additional quantities of water are added from time to time until the
soil is saturated. In order to be able to observe when this takes place
there is a slit at the lower end of the zinc tube which is closed with a
piece of glass. This slit should be about two centimeters broad and ten
centimeters long. The lower end of the zinc tube is set on a glass plate
to prevent evaporation.
As soon as the water shows itself at the lower end of the zinc tube, the
excess of water in the upper glass tube is at once removed by a pipette
and a stopper inserted through which a glass tube passes drawn out into
a fine point above. The object of this is to avoid evaporation on the
upper surface. The apparatus is then left at rest for thirty-six hours.
At the end of this time the clamps are removed and the column of moist
earth cut with a piece of platinum foil, and the two ends of the glass
tube, next to the zinc tube, covered with glass plates. It is then
weighed and the weight of moist earth determined by deducting the weight
of the tube and its glass covers. The moist earth is carefully removed
to a large porcelain dish and dried at 100°. Before weighing it is
allowed to stand twenty-four hours in the air. The data obtained are
used to calculate the water content to volume per cent.
The volume of the glass tubes should be determined by careful
calibration.
=151. Method of Heinrich.=[106]—The soil to the depth turned by the plow
is dug out and in the hole a lead vessel without bottom, twenty
centimeters in diameter and forty centimeters high, is placed. The soil
is then thrown back around and outside the lead vessel until the latter
appears buried in the fragments.
The rest of the soil is passed into the lead vessel, through a sieve
having four meshes to the centimeter, using for this purpose enough
water to thoroughly moisten it. Care should be taken not to use enough
water to cause any separation of the fine from the coarse particles.
By this process all coarse stones, sticks, etc., are separated. In sandy
soils the flask is left for a few hours while in clay soils a much
longer time is necessary. When the excess of water has disappeared the
lead cylinder is removed, and a piece cut out of the center of it placed
in a weighed drying flask and dried at 100°.
=152. Effect of Pressure on Water Capacity.=[107]—The increasing
capacity of soil to hold water developed by shaking or pressure, is
determined by Henrici in the following way:
Into a glass cylinder of twenty millimeters internal diameter are poured
twenty cubic centimeters of water. A given quantity of soil is next
added, and after standing until thoroughly saturated, the residual water
is measured by pouring off, or better, by graduations on the side of the
tube. The increase in the volume of the clear water is also measured,
after shaking, in the same way. The data of a determination made as
above described follow:
Water in cylinder 30 cubic centimeters.
Water and saturated soil 40 „ „
Volume of unsaturated soil = e = 10 „ „
Volume of saturated soil = e + w = 20.5 „ „
Water contained therein = w = 10.5 „ „
By repeated shaking the volume of e + w, the content of w therein, and
the relative values of e/w were found to be as follows:
Cubic Cubic Cubic Cubic
centimeters. centimeters. centimeters. centimeters.
e + w 20.5 16.0 15.7 15.0
w 10.5 6.0 5.7 5.0
─────────────────────────────────────────────────────────
w/e 1.05 0.60 0.57 0.50
If e′ represent the volume of the saturated soil then e′ = e + w, and
this gives the relation to the volume of dry earth represented by the
equation e′/e = 1 + w/e. This indicates that the relative volume of the
saturated soil is equal to unity increased by the relative content of
water.
=153. Coefficient of Evaporation.=—At an ordinary room temperature in
the shade, samples of soil, if they are subjected to experiment in
tolerably thin layers have nearly an equal coefficient of evaporation.
That is, the absolute quantity of water evaporated in a given time is
almost entirely conditioned upon the magnitude of the surface exposed
and the temperature of the surrounding air. Only when exposed in
conditions as nearly as possible natural in thin layers to the action of
the sunlight and shade do the soils show their peculiarities in respect
of the evaporation of moisture. In order to see these peculiarities,
samples of soil which have been previously examined must be subjected to
examination at the same time with the soil whose properties are to be
determined.
