ABSORPTION BY SOILS.
A property of soils, affecting profoundly the composition and
concentration of the soil solution, is absorption.[70] It is generally
recognized that soils are good absorbers for vapors, and this fact
finds practical expression in the common practice of burying things
with a disagreeable odor, such as animal carcasses, night-soil, etc. It
is also well-known that dissolved as well as suspended material can be
more or less completely removed from water by passing it through sand
or soil, and this fact finds important application in water supplies
for cities and towns, sewage disposal, etc. It was known as long ago
as Aristotle’s time that ordinary salt is partly removed from water by
passing through sand or soil. In recent times the practical as well
as theoretical importance of this phenomenon has led to considerable
study and experimental research, so that our knowledge of absorption
effects is now fairly extensive, though it can hardly be claimed that
it is satisfactory. The absorption of a dissolved substance from
solution by a soil may be one or more of at least three kinds of
phenomena. It may be a mechanical inclusion or trapping, distinguished
by the term _imbibition_, the most familiar and striking case being
the absorption of water itself by soil or sponge or similar medium. It
may be a partial taking up of the dissolved substance to form a new
compound or a _solid solution_,[71] as probably is the absorption of
phosphoric acid by lime or ferric oxide. Or it may be a condensation
or concentration of the dissolved substance on or about the surface
of the absorbing medium, a phenomenon known as _adsorption_. To prove
the existence of adsorption definitely and conclusively in any given
case is always difficult, if ever possible, but the existence of this
phenomenon is the most logical explanation of many observations, and is
generally admitted by chemists and physicists at the present time.[72]
It is by adsorption, probably, that potash and ammonia are held by the
soil when added in fertilizers.
[70] For a detailed discussion and citations of the literature, see:
Absorption of vapors and gases by soils, by H. E. Patten and F. E.
Gallagher, Bull. No. =51=; and Absorption by soils, by H. E. Patten
and W. H. Waggaman, Bull. No. =52=, Bureau of Soils, U. S. Dept.
Agriculture, 1908.
[71] That is, a homogeneous solid, which may be either crystalline or
amorphous. Probably the readiest criterion for distinguishing between
a definite compound and a solid solution, is that the former is stable
in contact with a liquid solution of its constituents over a measurable
range of concentrations, while the composition of the solid solution
changes with every change in the concentration of the liquid solution
in contact with it.
[72] A clear and apparently indisputable case of adsorption has been
noted by Patten (Some surface factors affecting distribution, Trans.
Am. Electrochem. Soc., =10=, 67-74, (1906). On adding powdered quartz
to an aqueous solution of gentian violet, there is a distribution of
the dye between the water and the quartz. A microscopic examination of
the latter showed that the dye was concentrated in thin layers upon the
surface of the quartz grains, from which it could be washed with water,
no change in the quartz grains being noticeable.
That absorption is dependent in some manner upon the solubility of the
dissolved substance in the particular solvent employed would seem to
be obvious. But what the relation may be, if it exists at all, is not
known. For instance, silk absorbs picric acid from solutions in water
and alcohol but not from solutions in benzene, although the solubility
of picric acid in benzene lies between its solubility in water and in
alcohol.[73]
[73] Absorption of dilute acids by silk, by James Walker and James R.
Appleyard, Jour. Chem. Soc., =69=, 1334-1349, (1896).
The absorption of any given dissolved substance from different solvents
is markedly different. Most soils absorb methylene blue from aqueous
solutions with great avidity, but washing out the absorbed dye with
water is an extremely tedious and unsatisfactory process, although the
dye can be readily and more or less completely removed from the soil
by alcohol. As might be anticipated from this, it is known that the
presence of one dissolved substance affects the absorption of another,
but in what way can not, generally, be anticipated, although it would
seem that the importance of this subject for manurial practice would
invite further research.
