Chapter VII: Introduction (6)

The fine earth should be placed in a flask with 100 to 200 cubic centimeters of the one-tenth to one-hundredth normal solution of the substance to be absorbed. The flask should be well shaken and allowed to stand with frequent shaking twenty-four to forty-eight hours at ordinary temperatures. The whole is then to be thrown upon a folded filter and an aliquot part of the filtrate taken for the estimation. The methods of determining the quantities of the substances used will be found in other parts of this manual. It is recommended to conduct a blank experiment with water under the same conditions in order to determine the amount of the material under consideration abstracted from the soil by the water alone. The difference in the strength of the solution as filtered from the soil, corrected by the amount indicated by the blank experiment, and the original solution will give the absorptive power of the soil for the particular substance under consideration.

If it should be desired to determine the absorptive power of the soil for all the ordinary chemical fertilizing materials at the same time, a larger quantity of the sample should be taken corresponding to the increased amount of the standard solutions used. About 500 cubic centimeters of the mixed salt solution should be shaken with 125 grams of the earth and the process carried on in general as indicated above. The absorption coefficient of an earth for any given salt according to Fesca,[96] is the quantity of the absorbed material expressed in milligrams calculated to a unit of 100 grams of the soil.

=133. Method of Pillitz and Zalomanoff.=—It is recommended by Pillitz and Zalomanoff[25] to reject the old method, _viz._, shaking the soil with the solution in a flask, and substitute the filtration method both because it gives a more natural process and because the results are more constant. The apparatus is shown in Fig. 15.

FIGURE 15.

ZALOMANOFF’S APPARATUS FOR DETERMINING ABSORPTION OF SALTS BY SOILS.
]

Two cylinders are placed vertically, one over the other. The lower cylinder is graduated in cubic centimeters, the upper cylinder is closed at each end by perforated rubber stoppers A and B through the openings of which the glass tubes _c_ and _d_ pass. Within the cylinder A the opening of the small tube _d_ is closed with a disk of Swedish filter paper. The lower part of the small tube is _d_ connected by means of a rubber tube carrying a pinch-cock C, with another small tube _e_ which passes through the stopper _f_. In carrying out the process the weighed quantity of soil is placed in the upper cylinder and afterwards the measured quantity of the solution, the whole thoroughly mixed and the cylinder closed. The valve C is then opened, a given quantity of the solution, but not all, is made to drop into the lower cylinder and the valve C is then closed. The liquid which has passed into the lower cylinder as well as that which remains in the upper cylinder, is thoroughly stirred and the quantity of the material remaining in both liquids determined and the absorbing power of the soil estimated from their difference. It does not appear that this method of estimation of the absorption power possesses any special advantages over the old and far simpler method of shaking in a flask.

FIGURE 16.

MÜLLER’S APPARATUS TO SHOW ABSORPTION OF SALTS BY SOILS.
]

=134. Method of Müller.=—The method of Müller[97] for illustrating absorption is carried out by means of the apparatus shown in Fig. 16. A glass cylinder A about 750 centimeters long and four to five centimeters wide is closed at each end with rubber stoppers with a single perforation. The cylinder A is for the reception of the soil with which the experiment is to be made. Before using, the lower part of it is filled with glass pearls or broken glass and above this a layer of glass wool is placed about one centimeter thick. The object of this is to prevent the soil from passing into the small tube below. As soon as the soil has all been placed in the cylinder A the upper part of the tube is also filled with glass wool. The cylinder A is connected with the pressure bottle B by means of a rubber tube and the small glass bulb tube shown in the figure. The bottle B should have a content of about two liters. It is filled with the standard solution of the material of which the absorption coefficient is to be determined. At _c_ the rubber tube is connected with a glass T one arm of which is provided with a piece of rubber tubing which can be closed by means of a pinch-cock. At _c_ a screw pinch-cock is placed which can be used to regulate the flow of the solution from B to A. By opening the pinch-cock at _e_ on the short arm of the T piece, a sample of the original liquid can be taken and this can be compared with the part which runs to _b_. If it is desired for instance, to show that potassium carbonate has been absorbed by the soil the two bulbs shown on the small glass tubes connecting with A can be filled with red litmus paper. This paper will at once be turned blue in the lower bulb while in the upper one it will retain its original color because the liquid in passing through the soil will have lost its alkaline reaction. The solutions used should be very dilute. The apparatus is designed for lecture experiments and not for quantitative determinations.

