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.