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:
Strength of solution. ⅒ Normal. ¹⁄₂₀ Normal.
Grams Grams
Salt Used K₂O absorbed. K₂O absorbed.
KCl 0.3124 0.1990
K₂SO₄ 0.3362 0.2098
K₂CO₃ 0.5747 0.3154
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:
Pine barrens 23,940 square feet.
Truck lands 74,130 „ „
Tobacco lands 84,850 „ „
Wheat lands 94,540 „ „
River terrace 106,260 „ „
Limestone subsoil 202,600 „ „
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.
[Illustration:
FIGURE 15.
ZALOMANOFF’S APPARATUS FOR DETERMINING ABSORPTION OF SALTS BY SOILS.
]
Two cylinders are placed vertically, one over the other. The lower
cylinder is graduated in cubic centimeters, the upper cylinder is closed
at each end by perforated rubber stoppers A and B through the openings
of which the glass tubes _c_ and _d_ pass. Within the cylinder A the
opening of the small tube _d_ is closed with a disk of Swedish filter
paper. The lower part of the small tube is _d_ connected by means of a
rubber tube carrying a pinch-cock C, with another small tube _e_ which
passes through the stopper _f_. In carrying out the process the weighed
quantity of soil is placed in the upper cylinder and afterwards the
measured quantity of the solution, the whole thoroughly mixed and the
cylinder closed. The valve C is then opened, a given quantity of the
solution, but not all, is made to drop into the lower cylinder and the
valve C is then closed. The liquid which has passed into the lower
cylinder as well as that which remains in the upper cylinder, is
thoroughly stirred and the quantity of the material remaining in both
liquids determined and the absorbing power of the soil estimated from
their difference. It does not appear that this method of estimation of
the absorption power possesses any special advantages over the old and
far simpler method of shaking in a flask.
[Illustration:
FIGURE 16.
MÜLLER’S APPARATUS TO SHOW ABSORPTION OF SALTS BY SOILS.
]
=134. Method of Müller.=—The method of Müller[97] for illustrating
absorption is carried out by means of the apparatus shown in Fig. 16. A
glass cylinder A about 750 centimeters long and four to five centimeters
wide is closed at each end with rubber stoppers with a single
perforation. The cylinder A is for the reception of the soil with which
the experiment is to be made. Before using, the lower part of it is
filled with glass pearls or broken glass and above this a layer of glass
wool is placed about one centimeter thick. The object of this is to
prevent the soil from passing into the small tube below. As soon as the
soil has all been placed in the cylinder A the upper part of the tube is
also filled with glass wool. The cylinder A is connected with the
pressure bottle B by means of a rubber tube and the small glass bulb
tube shown in the figure. The bottle B should have a content of about
two liters. It is filled with the standard solution of the material of
which the absorption coefficient is to be determined. At _c_ the rubber
tube is connected with a glass T one arm of which is provided with a
piece of rubber tubing which can be closed by means of a pinch-cock. At
_c_ a screw pinch-cock is placed which can be used to regulate the flow
of the solution from B to A. By opening the pinch-cock at _e_ on the
short arm of the T piece, a sample of the original liquid can be taken
and this can be compared with the part which runs to _b_. If it is
desired for instance, to show that potassium carbonate has been absorbed
by the soil the two bulbs shown on the small glass tubes connecting with
A can be filled with red litmus paper. This paper will at once be turned
blue in the lower bulb while in the upper one it will retain its
original color because the liquid in passing through the soil will have
lost its alkaline reaction. The solutions used should be very dilute.
The apparatus is designed for lecture experiments and not for
quantitative determinations.
=135. Method of Knop.=—For rapid determination of the absorption
coefficient of the soil Knop’s method may be used.[98]
The fine earth which is employed is that which passes a sieve with
meshes of half a millimeter. From 50 to 100 grams of this soil are mixed
with from five to ten grams of powdered chalk and with about twice the
weight of ammonium chlorid solution of known strength, _viz._, from 100
to 200 cubic centimeters. The ammonia solution should be of such a
concentration that the ammonia by its decomposition for each cubic
centimeter of the liquid evolves exactly one cubic centimeter of
nitrogen. This solution is prepared by dissolving in 208 cubic
centimeters of water one gram of ammonium chlorid. With frequent shaking
the solution is allowed to stand in contact with the soil for
forty-eight hours. The whole is now allowed to settle and the
supernatant clear liquid is poured through a dry filter. From the
filtrate twenty to forty cubic centimeters are removed by a pipette, and
evaporated to dryness in a small porcelain dish, with the addition of a
drop of pure hydrochloric acid. The ammonium chlorid remaining in the
porcelain dish is washed with ten cubic centimeters of water into one of
the compartments of the evolution flask of the Knop-Wagner azotometer.
