Chapter VI.. The Film Water.

THE FILM WATER.

When a relatively small quantity of water is added to an absolutely dry soil or other powdered solid, there is some shrinkage in the apparent volume of the soil or powder. The water spreads over the surfaces of the solid particles in a film, and a rise in temperature shows that a noticeable energy change accompanies the formation of the film. With further increments of water the apparent volume of the soil increases until a maximum is reached. The water content at which this maximum volume of soil can be attained is a definite physical characteristic for any given soil. What is popularly known as the “optimum water content” corresponds to this critical content. It is the point at which further additions of water will not increase the thickness of the moisture film on the soil grains, but will give free water in the soil interstices. Just as the apparent volume of a given mass of soil varies with the water content, and reaches a maximum at a critical moisture content, so do all the physical properties vary and have either a maximum or minimum value at this same critical moisture content. Thus the apparent specific gravity of a soil reaches a minimum, the force required to insert a penetrating tool becomes a minimum, while the rate at which a soil warms up reaches a maximum, and the ease with which aeration takes place reaches a maximum. In fine, this critical water content is that at which the soil can be brought into the best possible physical condition for the growth of crops. The practical significance of the optimum water content is far greater than would be supposed from the attention given it hitherto by students of the soil. It is the content of soil water which the greenhouse man should strive to maintain, and which the irrigation farmer should seek to provide, instead of the over-wetting so common to the practice of both. In general farming it is that moisture content at which the farmer will attain the best results in plowing and cultivating, and attain these results most readily.

See, in this connection, Energy changes accompanying absorption, by Harrison E. Patten, Trans. Am. Electrochem. Soc., 11, 387-407, (1907); see also the recent valuable research, Les dégagements de chaleur qui se produisent an contact de la terre sèche et de l’eau, par A. Muntz et H. Gaudechon, Ann. sci. agron. (3), 4, II, 393-443, (1909), where it is shown that probably a part of the heat is due to chemical combination between the water and the other soil components. To quote, “Ces diverses observations nous conduisent à penser, sans nous en donner toutefois la preuve absolute, que la fixation de l’eau sur les éléments terreux très fins et sur les matériaux organisés, est tout au moins, en partie, attribuable à une combinaison chimique qui se manifeste non seulement par un fort dégagement de chaleur, mais aussi par la soustraction de l’eau à des substances aux-quelles elle semble chimiquement liée.”

The moisture content and physical condition of soils, by Frank K. Cameron and Francis E. Gallagher, Bull. No. 50, Bureau of Soils, U. S. Dept. of Agriculture, 1908. See also Über physikalische Bodenuntersuchung, von H. Rodewald, Schriften Naturwiss. Vereins Schleswig-Holstein, 14, 397-399, (1909).

Heat transference in soils, by Harrison E. Patten, Bull. No. 59, Bureau of Soils, U. S. Dept. Agriculture, 1909.

With additions of water beyond the critical point, there is a presence of free water in the soil interstices accompanied by important changes in the soil structure. With continued additions, there is a more or less rapid decrease in the apparent volume; there is a tendency for the soil aggregates to break down and the “crumb structure” so greatly desired by agriculturists is less and less readily obtained, and working of the soil tends in some cases to produce that phenomenon known as “puddling.” However desirable the property of puddling may be to the potter or the brick maker, to the farmer it is a bane to be avoided above all things. To overcome it requires his best skill, and it usually takes several years of patient effort to restore a puddled soil to good tilth.

The force with which the film water is held against the soil grains has not been determined as yet with any degree of precision, but it is certainly very great. If a soil be saturated, that is, if so much water be added that further additions will cause a flow of free water, and the soil be then submitted to some mechanical device for abstracting the water, the moisture content of the soil can be readily diminished to the critical water content; but to diminish it further by mechanical means is not easy. The tenacity with which film water is held by the soil grains has been shown in several ways. In one of these, for instance, a semi-permeable membrane was precipitated in the walls of a porous clay cell, which was then filled with sugar solution having an osmotic pressure of about 35 atmospheres. When this cell was buried in a soil having a moisture content above the optimum, water flowed into the cell. On the contrary, when the cell was buried in another sample of the same soil having a moisture content well below the optimum, there was a marked flow of water from the cell. It would appear, therefore, that the attraction between the soil grains and the film-forming water was certainly greater than the solution pressure of the sugar. Again, by whirling wetted soils in a rapidly revolving centrifuge, fitted with a filtering device in the periphery, and developing a force equivalent on the average to 3,000 times the attraction of gravitation, the soils could not be reduced below the critical water content. From the results of Lagergren, Young, and Lord Rayleigh, it appears that the force holding a very thin moisture film on the soil grains would be of an order of magnitude from 6,000 to 25,000 atmospheres. This force, however, must greatly decrease with thickening of the film, as is shown by the fact that at the critical moisture content a small further addition of water produces no marked heat manifestation, though making a noticeable difference in the physical properties of the soil. Therefore, while recognizing that our knowledge of this force still lacks a desirable precision, it is nevertheless clear that the force is very great.

