
below the condenser as calcium borate. The contents of the distillation
flask are evaporated to dryness and ignited conveniently in the same
crucible in which the lime was burned. The increase in weight represents
the quantity of boric anhydrid, B₂O₂ obtained.
[Illustration:
FIGURE 92. GOOCH’S APPARATUS FOR BORIC ACID.
]
=519. Method Of Moissan.=—The principle of the method of Gooch, which
has just been described, is applied by Moissan in a slightly modified
manner.[345]
In this method the generating flask is made smaller than in the Gooch
apparatus, and the funnel at the top is oval and provided with a
ground-glass stopper. It is closed at the bottom with a glass stop-cock,
and the slender funnel-tube enters through a rubber stopper and ends
about the middle of the bulb of the flask. The delivery-tube is longer
than in Fig. 91, and is bent upward at its middle part in the form of an
obtuse angle. The receiving flask is connected with the condenser by
means of a tube-shaped funnel, which prevents any regurgitation into the
generating flask. The receiving flask also has attached to it a
three-bulb potash absorption tube, through which all vapors escaping
from the receiving flask must pass. The bulbs contain a five per cent
solution of ammonia. The receiving flask should be placed in a
crystallizing dish and kept surrounded with ice-water.
The boron which is to be estimated should be in the form of boric acid.
This can readily be accomplished by treating the residue to be analyzed
with nitric acid in a sealed tube. The mixture is introduced into the
generating flask, washing with a little nitric acid, and evaporated to
dryness. The heat is removed, and, by means of the funnel, ten cubic
centimeters of methyl alcohol added, and distillation is renewed. This
operation with methyl alcohol is repeated four times, taking care to
distill to dryness in each case before the addition of a fresh quantity
of alcohol. Afterwards, there is introduced into the apparatus one cubic
centimeter each of distilled water and nitric acid and the distillation
again carried to dryness. The treatment with methyl alcohol, as
described above, is then repeated three times. To determine whether all
the boric acid have passed over, the receiving flask at the bottom of
the condenser is disconnected and a drop of the alcohol taken from the
end of the condensing tube by means of a filament of filter paper. On
burning, the flame should not show any trace of green. In case a green
color is observed, the distillation with nitric acid and methyl alcohol
must be repeated.
The ammonia in the potash bulbs serves to arrest any of the vapors
carrying boric acid which might escape from the receiving flask. The
contents of the bulbs are to be mixed with the liquid in the receiving
dish, and the whole poured onto a known weight of recently ignited
calcium oxid contained in a platinum dish, and the mixture briskly
stirred. If the liquid be very acid the platinum dish should be kept in
ice-water to prevent heating. After fifteen minutes the liquid usually
becomes alkaline, and it is then evaporated at a temperature below the
boiling-point of methyl alcohol (66°). The mass, after the methyl
alcohol has disappeared, is dried at a gradually increasing temperature,
and finally, the dish is ignited over a blast, at first covered and
afterwards open. The dish is covered and weighed and again ignited until
constant weight is obtained.
The lime used should be specially prepared by igniting calcium nitrate
incompletely, and reigniting a portion of this to constant weight just
before beginning each analysis. The calcium oxid is then obtained in a
perfectly fresh state. It should be employed in considerable excess, for
each half gram of boric acid at least eight grams of the lime. The
operation is tedious but the results are quite accurate.
SPECIAL TREATMENT OF MUCK SOILS.
=520. General Considerations.=—Deposits of muck which are to be used as
soil for cultural purposes, or marsh lands, containing large quantities
of organic matter, require a special treatment in addition to the
general principles of examination illustrated in the previous pages.
These soils, essentially of an organic origin, do not permit of the same
treatment either chemical or physical as is practiced with soils of a
mineral nature. For instance, it would be useless to attempt a silt
analysis with organic soils, and the extraction of them with
hydrochloric acid for the purpose of determining the materials passing
into solution would prove of little utility. The object of the
examination is not only to obtain knowledge of the ultimate constitution
of the sample, but also, and this is the practical point, to gain some
idea of its stores of plant food and of the proper steps necessary to
secure a supply of the deficient nutrients. The final analytical
processes for the estimation of the constituents of a muck or vegetable
soil are the same as those already given, but the preliminary treatment
is radically different.
=521. Sampling.=—First of all the geologic and meterologic conditions of
the muck formations must be determined as nearly as possible. It is fair
to presume that these formations are of comparatively recent origin, in
fact that they are still in progress. The geologic formation in the
vicinity of the deposit should be noted. Information should be given in
respect of the character of the water, whether running or stagnant,
fresh, salt, or brackish, and changes of level to which it is subject,
should be noted. It should be particularly stated whether the vegetable
growth contributing to the formation be subject to frost or freezing.
The character of the growth is to be carefully noted, and observation
made of any changes in vegetation due to drainage preparatory to
cultivation. It is to the original vegetation that the chief vegetable
accretions in the muck must be accredited. In all cases, for purposes of
comparison, some samples must be taken from parts of the field which
have not been under cultivation or fertilization. The original
properties of the muck can thus be determined and compared with the
portions which have been changed under cultivation. If the vegetation in
different parts of the field vary it is an indication that the muck is
not homogeneous, and in such cases all the different kinds must be
separately sampled. Any alluvial deposit should be carefully separated
from the muck found _in situ_, for the two layers are radically
different in nature.
The sampling should be made by digging a pit, if possible to the bottom
of the muck formation, and taking the samples at depths of one foot from
one or all of the sides. The samples from sections of even depth are to
be mixed and about five kilograms of the well-mixed sample preserved.