From the same solution, different absorbents remove a dissolved
substance in different degrees. Speaking generally, paper absorbs dyes
more readily than do soils, while soils absorb bases more readily than
does paper. Hence the reddening of litmus paper when in contact with a
moist soil. Heavy soils or soils containing much hydrated ferric oxide
absorb bases more readily than do light soils, but this is probably
owing to relative amounts of surface exposed, for the same relation
holds true with respect to phosphoric acid. Soils rich in humus are
better absorbers than soils not so rich. But here again there is yet
doubt as to whether the explanation lies in the amount or in the kind
of surface acting.
From the same solvent different dissolved substances are absorbed quite
differently by any given absorbent. This can be readily illustrated
again by dyes. If an aqueous solution of a mixture of methylene
blue and sodium eosine, for instance, be shaken up with a soil, or
percolated through a column of soil, the methylene blue is absorbed the
more quickly and completely and a partial separation of the two dyes
can be readily effected, the separation being more or less complete
according to the conditions of the experiment. In the same manner two
salts in solution can be separated partially at least.[74] Soils absorb
potassium more readily than sodium; magnesium more readily than lime;
and ammonia more readily than any of these bases.[75]
[74] For a number of interesting examples, see, Ueber das Aufsteigen
von Salzlösungen in Filtrirpapier, von Emil Fischer und Edward
Schmidmer, Liebig’s Annalen der Chemie, =272=, 156-169, (1893).
[75] The prompt absorption of a base by soils is shown by the following
experiment: To some freshly boiled distilled water add several drops
of alcoholic phenolphthalein, and then just enough base to produce
a decided red color. If the solution be now passed through a short
column of soil, cotton, shredded filter-paper or similar absorbent, the
percolate will be perfectly colorless. The red color will be restored,
however, by adding a little of the base to the percolate.
The absorption from aqueous solutions of inorganic salts involves a
most interesting complication. Just as a mixture of two or more dyes
or salts in solution can be separated by the selective action of an
absorbent, so can an electrolyte itself be decomposed or resolved.
Thus, if a solution of potassium chloride be passed through a column
of soil, or cotton, or paper, or any similar absorbent, the filtrate
will not only be less concentrated than the original solution, but the
potassium will be found to have been absorbed to a greater extent than
the chlorine, that is, the percolate contains free hydrochloric acid.
The importance of this phenomenon for the conservation of the desirable
constituents of manurial salts, and the elimination or leaching out
of the less desirable constituents is obviously great. Equally great
perhaps, is the effect upon the reaction of the soil, whether it be
rendered temporarily alkaline or acid, an effect of the very greatest
importance for the growth of some of our common crop plants[76] and
for the lower soil organisms, such as the bacteria, molds, together
with ferments, enzymes, etc., many of which are very sensitive to the
reaction of the media in which they may be, and which in turn are of
undoubted importance in determining the fertility of the soil for
higher plants.
[76] See, The toxic action of acids and salts on seedlings, by F. K.
Cameron and J. F. Breazeale, Jour. Phys. Chem., =8=, 1-13, (1904). It
is quite conceivable, for instance, that if the drainage conditions
were not exceptionally good under a heavy type of soil, it might be
rendered temporarily unfit for clover or alfalfa by a heavy application
of potassium salts or of sodium nitrate. The idea put forward by some
authorities that too long continued or over fertilizing renders soils
acid, may have better foundation than their theoretical reasoning would
seem to warrant.
The absorption of a dissolved substance from solution by an absorbent
is in effect a distribution phenomenon and the simplest formula to
give quantitative expression to such a distribution is C/C¹ = K when C
is the concentration in the liquid phase and C¹ the concentration in
the solid phase, K being a characteristic constant for the particular
case under consideration. When a chemical reaction or a change of
state, chemical or physical, is involved in the absorption in either
dissolved substance or absorbent the formula becomes Cⁿ/C¹ = K when
_n_ is a function which may be very simple or very complex. Attempts
to develop a precise formula of this general type for the absorption
by some given soil, although such a formula would be desirable for
theoretical and practical reasons alike, have uniformly failed. A
sufficient reason for this failure seems to lie in the fact that most
dissolved substances produce an appreciable effect on the granulation
or flocculation of the soil particles, which is progressive with
the absorption so that a continual change of absorbing or effective
surface is taking place as the absorption proceeds.[77] Moreover, in
the case of an absorption, with the formation of a continuous film of
the dissolved substance, a new kind of absorbing surface is developed.
Hence _n_ is a function of so difficult a character as to defy thus far
any attempt at formulation.[78]
[77] That mineral fertilizers have a decided influence on the
granulation of soils and the properties dependent thereon, and that
this is of practical importance, is gradually coming to be recognized;
see, for instance, Ein Beitrag zur Kenntnis der Wirkung künstlicher
Dünger auf die Durchlässigkeit des Bodens für Wasser, von Edwin
Blanck, Landw. Jahrb., =38=, 863-869, (1909), and the literature there
cited. Dr. R. O. E. Davis in a yet unpublished investigation has shown
that the addition of soluble salts produces decided effects upon the
soil-moisture relations which affect crop production. The critical
moisture content is displaced, the penetrability, permeability,
specific volume, vapor tension, etc., are affected in measurable
degree, and it appears that the physical functions of mineral
fertilizers are much greater in amount and importance than has been
popularly assumed.
[78] The distribution of solute between water and soil, by F. K.
Cameron and H. E. Patten, Jour. Phys. Chem., =11=, 581-593, (1907).
We cannot therefore predict in any quantitative way what will be
the distribution of a soluble substance such as salts in commercial
fertilizers, for instance, between the solid soil particles and the
soil solution. Empirical experiments show, however, that with the
amount of a soluble salt present under normal conditions in a humid
climate, or as used in fertilizer practice, the absorption of ammonia,
lime, potassium or phosphoric acid is relatively very great, and in a
general way in about the order named.
Absorption is not an instantaneous process. However, the rate at
which a dissolved substance is absorbed is generally quite rapid.
That is, if a soil be stirred or mixed with an aqueous solution, the
absorption takes place very quickly, in the absence of any outside
disturbing influences. The law governing the rate of absorption by
soils has not therefore possessed any great practical interest and
has not been studied from a quantitative point of view, although it
is known qualitatively that the rate is increased by increasing the
concentration of the solution, or by increasing the amount of the
absorbent or at least its effective surface. Two rate equations are
of interest in this connection, and have been carefully studied. The
rate at which a salt or other dissolved substance will advance into an
absorbing soil from a solution is given by the same equation as that
describing the rate of advance of the water itself, _yⁿ_ = _kt_ where
_y_ is the distance and _t_ the time.[79] The constants _n_ and _k_
for the slower moving dissolved substance are different from those for
the water. This equation has probably little importance for ordinary
agriculture, for absorption by the soil from a large (and relatively
illimitable) mass of solution is unusual. That it may have considerable
importance in seepage, irrigation, and some soil engineering problems,
seems quite likely.
The rate at which a soil will absorb a dissolved substance from a
percolating solution is given by the equation
_dx_
————— = K(A - _x_),
_dt_
as has been pointed out above.[80] More interesting and important,
however, is the fact that this same equation describes the rate at
which an absorbed substance is removed from the soil by leaching. In
the case of soils in humid areas _dx_/_dt_ rapidly becomes exceedingly
small as _x_ approaches A, that is, when the amount of soluble material
in the soil becomes small, and is practically constant under such
conditions, as has been pointed out above when describing the removal
of potassium and phosphoric acid from soils by percolating waters. This
formula has a special interest in considering the reclamation of alkali
lands by underdrainage, a problem to which reference will be made later.
[79] See formula, page 28.
[80] See formula, page 47.
Both percolation experiments, as those cited above, and direct
absorption experiments made by shaking up soils with solutions of the
salts in question, show conclusively that the absorption phenomena
taking place in the soil are in harmony with the direct solubility
effects in tending to produce and maintain a solution of a normal
concentration as regards those constituents which it happens are also
derived from the soil minerals.[81] It is an interesting coincidence
that nitric acid (in combination with various bases of course) is very
little absorbed by most soils, and does vary in concentration, not
only in different soils but in the same soil, between wide limits, and
within short intervals of time.[82] The nitrates of the soil are not
derived from minerals, and should more properly be considered with the
organic constituents of the soil solution.
