Chapter VIII: Introduction (7)

=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.

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:

Depth in Date. Per cent Pounds per Increase or decrease.
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.

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.

The lysimeters of the other pair, which are the same size as the first, are filled in a different way. The lysimeters are first constructed and placed over vertical columns of soil _in situ_, which are obtained by digging away all the surrounding soil and leaving the columns standing. The shorter lysimeter is sunk in this way to within two inches of its entire length. It is then tipped over carrying the column of soil with it. Six inches of the subsoil are then removed, when the drainage tube and sand are put in, as in the first pair, and the bottom of the tube soldered in place. The lysimeter is thus filled with the natural soil in place. The longer box is in the same way filled, as far as possible, with the soil in place, but a gravelly nature of the soil may render it impossible to do the filling with a single column unbroken, so the gravel and sand from the lower portion of the soil are to be filled in separately. The drainage tube and bottom of sand are placed in the longer lysimeter in the same way as in the shorter.

FIGURE 23.

GROUND PLAN AND VERTICAL SECTION OF LYSIMETERS AND VAULTS SHOWING
POSITION OF THE APPARATUS.

1, 1, 1, 1, Lysimeters.
2, 2, 2, 2, Receiving bottles.
3, 3, Supplying apparatus.
4, 4, Skylights.
5, 5, 5, 5, Wall of vault.
6, 6, Brick walls.
7, Entrance Steps.
8, Vault.
]

The purpose of placing sand at the bottom of each lysimeter is to offer a porous stratum in which free water may collect and rise to the level of the perforated copper tube, which would prevent any further rise by conveying the surplus above into the vault as drainage water. The soil above the tube will therefore be constantly drained and the sand below constantly saturated, unless the water be drawn up by the capillary action of the soil as the result of evaporation from the surface.

By means of a proper arrangement within the vault, of a kind of Mariotte’s bottle, the water may be caused to flow back through the drainage tube into the lysimeter to take the place of that lost by evaporation, and thus maintain the level of free water just below the drainage tube. The water flowing back to the lysimeter, and the amount of drainage water, are carefully measured by a system of graduated tubes.

The lysimeters thus constructed represent tile-drained land; in one case the tile being three feet below the surface and in the other six feet below. The drainage waters collected in the receiving bottles can be measured and analyzed from time to time, as occasion may require, to determine the amount of plant food which is removed.

=179. Improved Method of Deherain.=[120]—Deherain’s earlier experiments were made in pots containing about sixty kilos of soil. These vases serve very well for some kinds of plants, but there are other kinds which do not grow at all normally when their roots are imprisoned. For instance, in pots, even of the largest size, wheat is always poor, beets irregular, maize never acquires its full development, and the conclusions which can be drawn from the experiments can not be predicated of the action of the plant under conditions entirely normal.

It is necessary therefore to carry on the work in an entirely different way, and to construct boxes so large as to make the conditions of growth entirely normal. The arrangement of these boxes is shown in Fig. 24.

They are placed in a large trench, two meters wide, one meter deep, and forty meters long. There are twenty boxes in this trench, the upper surface of each containing four square meters area. The boxes are one meter deep, and therefore can contain four cubic meters of soil. The sides and bottoms of the boxes are made of iron lattice work, covered with a cement which renders them impervious to water.

The bottom inclines from the sides towards the middle, and from the back to the front, thus forming a gutter which permits of the easy collection of the drainage. The drainage water is conveyed, by means of a pipe and a funnel, into a demijohn placed in the ditch in front of the apparatus, as shown in the figure. These receptacles stand in niches under the front of the cases, and are separated by the brick foundations. Access to them is gained by means of the inclined plane shown in the figure, and this plane permits the demijohns in which the drainage water is collected, to be removed with a wheelbarrow for the purpose of weighing. This apparatus is especially suitable for a study of the distribution of the nitrogen to the crop, the soil and the drainage waters. The loss in drainage waters of potash and phosphoric acid is insignificant in comparison with the loss in nitrogen.

The cases having been placed in position they are filled with the natural soil, which is taken to the depth of one meter, in such a way that the relative positions of the soil and subsoil are not changed.

While the soil is transferring to the cases it is carefully sampled in order to have a portion representing accurately the composition of both the soil and subsoil. These samples are subjected to analysis and the quantities of nitrogen, phosphoric acid, and potash contained therein carefully noted.

One or two cases should be left without crop or fertilizer to determine the relations of the soil and subsoil to the rain-fall. Three or four cases should be kept free of vegetation and receive treatment with different fertilizer, in order to determine the influences of these on the deportment of the soil to rain-fall. The rest of the cases should be seeded with plants representing the predominant field culture of the locality, and some of them should be fertilized with the usual manures used in farm culture.

FIGURE 24.

DEHERAIN’S APPARATUS FOR COLLECTING DRAINAGE WATER.
]

AUTHORITIES CITED IN PART THIRD.

Footnote 70:

Comptes rendus, Tome 112, p. 598.

Footnote 71:

Stockbridge, Rocks and Soils, p. 153.

Footnote 72:

Die Landwirtschaftlichen Versuchs-Stationen, Band 8, S. 40.

Footnote 73:

König, Untersuchung Landwirtschaftlich und Gewerblich Wichtiger
Stoffe, S. 48.

Footnote 74:

Methods of Swedish Agricultural Chemists, translated for author by F.
W. Woll.

Footnote 75:

Poggendorff’s Annalen, Fifth Series, Band 9, Ss. 102, et seq.

Footnote 76:

Pennsylvania Agricultural Experiment Station Report, for 1891, pp.
194, et seq.

Footnote 77:

Agricultural Science, Vol. 8, pp. 28, et seq. (Correction. For Fig.
13, second line from bottom of page 112, read Fig. 14.)

Footnote 78:

Haberland. Forschungen auf der Gebiete der Agricultur-Physik, 1878, S.
148.

Footnote 79:

Grundlagen zur Beurteilung der Ackerkrume, Weimar, 1882.

Footnote 80:

Vid. supra, 10.

Footnote 81:

These general principles are taken chiefly from a résumé of the
subject by Prof. H. A. Huston. Indiana Agricultural Experiment
Station, Bulletin 33, pp. 46, et seq.

Footnote 82:

Knop’s Agricultur Chemie, Abteil II.

Footnote 83:

Beiträge zur Frage der Bodenabsorption.

Footnote 84:

Henneberg’s Journal, 1859, S. 35.

Footnote 85:

Die Landwirtschaftlichen Versuchs-Stationen, Band 27, S. 107.

Footnote 86:

Die Bonitirung der Ackererde, S. 49.

Footnote 87:

Journal Chemical Society of London, 1868.

Footnote 88:

Landw. Central-Blatt, Band 11, S. 169.

Footnote 89:

bis Die Landwirtschaftlichen Versuchs-Stationen, Band 12, Ss. 21–50.

Footnote 90:

Jour. f. Landw., 1862, Band 3, Ss. 49–67.

Footnote 91:

Ann. d. Landw., Band 34, S. 319.

Footnote 92:

American Journal of Science, Vol. 14, p. 25.

Footnote 93:

bis (p. 122). Ms. communication to author.

Footnote 94:

Maryland Agricultural Experiment Station, Fourth Annual Report, p.
282.

Footnote 95:

Bulletin No. 4, U. S. Weather Bureau, p. 80.

Footnote 96:

bis (p. 125), Beiträge zur Agronomische Bodenuntersuchung, S. 31.

Footnote 97:

Zeitschrift für angewandte Chemie, 1889, S. 501.

Footnote 98:

Die Landwirtschaftlichen Versuchs-Stationen, Band 17, S. 85.

Footnote 99:

Ms. communication to author.

Footnote 100:

Proceedings of the Ninth Meeting of the Society for the Promotion of
Agricultural Science, p. 51.

Footnote 101:

Rocks and Soils, pp. 155 et. seq.

Footnote 102:

Anleitung zur Wissenschaftlichen Bodenuntersuchung, S. 137.

Footnote 103:

Analyse du Sol, p. 13.

Footnote 104:

Landwirtschaftliche Jahrbücher, Band 3, Ss. 771.

Footnote 105:

Forschungen auf dem Gebiete der Agricultur-Physik, 1885, Ss. 177, et
seq.

Footnote 106:

Vid. supra, S. 259.

Footnote 107:

Poggendorf, Annalen, Band 129, Ss. 437, et seq.

Footnote 108:

König, Untersuchung Landwirtschaftlich und Gewerblich Wichtiger
Stoffe, S. 59.

Footnote 109:

Vid. 37, S. 60.

Footnote 110:

Landwirtschaftliche Jahrbücher, Band 2, S. 383.

Footnote 111:

Forschungen auf dem Gebiete der Agricultur-Physik, 1880, S. 218.

Footnote 112:

Beurteilung der Ackerkrume, S. 222.

Footnote 113:

Wisconsin Agricultural Experiment Station, Seventh Annual Report, pp.
134, et seq.

Footnote 114:

Vid. supra, pp. 139, et seq.

Footnote 115:

Wisconsin Agricultural Experiment Station, Seventh Annual Report, p.
145.

Footnote 116:

Bulletin No. 4, Weather Bureau, pp. 13, et seq.

Footnote 117:

Philosophical Magazine, 1878.

Footnote 118:

Weather Bureau, Bulletin No. 4.

Footnote 119:

Forschungen auf dem Gebiete der Agricultur-Physik, 1891, S. 11.

Footnote 120:

Annales Agronomiques, Tome 16, p. 337; Tome 17, p. 49; Tome 18, p.
237; Tome 19, p. 69.

PART FOURTH.

MECHANICAL ANALYSIS OF SOILS.

THE FLOCCULATION OF SOIL PARTICLES.

=180. Relation of Flocculation to Mechanical Analysis.=—The tendency of the fine particles of silt to form aggregates, which act as distinct particles of matter, is the chief difficulty connected with the separation of the soil into portions of equal hydraulic value by the silt method of analysis. This tendency has been discussed fully by Johnson[121] and Hilgard.[122]

=181. Illustration of Flocculation.=—A sediment, consisting of particles of a hydraulic value, equal to one millimeter per second, is introduced into an ordinary conical elutriating tube placed vertically, in which the current of water entering below performs all the stirring which the particles receive.

A current of water corresponding to a velocity below one millimeter per second will, of course, not carry any of the particles out at the top of the cylindrical tube, but will keep them moving through the conical portion of the tube. If now the current be increased until its velocity is greater than one millimeter per second after having run at the slower velocity for fifteen or twenty minutes, very little of the sediment will pass over, although theoretically the whole of it should. Even at a velocity of five millimeters per second, much of the sediment will remain in the tube. This, of course, is due to the coagulation of the particles into molecular aggregates having a higher hydraulic value even than five millimeters per second. These aggregates can be broken up by violent stirring or moderate boiling, and the sediment reduced again to its proper value. The conclusions which Hilgard derives from a study of the above phenomena are as follows:

1. The tendency to coagulation is, roughly, in an inverse ratio to the size of the particles. With quartz grains it practically ceases when their diameter exceeds about two-tenths of a millimeter having a hydraulic value of eight millimeters per second. The size of the aggregates formed follows practically the same law as above. Sediment of 0.25 millimeter hydraulic value will sometimes form large masses like snow-flakes on the sides of the elutriator tube.

2. The degree of agitation which will resolve the aggregates into single grains is inversely as the size of the particles; or, more properly perhaps, inversely as their hydraulic value.

3. The tendency to flocculation varies inversely as the temperature. So much so is this the case that Hilgard at one time contemplated the use of water at the boiling point in the mechanical analysis of soils, in place of mechanical stirring.

4. The presence of alcohol, ether, and of caustic or carbonated alkalies, diminishes the tendency to flocculation, while the presence of acids and neutral salts increases it.

5. As between sediments of equal hydraulic value, but different densities, the tendency to flocculation seems to be greater with the less dense particles.

In regard to the mechanical actions which take place between the particles, Hilgard considers them as irregular spheroids, each of which can at best come in contact at three points with any other particle. The cause of aggregation cannot therefore be mere surface adhesion independent of the liquid, and the particles being submerged there is no meniscus to create an adhesive tension.

Since experiment shows that the flocculative tendency is measurably increased by the cohesion coefficient of the liquid, it seems necessary to assume that capillary films of the latter interposed between the surfaces of solids create a considerable adhesive tension even in the absence of a meniscus.

=182. Effect of Potential of Surface Particles.=—Whitney suggests that this is due to the potential of the surface particles of solids and liquids.[123] The potential of a single water particle is the work which would be required to pull it away from the surrounding water particles and remove it beyond their sphere of attraction. For simplicity, it may be described as the total force of attraction between a single particle and all other particles which surround it. With this definition, it will be seen that the potential of a particle on an exposed surface of water is only one-half of the potential in the interior of the mass, as half of the particles which formerly surrounded and attracted it were removed when the other exposed surface of water was separated from it. A particle on an exposed surface of water, being under a low potential, will therefore tend to move toward the center of the mass where the potential, _i. e._, the total attraction, is greater, and the surface will tend to contract so as to leave the fewest possible number of particles on the surface. This is surface tension.

If, instead of air, there is a solid substance in contact with the water, the potential will be greater than on an exposed surface of the liquid, for the much greater number of solid particles will have a greater attraction for the water particles than the air particles had. They may have so great an attraction that the water particle on this surface, separating the solid and liquid, may be under greater potential than prevails in the interior of the liquid mass. Then the surface will tend to expand as much as possible, for the particles in the interior of the mass of liquid will try to get out on the surface. This is the reverse of surface tension. It is surface pressure, which may exist on a surface separating a solid and liquid.