Blocks of the unbroken and unshattered material should also be taken
from each section for the purpose of determining permeability to water
and air. All living vegetable matter should be as fully as possible
removed before the sampling begins. The nature of the subsoil must be
observed, and it should be stated whether it be sand, clay, limestone,
etc. Fresh samples should be taken at various depths for the purpose of
determining the content of moisture in the manner described in paragraph
=65=. The tubes used are made sharp at the end to be inserted in the
soil, and so arranged as to cut cylinders of soil a trifle smaller than
their interior diameter. By this means the sample slips easily into its
place. The same care and judgment must be used in taking these samples
as are required in the case of common soils.
_Illustration._—Samples of muck soil taken at Runnymede, Florida.
(_a_) Formation. Littoral fresh-water lacustrine deposits, varying from
a few inches to four feet in depth, and from a few feet to half a mile
in width.
(_b_) Vegetation before drainage. Saw grass (_Cladium Mariscus_ or
effusum).
(_c_) Principal present vegetation (see pages 59–60).
(_d_) _Kinds of Soil._—The muck shows two distinct colors, black and
brown. The vegetation, however, seems to be the same in both cases. The
black muck has the appearance of being more thoroughly decomposed.
(_e_) _Geologic Formation._—This portion of the Florida peninsula is
covered generally with sand due to marine submergence during recent
geologic periods. The forest growth is pine. The drainage from the pine
land is towards the muck deposits. The pine land lies from four to ten
feet higher than the surface of the muck and is much less subject to
frost.
=522. Water Content.=—The capacity of a muck soil for retaining water is
very great. In a moist state these soils are heavy and apparently quite
firm. When dry they are light and fluffy and unsuited to hold the
rootlets of plants. Saturated to their greatest capacity they hold
considerably more than their own weight of water. Attention has already
been called to the danger of drying such samples at a high temperature.
As in most cases of drying exposure at the temperature of boiling water
until a constant weight is obtained is a perfectly safe way. It is hard
to say what comes off in addition to water at a higher temperature. All
that comes off even at the temperature of boiling water is not water.
The method of determination usually employed in this laboratory is the
following:
From four to five grams of the material are spread as evenly as possible
over the flat bottom of a circular aluminum dish, about seven
centimeters in diameter. The dish is exposed for three hours at the
temperature of boiling water and then kept for two hours in an air-bath
at 110°. At the end of this time constant weight is obtained. Additional
drying at 110° for five hours, usually gives no further loss of volatile
matter. The dish should be covered during weighing on account of the
hygroscopicity of the residue. When well sampled the dry matter thus
obtained serves as the basis of calculation for the general analytical
data.
_Results._—Samples of muck soil taken in brass tubes in March during the
dry season had the following contents of moisture:
Matter volatile at 110°, per cent.
Taken near the surface 61.60
„ one foot below the surface 84.35
„ two feet „ „ „ 81.52
It is thus seen that the normal content of moisture in such a soil
during the dry season, exclusive of the top layer, is about eighty per
cent.
=523. Organic Carbon and Hydrogen.=—The organic carbon and hydrogen in
muck soils are determined on the carefully dried sample by combustion
with copper oxid. This process gives not only the quantities of these
bodies combined as humus, but also those in a less advanced state of
decomposition and present as fatty bodies or resins. The method employed
is given on pages 319–20.
_Results._—The data obtained on a sample of muck soil from Florida are
as follow:
Per cent. carbon. Per cent. hydrogen.
One foot from surface 57.67 4.48
Two feet „ „ 47.07 5.15
Three „ „ „ 8.52 0.53
The last sample was largely mixed with sand, the muck at the point when
it was taken not being quite three feet deep.
=524. Total Volatile and Organic Matter and Ash.=—The ignition of the
sample should be very carefully conducted at the lowest possible
temperature. About five grams of the air-dried sample or double that
amount of the moist sample should be taken. In the latter case the
calculations should be made on the basis of the dry material. The
ignition should be continued with frequent stirring with a platinum wire
until all organic matter is destroyed. At the same time in a large dish
one or more kilograms of the sample should be ignited in order to secure
an ash for analysis. The ash should be quickly weighed to avoid
absorption of moisture.
=525. Sulfur.=—The sulfur present in muck is combined either in an
organic form or with iron. It may be estimated by the method of
Fleischer.[346]
From five to ten grams of the sample are ignited carefully in a hard
glass tube in a stream of air or better of oxygen. The sulfur compounds
escape as sulfuric or sulfurous acid.
The combustion is carried on in the apparatus shown in Fig. 93.
[Illustration:
FIGURE 93. APPARATUS FOR DETERMINING SULFUR.
]
The end of the tube A, next to B, is lightly stopped with a plug of
glass wool, the substance introduced and held in place by a second plug
of glass wool next to C. A is connected to the working flask C,
containing water, as is shown in the illustration. The chief object of
the flask is to control the rate of aspiration of the air or oxygen. A
is also connected with the bulb-tube B, as shown in the figure. B
contains potash-lye, free of sulfur. On the top of B is placed a drying
tube filled with glass pearls, moistened with potash-lye. This is
connected with the aspirator by a small bulb-tube bent at right angles,
as indicated. The bulb of this tube contains a little neutral litmus
solution, which must suffer no change of color during the progress of
this analysis. The tube, thus arranged, is placed in a combustion
furnace and gradually heated to redness, beginning with the part next to
B. A moderate stream of air or oxygen is passed through the tube during
the operation. Any product of the combustion collecting in the tube
before reaching B, is driven into B by careful heating. At the end of
the combustion the contents of B are acidified with hydrochloric acid,
and treated with bromin to convert the sulfurous into sulfuric acid. The
excess of bromin is afterwards removed by boiling, and the sulfuric acid
precipitated by barium chlorid and estimated in the usual way.