[81] An extreme case is worth citing in this connection. Mr. W. H.
Heileman in studying the influence of various kinds of alkali upon
plant growth, added from 3-4 per cent. of sodium carbonate to soils
known to be otherwise free from alkali. Wheat seedlings grown in the
soils so treated showed no ill effects from the added salt. When
distilled water was percolated slowly through the soils, or shaken up
with them, the resulting solution contained the merest traces of the
alkali.
The ordinary method of determining the lime requirement of a soil
by adding lime water until the solution shows an alkaline reaction,
is another obvious absorption phenomenon, and is not dependent, as
popularly supposed, upon the presence of acids in the soil. Soils which
by no possibility could contain any free acid, frequently absorb very
large amounts of lime in this manner.
[82] Usually, in the growing season, the soil solution has a much
higher concentration with respect to nitrates in the morning than it
has in the evening.
An important application of these views concerning absorption arises
in connection with certain widespread notions concerning soil acidity.
There is a popular belief that most soils are acid, that the soil
solution contains some free acid, mineral or organic, other than
dissolved carbon dioxide, and that a neutral or alkaline solution is
necessary to the successful production of most of our crops. This
belief is, however, unwarranted, for the vast majority of soils yield
an aqueous extract which is alkaline when boiled to expel carbon
dioxide, and some of our crops, for instance wheat, seem to thrive
better in a slightly acid medium. This popular fallacy seems to have
its origin in the fact that most soils when moistened and pressed
against blue litmus paper, redden it. This reddening may sometimes
be due to the actual presence of some acid, or to dissolved carbon
dioxide, but is undoubtedly due in the majority of cases to selective
absorption. Litmus is a red dye of an acid-like character, which forms
a soluble blue salt with the ordinary bases. But it has been shown
that most soils are better absorbents of bases than is paper, whereas
paper is a better absorbent of dye, speaking generally, than is a soil.
Consequently when moist soil is brought into contact with wetted blue
litmus paper the base is absorbed more readily by the soil, and the dye
by the paper, the latter therefore becoming reddened.
The reddening of blue or “neutral” litmus paper can be accomplished
with various absorbents. By pressing the litmus paper between moistened
wads of absorbent cotton the reddening can be readily accomplished,
usually in the course of ten minutes to a half hour. That the
phenomenon is not due to any adhering acid on the cotton can be shown
in the following way: A litmus solution is carefully prepared so that
there is a very small excess of base present over that required to
give the blue color. A wad of absorbent cotton is carefully washed by
repeatedly sousing it in distilled water from which carbon dioxide has
been expelled by boiling. When the cotton has been thoroughly washed,
it is stirred thoroughly in a portion of distilled water, free from
carbon dioxide, then withdrawn by some appropriate instrument and
allowed to drain for a few minutes. The litmus is added in fairly large
quantity to the drainings, which should then have a blue color. Again
stir the cotton in the water, and more or less quickly, depending on
the amount and purity of the litmus preparation as well as the quantity
of cotton used, the solution will become red. The only criterion for
determining surely that a soil is acid, is to make an aqueous extract,
expel the dissolved carbon dioxide by boiling, or by passing through
the solution an inactive gas, such as nitrogen, and then to test the
reaction of the solution. Acid soils undoubtedly do exist, but they are
by no means common or widespread, and are to be regarded as exceptional
and abnormal.
The phenomena of selective absorption suggest the important part which
surfaces play in modifying and changing chemical reactions.[83] For
instance, Becquerel[84] observed that a solution of copper nitrate or
cobalt chloride diffusing from a cracked test-tube placed in a solution
of sodium sulphide, led to the formation of the corresponding sulphide,
but in the crack the metal itself was precipitated. Experiments of
Graham[85] show that when a solution of silver nitrate is percolated
through charcoal, not only is there a selective absorption as is shown
by the percolate containing free acid, but there is a chemical reaction
involved, since the silver is deposited in metallic spangles in the
interstices of the absorbent. Graham has shown, and since his time
others, that often metals can be separated from solutions of their
salts by such absorbents as charcoal. Spring[86] has shown that at
bounding surfaces of dilute solutions, chemical action is increased.
