115. Method of Whitney and Marvin.^[77]—The thermometer devised by Whitney and Marvin is shown in Fig. 14. The principle on which this modification depends is as follows:

A mercurial thermometer of the ordinary construction is liable to give wrong indications of the temperature because it is difficult to determine the temperature of the column of mercury from the bulb to the surface of the ground. To avoid this source of error the thermometer figured was constructed.

The bulb of the thermometer is made quite small and a slender portion of the stem extends into its spherical portion. The top portion of the thermometer stem does not differ in any essential respect from the stem of an ordinary thermometer.

The bulb is almost wholly filled with alcohol, which acts as the principal thermometric fluid and has the advantages of a high coefficient of expansion. The thermometer bulb and the stem of the thermometer up to a point convenient for graduation, are filled with mercury. In the drawing the mercury is represented by the heavy black marking in and just above the small bulb. The peculiar construction at this point is for the purpose of retaining the mercury about the point of the slender capillary stem inside the bulb and preventing the entrance of alcohol into the stem when the thermometer is horizontal.

In order to register the maximum and minimum temperatures a short column of alcohol is placed in the upper portion of the stem, above the mercury, and within this are arranged two small steel indexes, so constructed that they will not slide in the tube of their own weight, but are easily pushed upward by the mercury column or pulled downward by the top meniscus of the alcohol column. The indexes are set by means of a small magnet, the one being drawn down upon the top of the mercurial column and the other raised up against the meniscus of the alcohol column.

The rise of the mercury carries its index upward, leaving it to register the highest point reached, while the alcohol meniscus withdraws the other index and leaves it at a point representing the minimum temperature. It remains only to mention that the graduations are fixed in the usual way, having reference only to the positions of the mercurial column. Beyond the highest point supposed to be reached by the mercury, say about 120°, the graduations are extended in an arbitrary manner. The scale numbers represent temperatures by the mercurial column and are continued in regular sequence beyond the 120°. On this plan the readings for minimum temperatures are on a purely arbitrary scale and are converted into true degrees of temperature by use of a table prepared for each thermometer, which table embodies as well all the corrections for instrumental error.

The arrangement of the alcohol columns above the mercurial column and the indexes are shown enlarged at one side of the illustration. The readings of the maximum temperature are made from the bottom end of the index next to the mercurial column. The minimum temperature is the reading of the top of the uppermost index. Thus in the figure the maximum temperature indicated is 76.5°, and the minimum 125.7°, which, by reference to the table of correction for this thermometer, No. 10, is found to be 53.3°.

The use of mercury in the stem of the thermometer not only admits of the use of the index for registering the maximum temperature, but possesses the additional advantage of reducing the error due to uncertain temperature of the stem to about one-sixth what it would be if alcohol were used. Moreover, if necessary, as in the case with thermometers for greater depths than that figured, the ungraduated portion of the stem can be made of very much finer bore than the graduated portion, the effect of which is to diminish the objectionable error to a comparatively unimportant quantity.

The chief objection to thermometers of this construction is the liability of alcohol getting from the bulb into the stem during the processes of construction, graduation and subsequent handling, and the difficulty of safely shipping them.

When once set up, however, there seems to be little or no possibility of derangement and the error common to mercurial thermometers due to rise of the freezing point with age does not apply owing to the high coefficient of expansion of the alcohol used in the bulb.

APPLICATIONS OF SOIL THERMOMETRY.

116. Estimation of the Absorption of Heat by Soils.—A cubical zinc box, six centimeters square, is filled with the sifted air dried soil. The box, one side of which is left open, is encased snugly in a wooden cover, exposing only the open end, and placed for a few hours in the direct rays of the sun. The temperature is then taken at a given depth. The box may be provided with thermometers at different depths, the bulbs thereof extending to the center. In this case the box should be covered with thick felt instead of wood. The temperature of the layers of soils of different depths can thus be read off directly. The air temperature directly above the box should be accurately noted while the experiment continues.