The zinc box, before described, should be protected with a well fitting
cover of thick paper, and the different samples of soil which are to be
tested placed therein. This should now be placed in a wooden box, the
top of which is exactly even with the top of the zinc vessel. This box
containing the vessel should be exposed to the sunlight. After
twenty-four hours the zinc boxes can be taken away from position and
their loss in moisture determined, and these weighings, according to the
condition of the atmosphere, can be continued from fourteen days to
three weeks, the temperature of the air of course being carefully
determined at each time. At first, all the different soils being
saturated with moisture, it will be observed that the loss of moisture
is proportionately the same for all. Soon, however, the rapidity of the
evaporation in the samples of soil rich in humus and clay will be
decreased as compared with the sandy soils, and in general, those which
possess a high capillary power capable of bringing the moisture rapidly
from the deeper layers to the surface. There soon comes a point when the
difference in evaporation is at its greatest; and then there will be a
gradual diminution until the samples lose no further moisture. This
point, for the different soils, can be determined by frequent weighings
of the vessel.
=154. Determination of Capillary Attraction.=—Long glass tubes graduated
in centimeters may be used for this determination, or plain tubes so
arranged as to admit of easy measurements with a rule. The tubes may be
from one to two centimeters internal diameter and about one meter long.
The fine earth should be evenly filled in little by little, with gentle
jolting. The lower end of each tube, before filling, is closed with a
piece of linen.
The tubes, after filling, are supported in an upright position by a
frame AE, Fig. 19, in a vessel B containing water in which the linen
covered ends D dip to the depth of two centimeters. The height of the
water in the several tubes should be read or measured at stated
intervals. The water contained in the supply vessel should be kept at a
constant height by a Mariotte bottle.
[Illustration:
FIGURE 19.
APPARATUS TO SHOW CAPILLARY ATTRACTION OF SOILS FOR WATER.
]
The observations may be discontinued after one hundred and twenty hours,
but even then the water will not have reached its maximum height.
It is recommended by some experimenters to cut the tubes, after the
above determination is completed, into pieces ten centimeters in length,
and to determine the per cent of water in each portion.
=155. Statement of Results.=—The following table illustrates a
convenient method of tabulating the observed data as given by
König.[108]
Number of sample 1 2 3 4 5 6
───────────────────────────────────────────────────────────────────────
Height of 24 hours. 27.3 38.0 16.7 36.4 8.0 28.8 centimeters.
moisture column
after:
„ 48 „ 35.9 50.8 24.5 49.2 11.9 40.5 „
„ 72 „ 41.5 59.5 30.0 57.9 15.2 49.1 „
„ 96 „ 44.4 66.2 33.5 63.8 17.5 55.2 „
„ 120 „ 46.7 70.0 36.3 68.5 19.2 60.5 „
=156. Inverse Capillarity.=—In tubes filled with fine earth, as
described in paragraph =154=, water is quickly poured, the same quantity
into each tube of the same diameter, or such quantities in tubes of
different diameters as would form a water column of the same depth over
the surface of the sample. The rate at which the water column descends
in each tube, the time of the disappearance of the water at the surface
and the final depth to which it reaches, are the data to be entered.
=157. Statement of Results.=—The points to be observed in the
determination of inverse capillarity are the number of hours required
for the total absorption of a column of water of a given height, the
depth of the moisture column at that moment, and the total depth to
which the moisture column finally reaches. The data of observations with
six samples with a water column four centimeters high are given by
König[109] as follows:
Number of sample 1 2 3 4 5 6
─────────────────────────────────────────────────────────────────────────
Number of hours required for
water to disappear 4.3 1.8 10.3 3.0 21.0 4.3
Depth of moisture at time of
disappearance of water 11.0 12.0 11.4 13.3 11.7 12.0 centimeters.
Total depth of moisture 13.0 18.1 13.0 19.0 12.0 16.5 „
=158. Determination of the Coefficient of Evaporation.=—The coefficient
of evaporation is the number of milligrams of water evaporated from a
square centimeter of soil surface in a given unit of time. It is evident
that this number will vary with the physical state of the soil, the
velocity of the wind, the saturation of the air with aqueous vapor and
the temperature. In all statements of analyses these factors should
appear.
The process may be carried on first (a) with soil samples kept
continually saturated with water and (b) with samples in which the water
is allowed to gradually dry out.
_Method a._—The determination may be made in the shade or sunlight.