[83] For references to the literature see, Bull. No. =30=, Bureau of
Soils, U. S. Dept. Agriculture, p. 61 _et seq._
[84] Note sur les réductions métalliques produites dans les espaces
capillaires, par M. Becquerel, Comptes rendus, =82=, 354-356, (1876).
[85] Effects of animal charcoal on solutions, by T. Graham, Quart.
Jour. Sci., I, 120-125, (1830).
[86] Über eine Zunahme chemischer Energie an der freien Oberfläche
flüssiger Körper, von W. Spring, Zeit. physik. Chem., =4=, 658-662,
(1889).
It has been shown that the amount and kind of surface has a marked
influence on the decomposition of hypochlorous acid, carbon dioxide,
phosphine, arsine, and other compounds. Meyer and his associates, as
well as a number of other investigators, have shown that the character
of the surface of the containing vessel greatly affects the combination
of hydrogen and oxygen. Many reactions have been investigated by van’t
Hoff, who concludes that both the nature and amount of surface exposed
have an influence. The inversion of sugar is affected by the nature
of the walls of the containing vessel, and its reduction by Fehling’s
solution is affected both by the walls of the vessel and the amount
of cuprous oxide formed in the reaction. Alteration in the character
as well as degree of a number of reactions by having them take place
in capillary spaces has been observed by Liebreich, Becquerel,
Lieving and other investigators. So-called “contact reactions,” as in
the production of sulphuric acid, are now familiar processes finding
commercial applications. And the solubility of some substances at
least, notably gypsum, has been shown to vary considerably with the
size and consequent shape of the particles in the solid substance in
contact with its solution.[87]
[87] See especially, Beziehungen zwischen Oberflächenspannung und
Löslichkeit, von G. A. Hulett, Zeit. Phys. Chem., =37=, 385-406,
(1901). Löslichkeit und Löslichkeits Beeinflussung, von V. Rothmund, p.
109, (1907); Principles théoretiques des methodes d’analyse minerale,
par G. Chesneau, p. 16-25, (1906).
It has been shown that some soils will at times produce the blue
coloration in alcoholic solutions of guiac, which is characteristic
of oxidases, and yellow aloin solutions are sometimes colored red.
Hydrogen peroxide is decomposed by some soils even after they have been
thoroughly ignited to get rid of all organic matter. But in how far
these effects may be due to surface influences can not be positively
stated; yet uncompleted investigations by Dr. M. X. Sullivan indicate
that some of these phenomena at least must be attributed to specific
influences (although probably of catalytic character) of particular
soil components, such possibly as manganous oxide or ferric oxide; but
the mechanism of the reactions is as yet largely speculative.
The soil is composed in large part of very fine particles of rounded
shape, exposing relatively an enormous surface to the soil solution,
and normally this solution is mainly under capillary conditions, so
that we should expect that many reactions would take place quite
differently in the soil from the way they would in a beaker or flask.
This fact has been generally overlooked or ignored, and is probably
the explanation of many of the apparently anomalous results hitherto
reported in chemical investigations of soils. Abnormal solubilities,
precipitations, oxidations or reductions are frequently found in the
literature, and when their abnormality is noted at all, they are too
often and with slight show of reason ascribed to indefinite bacterial
action or more mysterious vital agencies. Many of them are undoubtedly
the results of surface actions. Unfortunately, aside from some few
studies of absorption phenomena, surface effects have received little
or no attention from soil investigators, although obviously one of the
most important and apparently fruitful fields, requiring immediate
attention. Enough is known to justify the statement that the chemistry
of the soil need not be, and probably is not, the chemistry of the
beaker.