Any other kind of box well protected against all heat save the direct sunlight on the open surface of the soil will answer as well as the one described.

To determine the action of moist earth in similar conditions the soil may be previously moistened; the per cent of moisture being determined in a separate portion of the soil or the amount of water added to the air-dried soil being noted.

117. Estimation of the Conductivity of Soils for Heat.—The bulb of a thermometer is placed in the middle of a mass of fine earth which is then exposed, best in a metallic box painted with lamp black, in a warm place. The time required for the thermometer to reach a certain degree is noted. By reversing the experiment and placing the mass of earth heated to a given degree in a cool place the conductivity can be determined by the time required for the mercury in the thermometer to fall to any given point.

The experiment may also be made by packing the soil by gently jolting it in a glass tube six to eight centimeters in diameter. One end of the tube is closed with a piece of metal or fine wire gauze painted with lamp black and is exposed to the source of heat. The bulb of a thermometer is placed at a given distance from the end of the tube and the time for the mercury to be affected observed.

COHESION AND ADHESION OF SOILS.

118. Behavior of Soil After Wetting.—The deportment of a soil when thoroughly wet in respect of its physical state on drying out is a matter of great practical concern to the agronomist. Some soils on becoming dry fall into a pulverulent state and are easily brought into proper tilth; others become hard and tenacious, breaking into clods and resisting ordinary methods of pulverization. The physical laws which determine these conditions depend largely on the principles of flocculation soon to be described. The present task is to describe briefly some of the methods of estimating the force of cohesion and adhesion.

119. General Method.—The fine earth, air-dried, is mixed with enough water to make a paste and molded into forms suitable for trial in a machine for testing strength of cement, etc. The forms most used are cakes three to five centimeters in length and one to two centimeters thick. These are used for determining crushing power. For longitudinal adhesion the paste may be molded in prismatic or cylindrical shape.^[78] The prisms should show one to two centimeters in cross section or the cylinder be one to two centimeters in diameter. Before use they are to be exposed for several days until thoroughly air-dried. The force required to separate or crush these prepared pieces will measure the adhesive or cohesive property of the sample. A great number of trials should be made and the mean taken.

120. Method of Heinrich.^[79]—This process consists in mixing the air-dried earth with water until its aqueous content is fifty per cent of the highest water capacity determined by experiment. The sample is next placed between two pieces of sheet iron of ten centimeters square, each of which in its middle point is provided with a hook. The thickness of the layer between the two pieces of iron should be about five to ten centimeters. The exuding particles of soil are cut off with a knife. The upper piece of sheet iron is next suspended by a cord in such a way that the iron piece occupies a horizontal position. A small basket is attached to the lower surface and sand added thereto, little by little, until the column of earth is separated. The sand basket and iron plate are weighed, and the total weight gives the power necessary to separate a column of soil ten centimeters square in cross section. The iron plates may be roughened so that the adhesion thereto of the soil is greater than its cohesive force.

121. Adhesion of Soil to Wood, Iron, Etc.—The adhesive power of moist soil for wood, iron, etc., is measured by Heinrich^[80] in the following way: The soil is mixed with water, as above, until it contains just fifty per cent of its total water-holding content. It is then placed in a large vessel and the upper surface made as smooth as possible. A plate of wood, iron, etc., of ten centimeters square is then pressed on the surface until a complete contact is secured. This plate, by means of a hook and cord passing over a pulley, is then subjected to stress by weighting the cord which carries a basket for that purpose. The basket should be of the same weight as the plate in contact with the soil. The weight added to the basket necessary to separate the plate from the soil is taken to represent the cohesive force. The author of the method appears to take no account of the pressure of the air on the plate caused by the exclusion of the air from its under surface.

THE ABSORPTIVE POWER OF SOILS FOR SALTS IN SOLUTION.

122. General Principles.^[81]—It is a fact of every-day observation that soils have a particular property of absorbing certain materials with which they come in contact. If it were not for this property all our wells would soon become unwholesome from the reception of decayed animal and vegetable matter carried to them in the drainage water from the surface. It is also a well-known fact that burying dead bodies prevents the gaseous products of decomposition from reaching and vitiating the atmosphere.

Besides this well-known power of soils to absorb the decomposition products of animal and vegetable matter, they also possess a property which is of far greater importance in plant economy; that is, the power of withdrawing and retaining certain mineral constituents from their solutions.

As far back as the sixteenth century mention is made by Lord Bacon of a process for obtaining pure water on the seashore by simply digging a hole in the sand and allowing it to fill with filtered sea water, which by this means is deprived of its salt. Although certain facts were observed by some of the earlier writers in regard to soil absorption, no systematic researches were conducted with a view of demonstrating the extent and cause of this power until within a comparatively few years.

In 1850 Prof. Way published in the Journal of the Royal Agricultural Society of England, the results of a thorough and most excellent investigation of the subject. Since then many distinguished chemists, such as Henneberg, Stohmann, Peters, Heiden, Knop, Ullik, Pillitz, Biedermann, Tuxen, and others have given their attention to this matter.

123. Summary of Data.—If a solution of a soluble sulfate, chloride or nitrate of an alkali or an alkaline-earth metal be placed in contact with a soil, the result is that the soil takes up a part of the base but none of the acid. This absorption of base is attended with the liberation of some other base from the soil which combines with the acid of the solution. Any alkali or alkaline earth base has the power of replacing any other such base. However, if soluble phosphates and silicates of these bases be placed in contact with the soil both the base and the acid are removed from the solution.

Peters^[82] has shown that the amount of absorption depends upon the concentration of the solution, the relation between the quantity of solution and the soil and the kind of salt used. He treated 100 grams of earth with 250 cubic centimeters of solutions of different potash salts with the following results:

Biedermann^[83] proves that, for phosphoric acid at least, the absorption increases with the temperature.

It has also been found that the amount of absorption depends upon the time of contact between the soil and solution. Way found that the absorption of ammonia was complete in half an hour, while Henneberg and Stohmann^[84] noticed that the phosphoric acid continued to be fixed after the expiration of twenty-four hours.

It is a very important fact that the absorption of a base is never complete; no matter how dilute the solution it will still carry a small portion of the base with it. Peters states that it requires about 28,000 parts of water to remove one part of absorbed potash and Stohmann found that it required about 10,000 parts of water to remove one part of absorbed ammonia. With phosphoric acid, the resulting compound seems to be much more insoluble.

According to Tuxen^[85] the presence of salts of soda and potash in solution decreases the power of a soil to absorb ammonia compounds and the presence of sodium salts decreases the power of a soil to absorb potash. On the other hand the presence of potassium compounds considerably increases the absorption of phosphoric acid. He further affirms that the compounds of potash, phosphoric acid, etc., formed in the soil, are decidedly more soluble in sodium salts than in pure water.

124. Cause of Absorption.—The withdrawing and fixing of phosphoric acid from solutions by the soil is not very difficult to understand as this acid forms insoluble compounds of iron, lime, and magnesium, some or all of which are present in all soils. As to the absorption of the alkalies, the explanation is far more difficult as nearly all of their ordinary compounds are readily soluble in water.

As lime is usually found combined with the acid part of an alkali salt, from which the base has been absorbed by the soil, it might naturally be supposed that the absorptive power of the soil would depend upon the amount of lime present. Way found, however, that the addition of chalk in no way influenced the absorption of ammonia by a soil which contained but a small amount of lime. This fact was also confirmed by Knop^[86] who found that chalk exerted no influence on the absorption of ammonia salts. These facts would seem to point to the conclusion that lime was present in sufficient quantity in these experiments, or that it is not essential to the phenomena of absorption. However, as any alkali or alkaline-earth base can replace any other such base, the presence of lime in the filtrate is probably more of an accidental occurrence, owing to the comparatively large amount of that substance in most soils, than a necessary condition, as any other base would doubtless answer in the absence of lime.

125. Warington^[87] has shown that hydrated oxides of iron and aluminum, and especially the former, are capable of absorbing potash and ammonia, and as more or less of these hydrates exist in nearly all soils, a part, at least, of absorptive phenomena is to be ascribed to them.

126. Way tried to determine which of the constituents of a soil exercised chiefly the absorptive power. He passed a solution of ammonia through tubes containing pure sand and found that it came through apparently unaltered from the first, while a soil treated in the same way removed the ammonia for a considerable time. He concluded from this that the absorptive power does not exist in the sand. He next oxidized the organic matter in a soil with nitric acid and then treated it with ammonia in the same way. The first portions of the filtrate showed no ammonia in any form, hence he concluded that organic matter is not essential to the act of absorption. He further showed that clay alone is capable of causing absorption phenomena, by treating powdered clay tobacco pipes with ammonia.

Having shown that clay was the main constituent in a soil which caused the absorption of alkalies, he tried next to trace out the particular compound which caused the absorption. Having tried various natural silicates he at last succeeded in producing a hydrated silicate of aluminum and soda which exhibited displacement and absorptive properties very similar to those shown by the soil.

As Way had succeeded in producing an artificial hydrated silicate possessing absorptive properties, Eichorn^[88] thought of trying natural hydrated silicates or zeolites and found that they exhibited the same power as Way’s artificial preparation. It has also been shown by Biedermann,^[89] Rautlenberg,^[90] and Heiden^[91] that the absorptive power bears a close relation to the amount of soluble silicates present.

In view of these facts it is now generally accepted that the absorption of salts of the alkalies, accompanied by the change of base, is due chiefly to the presence of decomposed zeolite minerals in the soil.

Besides the purely chemical absorption of salts by the soil, we have a physical absorption of various substances similar to the action of charcoal when used as a filter.

127. Conclusions of Armsby.—The data connected with the absorption of bases by a soil have also been reviewed by Armsby.^[92] He shows that the absorption is accompanied by a chemical reaction between the salt whose base is absorbed and some constituent of the soil, and this change seems to be due particularly to certain zeolitic silicates, although Liebig and others were disposed to credit this absorption largely to physical causes.

Knop advances the idea that the soil has the power of disintegrating salts in the presence of some substances like calcium carbonate which can unite with the acid. In experiments made with hydrous silicates it was shown that the absorption resembled in all cases like phenomena in the soil; hence the supposition already advanced in regard to the influence of such silicates is doubtless true.

In respect of absorption in general, the following conclusions were reached:

1. The absorption of combined bases by the soil consists in an exchange of bases between the salt and the hydrous silicates of the soil.

2. This exchange, which is primarily chemical, is only partial, its extent varying

(a) with the concentration of the solution, and

(b) with the ratio between the volume of the solution and the quality of soil used.

3. The cause of these variations is probably the action of mass or the tendency of resulting compounds to re-form the original bodies, the absorption actually found in any case marking the point where the two forces are in equilibrium.

128. Selective Absorption of Potash.—As a rule more potash is absorbed from the sulfate than from the chlorid. This fact would seem to point to the advisability of using sulfate as a fertilizer in preference to chlorid. However, as with the exception of nitrates, the absorptive power of a soil, for the salts used as fertilizers, is many times greater than it is ever called upon to exert in fixing applied fertilizers, we need not trouble ourselves in regard to the absorption of phosphoric acid, potash or ammonia, in so far as the practical side of the matter is concerned. For example, an acre of soil to the depth of nine inches weighs about 900 tons. Now it has been found by Huston,^[93] that 100 parts of a soil experimented upon absorbed over 0.25 part of P₂O₅, hence 900 parts would absorb over 2.25 parts of P₂O₅; or an acre of this soil to the depth of nine inches would absorb over two and one-fourth tons of phosphoric acid. 500 pounds per acre is a large dressing of a phosphatic fertilizer for field crops and 500 pounds of a high grade fertilizer would contain about 100 pounds of P₂O₅; hence the power of such a soil to absorb phosphoric acid is more than forty-five times as great as it is ever likely to be called upon to exert in fixing the phosphoric acid added to it as a fertilizer.

Huston has further shown that an acre of soil nine inches deep will absorb more than 2.7 tons of potash (K₂O) from potassium chlorid from which salt less potash is absorbed than from the sulfate. Now one-tenth ton of potassium chlorid per acre would be a large dressing of potash, hence this soil possesses the power of absorbing more than twenty-seven times as much potash as is ever likely to be applied as a fertilizer.

In like manner it may be shown that the power of an acre of soil nine inches deep to absorb ammonia from ammonium sulfate is more than thirty-two times as great as it would be called upon to exert in fixing the ammonia from a dressing of one-quarter ton of ammonium sulfate per acre.

With sodium nitrate, however, there is no absorption; hence great care is necessary in the application of nitrogen as a nitrate, for, if it be put on in large quantities, at a season when the plant is not prepared to assimilate it, or during a period of heavy rains, there must unavoidably result loss from drainage. The best time to apply a nitrate is evidently during the active growing season.

129. Whitney^[94] places great emphasis on the surface area of soil particles in respect to their power to absorb solutions of salts. The approximate surface area of a cubic foot of each of the different typical soils of Maryland is as follows:

It will be seen that there are about 24,000 square feet of surface area in a cubic foot of the subsoil of the pine barrens, no less then 100,000 square feet or two and three-tenths acres of surface area in a cubic foot of the subsoil of the river terrace, and 200,000 square feet of surface area in a cubic foot of the limestone subsoil.

These figures seem vast, but they are probably below rather than above the true values, on account of the wide range of the diameters of the clay group. This great extent of surface and of surface attraction, which has been described as potential, gives the soil great power to absorb moisture from the air, and to absorb and hold back mineral matters from solution. A smooth surface of glass will attract and hold, by this surface attraction, an appreciable amount of moisture from the surrounding air. A cubic foot of soil, having 100,000 square feet of surface, should be able to attract and hold a considerably larger amount of moisture.

It might have been added that if the potential of the surface, separating the solution from the soil, be greater than the potential in the interior of the liquid mass, there will be a tendency to concentrate the liquid on this surface of separation. It has been shown that certain fluids have greater density on a surface separating the fluid from a solid. On the other hand, if the potential were low there might be no tendency for this concentration, and even the reverse conditions would prevail and the soluble substance could be readily washed out of the soil.

130. Removal of Organic Matters.—It is probably largely due to this straining power that organic matters are removed from solutions in percolating through the soil. Whitney^[95] has observed that the organic matter may be coagulated and precipitated from solution by the soil constituents, and held in the soil in loose flocculent masses, while the liquid passes through nearly free of organic matter.

131. Importance of Soil Absorption.—The importance of the absorptive power of the soil can hardly be overestimated. By means of this power those mineral ingredients of plant food, of which most soils contain but little, are held too closely to allow of rapid loss by drainage, and still sufficiently available to answer the needs of vegetation, provided the store is large enough. The only important plant food liable to be deficient in the soil which does not come under the influence of absorption is nitrogen in the form of salts of nitric acid, and nature has made a wide provision for this element by binding it in the form of organic bodies which nitrify but slowly, and by supplying each year a small quantity from the atmosphere.

By means of the absorptive power of soils the farmer, if he puts on an excess of potash or phosphoric acid as a fertilizer, does not lose it but is able to reap some benefits from it in the next and even in succeeding crops. If it were not for this power the best method for applying fertilizers would be a much more complicated problem than it is at present; and it would be necessary to apply them at just the proper season and in nicely regulated amounts to insure against loss.

132. Method of Determining Absorption of Chemical Salts.—The soil which is to be used for this experiment should be treated as has been indicated and passed through a sieve the meshes of which do not exceed half a millimeter in size. From twenty-five to fifty grams of the fine earth may be used for each experiment.

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.

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.

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:

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:

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.: