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.