The chemistry of the soil as related to crop production, by Milton Whitney and Frank K. Cameron, Bull. No. 22, Bureau of Soils, U. S. Dept. Agriculture, 1903, p. 54.

The moisture equivalent of soils, by Lyman J. Briggs and John W. McLane, Bull. No. 45, Bureau of Soils, U. S. Dept. Agriculture, 1907.

Über die beim Benetzen fein verteilter Körper auftretende Wärmetönung, von Lagergren, Bihang till K. sv. Vet.-Akad., Handl., 24, Afd. II, No. 5, (1898).

Hydrostatics and elementary hydrokinetics, George M. Minchin, p. 311, 1892.

On the theory of surface forces, by Lord Rayleigh, Phil. Mag. (5), 30, 285-298, 456-475, (1890).

The function of the film water in maintaining the soil structure is undoubtedly important. A soil in good tilth, or good condition for crop growth, shows a peculiar structural arrangement of the individual soil grains or soil particles, which it is very difficult to describe in precise terms, but which is readily recognized in practice. This condition is usually described as a “crumb structure,” either because of its appearance or because of the peculiar crumbly feeling which a soil in this condition gives when rubbed between the fingers. The individual grains of soil are gathered into groups or floccules. While other causes may be more or less operative in particular cases, it seems very probable that the film water is primarily the agency holding together the grains in these floccules. The obvious explanation is that the film is exerting a holding power because of its surface tension. It follows, therefore, that anything which affects the surface tension of water should affect the structure of the soil; that is, the flocculation or granulation of the particles. But certain agents which produce respectively flocculation or deflocculation, nevertheless modify the surface tension of the solution in the same direction, and in not widely varying degree. Similar difficulties arise in attempting to correlate “crumbing” phenomena with the viscosity of the film water, and it must be admitted frankly that present views on this subject are very unsatisfactory, and that more careful investigation is urgently needed on this fundamental and important problem. Not only is the absence of a satisfactory theory embarrassing in considering the problems of soil structure and a rational control, but the difficulties are no less in the equally important problems of the movement of film moisture, and the distribution of moisture in a soil.

Equally unsuccessful is the attempt to correlate flocculating agents with changes in the density of water. See, The condensation of water by electrolytes, by F. K. Cameron and W. O. Robinson, Jour. Phys. Chem., 14, 1-11, (1910).

The movement of moisture into a soil from an illimitable supply is a comparatively simple phenomenon, controlled by a rate law which may be expressed by the equation yⁿ = kt when y is the distance through which the movement has taken place; t is the time, and k and n are characteristic constants for the particular soil and solution. This expression may be more readily recognized as a rate formula when written dy/at = Ayᵐ, where A and m are constants for the particular system. The first form of the equation promises to be the more useful. This formula also describes the rate of advance of a dissolved substance into the soil.

Owing to irregularities in the soil column this equation is more readily studied with capillary tubes or with such absorbents as filter-paper or blotting paper. The following tables will, however, give an idea as to its validity for soils.

ALLUVIAL SOIL, GILA RIVER.

===============+====================+================= Time,t min. | Height,y inches | k (n = 1.86) ---------------+--------------------+----------------- 2 | 1.5 | 1.05 5 | 2.4 | 1.02 10 | 3.6 | 1.08 15 | 4.3 | 1.01 30 | 6.3 | 1.05 60 | 9.2 | 1.07 ---------------+--------------------+-----------------

DISTILLED WATER IN PENN. LOAM (t = 21° C).