=135. Method of Knop.=—For rapid determination of the absorption coefficient of the soil Knop’s method may be used.[98]

The fine earth which is employed is that which passes a sieve with meshes of half a millimeter. From 50 to 100 grams of this soil are mixed with from five to ten grams of powdered chalk and with about twice the weight of ammonium chlorid solution of known strength, _viz._, from 100 to 200 cubic centimeters. The ammonia solution should be of such a concentration that the ammonia by its decomposition for each cubic centimeter of the liquid evolves exactly one cubic centimeter of nitrogen. This solution is prepared by dissolving in 208 cubic centimeters of water one gram of ammonium chlorid. With frequent shaking the solution is allowed to stand in contact with the soil for forty-eight hours. The whole is now allowed to settle and the supernatant clear liquid is poured through a dry filter. From the filtrate twenty to forty cubic centimeters are removed by a pipette, and evaporated to dryness in a small porcelain dish, with the addition of a drop of pure hydrochloric acid. The ammonium chlorid remaining in the porcelain dish is washed with ten cubic centimeters of water into one of the compartments of the evolution flask of the Knop-Wagner azotometer. It is decomposed with fifty cubic centimeters of bromin lye and the nitrogen estimated volumetrically. The difference between the amount of nitrogen in this material and that of the original material will give the amount of absorption exercised by the fine earth. This number, without any further calculation, can be taken as the coefficient of absorption.

=136. Method of Huston.=—The salt solutions recommended by Huston[99] are sodium phosphate (Na₂HPO₄), potassium chlorid, potassium sulfate, ammonium sulfate and sodium nitrate.

The solutions should be approximately tenth normal, the actual strength in each case being determined by analysis. The phosphorus is determined as magnesium pyrophosphate in the usual way, the potash as potassium platinochlorid, the ammonia by collecting the distillate from soda in half normal hydrochloric acid and titrating with standard alkali, and the nitrate by Warington’s modification of Schlösing’s method for gas analysis. The details of these methods of determination will be given later. One hundred grams of the sifted, air-dried soil are placed in a rubber stopped bottle and treated with 250 cubic centimeters of the solution to be tested. The digestion is continued for forty-eight hours in each case, the bottles being thoroughly shaken at the end of twenty-four hours. At the end of the treatment the solutions are filtered and the salts determined in aliquot portions. The details of this method are essentially those already described.

=137. Statement of Results.=—Duplicate analyses should be made and the tabulation of the data is illustrated in the following analyses by Huston:

Na₂HPO₄ cubic Weight of Weight of P₂O₅ Salt
centimeters Mg₂P₂O₇ in Mg₂P₂O₇ in absorbed removed
filtrate taken. twenty-five filtrate. by 100 per
cubic grams cent.
centimeters of soil.
the solution.
(a) 25 0.1368 gram 0.0962 gram
(b) 25 0.0963 „ 0.2589 29.6
gram
——————
Mean 0.0963 „

KCl cubic Weight of Weight of K₂O Salt
centimeters K₂PtCl₆ in K₂PtCl₆. absorbed removed
filtrate taken. twenty-five by 100 per
cubic grams cent.
centimeters of soil.
solution.
(a) 25 0.6154 gram 0.4505 gram
(b) 25 0.4540 „ 0.3161 26.5
gram
——————
Mean 0.4523 „

K₂SO₄ cubic Weight of Weight of K₂O Salt
centimeters K₂PtCl₆ in K₂PtCl₆. absorbed removed
filtrate taken. twenty-five by 100 per
cubic grams cent.
centimeters of soil.
solution.
(a) 25 0.6113 gram 0.4426 gram
(b) 25 0.4371 „ 0.3324 28.0
gram
——————
Mean 0.4399 „

(NH₄)₂SO₄ cubic Number cubic Half normal acid N absorbed Salt
centimeters centimeters neutralized. by 100 absorbed
filtrate taken. one-half normal grams per
acid neutralized soil. cent.
by fifty cubic
centimeters of
solution.
(a) 50 10.00 7.25 grams
(b) 50 7.25 „ 0.0964 27.5
gram
————
Mean 7.25 „

NaNO₃ cubic Number cubic Cubic centimeter N absorbed Salt
centimeters centimeters N₂O₂ N₂O₂ at 0° and by 100 absorbed
filtrate taken. afforded by ten 1000 grams per
cubic millimeters. soil. cent.
centimeters of
solution at 0°
and 1000
millimeters
pressure.
(a) 10 16.63 16.77 grams
(b) 10 16.70 „ none 00.00
—————
Mean 16.73 „

Upon comparing the figures it will be found that the absorption, passing from the greatest to the least, is as follows: phosphoric acid (P₂O₆), potassium sulfate, ammonium sulfate, potassium chlorid and sodium nitrate.

It will be seen that there was no absorption in the case of the nitrate, while with each of the other salts there was quite a marked absorption. It will also be noticed that the percentages of absorption are not very different, and especially is this true of the potassium and ammonium salts, the P₂O₅ being somewhat higher. Whether this fact is merely an accidental occurrence or is due to the law of combination by equivalents could hardly be predicted from the single soil experimented upon; but taking into consideration the possibility of difference in solubility of the resulting compounds in the saline solutions used, and of other varying conditions, the percentages are evidently not far enough apart to exclude the possibility of the bases uniting in equivalent proportions.

=138. Preparation of Salts for Absorption.=—The salts employed in the foregoing determinations are conveniently prepared, in fractional normal strength.

In grams per liter the following quantities in grams are recommended, _viz._, 5.35 g NH₄Cl; 10.11 g KNO₃; 16.40 g Ca(NO₃)₂; 24.60 g MgSO₄ + 7H₂O; 23.4 g CaH₄(PO₄)₂, etc.

The ammonium chlorid, potassium nitrate and magnesium sulfate can be weighed as chemically pure salts and the standard solution be directly made up. Calcium nitrate is so hygroscopic that a stronger solution must be made up, the calcium determined and the proper volume taken and diluted to one liter.

Monocalcium phosphate is prepared as follows:

A solution of sodium phosphate is treated with glacial acetic acid and precipitated with a solution of calcium chlorid. It is then washed with water until all chlorin is removed. The fresh precipitate is saturated with pure, cold phosphoric acid of known strength. After filtering the solution is placed in a warm room and left for two or three weeks until crystallization takes place.

The crystals are pressed between blotting papers and finally dried over sulfuric acid and washed with water-free ether, and again dried. Since this salt is decomposed in strong solutions it should be used only in one hundredth normal strength, viz., 2.34 grams per liter.

POROSITY AND ITS RELATIONS TO MOISTURE.

=139. Porosity.=—The porosity of a soil depends upon the state of divisibility and arrangement of its particles, and upon the amount of interstitial space within the soil. If a soil be cemented together into a homogeneous mass, its porosity sinks to a minimum; if it be composed, however, of numerous fine particles, each preserving its own physical condition, the porosity of the soil will rise to a maximum. The porosity of a soil may be judged very closely by the percentage of fine particles it yields by the process of silt analysis to be described further on. In general, the more finely divided the particles of a soil, the greater its fertility. This arises from various causes; in the first place, such a soil has a high capacity for absorbing moisture and holding it; thus the dangers of excessive rain-falls are diminished, and the evil effects of prolonged drought mitigated. In the second place, a porous soil permits a freer circulation of the gases found in the soil. The influence of lime in securing the proper degree of porosity of a soil is very great, especially in alluvial deposits and other stiff soils. It prevents the impaction which will necessarily follow in a soil which is too finely divided. In general, the porosity of the soil may be said to depend on three factors, _viz._: 1. Upon the state of divisibility or the number of particles per unit volume; 2. Upon the nature and arrangement of these particles; 3. Upon how much interstitial space there is in the soil.

=140. Influence of Drainage.=—Good underdrainage increases the porosity of a soil by removing the excess of water during wet seasons and rendering the soil more suitable to capillary attraction which will supply moisture during dry seasons. The influence of tile drainage on the production of floods has been carefully studied by Kedzie,[100] who shows that surface ditching in conjunction with deforesting may increase floods and contribute to droughts, and that tile-draining may increase flood at the break-up in spring, when the water accumulated in the surface soil by the joint action of frost and soil capillarity during the winter, and the surface accumulations in the form of snow are suddenly set free by a rapid thaw.

He also points out that during the warm months tile-draining tends to prevent flood by enabling the soil to take up the excessive rain-fall and hold it in capillary form, keeping back the sudden flow that would pass over the surface of the soil if not absorbed by it, and it mitigates summer drought by increased capacity of the soil to hold water in capillary form and to draw upon the subsoil water supply.

=141. Soil Moisture.=—The capacity of a soil to absorb moisture and retain it depends on its porosity and is an important characteristic in relation to its agricultural value.

The following general principles relating to soil moisture are adapted from Stockbridge:[101]

During dry weather plants require a soil which is absorptive and retentive of atmospheric moisture. The amount of this retention is generally in direct ratio to two factors, _viz._, the amount of organic matter and its state of division. The capillary water of the soil is very closely related to its percolating power, since all waters in the soil are governed in their movements by what is known as capillary force. Liebenberg has shown that this movement may be either upwards or downwards, according as the atmosphere is dry or supplies soil-saturating rain. The water absorbed by the roots passes into the plant circulation, and the greater part is evaporated from the leaves. Where the supply of water is insufficient, the plant wilts, and if the evaporation long continue in excess of the supply obtained from the soil, the plant must die. The experiments of Hellriegel have shown that any soil can supply plants with all the water they need, and as fast as they need it, so long as the moisture within the soil is not reduced below one-third of the whole amount that it can hold. The quantity of water required and evaporated by different agricultural plants during the period of growth has been found to be as follows:

One acre of wheat exhales 409,832 pounds of water.
„ „ „ clover „ 1,096,234 „ „ „
„ „ „ sunflowers „ 12,585,994 „ „ „
„ „ „ cabbage „ 5,049,194 „ „ „
„ „ „ grape-vines „ 730,733 „ „ „
„ „ „ hops „ 4,445,021 „ „ „

Dietrich estimates the amount of water exhaled by the foliage of plants to be from 250 to 400 times the weight of dry organic matter formed during the same time. Cultivation conserves soil moisture. It must be remembered that this water contains soil ingredients in solution. Hoffmann has estimated that the quantity of matter dissolved from the soil by water varies from 0.242 to 0.0205 per cent of the dried earth. The experiments of Humphrey and Abbott have shown that about one-sixth of the total sediment of the Mississippi river is soluble in water.

=142. Determination of the Porosity of the Soil.=—The porosity of the soil is fixed by the relative volume of the solid particles as compared with the interstitial space. It is most easily determined by dividing the apparent by the real specific gravity.

Let the real specific gravity of a soil be 2.5445 and the apparent specific gravity of the same soil be 1.0990.

The porosity is then calculated according to the following ratios, _viz._:

2.5445 : 1.099 = 100 : X

Whence X = 43.2 = per cent volume occupied by the solid particles of the soil.

The per cent volume occupied by the interstitial space is therefore 56.8.

=143. Method of Whitney.=—The total volume of interstitial space within the soil, in which water and air can enter, is best determined by calculation from the specific gravity and the weight of a known volume of soil. To determine this in the soil in its natural position in the field, a sample is taken in the following way: A brass tube, about two inches in diameter and nine inches long, has a clock spring securely soldered into one end, and this end turned off in a lathe to give a good cutting edge of steel. The area enclosed by this steel edge is accurately determined, and a mark is placed on the side of the tube exactly six inches from the cutting edge. A steel cap fits on top of the brass cylinder to receive the blows of a heavy hammer or wooden mallet. The cylinder is driven into the ground until the six-inch mark is just level with the surface. The whole is then dug out, care being taken to slip a broad piece of steel under the cylinder before it is removed, so as to prevent the soil which it contains from falling out. The cylinder is then carefully laid over on its side, and the soil is cut off flush with the cutting edge of steel. The soil is then removed from the cylinder, carried to the laboratory and properly dried and weighed. The object of the steel inserted in one end of the cylinder is to reduce the friction on the inside of the tube to a minimum, and thus prevent the soil inside the cylinder being forced down below the level of the surrounding earth. The volume of the soil removed with this sampler can readily be determined by calculation, as the area of the end of the tube is known and the sample is six inches deep. In a sampler, such as described here, this volume is about 300 cubic centimeters. From the weight of soil and the volume of the sample, the volume of interstitial space may be found by the following formula:

S = ([V − W/ω] × 100)/V

S is the per cent by volume of interstitial space, V is the volume of the tube in cubic centimeters, W is the weight of soil in grams, and ω is the specific gravity of the soil. The specific gravity can be determined for each soil, or the factor 2.65 can be used, which is sufficiently accurate for most work.

The per cent by volume of interstitial space in the undisturbed subsoil is found to range from about thirty-five for sandy land, to sixty-five or seventy for stiff clay lands.

For the determination of the amount of water an air-dried soil will hold, if all the space within it is completely filled with water, an eight-inch straight argand lamp chimney, with a diameter of about two inches, can be conveniently used. A mark is placed on the side of the tube, six inches from one end, and the volume of the tube up to this mark is found by covering the end with a piece of thin rubber cloth, or by pressing the chimney down firmly on a glass plate, and making a water-tight joint with paraffin or wax. Water is then poured into the tube up to the six-inch mark, and the weight or volume of water determined. The tube can then be dried, a piece of muslin tied tightly over the top and the whole then weighed. Soil is carefully poured in and the tube gently tapped on a soft support until the soil is six inches deep in the tube, and has the desired degree of compactness. The weight and volume of the soil can thus be determined, and the volume of the interstitial space from the formula already given. This can also be determined directly by introducing water from above, or by immersing the cylinder of soil up to the six-inch mark in water, and allowing the water to enter the soil from below. With such a short depth of soil, very little water will flow out when the cylinder is suspended in the air. The amount which will flow out when the cylinder is thus suspended, will depend both upon the texture and the depth of soil. It is impossible, however, by this method, to completely remove the air or to completely fill the space within the soil with water; for as the water enters the soil, a considerable amount of air becomes entangled in the capillary spaces, and this could not be removed except by boiling and vigorous stirring, which would altogether change the texture of the soil. The amount of water held by the soil, or the amount of space within the soil into which water and air can enter, will evidently depend upon the compactness of the soil, and this is best expressed in per cent by volume of space.

=144. Capacity of the Fine Soil for Holding Moisture.=—The soil, as it is taken from the field, may have quite a different water coefficient from the same soil after it has been passed through a fine sieve or been dried at air temperatures or at 100° or 110°. The method of determination which depends upon adding excess of water to a given weight of fine earth, and afterwards eliminating the excess by percolation or filtration, is apt to give misleading results. If, however, the results are obtained by working on the same weight of soil, and in the same conditions, they may have value in a comparative way. The comparison between soils must be made with equal weights, in like apparatus and with the same manipulation, to have any value. These determinations, however, cannot have the same practical value as those made in the samples in a natural condition as has just been described.

=145. Method of Wolff Modified by Wahnschaffe.=[102]—A cylindrical zinc tube (Fig. 17), sixteen centimeters long and four centimeters internal diameter, is used, the cubical capacity of which is 200 cubic centimeters.

The cylinder is graduated by placing the moist linen disk on the gauze and tying a piece of rubber cloth over the bottom. Water is now poured in until the level is even with the gauze bottom. Add then exactly 200 cubic centimeters of water, mark its surface on the zinc, throw out the water, and file the zinc down to the mark.

The bottom of the tube is closed with a fine nickel-wire gauze. Below this a piece of zinc tubing, of the size of the main tube, is soldered; pierced laterally with a number of holes.

Before using, the gauze bottom of the cylinder is covered with a moist, close fitting linen disk, and the whole apparatus weighed. It is then filled with the fine earth, little by little, jolting the cylinder on a soft substance after each addition of soil to secure an even filling. When filled even full the whole is weighed, the increase in weight giving the weight of soil taken.

FIGURE 17.

CAPACITY OF THE FINE SOIL FOR HOLDING MOISTURE. METHOD OF WOLFF
MODIFIED BY WAHNSCHAFFE.
]

A large number of cylinders can be filled at once and placed in a large glass crystallizing dish containing water and covered with a bell jar (Fig. 17). The water should cover the gauze bottoms of the cylinders to the depth of five to ten millimeters. More water should be added from time to time as absorption takes place. The cylinders should be left in the water until when weighed at intervals of an hour no appreciable increase in weight takes place. The temperature and barometer reading should be noted in connection with each determination. With increasing temperature the water coefficient is diminished.

The method of Wolff, as practiced in the laboratory of the Chemical Division of the U. S. Department of Agriculture, has given very concordant results. Five determinations were made on a sample of vegetable soil with the Wolff cylinders, which were weighed at intervals of ten, twenty, and thirty days, with the following results:

No. 1. Water absorbed after ten days 106.25 per cent
„ 2. „ „ „ „ „ 105.68 „ „
„ 3. „ „ „ „ „ 105.86 „ „
„ 4. „ „ „ „ „ 106.11 „ „
„ 5. „ „ „ „ „ 105.83 „ „
——————
Mean 105.95 „ „

No. 1. Water absorbed after twenty days 106.44 per cent
„ 2. „ „ „ „ „ 105.98 „ „
„ 3. „ „ „ „ „ 106.56 „ „
„ 4. „ „ „ „ „ 106.52 „ „
„ 5. „ „ „ „ „ 106.38 „ „
——————
Mean 106.38 „ „

No. 1. Water absorbed after thirty days 108.35 per cent
„ 2. „ „ „ „ „ 107.60 „ „
„ 3. „ „ „ „ „ 108.32 „ „
„ 4. „ „ „ „ „ 107.86 „ „
„ 5. „ „ „ „ „ 107.87 „ „
——————
Mean 108.00 „ „

The data obtained show that there was a very slight increase in the amount of moisture absorbed after the tenth day.

As will be seen, however, from the following data, the soil within the cylinder does not contain in all parts the same percentage of moisture, the lower portions of the cylinder containing notably larger proportions than the upper parts. The cylindrical soil column was divided into four equal parts and the moisture determined in each part. Beginning with the top quarter the percentages of moisture were as follows:

First quarter 97.52 per cent
Second „ 105.91 „ „
Third „ 112.83 „ „
Fourth „ 116.48 „ „

=146. Method of Petermann.=[103]—The method of Wolff as practiced by the Belgian Experiment Station, at Gembloux, is essentially the same as described above.

Petermann recommends the use of tared cylinders twenty to twenty-five centimeters long and six to eight centimeters in diameter. The cylinder is to be filled with the fine earth, little by little, with gentle tapping after each addition. The bottom of the cylinder is closed with a perforated rubber stopper on which is spread a moistened disk of linen. The cylinder, thus prepared and filled, is weighed and afterwards placed in a vessel containing distilled water, to such a depth as to secure a water level about two centimeters above the lower surface of the soil in the cylinder. The level of the water is kept constant as the contents of the cylinder are moistened by capillarity. When the earth appears to be thoroughly moistened, as can be told by the appearance of the upper surface, maintain the contact with water for about five or six hours.

The cylinder is then removed, the upper surface covered to avoid evaporation, allowed to drain for a few hours, wiped and weighed. The cylinder is again placed in water to see if any increase in weight takes place. The weight of the fine earth and of the absorbed water being known, the percentage of absorption is easily calculated.

=147. Method of A. Mayer.=[104]—A glass tube, one and seven-tenths centimeters in diameter, composed of two pieces, seventy-five centimeters and twenty-five centimeters in length, is united by a piece of rubber tubing. The lower free end of the seventy-five centimeter piece is closed with a piece of linen. The tube is filled, with gentle jolting, to the depth of one meter with fine earth, the earth column thus extending twenty-five centimeters above the point of union of the two pieces. Thus prepared, a quantity of water is poured into the upper tube sufficient to temporarily saturate the whole of the soil.

During the sinking of the water in the tube there is thus effected a moistening of the material before it is wholly filled with water. After waiting until the water poured on top has disappeared the tube is separated at the rubber tube connection and a sample of the moist soil taken at that point. This is at once weighed and then dried at 100°. The loss in weight gives the water absorbed.

The number thus obtained is calculated to the standard by volume, by use of the number representing the apparent specific gravity of the fine earth.

For sand of different degrees of fineness the following numbers were found:

Degree of fineness 2 3 4
Per cent water absorbed 7.0 13.7 44.6

The numbers thus obtained are taken to represent the absolute water capacity of a mineral substance in powder.

The full water capacity, _i. e._, the power of holding water when the powder is immersed in water, the excess of which is then allowed to flow away is much greater than the absolute number.

This difference is shown in the following data:

Quartz, size three. Clay, size three.
Full water capacity 49.0 per cent 46.8 per cent
Absolute water capacity 13.7 „ „ 24.5 „ „

In general the absolute is markedly inferior to the full water capacity. Only in the finest dust do the two numbers approach each other.

=148. Volumetric Determination.=—A convenient apparatus for this determination has been devised by Mr. J. L. Fuelling, of the Chemical Division, Department of Agriculture. It is shown in Fig. 18.

It consists of an ordinary percolator the diameter of which decreases slightly towards the lower end, a thick-wall rubber tube and an ordinary burette, divided in tenths. A rubber stopper is fitted to the mouth of the percolator and perforated twice—in the middle and at the side, the former for a small tube provided with pinch-cock and the latter for the neck of a small funnel. The whole is supported on a convenient stand, the clamp holding the percolator being placed above that supporting the burette, both clamps arranged to slide on the stand-rod.

FIGURE 18.

FUELLING’S APPARATUS.
]

The method is as follows:

A mark is placed upon the projecting tube at the lower end of the percolator, and the tube at this point may be drawn out sufficiently to decrease the width of meniscus to one-eighth inch. Into the percolator is first introduced a small disk of wire gauze or perforated porcelain, with heavy wire pendant in the tube. Through the rubber stopper a small glass tube is passed and its lower end pressed firmly upon the wire or porcelain disk, its upper end being curved and supplied with a pinch-cock. Into the percolator is now poured one inch of fine shot (No. 20) and then one inch of fine sand which has been previously digested with hydrochloric acid and well cleaned of dust by washing.

_The zero._—After the shot and sand have been shaken even, the burette is filled with water and raised above the level of the sand, wetting the percolator for four inches of its length. The burette is lowered and the shot and sand bed allowed to drain by opening the pinch-cock of the inner tube. The burette is raised and the shot-sand flooded repeatedly until, by lowering the burette until the zero mark of the percolator tube is reached, a uniform reading on the burette is secured. Thus the shot-sand bed is completely charged with water. The water level is now made zero on the percolator stem, the burette filled to its zero mark and the apparatus is prepared for introduction of the soil.

_The Determination._—From 100 to 200 grams of soil, previously dried free of moisture, are weighed, the burette raised until the water level is three inches above the sand, and the soil gently dropped through a funnel into the water. When the soil has been introduced and wetted completely the water level is raised above the soil and allowed to remain thus two hours. The burette is then lowered and the water allowed to drain from the wetted soil. Four to six hours are usually given the draining, the reading taken on the burette after establishing the zero on the percolator stem, the volume of absorbed water thus ascertained and divided by the weight of soil multiplied by 100; the result expresses the water absorbed per hundred of soil.

Example:

Water required to saturate disk, etc. 0.50 cubic centimeter. Weight of air-dried soil taken 20.00 grams. Moisture at 105° therein 14.25 per cent. Weight water in soil 2.85 grams. Reading of burette after saturation 10.75 cubic centimeters. Less water required for disk, etc 9.25 „ „ Temperature 20°.00 Weight of 9.25 cubic centimeters H₂O at 20° 9.22 grams. Total weight of water retained by soil 12.07 „ Per cent water retained by soil 60.35 per cent.

For general analytical work the correction for variations in the weight of water for different temperatures is of no practical importance.

=149. Accuracy of Results.=—A sample of soil from the beet sugar station, in Nebraska, gave the following duplicate results:

First trial 45.75 per cent water.
Second trial 44.85 „ „ „

Muck soils from Florida, containing varying proportions of sand, gave the following numbers:

Soil number one, 144.85 per cent, and 145.43 per cent; soil number
two, 109.13 per cent, and 107.93 per cent; soil number three (very
sandy), 46.86 per cent, and 46.51 per cent.

=150. Method of Wollny.=[105]—A zinc tube, ninety centimeters long and four centimeters internal diameter, carries at each end, at right angles to the axis, a flattened rim 1.5 centimeters broad. The lower end of the tube is closed with a strong piece of coarse linen. The soil to be examined is then filled in little by little, with gentle tamping.

On the upper end two glass tubes are placed, each ten centimeters long and four centimeters internal diameter. These tubes are furnished at each end with cemented brass cylinders which are expanded to a circular, evenly ground rim, 1.5 centimeters wide, also at right angles to the axis of the main tube. These rims are greased and placed together, one on the other, and held together by wooden clamps.

The glass tube in immediate connection with the zinc tube is also firmly filled with the soil sample, while the second tube is only partly filled, so that any settling which may take place in the soil on the addition of water may still find the first glass tube full of the sample.

The empty part of the upper glass tube is now filled with water and 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.

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.

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.