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.