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.[27] 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.[28] 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,[29]
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.
[27] 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.”
[28] 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).
[29] 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.[30] Again, by whirling wetted soils in a rapidly revolving
centrifuge,[31] 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,[32] Young,[33]
and Lord Rayleigh,[34] 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.
[30] 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.
[31] The moisture equivalent of soils, by Lyman J. Briggs and John W.
McLane, Bull. No. 45, Bureau of Soils, U. S. Dept. Agriculture, 1907.
[32] Über die beim Benetzen fein verteilter Körper auftretende
Wärmetönung, von Lagergren, Bihang till K. sv. Vet.-Akad., Handl., 24,
Afd. II, No. 5, (1898).
[33] Hydrostatics and elementary hydrokinetics, George M. Minchin, p.
311, 1892.
[34] 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,[35] 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.
[35] 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.[36]
This expression may be more readily recognized as a rate formula
when written _dy/at_ = A_yᵐ_, 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.[37]
===============+====================+=================
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
----------+--------------+----------------
[36] 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.
[37] 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[38] 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.[39]
[38] Sur la diffusion des engrais salins dans le terre, par Muntz et
Gaudechon, Comptes rendus, =148=, 253-258, (1909).
[39] 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.