Muddy water may remain turbid for an indefinite time, but if a trace of lime or salt be added to the water the grains of clay flocculate, that is, they come together in loose, light flocks, like curdled milk, and settle quickly to the bottom, leaving the liquid above them clear. Ammonia and some other substances tend to prevent this and to keep the grains apart if flocculation has already taken place.

If two small grains of clay, suspended in water, come close together they may be attracted to each other or not, according to the potential of the water particles on the surface of the clay. If the potential of the surface particle of water is less than that of the particle in the interior of the mass of liquid, there will be surface tension, and the two grains will come together and be held with some force, as their close contact will diminish the number of surface particles in the liquid. If, on the other hand, the potential of the particle on the surface of the liquid is greater than of the particle in the interior of the mass, the water surface around the grains will tend to enlarge, as there will be greater attraction for the water particles there than in the interior of the mass of liquid, and the grains of clay will not come close together and will even be held apart, as their close contact would diminish the number of surface particles in the liquid around them.

=183. Influence of Surface Tension.=—Hilgard supposes that the surface tension which is assumed to exist between two liquid surfaces must exert a corresponding influence between the surfaces of solids and liquids, apart from any meniscal action.

It is then to be expected that the adhesion of the particles constituting one of these floccules will be very materially increased whenever the formation of menisci between them becomes possible by the removal of the general liquid mass. Suppose one of the floccules to be stranded, it will, in the first place, remain immersed in a sensibly spherical drop of liquid. As this liquid evaporates, the spherical surface will become pitted with menisci forming between the single projecting particles, and as these menisci diminish their radius by still further evaporation, the force with which they hold the particles together will increase until it reaches a maximum. As the evaporation progresses beyond this point of maximum, the adhesion of the constituent particles must diminish by reason of the disappearance of the smaller menisci, and when finally the point is reached when liquid water ceases to exist between the surfaces, the slightest touch, or sometimes even the weight of the particles themselves, will cause a complete dissolution of the floccule, which then flattens down into a pile of single granules.

In regard to natural deposits from water, Hilgard supposes that they are always precipitated in a flocculated state. The particles of less than two-tenths millimeter diameter are carried down with those of a larger diameter having much higher hydraulic value. Thus the deposition of a pure clay can take place under only very exceptionable circumstances.

Whitney, on the other hand, suggests that grains of sand and clay carry down mechanically the particles of fine silt and clay as they settle in a turbid liquid in a beaker; and it is often difficult to wash out a trace of fine material from a large amount of coarse particles, for this reason, although there may be no trace whatever of flocculation.

=184. Destruction of Floccules.=—The destruction of the natural floccules is seen in the ordinary process of puddling earth or clay. It is also the result of violent agitation of water or of kneading or boiling, or, finally, to a certain extent, of freezing. All these agencies are employed by the workers in clay for the purpose of increasing the plasticity which depends essentially upon the finest possible condition of the material to be worked. As an illustration of this, Hilgard cites the fact that any clay or soil which is worked into a plastic paste with water, and dried, will form a mass of almost stony hardness. If, however, to such a substance one-half per cent of caustic lime be added, a substance which possesses in an eminent degree the property of coagulating clay, the diminution of plasticity will be obvious at once, even when in a wet condition. If now the mass be dried, as in the previous case, it is easily pulverized. This is an illustration of the effect of lime upon stiff lands, rendering them more readily pulverulent and tillable. The conversion of the lime into a carbonate in the above experiment by passing bubbles of carbonic acid through the mass while still suspended in water does not restore the original plasticity, thus illustrating experimentally the fact known to all farmers that the effect of lime on stiff soil lasts for many years, although the whole of the lime in that time has been converted into carbonate.

=185. Practical Applications.=—The practical application of this is, according to Hilgard, that the loosely flocculated aggregation of the soil particles is what constitutes good tilth. For this reason the perfect rest of a soil, if it is protected from the tamping influence of rains and the tramping of cattle, may produce a condition of tilth which cannot be secured by any mechanical cultivation. As an illustration of this, the pulverulent condition of virgin soils protected in a forest by the heavy coating of leaves may be cited. On the contrary, as pointed out by Hilgard, there are some kinds of soil in which a condition of rest may produce the same effect as tamping. These are soils which consist of siliceous silt without enough clay to maintain them in position after drying. In such a case, the masses of floccules collapse by their own weight or by the least shaking, and fall closely together, producing an impaction of the soil. This takes place in some river sediment soils in which the curious phenomenon is presented of injurious effects produced by plowing when too dry, which is the direct opposite of soils containing a sufficient amount of clay and which are injured by plowing too wet.

It is further observed that the longer a soil has been maintained in good tilth, the less it is injured by wet plowing. This is doubtless, according to Hilgard, due to the gradual cementation of the floccules by the soil water which fixes them more or less permanently.