==========+==============+================ Time,t | Height,y | k min. | cm. | (n = 2.25) ----------+--------------+---------------- 1 | 1.15 | 1.37 2 | 1.54 | 1.33 3 | 1.85 | 1.33 4 | 2.08 | 1.30 5 | 2.28 | 1.28 7 | 2.59 | 1.21 10 | 2.97 | 1.16 15 | 3.47 | 1.10 20 | 3.90 | 1.07 30 | 4.67 | 1.06 40 | 5.39 | 1.11 50 | 5.90 | 1.09 60 | 6.47 | 1.12 75 | 7.20 | 1.13 90 | 8.03 | 1.21 105 | 8.72 | 1.25 ----------+--------------+----------------

See Bull. No. =30=, Bureau of Soils, U. S. Dept. Agriculture, p. 50 et seq.; also, The flow of liquids through capillary spaces, by J. M. Bell and F. K. Cameron, Jour. Phys. Chem., =10=, 659, (1906); See also, Wo. Ostwald, 2 Supplementheft Zeitschrift Kolloidchemie, 1908, 20.

Computed from observations by Loughridge, Report Agr. Expt. Sta., University California, 1893-94, p. 93.

INDIGO CARMINE IN PENN. LOAM SOIL (t = 21° C.).

Solution contained 2 grains dye per liter. =========+============+==============+================+============= Time,t| Height,y | k for water| Height colored | k for dye min. | wet cm. | (n = 2.25)| cm. | (n = 2.25) ---------+------------+--------------+----------------+------------- 1 | 1.28 | 1.75 | 0.64 | 0.37 2 | 1.67 | 1.59 | 0.90 | 0.39 3 | 2.05 | 1.68 | .. | .. 4 | 2.26 | 1.56 | .. | .. 5 | 2.49 | 1.56 | 1.02 | 0.21 7 | 2.74 | 1.38 | .. | .. 10 | 3.20 | 1.40 | .. | .. 15 | 3.72 | 1.29 | .. | .. 20 | 4.28 | 1.32 | 1.92 | 0.22 30 | 5.10 | 1.31 | .. | .. 40 | 5.77 | 1.29 | 2.69 | 0.23 50 | 6.41 | 1.26 | 3.20 | 0.28 60 | 6.90 | 1.29 | .. | .. 75 | 7.46 | 1.23 | .. | .. 90 | 8.74 | 1.46 | 3.59 | 0.20 105 | 9.00 | 1.33 | .. | .. ---------+------------+--------------+----------------+-------------

It has also been shown repeatedly by experiment that the movement of moisture is relatively rapid when the moisture content of the soil is above the optimum, but that the movement is exceedingly slow when the soil has a lower water content than the optimum; that is, the point at which the water is entirely in the form of film water. For instance, if a moderately wet sample of soil be brought into intimate contact with an air-dry sample of the same soil, there will, at first, be a relatively rapid movement of the moisture, but as soon as the wetted portion has been brought to the “optimum” condition, no further movement can be detected, although the experiment has been tried of leaving such samples together for months and with a difference of water content amounting, in the case of clay soils, to 15 or 20 per cent. Since the drought limit, or the soil moisture content at which plants wilt, is, for most soils, considerably below the optimum water content, the movement of film water is obviously a problem of the first importance from a practical point of view as well as of the highest theoretical interest.

The movement of water vapor, or its distillation from place to place in the soil, is another problem often confused with the above. Its importance is not yet clear, although according to some investigators it would appear that the addition of soluble fertilizer salts by causing a lowering of the vapor pressure of the water induces a distillation to that region from other regions of the soil as well as from the atmosphere above. This brings up the problem of the diffusion of water and other vapors through the soil. It has been shown that the soil “plug” retards the rate at which diffusion takes place but induces no other effect in the ordinary phenomenon of free diffusion. This fact is obviously of the first importance in the theory of mulches, but requires no further consideration here.

Sur la diffusion des engrais salins dans le terre, par Muntz et Gaudechon, Comptes rendus, =148=, 253-258, (1909).

See, Contribution to our knowledge of the aeration of soils, and Studies of the movement of soil moisture, by Edgar Buckingham, Bulls. Nos. =25=, 1904, and =33=, 1907, Bureau of Soils, U. S. Dept. of Agriculture.