It is decomposed with fifty cubic centimeters of bromin lye and the
nitrogen estimated volumetrically. The difference between the amount of
nitrogen in this material and that of the original material will give
the amount of absorption exercised by the fine earth. This number,
without any further calculation, can be taken as the coefficient of
absorption.
=136. Method of Huston.=—The salt solutions recommended by Huston[99]
are sodium phosphate (Na₂HPO₄), potassium chlorid, potassium sulfate,
ammonium sulfate and sodium nitrate.
The solutions should be approximately tenth normal, the actual strength
in each case being determined by analysis. The phosphorus is determined
as magnesium pyrophosphate in the usual way, the potash as potassium
platinochlorid, the ammonia by collecting the distillate from soda in
half normal hydrochloric acid and titrating with standard alkali, and
the nitrate by Warington’s modification of Schlösing’s method for gas
analysis. The details of these methods of determination will be given
later. One hundred grams of the sifted, air-dried soil are placed in a
rubber stopped bottle and treated with 250 cubic centimeters of the
solution to be tested. The digestion is continued for forty-eight hours
in each case, the bottles being thoroughly shaken at the end of
twenty-four hours. At the end of the treatment the solutions are
filtered and the salts determined in aliquot portions. The details of
this method are essentially those already described.
=137. Statement of Results.=—Duplicate analyses should be made and the
tabulation of the data is illustrated in the following analyses by
Huston:
Na₂HPO₄ cubic Weight of Weight of P₂O₅ Salt
centimeters Mg₂P₂O₇ in Mg₂P₂O₇ in absorbed removed
filtrate taken. twenty-five filtrate. by 100 per
cubic grams cent.
centimeters of soil.
the solution.
(a) 25 0.1368 gram 0.0962 gram
(b) 25 0.0963 „ 0.2589 29.6
gram
——————
Mean 0.0963 „
KCl cubic Weight of Weight of K₂O Salt
centimeters K₂PtCl₆ in K₂PtCl₆. absorbed removed
filtrate taken. twenty-five by 100 per
cubic grams cent.
centimeters of soil.
solution.
(a) 25 0.6154 gram 0.4505 gram
(b) 25 0.4540 „ 0.3161 26.5
gram
——————
Mean 0.4523 „
K₂SO₄ cubic Weight of Weight of K₂O Salt
centimeters K₂PtCl₆ in K₂PtCl₆. absorbed removed
filtrate taken. twenty-five by 100 per
cubic grams cent.
centimeters of soil.
solution.
(a) 25 0.6113 gram 0.4426 gram
(b) 25 0.4371 „ 0.3324 28.0
gram
——————
Mean 0.4399 „
(NH₄)₂SO₄ cubic Number cubic Half normal acid N absorbed Salt
centimeters centimeters neutralized. by 100 absorbed
filtrate taken. one-half normal grams per
acid neutralized soil. cent.
by fifty cubic
centimeters of
solution.
(a) 50 10.00 7.25 grams
(b) 50 7.25 „ 0.0964 27.5
gram
————
Mean 7.25 „
NaNO₃ cubic Number cubic Cubic centimeter N absorbed Salt
centimeters centimeters N₂O₂ N₂O₂ at 0° and by 100 absorbed
filtrate taken. afforded by ten 1000 grams per
cubic millimeters. soil. cent.
centimeters of
solution at 0°
and 1000
millimeters
pressure.
(a) 10 16.63 16.77 grams
(b) 10 16.70 „ none 00.00
—————
Mean 16.73 „
Upon comparing the figures it will be found that the absorption, passing
from the greatest to the least, is as follows: phosphoric acid (P₂O₆),
potassium sulfate, ammonium sulfate, potassium chlorid and sodium
nitrate.