_In the Shade._—A zinc cylinder (Z Fig. 20), fifteen centimeters in
diameter and 7.5 centimeters high, with a rim one centimeter wide and
one centimeter from top, is covered at one end with linen or cotton
cloth and filled with fine earth, with gentle jolting, until even with
the top. It is then placed in a zinc holder H, into the circular opening
of which it snugly fits as in A. This holder is twenty centimeters in
diameter and 7.5 centimeters deep. It has an opening at O through which
water can be added until it is filled so as to wet the bottom of Z when
in place. As the water is absorbed by the soil more is added and, the
top being covered, the apparatus is allowed to stand for twenty-four
hours. At the end of this time the soil in the zinc cylinder is
saturated with water to the fullest capillary extent.
The whole apparatus, after putting a stopper in O, is now weighed on a
large analytical balance and placed in an open room, with free-air
circulation, for twenty-four hours. At the end of this time it is again
weighed and the loss of weight calculated to milligrams per square
centimeter. Where large and delicate balances can not be had, the
apparatus can be constructed on a smaller scale suitable for use with a
balance of the ordinary size.
_In the Sunlight._—The apparatus described above is enclosed in a wooden
box having a circular opening the size of the soil-zinc cylinder. In the
determination of the rate of evaporation, the apparatus, charged and
weighed as above described, is exposed to the sun for a given period of
time, say one hour. On the second weighing the loss represents the water
evaporated. The time of year, time of day, velocity of wind and
temperature, and degree of saturation of the air with aqueous vapor,
should be noted. The data obtained can then be calculated to milligrams
of water per square centimeter of surface for the unit of time.
_Method b._—As in method a the determination may be made in the shade or
in the sunlight. The rate of evaporation is, in this method, a
diminishing one and depends largely on the reserve store of water in the
sample at any given moment.
The same piece of apparatus may be used as in the determinations just
described. After charging the sample with moisture all excess of water
in the outer zinc vessel is removed and the rate of evaporation
determined by exposure in an open room or in the sunlight, as is done in
the operations already described.
_Alternate Method._—The zinc cylinders used in determining saturation
coefficient, paragraph =145=, may also be employed in determining the
rate of evaporation. Each cylinder should be wrapped with heavy paper or
placed in a thick cardboard receptacle, and all placed in a wooden box,
the cover of which is provided with circular perforations, just
admitting the tops of the cylinders, which should be flush with the
upper surface of the cover.
Arranged in this way the cylinders previously weighed are exposed in the
shade or to direct sunlight and reweighed after a stated interval. On
account of the small surface here exposed in comparison with the total
quantity of soil and moisture it is recommended to weigh the cylinders
once only in twenty-four hours. The weighings may be continued for a
fortnight or even a month.
In soils fully saturated with water the rate of evaporation is at first
nearly the same on account of the surface being practically that of
water alone. As the evaporation continues, however, the rate changes
markedly with the character of the soil.
=159. Rapid Method of Wolff.=—In order to expose a larger surface to
evaporation and to secure the results in a shorter period of time,
Wolff[110] fills square boxes, having wire-gauze bottoms, with fine
earth, and after saturating with moisture weighs and suspends them in
the open air. The wire-gauze bottoms are previously covered with filter
paper to prevent loss of soil.
[Illustration:
FIGURE 20.
APPARATUS FOR DETERMINING COEFFICIENT OF EVAPORATION.
]
=160. Estimation of Water Given up in a Water-Free Atmosphere.=—The
air-dried sample, in quantities of from five to ten grams in a thin
layer on glass, is placed over a vessel containing strong sulfuric acid.
It is then placed on a ground glass plate and covered with a bell jar.
The sample is weighed at intervals of five days until the weight is
practically constant.
This method is valuable in giving the actual hygroscopic power of a soil
depending on its structure alone.
=161. Estimation of the Porosity of the Soil for the Passage of
Gases.=—Some further notion of the physical state of the soil known as
porosity, may also be derived by a study of the rate at which it will
admit of the transmission of gases. A method for estimating this has
been devised by Ammon.[111]
Air is compressed in two gas holders by means of a column of water of
proper height to give the pressure required.
The tubes through which the air passes out of the gas holders are each
furnished with a stop-cock and united with a glass tube having a side
tube set in at right angles for carrying off the air.
The use of two holders makes it possible to carry on the experiment as
long as may be desired, one holder being filled with air while the other
is emptying.
The common conducting tube is joined with a meter which is capable of
measuring, to 0.01, the volume of air passing through it. The pressure
is regulated by means of the stop-cocks. The air passing from the meter
is received in a drying tube filled with calcium chlorid.