The total sulfuric acid having thus been determined, the sample is
extracted with water and the sulfuric acid estimated in the residue.
The sulfuric acid in a muck which is injurious to vegetation is
classified by Fleischer, as follows:
(1) Free sulfuric acid. (The residue which is obtained by calculation as
sulfates of the bases in the water extract.)
(2) The sulfuric acid contained as copperas (calculated from the iron
oxid content of the aqueous extract).
(3) Sulfuric acid arising from the oxidation of pyrites (calculated from
the sulfuric acid obtained by treatment of the water-extracted sample).
A better idea of the distribution of the sulfur in the sample can be
obtained by estimating it according to the method given in paragraph
=385=.
=526. Phosphoric Acid.=—The method for determining the phosphorus in
muck is given in paragraph =382=. The process given in =378= may also be
used.
The method of extraction with hydrochloric acid is wholly unreliable as
a means of determining the available phosphoric acid in muck.
There are some vegetable soils which contain so much iron and lime that
the whole of the acid ordinarily used would be consumed thereby. This
fact has been clearly pointed out by Wiklund in determining the
phosphoric acid in a large number of peaty soils.[347] His experiments,
were made with acid of only four per cent strength. In some cases,
however, it may be found useful to determine the quantity of phosphoric
acid which can be extracted with hydrochloric acid, and afterwards to
separate the humus and determine the content of phosphoric acid therein.
=527. Humus.=—In this laboratory the humus is estimated by the method of
Huston and McBride, as given in paragraph =312=. In samples so rich in
organic matter the method of Grandeau does not give as good results.
Often more than half the weight of the dry substance is soluble in
ammonia after treatment with acid. The nitrogen in the original sample
and the separated humus should be estimated by moist combustion with
sulfuric acid in the usual manner.
=528. The Mineral Contents of Humus.=—The material obtained by
precipitating the alkaline extract of a vegetable earth with an acid
does not consist alone of oxygen, carbon, hydrogen, and nitrogen. The
complex molecules which make up this mixture contain certain quantities
of iron, sulfur, and phosphorus in an organic state. These bodies are
left as inorganic compounds on ignition, provided there is enough of
base present to combine with all the acid elements. Much of the sulfur
and phosphorus, however, in these compounds might be lost by simple
ignition. In such cases moist oxidation of these bodies must be
practiced, or the gases of combustion passed over bodies capable of
absorbing the oxidized materials in order to detect and determine them.
The proper methods of accomplishing this have already been pointed out
for vegetable soils, and the same processes are applicable in the case
of extracted and precipitated humus.
Another proof that both phosphorus and sulfur are present in humus in an
organic state is found in the fact pointed out by Eggertz and Nilson,
that the ash of muck soils is always richer in sulfuric and phosphoric
acids than the solution obtained therefrom by hydrochloric acid.[348]
In a sample of muck examined by them there was found in the ash 1.46 per
cent SO₃, and in the acid extract only 0.05 per cent SO₃; and in the ash
0.3 per cent P₂O₅, while in the extract only 0.04 per cent P₂O₅.
=529. Combustion of the Humus.=—The percentage composition of the
extracted humus can be determined, after drying to constant weight, by
combustion with copper oxid. There is little use in trying to assign
definite chemical formulas to any of the components of the complex which
we call humus. Some of the supposed formulas have been given on pages 61
and 62.
=530. Ether Extract.=—Most peaty soils, when very dry, are not easily
moistened with water. This is due to a superficial coating of fatty or
resinous bodies which prevents the water from reaching the muck
particles. In such cases water will pass between the particles and
percolate to a considerable depth, but without wetting. This oily matter
can be removed by treating the dry material with ether in any approved
extraction apparatus. For the separation of the more purely fatty
bodies, light petroleum may be used, while the total of such matters is
extractable with sulfuric ether. The extracted bodies should be examined
to determine their nature, whether fatty, resinous, or of other
materials soluble in ether. The quantity of this material in some muck
soils is remarkably high. In a Florida muck, examined in this
laboratory, 18.95 per cent in the air-dried substance, which contained
still 41.83 per cent of water, or about 32.5 per cent of the water-free
material were found to be soluble in ether.
The color of the ether extract may be almost black, showing the
extraction of a part of the humus or coloring matter in the muck. This
extractive coloring matter may also be a partial oxidation product of
the original chlorophyl of the plant.
=531. Further Examination of the Ether Extract.=—The ether extract
should be first treated with petroleum ether, unless this substance be
used first in extraction. Afterwards, it is to be exhausted with strong
alcohol, and the quantities of material soluble in the three reagents
separately determined.
The nitrogen is further to be determined in the several extracts, and,
for control, in the residue of the muck.
The method of procedure practiced in this laboratory is to first extract
the sample with petroleum ether, which will yield any free fat acids,
fats, or oils, waxes, and possibly some resinous matter. A weighed
portion of the sample, about two grams, is extracted quantitively by one
of the methods which will be described in the second volume of this
manual.
From two to five kilograms of the sample are then extracted in bulk for
the purpose of securing a sufficient quantity of the material to use for
further analysis.[349]
In each case the petroleum is followed by pure ether, and in this way
the chlorophyl, resins, etc., are obtained. This extract is examined
also for its several proximate constituents.[350]
The treatment with ether is followed by extraction with absolute alcohol
for the removal of tannins and other glucosides, resins insoluble in
ether, etc., and the extract subjected to the usual examination.[351]
Instead of absolute alcohol a spirit of ninety-five per cent strength,
or even of eighty per cent, may be used. The final residue should be
subjected to the usual determination for nitrogen, volatile matter, ash,
etc., in the manner already described. The large amount of resinous and
other matters soluble in petroleum and ether, which is found in the
Florida muck soils, is probably due to the proximity of pine forests,
the débris of which, sooner or later, find their way to these littoral
deposits. Considerable portions of organic humic acids and even humus
itself, may also be removed by ether and alcohol and in every case
nitrogen should be determined in these extracts.
RARE CONSTITUENTS OF SOILS.
=532. Estimation of Copper.=—The natural occurrence of copper in many
vegetables has acquired additional significance by reason of its
relation to added copper in canned peas and other preserves. Formerly,
copper was not regarded, in any sense, as a plant food. Even now it can
scarcely be considered as more than an accidental and non-essential
constituent of vegetable matter. It is by no means certain, however,
that copper may not be, in some sense, in organic combination, as
phosphorus and sulfur often are. It is said, also, to be found in
certain animal organisms, notably in the oyster. In the estimation of
copper in soils, there is first made a hydrochloric acid solution of the
sample. The solution is treated with well-washed hydrogen sulfid until
well saturated. The precipitate is collected at once on a gooch, and
washed with water containing the precipitating reagent. The filtrate is
dried, gently ignited or roasted, and dissolved in aqua regia. After
evaporating to dryness on a steam-bath, water and hydrochloric acid are
added, and the copper reprecipitated in the manner described above.
If zinc be present in the sample the solution should be made very
strongly acid with hydrochloric before the treatment with hydrogen
sulfid, otherwise some zinc may be carried down with the copper.[352] If
lead be present it is also precipitated with the copper and can be
separated as described below. In the filtrate from the solution in
nitric acid after the second precipitation the copper is precipitated as
hydroxid by potash, collected in a porcelain gooch, dried, ignited, and
weighed as CuO. Or the copper may be secured as sulfate and estimated
electrolytically in the manner described in volume second for the
gravimetric estimation of sugar.
=533. Estimation of Lead.=—If the soil contain lead this metal will be
thrown down with the copper as sulfid in the manner described above. In
this case the mixed sulfids are dissolved in nitric acid, diluted with
water, filtered, and washed. The filtrate is treated with sulfuric acid
in considerable excess, and evaporated until all the nitric acid has
passed off and the sulfuric acid begins to escape. After cooling, water
is added and the lead sulfate collected on a porcelain gooch and washed
with water containing sulfuric acid. Finally it is washed with alcohol,
dried, ignited, and the lead weighed as PbSO₄.
=534. Estimation of Zinc.=—If zinc be present in the hydrochloric acid
extract of a soil it may be estimated as carbonate after freeing it
carefully of iron. The principal part of the iron should first be
separated in the usual way by sodium acetate. In the warm solution (acid
with acetic) the zinc is precipitated by hydrogen sulfid in excess. The
beaker in which the precipitation takes place should be left covered in
a warm place at least twelve hours. After collecting the zinc sulfid on
a filter it is washed with water saturated with hydrogen sulfid. In
order to free the zinc from every trace of iron it is better to dissolve
the precipitate in hot dilute hydrochloric acid and reprecipitate as
above, and, after boiling with some potassium chlorate, saturate it with
ammonia. Any remaining trace of iron is precipitated as ferric hydroxid
while the zinc remains in solution. The ferric hydroxid is separated by
filtration and the filtrate, after acidifying with acetic, is treated
with hydrogen sulfid as above. The zinc sulfid is dissolved again in hot
hydrochloric acid, oxidized with potassium chlorate, the acid almost
neutralized with soda and the zinc precipitated as carbonate with the
sodium salt. After precipitation, the contents of the beaker are boiled
until all free carbon dioxid is expelled, the carbonate collected on a
filter, washed with hot water, dried, ignited, and weighed as ZnO.
=535. Estimation of Boron.=—Boron has been found in the ashes of many
plants and agricultural products. Whether or not it be an essential or
only accidental constituent of plants has not been determined. Its
occurrence in the soil, nevertheless, is a matter which the agricultural
chemist can not overlook. The boron should be dissolved from the soil by
gently heating with dilute nitric acid followed by washing with hot
water. Boiling should be avoided on account of the volatility of boric
acid. In the solution thus obtained, concentrated on a bath at a
moderate temperature to a convenient volume, the boron is to be
estimated by the method given in paragraphs =518= and =519=.
AUTHORITIES CITED IN PART EIGHTH.
Footnote 343:
Bulletin 13, Chemical Division, Department of Agriculture, p. 1021.
Footnote 344:
Sanitary and Technical Examination of Water, p. 60.
Footnote 345:
Bulletin de la Société Chimique, [3], Tomes 11–12, p. 955.
Footnote 346:
Anleitung zur Wissenschaftlichen Bodenuntersuchung, S. 126.
Footnote 347:
Mitteilungen über die Arbeiten der Moor Versuchs-Station in Bremen;
dritter Bericht, S. 540.
Footnote 348:
Biedermann’s Centralblatt, 1889, S. 664.
Footnote 349:
Dragendorff’s Plant Analysis, translation by Greenish, pp. 8, et seq.
Footnote 350:
Vid. op. cit. supra, pp. 31, et seq.
Footnote 351:
Vid. op. cit. 7, pp. 38, et seq.
Footnote 352:
Journal für praktische Chemie, Band 73, S. 241.
NOTE.—On page 557, paragraph =500=, ninth line, read “red-yellow”
instead of “blue.”
INDEX.
A
Absorption, cause in soils, 119
determination, 287
of heat, by soils, 115
water, by soils, determination, 136–143
Acetic acid, solvent for soils, 344
Acid phenyl sulfate, reagent for nitric acid, 554, 555
soluble materials, extraction, 455
Adobe, analyses, 58
soils, 57
Aeolian rocks, 38
Air, absorption, 286
action, 51
Albuminoid ammonia, estimation, 572
Alkali salts, composition, 56
Alkalies and alkaline earths, estimation, 384
Alkaline soils, 53–55
Alumina, estimation, 354, 357, 362
Aluminum, 17
-mercury couple for nitric acid, 542
microchemical examination, 266
Ammonia, determination of free and albuminoid, 570–573
estimation, 448–452
formation in soils, 429
magnesia distillation process, 450
nitrification, 466
production, by microbes, 464
Ammonium chlorid, solvent for soils, 343
Apocrenic acid, 62
Apparatus for soil sampling, 82–86
Aqueous rocks, 32–38
vapor absorption, 283, 284
Armsby, soil absorption, 121
Assimilable phosphoric acid, method of Dyer, 410
Atomic masses, table, 3
Atwater, fish nitrogen, 14
Authorities cited in Part Eighth, 593
Fifth, 300
First, 63, 64
Fourth, 279–281
Second, 93, 94
Seventh, 573–575
Sixth, 456–458
Third, 169, 170
B
Bacteria, action, 50
Barium, 23
microchemical examination, 265
Barus, theory of flocculation, 177–180
Beaker elutriation, comparison with Hilgard’s method, 239
method, comparison with Schloesing’s, 241
Belgian methods for soil extracts, 361–363
Bennigsen, method of silt analysis, 194
Berlin-Schöne method, 194
Bernard, calcimeter, 339
Berthelot and André, determination of residual water, 308
method of water determination, 305
nature of nitrogen in soils, 430–434
odoriferous matters in soils, 97
phosphoric acid in soils, 411
Bigelow, solubility of digestion vessels, 348
Boric acid, 580, 581
Boron, 17
estimation, 593
Boussingault and Lewey, method of determining absorption, 290
method for nitric acid, 524–526
Braun’s separating liquid, 271
Bréon’s method, 272
Brewer, chemical action, 177
Brögger’s apparatus, 276
Brucin, reagent for nitric acid, 557
Bulk analysis, 365–367
C
Calcium, 18
microchemical examination, 264
Caldwell, preliminary treatment of soil samples, 88
Capillary attraction, determination, 145
movement of water, 153
Carbazol, reagent for nitric acid, 548
Carbon, 5
comparison of methods for estimating, 321
dioxid, diffusion in soils, 297
estimation in water, 579
in soils, apparatus for estimating, 293
occurrence in soils, 289
solvent for soils, 343
estimation of organic, 315
oxidation with chromic acid, 316
permanganate, 318
Carbonates, Belgian method, 342
deficiency in soils, 340
estimation, 337
Carnot, method for manganese, 397
phosphoric acid, 403
Chabrier, method for nitrous acid, 565
Chemical analysis of soils, 301–575
order of examination, 302
preliminary considerations, 301
elements in soils, 2
Chevreul, ammonium phosphate in guanos, 7
Chile, nitrate deposits, 15, 16
Chlorin, 6
estimation, 422
in water, 577
Mohr’s method, 424
Petermann’s method, 424
Wolff’s method, 423
Citric acid, solvent for soils, 344
Clarke, relative abundance of elements, 23
Classification of soils, 52
Clay, chemical nature, 232
colloidal, 231
mechanical determination, 242
properties, 223
separation, 230
suspension, 176
Clayey soils, effect of boiling on texture, 244–246
elutriation, 239
Cleavage of soil particles, 262
Coefficient of evaporation, determination, 144–146
Cohesion and adhesion of soils, 116, 117
Colloidal clay, estimation, 231
Color of rocks, 31
soil, determination, 97
Colorimetric comparison, delicacy, 548, 559
Compact soils, 90
Conductivity of soils, 115
Copper, estimation, 591
-zinc couple for nitric acid, 540, 542
Crenic acid, 61
Crum-Frankland process, 518
Crystal angles, measurement, 259
Culture media, composition, 468, 473, 474, 476, 479, 481, 483, 484, 486
solid, 479, 481
D
Darton, Florida phosphates, 9
Davidson, origin of Florida phosphates, 7
Decay of rocks, 43–52
Deherain, measurement of percolation, 167–169
Desiccator, drying, 309
Devarda’s method for nitric acid, 534
variation of Stoklassa, 535
Diffusion of gases, general conclusions, 299
Dietrich’s elutriator, 209
Digestion of soil, 456
vessels, 347
Dilution method, experiments, 483, 485
Diphenylimid, reagent for nitric acid, 549
Diphenylamin, reagent for nitric acid, 553
Distillation, prevention of bumping, 441
Dobeneck, method of determining absorption, 287–290
Drainage, influence on porosity, 132
Durham, clay suspension, 176
Dyer, citric acid solution, 344
Dyer, method for assimilable phosphoric acid, 410
E
Earth worms, action, 49
Eldridge, Florida phosphates, 10
Elements, different, simultaneous estimation, 425
relative abundance, 23
Elutriating tube, 236
Eruptive rocks, 41, 42
Estimation of gases in soils, 282–299
F
Ferric oxid, estimation, 353, 356, 357, 362, 399, 401
Fine soil, capacity for holding water, 135
Fish as fertilizer, 14
Flocculation, 171
effect of chemical action, 177
theory, 177
Floccules, destruction, 175
Florida phosphates, origin, 7–12
Fluorin, 17–24
Forchhammer, agricultural value of fucoids, 13
Frear, method of determining soil temperatures, 111
Freezing and thawing, 44
Fuchs and De Launy, origin of potash deposits, 21
Fuelling, determination of water absorption, 139
G
Gases, collection, 291
methods of study, 283
passage through the soil, 149, 150
determination, 150
relation to soil composition, 282
Gasparin, method of silt analysis, 195
Gautier, occurrence of oldest phosphates, 7
Gelatin, culture, 471–473
mineral, 473, 474
Gembloux station, method of soil solution, 350
German experiment stations, method of soil solution, 349
Glaciers, action, 45
Glucinic acid, 62
Goessmann, analysis of sea-weeds, 13
Gooch and Gruener, method for nitric acid, 546
estimation of boric acid, 580
Goss, method for phosphoric acid, 416–418
Grandeau, method of estimating humus, 324
H
Hands, sterilization, 489
Hannén, diffusion of carbon dioxid in soils, 297
Harada’s apparatus, 275
Heinrich, determination of passage of gases through soils, 150
water absorption, 143
method of determining cohesion, 116, 117
Henrici, determination of water capacity of soils, 143
Hilgard, alkaline soils, 55
digestion vessels, 347
humus estimation, 324
hygroscopic coefficient, 284
influence of surface tension, 174
method for soil solution, 348
of silt analysis, 225
methods of analysis of soil extract, 356–361
preliminary examination of soils, 87
Hilgard’s elutriator, 226
method, comparison with Osborne’s, 239
Hooker’s method for nitric acid, 548
Humic acid, 61, 62
estimation, 331
Humus, 60
combustion, 589
estimation, 324
German method, 333
method of Pasturel, 336
Raulin, 334
Van Bemmelén, 332
summary of results, 330
mineral contents, 589
Huston and McBride, humus estimation, 326
method of determining soil absorption, 128
Hydrochloric acid, solvent for soils, 344
strength, 345
time of digestion, 345
treatment of soil with cold, 350
Hydrofluoric acid, solvent for soils, 352
Hydrogen, 5
estimation of organic, 323
Hygroscopic coefficient, determination, 284
I
Ignition, loss, 307
Ilosvay, nessler reagent, 573
Indiana, lysimeter of agricultural experiment station, 165
Indigo method for nitric acid, 524–531
Insoluble residue, analysis, 363
Belgian method, 364
Wolff’s method, 363
Interstitial space, determination, 134
Inverse capillarity, 146
Iron, 22
and alumina, separation from phosphoric acid, 414
French method, 399, 400
method of Sachsse and Becker, 401–403
microchemical examination, 266
J
Jenkins, analysis of sea-weeds, 13
Johnson, method for nitric acid, 556
K
Kaolin, estimation, 426–428
Kedzie, digestion vessels, 347
influence of drainage, 132
King, apparatus for soil sampling, 82
capillary movement of water, 153
methods of water movement, 151
Knop, method of determining soil absorption, 128
Knop’s silt cylinder, 189
Knorr, apparatus for carbon dioxid, 337
Kostytchoff, origin of humus, 60
Kühn’s silt cylinder, 189
L
Lateral capillary flow, 153
Latitude, effect on decay of rocks, 46
Lead, estimation, 592
Lime, assimilable, 392
estimation, 354, 357, 362, 365, 384, 386, 388–394
of active, 389
available, 390
French method, 386, 388
method of Halle station, 393, 394
Russian method, 391
Lithium, microchemical examination, 264
Logarithmic constants, 254
Loges, humus estimation, 333
Loose soils, 89
Loughridge, time of digestion, 345, 346
Lunge and Lwoff, method for nitrous acid, 563
Lunge’s nitrometer, 519
improved, 520–524
Lysimetry, 158, 165
Mc
McGowan’s method for nitric acid, 543
M
Magnesia, estimation, 354, 360, 362, 365, 384, 394, 396
method of Halle station, 396
Magnesium, 18
microchemical examination, 264
Magnet, separation of silt particles, 278
Manganese, 23
estimation, 354, 360, 396, 397
French method, 397–399
Marx, method for nitric acid, 526–528
Masure’s silt apparatus, 210
Matiére noire, 324
Mayer, determination of water absorption, 138
Mayer’s modification of Schöne’s method, 220
Mechanical analysis of soils, 171–179
Mercury and sulfuric acid method, Noyes’ modification, 519
Warington’s modification, 518
Metamorphic rocks, 39
Metaphenylenediamin, reagent for nitrous acid, 557, 559
Microchemical examination of silt, 262, 266
Microscopic apparatus, 495
examination of silt separates, 256
Microscopical structure of rocks, 28, 29
Mineralogical examination of silt separates, 254–278
Minerals, classification, 26
in rocks, 24–26
machine for making sections, 267
Möckern, reduction method, for nitric acid, 533
Moissan, estimation of boric acid, 581
Moisture, effect on soil temperature, 102
estimation, 454
Moore, modification of silt analysis, 192
Muck soils, alcohol extract, 590
estimation of humus, 589
phosphoric acid, 588
sulfur, 587
ether extract, 590
organic carbon and hydrogen, 586
petroleum ether extract, 590
phosphoric acid, 415
sampling, 584
special treatment, 583
total volatile and organic matter, 586
water content, 585
Mulder, humic acid, 62
Müller, method of determining soil absorption, 126
Müntz and Marcano, origin of nitrate deposits, 14
N
Naphthylamin, reagent for nitrous acid, 560
Nessler’s process, 570
reagent, 570
Nitrate deposits, 14–16
Nitrates, estimation, 435
Nitric acid, classification of reduction methods, 531, 532
development in soils, 461
estimation by colorimetric comparison, 548–559
oxidation of a colored solution, 524–531
reduction to ammonia, 531–543
classification of methods, 496–498
in presence of nitrous acid, 557
extraction from soil, 498–500
ferrous salt process, 500–518
iodometric estimation, 543–548
mercury and sulfuric acid method, 518
methods, relative merit, 498
nitric oxid process, 500–524
reduction by electric current, 540–543
solvent for soils, 351
ferment, isolation, 471, 477, 481
Nitrification, apparatus and manipulation, 468
effect of potassium salts, 463
general conclusions, 496
necessary conditions, 461–463
preparation of seed, 468
progress, 469
statement of results, 478
test of commencement, 469
Nitrifying organisms, classification, 487
distribution, 470, 471
microscopic examination, 480, 484
occurrence, 467
Nitrites, destruction, 478, 558
Nitrogen, 12
active soil, 434
Arnold and Wedemeyer’s method, 440
Dumas’ volumetric method, 446–448
economic value, 395
estimation in soils, 428–456
of amid, 451
volatile compounds, 452–454
Hilgard’s method, 436
Methods of Official Agricultural Chemists, 434
Müller’s method 438–440
soda-lime method, 445
nature in soils, 430–434
order of oxidation, 465
organic, in soils, 459–461
oxidized, estimation in soils, 459–570
soda-lime method, 441–444
in presence of nitrates, 444
Nitrous acid, development in soils, 461
estimation, by coloration of a ferrous salt, 567
colorimetric comparison, 559
potassium ferrocyanid, 567
classification of methods, 496–498
iodometric method, 564–567
ferment, isolation, 471, 477
Nöbel’s apparatus, 207
Nöllner, nitrate deposits, 16
Norwacki and Borchardt, auger for soil sampling, 83
O
Odoriferous matters, determination, 98
in soils, 97
Official Agricultural Chemists, latest methods, 454–456
method for reduction of nitric acid, 532, 533
of analysis of soil extract, 353–356
soil solution, 349
Organic matter, estimation, 314
influence on absorption, 124
total, 315
Origin of soils, 43
Orth, classification, 185
Osborne, Berlin-Schöne method, 224
method, comparison with Schloesing’s, 241
of silt analysis, 196
-Schöne method, 219
comparisons, 223
Oxygen, 3
absorption, 286
estimation of organic, 324
P
Packard, separation of silt particles, 273
Pasturel, estimation of humus, 336
Peligot, preliminary treatment of samples, 91
Percolation, measurement, 161
soils _in situ_, 164
Petermann, determination of water absorption, 138
method for phosphoric acid, 409
preliminary treatment of samples, 92
Petrographic examination of silt particles, 266
microscope, 256
Pfaundler, specific heat method, 104–110
Phenylsulfuric acid, reagent for nitric acid, 554
Phosphoric acid, Carnot’s method, 403
estimation, 354, 362, 403–406, 409–416
French method, 406–409
Halle method, 404, 405
in muck soils, 415
method of Goss, 416–418
Hilgard, 413
Petermann’s method, 409
Russian method, 412
separation from iron and alumina, 414
Phosphorus, 6
microchemical examination, 266
state of existence in soils, 411
Piccini method for nitric acid, 558
nitrous acid, 567
Pillitz and Zalomanoff, method of determining soil absorption, 125
Pissis, nitrate deposits, 16
Polarized light, examination, 261
Porosity, 131
determination, 133
Potash, condition in soils, 367
estimation, 355, 358, 361, 365, 368, 370–372, 375–378, 381, 382
international method, 381
Italian method, 377
method of German experiment stations, 375
Tatlock and Dyer, 382
Russian method, 376
salts, deposits, 20
Smith’s method, 378–381
soluble in cold dilute acid, 370
concentrated acids, 368
Potassium, 18
microchemical examination, 263
Pratt, bowlder phosphates, 9
Q
Qualities of soils, 52, 53
R
Rain water, method of collecting, 569
preparation for analysis, 569
Raulin, method of estimating humus, 334
potash, 375
Reaction of a soil, 303
Refractive index, determination, 259
Rideal, method for nitric acid, 553
Rocks, aeolian, 38
aqueous, 32
chemical composition, 30
color, 31
composition, 43
decay, 43–52
eruptive, 41–42
kinds, 32
metamorphic, 39
microscopical structure, 28
minerals, 24–26
sedimentary, 35
types, 28
Rohrbach’s solution, 271
Rowland, fall of particles in liquid, 180
S
Sachsse and Becker, method for iron, 401–403
Salts, preparation for absorption, 130
Samples for moisture, 74
permeability, 74
staple crops, 75
preparation, 454
for elutriation, 229
Sampling, general directions, 66
method of Caldwell, 71
German experiment stations, 69
Grandeau, 77
Hilgard, 67
Lawes, 80
Official Agricultural Chemists, 79
French Commission, 69, 80
Peligot, 72
Richards, 69
Royal Agricultural Society, 76
Wahnschaffe, 72, 81
Whitney, 68, 73
Wolff, 81
Sandstone, 35
Schaeffer and Deventer, method for nitrous acid, 567
Schloesing method, comparison with Beaker, 241
DeKonick’s modification, 514–516
for nitric acid, 500
French modification, 500–505
of collecting soil gases, 291
silt analysis, 200
Schmidt’s process, 516
Schulze-Tiemann modification, 510–514
Spiegel’s modification, 509
Warington’s modification, 505–508
Schmidt’s method for nitric acid, 539
Schöne’s elutriator, 212
Schulze-Tiemann method, 510–514
Sea-weeds, analysis, 13
Sedimentary rocks, classification, 35
Sediments, separation of fine, 233
weighing, 235
Seeding, method employed, 475
Selective absorption of soils, 122
Separation of silt particles, 272
Shaler, phosphatic limestones, 12
Sieve analysis, classification, 185
German experiment stations, 183
separation, 182
Sievert’s method for nitric acid, 536
Sifting with water, 183
Silica, 4
direct estimation, 424
estimation, 356, 361, 365
Silt analyses, value, 279
analysis, Belgian method, 204
classification of results, 235, 236
interpretation, 251
Italian method, 195, 206
method of Hilgard, 225
Osborne, 196
Schöne, 212–220
methods, 185
Moore’s modification, 192
Schloesing’s method, 200
statement of results, 194
subsidence of soil particles, 186–188
Wolff’s method, 192
classes, illustrations, 258
particles, color and transparency, 278
examination, with polarized light, 261
forms and dimensions, 257
microchemical examination, 262–266
petrographic examination, 266
separation by specific gravity, 268–277
staining, 261
percentage in soils, 249
separates, microscopic examination, 256
Mineralogical examination, 254–278
Siphon silt cylinder, 190
Soda, estimation, 355, 358, 361, 365, 374, 384
Sodium, 22
amalgam process, 537–539
microchemical examination, 263
Soil absorption, importance, 124
method of determining, 125
analyses, special methods, 367–428
definition, 1
extracts, method of preparing, 158
gases, 149, 150
heat, sources, 102
ingredients, distribution, 247
moisture, 132
particles, number, 251–253
standard sizes, 181
surface area, 253
samples, air drying, 88
preliminary examination, 87–93
treatment in laboratory, 87
sampling, general principles, 65
solutions, analyses, 352–367
temperatures, method of determining, 111
thermometry, 111–116
Soils, absorptive power, 117–131
and subsoils, 62, 63
as a mass, 95
carbon dioxid, 293
chemical elements, 2
classification; according to deposition, 52
cohesion and adhesion, 116, 117
color, 95
compact, 90
composition, relation to gases, 282
conductivity for heat, 115
deficient in carbonates, 340
digestion with solvents, 343–352
estimation of carbonates, 337
loose, 89
mechanical analysis, 171–179
method of estimating absorption of heat, 115
nitrifying power, 467
origin, 1, 43
preliminary treatment, 87–93
qualities, 52–53
reaction, 303
selective absorption, 122
specific gravity, 98
determination, 99
unusual constituents, 591–593
volume, 100
weight of one hectare, 102
Solar heat, absorption, 103
Specific gravity, 30
of soils, 98–110
apparent, 101
determination of apparent, 101
heat of soils, 102
determination, 104–110
variations, 110
Spencer, photomicrographs, 259
Squanto, manurial value of fish, 14
Staining organisms, method, 487
silt particles, 261
Stassfurt potash salts, 20
Sterilization, 489
by heat, 490
high pressure steam, 492
Sterilized soil, nitrification, 487, 488
Sterilizing apparatus, 490, 493
oven, 491
Stockbridge, composition of humus, 61
soil moisture, 132
Strontium, microchemical examination, 265
Stutzer’s method for nitric acid, 537
Subcultures, method, 479
Subsidence, physical explanation, 180
Sulfanilic acid, preparation, 562
reagent for nitrous acid, 560
Sulfur, 51
state of existence in soils, 419
Sulfuric acid, estimation, 355, 358, 361
French method, 419
Italian method, 422
method of Berthelot and André, 419
Von Bemmelén, 420–422
Wolff, 422
Surface area, influence on absorption, 123
particles, effect of potential, 172
tension, influence, 174
method of estimating, 157
of fertilizers, 156
T
Thenard, humic acid, 62
Thermometry, general principles of soil, 111
Thermostats, 494
Thoulet’s solution, 268
U
Ulmic acid, 61
Ulsch’s method for nitric acid, 539
V
Von Bemmelén, determination of water, 310
method of humus estimation, 332
Vegetable life, action, 48
soils, 58–60
Volatile matter, estimation, 455
W
Wahnschaffe, preliminary treatment of samples, 91
Warington, absorption of potash and ammonia, 120
and Peake, oxidation of carbon, 316
experiments in nitrification, 468–470, 485–488
indigo method, 528–531
Schloesing method, 505–508
Water absorption by soils, determination, 136–143
capacity of soils, effect of pressure, 143
determination at 110°, 306
general conclusions, 313
estimation after air drying, 305
in water-free atmosphere, 149
in fresh samples, 304
soils, determination, 303–314
movement, causes, 155
in soils, 151–169
methods, 151
relative flow, 159
residual amount, dried at 110°, 308
solvent, action, 47
for soils, 343
special examination, 576–583
total solid matter, 576
Way, absorptive power of soils, 118, 120
Weight of soil, 102
Welitschowsky, measurement of percolation, 161–163
Wheeler and Hartwell, analysis of sea-weeds, 13
Whitney and Marvin, method of determining soil temperatures, 112–115
causes of water movement, 155
determination of interstitial space, 134
effect of potential, 172
influence of surface area on absorption, 123
measurement of percolation, 163
relative flow of water, 159
surface tension of fertilizers, 156
theory of subsidence, 180
Williams, machine for mineral sections, 267
-Warington method for nitric acid, 540, 541
Winogradsky, experiments in nitrification, 471–483
Wolff and Wahnschaffe, method of determination of water absorption, 136
determination of coefficient of evaporation, 148
method for silt analysis, 192
preliminary treatment of soil samples, 88
Wollny, determination of water absorption, 142
occurrence of carbon dioxid, 282
Wülfing’s apparatus, 277
Wyatt, phosphate deposits, 8
X
Xylic acid, 62
Z
Zinc, estimation, 592
------------------------------------------------------------------------
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