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.