From the drying tube the air enters a drying flask filled below with
concentrated sulfuric acid and above with pumice stone saturated
therewith. Next the dried air passes through a worm, eight meters long,
surrounded with water at a given temperature. The dried air of known
temperature next enters the experimental tube. This tube is made of
sheet zinc 125 centimeters in length and five centimeters in internal
diameter. It is placed in an upright position, and about six centimeters
from its upper end carries a small tube at right angles to the main one
for connection with a water-filled manometer.
The upper and lower ends of the tube are closed with perforated rubber
stoppers carrying tubes for the entrance and exit of the air.
In the inside of the zinc tube are found two close-fitting but movable
disks, of the finest brass wire gauze, between which the material to be
experimented upon is held.
The layer of fine soil is held between these disks and may be of such a
depth as is required for the proper progress of the experiment. With
soils of firm texture opposing a great resistance to the passage of the
air the column of earth tested should be shorter than with light and
very permeable soils. The experimental tube is surrounded with a water
jacket, which may also be made of sheet zinc, carrying small tubes
directed upwards for holding thermometers. The water jacket should be
kept at the same temperature as the air which is used in the experiment.
The process of filling the tube, the amount of pressure to be used and
the air and soil temperature, will naturally vary in different
determinations.
The volume of air at a given pressure and temperature which passes a
column of soil of a given length in a unit of time will give the
coefficient of permeability.
=162. Determination of Permeability in the Open Field.=—A method for
determining the rate of transmission of a gas through the soil in the
field has been devised by Heinrich.[112]
A box C (Fig. 21) is made of strong sheet iron and has an opening below,
ten centimeters square, and a height of about twenty centimeters. At
exactly ten centimeters from the bottom, the box has a rim at right
angles to its length so that it can be placed only ten centimeters deep
in the soil. The box holds a volume of earth equal to 1,000 cubic
centimeters.
[Illustration:
FIGURE 21.
METHOD OF HEINRICH.
]
The part of the box above ground is connected with the bottle B by a
glass tube as indicated in the figure. The bottle B should have a
capacity of about ten liters. The air in B is forced out through C by
water running in from the supply A and the pressure in B is recorded by
the manometer D. The experiment should be tried on a soil thoroughly
moist.
In measuring the pressure in B the water pressure should be cut off by
the pinch-cock between A and B, and the pressure on the manometer
observed after the lapse of one to two minutes.
MOVEMENT OF WATER THROUGH SOILS: LYSIMETRY.
=163. Porosity in Relation to Water Movement.=—The intimate relation
which water movement in a soil bears to fertility makes highly important
the analytical study of this feature of porosity. A soil deficient in
plant food, in so far as chemical analysis is concerned, will produce
far better crops when the flow of moisture is favorable than a highly
fertile soil in which the water may be in deficiency or excess.
Aside from the actual rain-fall the texture of the soil, in other words
its porosity, is the most important factor in determining the proper
supply of moisture to the rootlets of plants. Even where the rain-fall
is little, a properly porous soil in contact with a moist subsoil will
furnish the moisture necessary to plant growth. This fact is well
illustrated by the beet fields in Chino Valley, California. In this
locality most excellent crops of sugar beets are produced without
irrigation and almost without rain.
=164. Methods of Water Movement=—The translocation of soil water is
occasioned in at least two ways; namely,
1. By changing the porosity of a given stratum of soil.
2. By changing the amount of water a given stratum contains.
The following experiment by King[113] illustrates a convenient method of
studying this movement of water:
On a rich fallow ground of light clay soil, underlaid at a depth of
eighteen inches by a medium-grained sand, water, to the amount of two
pounds per square foot on an area of eight by eight feet, was slowly
added with a sprinkler, samples of soil having been previously taken in
six-inch sections down to a depth of three feet. The samples were taken
along a diagonal of the square under experiment and one foot apart. The
middle sample of the line being from the center of the area. The
sampling and wetting occurred between one and three P. M., on July 22,
and on the evening of the 23 a corresponding series of samples was taken
along a line parallel to the first but eight inches distant. The changes
in the percentages of water in the soil are given in the following
table, showing the translocation of water in soil due to wetting the
surface:
PER CENT OF WATER. DIFFERENCE.
Inches. Before After In per cent. In pounds per
wetting. wetting. cu. ft.
0–6 14.00 22.23 +8.23 +2.873
6–12 15.14 15.71 +0.57 +0.199
12–18 16.23 15.75 –0.48 –0.213
18–24 17.70 16.92 –0.78 –0.347
24–30 16.76 14.41 –2.35 –1.032
30–36 15.51 15.21 –0.30 –0.132
The figures given in the last column of the table are computed from the
absolute dry weights of the upper three feet of soil as determined in a
locality some rods from the place of experiment, and are therefore only
approximations, but the error due to this cause is certainly small. It
will be seen that while only two pounds of water to the square foot were
added to the surface, the upper six inches contained 2.87 pounds per
square foot more than before the water was added, and the second six
inches contained 0.199 pound more, and this too in the face of the fact
that the evaporation per square foot from a tray sitting on a pair of
scales close by, was 0.428 pound during the interval under
consideration. Similar experiments were made by taking the samples of
soil at 5.30 P. M. in one-foot sections down to four feet, at four
equally distant places along the diagonal of a square, six by six feet,
and having the ground sprinkled. At the same time four similar sets of
samples were taken on lines vertical to each of the sides of the square
but four feet distant from them. The amount of water the soil contained
was then determined, and at 11.30 A. M., nineteen hours later, another
series of samples was taken at points about four inches distant from the
last and the amount of water determined with the result given below.
TRANSLATION OF WATER OCCASIONED BY WETTING THE SURFACE.
─────────────┬───────────────────────────────────────────────────────
Depth of │
samples. │ WET AREA.
─────────────┼───────────────────────────╥───────────────────────────
„ │ Before wetting. ║ After wetting.
─────────────┼─────────────┬─────────────╫─────────────┬─────────────
„ │ │ Pounds of ║ │ Pounds of
│ Per cent of │ water per ║ Per cent of │ water per
│ water. │ cubic foot. ║ water. │ cubic foot.
─────────────┼─────────────┼─────────────╫─────────────┼─────────────
0–12 inches │ 16.86│ 11.78║ 20.15│ 14.06
12–24 „ │ 17.76│ 15.79║ 19.71│ 17.52
24–36 „ │ 16.76│ 14.73║ 17.72│ 15.58
36–48 „ │ 15.01│ 14.03║ 16.47│ 15.40
─────────────┼─────────────┼─────────────╫─────────────┼─────────────
Averages │ 16.59│ 14.08║ 18.51│ 15.64
│ │ ║ │
Total amount │ │ ║ │
of water │ │ 56.33║ │ 62.56
Amount of │ │ ║ │
change │ │ ║ │ +6.23
─────────────┴─────────────┴─────────────╨─────────────┴─────────────
─────────────┬───────────────────────────────────────────────────────
Depth of │
samples. │ AREA NOT WET.
─────────────┼───────────────────────────╥───────────────────────────
„ │ First samples. ║ Second samples.
─────────────┼─────────────┬─────────────╫─────────────┬─────────────
„ │ │ Pounds of ║ │ Pounds of
│ Per cent of │ water per ║ Per cent of │ water per
│ water. │ cubic foot. ║ water. │ cubic foot.
─────────────┼─────────────┼─────────────╫─────────────┼─────────────
0–12 inches │ 17.72│ 12.38║ 18.27│ 12.75
12–24 „ │ 19.18│ 17.05║ 19.94│ 17.72
24–36 „ │ 16.97│ 14.92║ 17.52│ 15.40
36–48 „ │ 15.49│ 14.48║ 15.16│ 14.17
─────────────┼─────────────┼─────────────╫─────────────┼─────────────
Averages │ 17.34│ 14.71║ 17.71│ 15.01
│ │ ║ │
Total amount │ │ ║ │
of water │ │ 58.83║ │ 60.04
Amount of │ │ ║ │
change │ │ ║ │ +1.21
─────────────┴─────────────┴─────────────╨─────────────┴─────────────
The above data show sufficiently well the method of investigation to be
pursued in studies of this kind.
=165. Capillary Movement of Water.=—The method of investigation proposed
by King[114] consists in taking samples of soil at intervals of one,
two, three, or four feet in depth, and determining the amount of
moisture in each in connection with the amount of rain-fall during the
period. The quantity of water contained in a given soil, at various
depths and on different dates, is shown in the following table: