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ESTIMATION OF COLLOIDAL MATERIAL IN SOS BY ADSORPTION
By
P. L. GILE, in Charge, Soil Chemical Investigations ; H. E. MIDDLETON, Scientist in Soil Physical
Investigations ; W. 0. ROBINSON, W. H. FRY, and M. S. ANDERSON, Scientists in
Soil Laberatory Investigations, Bureau of Scils.
WASHINGTON
GOVERNMENT PRINTING OFFICE
1924
UNITED STATES DEPARTMENT OF AGRICULTURE
BULLETIN No. 1193
February 15, 1924
Washington, D. C.
ESTIMATION OF COLLOIDAL MATERIAL IN SOILS BY ADSORPTION.
By P.L. Gre, in Charge, Soil Chemical Investigations, H. E. Mippieton, Scientist in
Soil Physical Investigations, W. O. Ropinson, W. H. Fry, and M.S. ANDERSON,
Scientists in Soil Laboratory Investigations, Bureau of Soils.
CONTENTS.
Page
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INTRODUCTION.
This bulletin is a report of progress made in developing an adsorp-
tion method for determining the colloidal material in soils. It is
anticipated that the method described can be improved as knowledge
of the soil colloids is increased, but even in its present imperfect
state it offers advantages over methods previously proposed and
should, therefore, prove useful in soil investigations.
Many of the most important properties of soils in general and
many of the special characteristics of particular soils are doubtless
associated with the colloidal matter present. It has been shown in a
previous publication that practically all the adsorptive power of a
soil, for certain substances at least, is localized in the colloidal material
(Anderson, 11). The colloidal material with its high adsorptive
ower probably, therefore, acts to a considerable extent as a regu-
ator of the character of the soil solution, influencing the kind and
concentration of the various bases in true solution and affecting the
rate at which the soil minerals decompose. The plasticity (Davis, 6)
and coherence of soils are evidently functions of the colloidal matter
or of some particular part of it. It has been generally recognized
that part of the effects produced by different methods of cultivation
1 Reference is made by number (italic) to ‘‘ Literature cited,” p. 39.
57580—24——1
2 BULLETIN 1198, U. S. DEPARTMENT OF AGRICULTURE.
and by the addition to the soil of various substances, such as lime,
fertilizers, and manures, are to be attributed to the influence of these
treatments on the condition of the colloidal matter present (Ehren-
berg, 8). In order properly to evaluate the part played by colloids
in various soil phenomena, a method for determining the total quan-
tity of colloidal material present is greatly needed.
COLLOIDAL AND NONCOLLOIDAL SOIL PARTICLES.
A fairly sharp distinction can be made between material in true
solution and that in colloidal suspension, but a sharp demarcation
between colloidal material and that in a coarser state of subdivision
is bound to be more or less arbitrary. The negative characteristics
of colloidal material, slight influence on the boiling or freezing point
of the Pepoine medium, and hardly sensible osmotic pressure
serve to differentiate colloidal suspensions from true solutions. The
more positive characteristics of colloidal material, those associated
with small size, such as the property of remaining in suspension in the
dispersing medium, Brownian movement, and high adsorptive
capacity are rather unsatisfactory criteria for a rigid separation of
colloidal and larger particles.
The criterion of colloidal condition most generally adopted is that
of size of the dispersed particles, the upper limit of pied particles
usually being set at 0.1 micron.? The selection of exactly 0.1 micron
as the upper limit of colloidal size is of course purely arbitrary.
There are no grounds for assuming that the adsorptive power, Brown-
ian movement, or reactivity of dispersed particles shows a sharp.
break at this particular poimt. Moreover, in the case of emulsoid
colloids in the gel condition, the classification on the basis of size
is not determinate. The division at 0.1 micron does, however, throw
the colloidal particles into the submicroscopic region.
In the case of soil constituents, also, a rigid distinction can not be
made between colloidal and noncolloidal material. Nevertheless,
there will probably be little disagreement concerning the colloidality
of most of the soil material which is classified as colloidal in this paper.
In the investigation described here all the organic matter of the
soil is regarded as colloidal. A considerable part of the organic
matter in normal soils can be readily peptized or dispersed into par-
ticles 0.3 micron or less in diameter, and the part of the organic
matter which is not so readily dispersable—the less humified material,
such as partially decomposed roots and plant residues—is doubtless
just as strictly colloidal as the material in a more advanced stage of
decomposition.’ The celluloses and structural plant substances, as
well as most of the intracellular material, is generally conceded to be
of a colloidal nature, although in the dried state these substances.
can not readily be converted into the sol condition.
In this investigation all inorganic material was classified as colloid
which could be dispersed into particles less than 1 micron in diameter
without subjecting the soil to a drastic chemical or physical treat-
ment that would disintegrate the mineral particles. One micron
2 Freundlich (10, p. 2) selects 0.5 micron as the upper timit. On the other hand, many soil investigators.
have used 2 microns as the dividing line between colloid and noncolloid (Atterberg, 3). i
’ An unextracted peat soil had just as high an adsorptive capacity for water as the material extracted
from it, which was composed of particies less than 0.3 micron in diameter. The extracted colloid was:
slightly less adsorptive of ammonia and more adsorptive of malachite green than the unextracted peat.
— oo —— a ~~ et
I
ESTIMATION OF COLLOIDAL MATERIAL IN SOILS. 3
was selected as the upper limit for colloidal particles rather than the
more conventional 0.1 micron, since a fairly sharp separation of par-
ticles can be made at 1 micron by subsidence or by centrifuging and
the separation can readily be checked by microscopical observation.
At 0.1 micron the control of size is inaccurate when dealing with
mixtures of colloidal material, and a sharp separation by subsidence
or centrifuging is exceedingly difficult. Soil particles 1 micron in
diameter show a distinct Brownian movement, which is usually
considered a colloidal characteristic. Also, some earlier investigators
have used 1 to 2 microns as the upper limit of size of discrete
“primary”’ or “ultimate” particles. It seems advisable, however,
to place the limit of size for unaltered soil particles as low as can be
accurately estimated, 1. e., at 1 micron.
The third kind of material classified as colloidal is that of certain
aggregates or lumps of material which may occur in sizes up to 50
microns or even more in diameter. With the ultramicroscope it can
be seen that these lumps are merely aggregates of small particles
which are less than a micron in diameter. The constituent particles
were not dispersable, however, by the methods we employed for this
purpose. These lumps doubtless exist as such in the soil, although
it is possible that they have been formed in some cases by the
mechanical treatment adopted for fractionation of the soil.
Distinctly mineral particles which are larger than 1 micron in diam-
eter are here treated as constituting the noncolloidal part of the soil.
PREVIOUS METHODS OF ips re THE COLLOIDAL MATERIAL IN
SOILS.
The first to call particular attention to the colloidal matter in soils
were Schloesing and Hilgard. Schloesing (31, 32) rubbed up clays
and agricultural soils with considerable quantities of water, separated
the coarser sands by decantation, treated the washings containing
the finer soil particles with acid to dissolve CaCO,, and then resus-
pended the fine material in water containing a little ammonia. The
fine material from 5 grams of soil which remained in suspension for
24 hours in 2 liters of water he called ‘‘clay.’’ He reported (32)
16 to 20 per cent of such material in heavy soils.
According to Schloesing (35, p. 476, and 34, p. 67), this clay frac-
tion, as well as a natural clay, contained two classes of substances:
Very fine sands devoid of cohesion, and an amorphous substance,
which he called “Vargile colloidale,” that cemented the sand grains
and was responsible for the plasticity of soils. This “colloidal clay”’
was very coherent, had a horny appearance, and was made up of
formless (possibly submicroscopic) particles which would remain in
suspension indefinitely. The colloidal clay was separated from the
other material in the clay fraction by long-continued subsidence
lasting several weeks or several months, with occasional decantations
to get rid of noncolloidal clay which settled out. The separation of
_ colloidal clay from the very fine sands was considered complete when,
“le microscope ne permet plus de découvrir dans le liquide aucun
élément figuré.”” (34, p. 67.) He stated that clays or soils con-
tained very rarely more than 1.5 per cent of l’argile colloidale.
Schloesing’s statement that the colloidal clay was free from other
material when no shaped particles were visible microscopically
4 BULLETIN 1193, U. S. DEPARTMENT OF AGRICULTURE.
showed that his argile colloidale was probably very similar to the
colloid we separated by the supercentrifuge; at least his largest
colloidal clay particles could not well have been smaller than the
ovine passing the supercentrifuge. Schloesing evidently based
is conclusions concerning the small quantities of colloidal clay
present in soils on the quantities he succeeded in isolating. His
error, which has since been made by several other investigators, lay
in assuming that his means of extraction and purification were sub-
stantially quantitative. As a matter of fact, he probably dispersed
only part of the colloidal matter in his preliminary treatment of the
soil and then lost colloid in the residues from which he repeatedly
decanted.
About the same time as Schloesing, Hilgard (/6) separated a
“‘colloidal clay” from the soil. This colloidal clay he believed con-
tained a ‘‘true clay substance,’ with a chemical composition similar
to kaolinite, which was responsible for the plastic and adhesive
properties of the soil. Hilgard classified as colloidal clay those
particles that would ‘‘fail to settle in the course of 24 hours through
a column of pure water 8 inches high” (15). He stated (i4 p. 333)
that the percentage of this material ‘‘seems rarely to reach 75 in the
purest natural clays, 40 to 47 in the heaviest of clay soils, and 10 to
20 in ordinary loams.”’
Apparently Hilgard believed that this fraction would contain,
besides ‘‘the true clay substance,’’ other colloidal material, such as
ferric, silicic, and aluminic hydrates, which he assumed would
impart no plasticity to soils. The material he obtained, however,
seems to have had the same properties as that of Schloesing. Hil-
gard’s method of extraction was possibly more thorough than that of
Schloesing, but he doubtless included in his colloidal clay, particles
that Schloesing graded out of his argile colloidale.
The very painstaking method of mechanical analysis reported by
W. R. Williams in 1895 (39)was evidently somewhat more effective
than that of Hilgard in extracting the colloidal matter of soils, in
that the fractions of coarser soil particles were more thoroughly
worked to rid them of colloidal material, and the method probably
gave a more complete separation of colloidal and noncolloidal
material.
The “‘schlamm”’ fraction secured by Williams, similar to the “clay”’
fraction of Hilgard, contained only those particles which would
remain in suspension for 24 hours in a column of water 10 centimeters
high. Williams stated that these particles, which were less than 1.5
microns in diameter, had very different properties from the larger
particles. The schlamm material gave a good suspension of formless
particles that showed an active Brownian movement. On drying,
the material shrank greatly, becoming a horny mass of conchoidal
fracture that would adhere strongly to the tongue. Williams
attributed the coherence, plasticity, and adsorptive power of soils to
the schlamm material, and stated that this material constituted 1.5
per cent to 40 per cent of the soil (39 p. 301).
The materials isolated by Schloesing, Hilgard, and Williams were
evidently all about the same and were chiefly colloidal. Apparently,
Ehrenberg (8 p. 99) does not recognize this, as in his recent com-
pendium of colloidal phenomena in soils he agrees with Schloesing that
soils contain only 0.5 per cent to 1.5 per cent of colloidal clay. Ehren-
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ESTIMATION OF COLLOIDAL MATERIAL IN SOILS. 5
berg evidently labors under the same misconception as Schloesing in
believing that the material which remains longest in suspension
differs essentially from material which does not remain so long in
suspension.
The chief object of much of the earlier work, in which colloidal
material was isolated from the other soil constituents, was not to
make a determination of all the colloidal material present in the soil,
but merely to separate the soil into different-sized particles. It was,
however, clearly recognized by Hilgard and Williams that one of the
chief problems in such a separation was the disintegration of lumps
of material into the individual particles of which they were formed.
The basis of the separation was entirely one of size.
As a matter of fact, under normal moisture conditions probably no
soil contains more than a trace of colloidal material in the sol con-
dition. The colloidal.material must be virtually all in a coagulated
or gel form, since there is not sufficient water present to form a
disperse medium. The problem of separating the colloidal material
by a method of mechanical analysis, therefore, becomes largely one
of converting the colloidal material from the gel to the sol form.
Since a considerable part of the gels in most soils is in a more or less
indurated condition that is difficultly dispersable, it might be expected
that the exact character of the suspension obtained, its stability, and
the size of the particles would be a more or less fortuitous matter,
depending on the means of dispersion employed. This seems especially
probable from a consideration of the well-known coagulating, peptiz-
ing, and stabilizing effects of various concentrations of different
ions.
From this point of view, the ideas of Schloesing and Ehrenberg, that
the particular preparations they obtained represented the only
truly colloidal matter in the soil, appear illogical. It is of course
true that size is one of the most dependable criteria for colloids, but
in the case of many preparations the question arises, size of what?
In the case of a coagulated suspensoid colloid, such as colloidal gold,
the size of the floccules is not the criterion, but the size of the indi-
vidual particles that are coagulated. But in the case of coagulated
emulsoid colloids or of partially dehydrated emulsoid gels, structure
or size of the molecular aggregates is evidently the criterion, and
size in this case can not be determined by direct observation.
In recent years a number of rapid methods have been proposed for
estimating the comparative amounts of colloids in soils. Many of
these methods are based on the fundamental misconception that by
some simple treatment of the soil, such as by mere agitation with
water, all the colloidal matter, or some definite part of it, is at once
brought into suspension. Among such methods may be mentioned
the method of Sokol (35) based on the relative volumes occupied by
the colloid when first it is flocculated and when it has settled for 24
hours: and the method of Scales and Marsh (30) based on the turbidity
of the suspension obtained on shaking the soil for five minutes wit
distilled water.
These methods fail to take into account that the quantity of
colloidal matter brought into suspension by mere agitation of a soil
with water may vary greatly according to a transitory condition of
the soil. Our experience shows that some soils yield practically no
colloidal matter on the first agitation with water, but after decanta-
6 BULLETIN 1198, U. S. DEPARTMENT OF AGRICULTURE.
tion of the water a second treatment yields a heavy colloidal suspen-
sion. In such cases the first treatment with water doubtless removes
a certain amount of a soluble salt which has a flocculating effect.
Konig and Hasenbiumer (17) showed that after addition of aluminum
ehloride to a soil, agitation with ten fresh portions of water failed to
bring any colloid into suspension; but the eleventh treatment
with water containing a small amount of ammonia produced a heavy
suspension.
Other methods which are based on adsorption—a positive property
of the colloids—have been proposed for indicating the relative
colloidal contents of soils. Ashley’s method (2), involving a deter-
mination of the quantity of malachite green adsorbed, has been
most widely used on ceramic clays, which may be regarded as a
special type of soil. Our results show, however, that while Ashley’s
method might indicate differences in the plasticities of ceramic
clays, it does not indicate the relative colloidal contents of agricul-
tural soils, since the colloids extracted from different soils vary
greatly in their adsorptive capacities for this dye.
Mitscherlich (21, p. 476) has suggested that the amount of water
vapor adsorbed by soils under standard conditions is probably to be
attributed almost entirely to the colloidal matter present, but that
the water adsorption values of different soils probably do not indicate
the relative amounts of colloids present, since different soils contain
different colloids which vary in their adsorptive capacities. Our
results would indicate that while colloids extracted from different
soils may vary considerably in their adsorptive capacities for water
vapor when in equilibrium with the partial pressure of water vapor
afforded by 10 per cent sulphuric acid they have a relatively constant
adsorptive capacity when in equilibrium with 3.3 per cent sulphuric
acid.
Stremme and Aarnio (36) applied the Van Bemmelen method of
determining colloids by digestion with sulphuric and hydrochloric
acids to various rocks, and they concluded that the method was
subject to error in that certain noncolloidal minerals were dissolved.
Van der Leeden and Schneider (18) conclude that the dye adsorp-
tions of soils do not correspond to the colloidal contents, that
unweathered silicates as well as colloidal material are dissolved by
digestion of the soil with hydrochloric acid, and that there is not an
exact parallelism between the quantities of water and dye adsorbed
by soils.
Tempany (37) has recently proposed that the colloidal content of a
soil might be estimated from the shrinkage of the soil on drying. It
is yet to be proved, however, that the colloidal materials in all soils
have the same shrinkage coefficient.
In the earlier investigations, and in much of the modern work, the
attempt has been made to determine the colloidal matter in soils
through the methods of mechanical analysis; that is, by actual separa-
tion of the colloidal and nencolloidal soil constituents. Proceeding
in this manner, it was, of course, necessary to get the colloidal matter
in suspension before it could be recognized as colloid and size of the
particles became the sole criterion of judging what was colloid.
Such a separation would be definite and entirely satisfactory for the
interpretation of soil phenomen2 if all the soil particles were of one
kind; that is, if the particles were all fragments of crystalline minerals
ESTIMATION OF COLLOIDAL MATERIAL IN SOILS. 7
differing only in size. The uncertainty of such separations and the
unsatisfactoriness of the results obtained are due to the presence of
colloidal material which it is difficult to disperse into discrete particles
of any definite size. A determination of the colloidal material in
soils is, therefore, needed to supplement the results afforded by any
system of mechanical analysis based on size alone, which does not
disperse colloidal aggregates (Davis, 7).+
ADSORPTION METHOD OF ESTIMATING COLLOIDAL MATERIAL IN
SOILS.
DESCRIPTION OF METHOD.
The earlier investigators in estimating the colloidal material in
soils actually isolated at least a part of the colloids, but they did not
utilize the marked adsorptive properties of the material as a means
of characterization nor as a test for complete separation. Later
investigators recognized high adsorptive capacity as a distinguishing
characteristic of the colloidal material, but failed to isolate the
material. Knowing the adsorptive capacity of the kind of colloidal
material present in a given soil and the adsorptive capacity of the
whole soil, one should be able to estimate the quantity of colloidal
material present. If the adsorptive power of the soil is due whoily
to the colloidal material, the ratio,
adsorption per gram of soil
adsorption per gram of colloid EOD
should give the percentage of colloidal material in the soil.
Moore, Fry, and Middleton, of this Bureau (22), suggested this
method of estimating the colloidal material in soils. They deter-
mined the adsorption ratio in the case of one soil, measuring the-
quantities of malachite green and ammonia gas adsorbed by the soil
and by a sample of colloidal material extracted from the soil. The
adsorption ratio obtained by malachite green agreed exactly with the
ratio obtained by ammonia.
ps It might seem that adsorption by noncolloidal soil particles would
constitute a fundamental error in this method, making the ratio of
the adsorptive capacities of soil and colloid indicate a higher per-
centage of colloid in the soil than was actually present. However, a
subsequent investigation (Anderson, /) showed that in the case of
ordinary soils more than 95 per cent of the total adsorptive capacity
of the soil for malachite green, water, and ammonia is located in the
colloidal constituents. Only in the case of highly micaceous soils
is the adsorption by noncolloidal particles significant.
Since the adsorption method of determining the colloidal content of
a soil would not, as a rule, be seriously affected by noncolloidal
constituents, it seemed advisable to try the method on a number of
different soils, 32 in all. It also seemed advisable to determine the
adsorptive capacities of the soils and colloids for water vapor as well
as for malachite green and ammonia. Theoretically, any substance
which is not appreciably adsorbed or acted upon by the noncolloidal
soil particles, but which is adsorbed by the colloidal material, might
be used for determining the colloidal content of the soil by this method.
4 Itis doubtful if the method of mechanical analysis proposed by Sven Odén (23, 24) attains as complete
@ deflocculation of the colloidal material as the older method of Williams, since the residues apparently
are not ‘‘worked”’ to the same extent.
8 BULLETIN 1198, U. S. DEPARTMENT OF AGRICULTURE.
It was expected that some evidence concerning the reliability of
the method would be afforded by the closeness with which the ratios,
adsorption per gram of soil
adsorption per gram of colloid
obtained by the adsorptions of dye, water, and ammonia agreed with
each other.
EXPERIMENTAL PROCEDURE.
SOILS USED.
The soils selected for a trial of the proposed method of colloid
determination were some of the most important agricultural types.
They varied in texture from sands to such heavy clays as the Simei
clay and the Stockton clay adobe soils. In most cases the surface
soil represented the first 8 to 12 inches of material and the subsoil
samples were taken to a depth of 2 to 3 feet. The samples were
received in a fairly moist condition and were kept moist until ready
for use, since in previous work there had been some evidence that
thorough drying was likely to make extraction of the colloid more
dificult. M. H. Lapham, J. E. Lapham, and W. Edward Hearn of
The Soil Survey collected the selicepe
ISOLATION OF A SAMPLE OF THE COLLOIDAL MATERIAL.
No attempt was made in the first part of the work described here
to extract all the colloidal matter in the soils. The idea was to isolate
from each soil enough of the colloidal material for chemical analysis
and for determination of the adsorptive capacity for malachite green,
water vapor, and ammonia gas. The samples of colloid for the
‘determination reported in Table 1 were extracted virtually as de-
scribed in the preliminary paper by Moore, Fry, and Middleton (22),
although in the subsequent work this procedure was altered some-
what.
One hundred pounds of each soil were agitated in barrel churns
with 500 pounds of distilled water for two hours. After standing
for 18 hours the turbid liquid was siphoned off and the soil was again
agitated with water to obtain a further yield of suspended matter.
Frequently the second treatment of the soil gave a heavier suspension
than the first, although a few soils. failed to give any appreciable
quantity of suspended matter on three such treatments. In such
eases a fourth treatment with water containing approximately 1 part
of ammonia to 3,000 parts of water was very effective in producing
a suspension. The turbid extract from the soils was run through a
high power centrifuge (known in the trade as a supercentrifuge) driven
at the rate of 17,000 revolutions per minute, which threw out a large
part of the suspended -matter. In this process the particles were
exposed to a force of approximately 17,000 gravity for about 3
minutes.
The colloidal suspension which passed through the centrifuge was
first concentrated by sucking the water off through batteries of
Chamberland-Pasteur filters (bogie F), the colloidal matter collecting
on the outside of the filters.» When the colloidal matter had thus
® The slimy film of colloid which collected on the outside of the filters was readily removed from time to
time by releasing the suction and blowing air into the tubes.
ESTIMATION OF COLLOIDAL MATERIAL IN SOILS. 9
been concentrated in a moderate volume of water it was taken down
to a nearly air-dry condition on the steam bath. The amount of
colloid thus isolated amounted to about 1 per cent of the weight
of the whole soil.
Samples were taken for the adsorption of malachite green when the
material had reached a thin, pasty condition,® while the water adsorp-
tion was determined on the air-dried material and the ammonia
adsorption on the material dried at 110° F.
Suspensions of colloidal material which passed through the super-
centrifuge were examined microscopically. Practically no particles
were visible in the ordinary microscope. Examination was also
made, using a 3-millimeter objective, 12.5 X eyepiece, and a dark
field illumination. Under these conditions the apparent size of the
particles seldom exceeded 0.3 micron in diameter.’ The unaggre-
gated particles had a most active Brownian movement and were
readily coagulated by electrolytes. The suspended matter in most
cases did not settle out appreciably on standing for many months.
As the concentrated colloidal material was dried, it shrank greatly,
finally becoming a horny or resinlike mass that had a conchoidal
fracture. It would polish readily and adhere strongly to the tongue.
This material was doubtless just as pronouncedly colloidal as that
extracted. by Schloesing (34) or that described by Ehrenberg (8,
p. 102). It also seemed ta have the same properties as the materials
described by Hilgard (16) and by Williams (39).
METHODS OF DETERMINING QUANTITIES OF MALACHITE GREEN, WATER, AND AMMONIA
ADSORBED,
The adsorption of malachite green was determined as follows:
Approximately 0.25 gram of colloid or 1 gram of soil was shaken over
night with 25 cubic centimeters of water in an end-over-end shaker.
Enough N/10 sodium oxalate was then added to precipitate any sol-
uble calcium present. After shaking for 15 minutes, a 0.2 per cent
solution of malachite green oxalate was added, with repeated shaking
until the depth of color in the supernatant liquid was about the same
as that of a check solution containing 0.0004 gram dye per cubic
centimeter. Shaking was continued for 1 hour to insure complete
adsorption. About 5 cubic centimeters of normal sodium chloride
solution was then added to coagulate the colloid, and after centrifug-
ing to throw down the coagulated material, the supernatant liquid
was read colorimetrically against the check solution. The quantity
of dye adsorbed was calculated from the amount added and the
amount left unadsorbed.
The determinations reported in Table 1 were all made on the moist
colloid and on the air-dried soil. Several tests showed that the 18-
hour shaking of the sample preliminary to determining the adsorption
was needed to bring the material in the most adsorptive condition.
This was particularly important in the case of soils. Duplicate
_§ In subsequent work reported here the air-dried colloidal material was used for determining the adsorp-
tion of malachite green. :
7 Of course, some aggregates of larger size were seen, but these were susceptible of resolution into particles
of the size mentioned. Part of the material may well have been invisible. This refers not only to exceed-
ingly small particles, but also to colorless particles of any size which had a refractive index sufficiently
near that of water.
57580—24
2
10 BULLETIN 1193, U. S. DEPARTMENT OF AGRICULTURE.
determinations made by this method usually agreed well, variations
between different determinations averaging about 5 per cent of the
values obtained.
In determining the adsorption of ammonia, 5 to 10 gram samples
of soil or 2 to 5 gram samples of colloid which had previously been
dried for 18 hours at 110° C. were used. The sample was placed in
a U-tube which was evacuated for 15 minutes while immersed in
boiling water. After replacing the boiling water about the U-tube
by an ice-pack, ammonia gas was let into the U-tube through a dry-
ing train for two to four hours. The pressure of ammonia gas
during this time was maintained at 1 centimeter above that of the
atmosphere by a mercury manometer. When the mercury level in
the manometer remained without perceptible change for 30 minutes
with the ammonia cut off, the adsorption was regarded as complete.
Two bottles containing a saturated solution of boric acid were then
attached to the U-tube, the ice-pack was removed, the pressure in
the apparatus was kept reduced by about 10 centimeters of mercury
in the manometer by means of a filter pump, air was let in through
a soda-lime tower, and boiling water was again placed around the
U-tube. In this way all the ammonia was swept out of the appara-
tus within 30 to 40 minutes. The ammonia in the boric acid solu-
tion was titrated, using methyl orange as an indicator. The quantity
of ammonia thus obtained was corrected for the blank of unadsorbed
ammonia in the apparatus. This method gave the amount of am-
monia adsorbed at 0° C. which was driven off at 100° C. Variations
in duplicate determinations by this method were usually not greater
than 3 per cent of the values obtained.
The adsorption of water vapor was determined as follows: About 2
grams of colloid or 2 to 4 grams of soil, previously air dried and passed
through a 100-mesh sieve, were weighed into shallow weighing bot-
tles. The uncovered weighing bottles were placed in a desiccator
containing a large amount of 3.3 per cent sulphuric acid, and the
desiccator was then evacuated to 50 millimeters or less of mercury.
The desiccator was kept in a thermostat maintained at 30° C. for
five days; at the end of which time the adsorption of water va-
por was practically complete. The samples were then weighed
moist and weighed again after drying for 18 hours at 110° C. The
difference between the two weights represented the quantity of
water adsorbed by the sample. A more detailed description of the
procedure is given in a previous paper (Robinson, 26).
The adsorption of the soil and of the corresponding colloid were
determined in the same run. Under these conditions duplicate de-
terminations usually agreed within 2 per cent of the values obtained.
Duplicate determinations made in different runs might show con-
siderably larger variations.
EXPERIMENTAL RESULTS.
THE ADSORPTIVE CAPACITIES OF SOILS AND EXTRACTED COLLOIDAL MATERIAL,
The adsorptive capacities of 32 soils and of samples of the colloidal
matter extracted from these soils are given in Table 1. In each case
the adsorptive capacity is expressed as the weight of dye, water, or
ammonia adsorbed by 1 gram of the material.
= a
ESTIMATION OF COLLOIDAL MATERIAL IN SOILS. ial
: TaBLe 1.—Adsorption of malachite green, water, and ammonia by soils and soul colloids.
Adsorbed per gram of soil. material.
'
|
{
'
| | Adsorbed per gram of colloidal
|
Type of soil or colloidal material.
Dye. | H:0. NH. | Dye. HO. NHs3.
Carrington loam: Gram. Gram. Gram. | Gram. Gram. Gram.
Se ee te ee ce re Lee ae ee / 0.0531 0. 1005 0. 0168 0.2744 | 0.3102 | 0. 0523
SUES ae te NS . 0504 | . 0978 - 0170 | . 3210 3327 | . 0665
Cecil clay loam: | Ror et |
Sa@tpese perth it aCe rity eee eels . 0078 . 0341 | - 0034 | . 90770 | . 3359 . 023 0
SSS o Ga PS ae eles meee ose . 0402 . 0903 . 0050 . 1582 | . 2870 | . 0192
Chester loam:
Oth Seto eseh se. LEGS 5 eh eas. . 0148 . 0203 . 0049 - 1348 | . 2398 . 0293
SSF TSTn ee ee eee eee ae ee | . 0239 . 0676 . 0087 . 1892 . 2723 . 0256
Clarksville silt loam: |
SOL ee a ee ee ee . 0353 | . 0789 . 0077 1424 | . 3390 - 0359
SOE DSE 2 Seed bs Se ap il ee Sl he . 0506 . 0804 . 0106 - 2154 | . 3147 . 0408
Hagerstown loam:
Sal 5 epee eer See penne | . 0213 | . 0434 . 0066 | . 1584 . 2592 . 0278
SOUS 10S See ES Pe FESR ERE ee ere eee . 0314 | . 0857 - 0089 | - 1704 | . 2675 . 0299
Huntington loam: |
Rene, eee AS, CoA Se | . 0236 | . 0632 . 0104 | . 1750 | . 2739 . 0319
Smisoueren. 2.5258 seer stkew- -<.cd -- . 0182 | . 0504 . 0083 | .1538 ;° _ £2903 . 0274
Manor loam:
bees es. 43. FROST AE ett . 9188 | - 0520 | . 0064 | . 1640 . 2822 . 0308
‘SioT SS | Ae ene ares See PER, Sees | . 0223 | . 0632 . 0057 | . 1838 | . 3402 . 0264
Marshall silt loam: | |
Me Py. Bexs tae fxsece 24: - -eprg- be wees 5 ft . 0782 - 0803 | - 9180 | - 2978 | . 2940 - 0536
SAFIN OT! aes Ree ere “==
Vega Baja clay loam, soi::
Insp. iraction.or colloids. -2 03.2 5.2... 16.6 | .0584 |] .3125 | .0206 54.6 | 100] 100 100
Second fraction of colloid............... 12.7) .0463 | .2749 | .0165 41.7 79 88 80
Third fraction of colloid ................ Lol . 0494 . 3298 . 0182 3.6 85 106 88
Total quantity colloid extracted......| 30.4 | .0530 | .2974 | .0188 | 100 91 95 91
On the whole, the different colloidal fractions isolated from the
same soil were very similar in adsorptive capacity to the first sample
extracted. In only two or three cases did any of the fractions differ
from the first fraction by more than 25 per cent. In the case of the
Marshall soil and the Huntington subsoil there was a gradual decrease
in the adsorptive capacities of the lots of colloid successively extracted ;
but this does not hold for the other three soils. Chemical analyses
made of the different colloidal fractions also showed that there was
no marked variation between the first two or three fractions in
ultimate chemical composition. The content of iron, aluminum, and
silica was very constant in all fractions except the last; this in some
cases was higher in silica and lower in iron and alumina than the
|
:
t
{
|
f
ESTIMATION OF COLLOIDAL MATERIAL IN SOILS. 19
preceding fractions. The chemical composition is discussed in detail
in an unpublished report (Robison and Holmes, 27).
So far as the adsorption method of determining the colloidal
material in soils is concerned, the important comparison is between
the adsorptive capacity of the first fraction of colloid and that of the
total quantity extracted. It will be noted that all the fractions of
colloid taken together (‘‘ total quantity colloid extracted’’) had prac-
tically the same adsorptive capacity as the first fraction. The differ-
ences are less than 10 per cent, except in the case of the Huntington
subsoil. It is significant in connection with the cause of the disa-
greement between the dye, water, and ammonia ratios given in
Table 2, that the slight variations between the adsorptive capacities
of the first fraction and the combined fractions were practically the
same for malachite green as for water or ammonia.
Although this comparison of the samples of colloid successively
extracted from a soil was not undertaken with a view to determining
quantitatively the total amount of colloidal material that it was
possible to isolate by the procedure outlined, nevertheless the total
quantity of colloidal material recovered is a matter of considerable
interest. It will be noted that the quantity of colloidal material
actually extracted from the different soils was 6.19 to 38.5 per cent
of the weight of the whole soil. This is highly important in bearing
on the opinion held by Schloesing 40 years ago, and still widely
current, that soils contain less than 2 per cent of truly colloidal
material. It is improbable that Schloesing was dealing with a prod-
uct in a finer state of dispersion than that obtained in this separation
inasmuch as he was obliged to rely on time of subsidence for grading
his material. As mentioned on page 9, suspensions passing through
the supercentrifuge in this work apparently contained very few
particles larger than 0.3 micron in diameter.
The quantities of colloidal material separated from the Sassafras
subsoil and Marshall soil were doubtless considerably less than the
quantities of this class of material actually present in the soil. As
already pointed out, considerable losses of the fine material of these
soils occurred in preparing the samples of colloidal material. In the
eases of the Huntington subsoil, the Sharkey soil, and the Vega Baja
soil the quantities of supercentrifuge colloid isolated represent prac-
tically all this class of material that it was possible to separate by
the means employed.
To return to the question of the sample of colloidal material as a
factor affecting the accuracy of colloid determinations by the adsorp-
tion method, it appears that so far as adsorptive capacity is concerned
a small sample of extractable colloidal material differs by approxi-
mately 10 per cent from all the colloidal material which is isolatable
by the methods described above. Of course the colloid which it was
impossible to extract with the methods employed might have a
different adsorptive capacity for dye, water, or ammonia than the
colloid which was extracted. However, to account for the disa-
greement in the ratios on the basis of an unfair sample of the colloid
the degree of difference between the adsorptive capacities of the
extractable and unextractable colloidal material would have to be
different with respect to dye, water, and ammonia. Evidence on
this point is given in the following section.
Comparison of the extractable and unezxtractable colloidal material.—
The adsorptive capacities of the extracted and the unextracted
20 BULLETIN 1193, U. S. DEPARTMENT OF AGRICULTURE.
colloidal material can not be directly compared, but an indirect
comparison of the two kinds of colloid is possible. If the extractable
colloidal material is completely separated from the soil as in the work
just described, the unextracted colloidal material will all be present
in the residual fractions of fine and coarse material, mixed, of course,
with the mineral fragments above colloidal size.
A comparison, then, of the adsorptive capacity of the fine or
coarse fraction for dye, water, and ammonia with the adsorptive
capacity of the extracted colloidal material for these three sub-
stances will show whether the unextracted colloid differs from the
extracted colloid more in its adsorption of dye than in its adsorption
of water or ammonia. ‘This comparison would show only the rela-
tion between the dye, water, and ammonia adsorptive capacities of
the extracted and unextracted colloids. It would not show, for
example, whether the unextracted colloid was 80 per cent or 40 per
cent as adsorptive for all three substances as the extracted colloid.
However, the adsorption ratios of the fine and coarse fractions,
adsorption per gram of fraetion
adsorption per gram of extracted colloid
will, other things being equal, show the absolute quantities of colloid
in the fractions in proportion as the adsorptive capacities of the
extracted and unextracted colloids are alike. Now it is possible to
determine the quantities of colloid in the fine and coarse fractions
by microscopical observation with considerable accuracy (Fry, 13).
Hence a comparison of the quantities of colloid found by microscopical
observation with the quantities indicated by the adsorption ratios
will show biaccimnately the relative adsorptive capacities oi the
extracted and unextracted colloidal materials. If the adsorption
ratio shows a smaller percentage of colloidal material than the
microscopical determination, it indicates that the unextracted col-
Joidal material has a lower adsorptive capacity than the sample of
extracted colloid and vice versa.
The soil fractions prepared in order to compare the successive
fractions of extractable colloidal material were not well suited for
a comparison of the extractable and unextractable colloid since
considerable portions of the fine and coarse fractions were lost,
except in the case of the Vega Baja soil. Accordingly new frac-
tionations of soils were made, taking care to avoid losses of colloidal,
fine, or coarse material.
The methods of extracting and concentrating the colloidal material
and of separating the fine and coarse residues were essentially the
same as those described on pages 16 to 17... The important difference
was that in this work a bottle centrifuge was used for separating the
colloidal material from the noncolloidal particles instead of the
supercentrifuge, the speed and time of centrifuging being so regulated
as to leave in suspension all particles less than 1 micron in diameter.
The centrifuge used had a diameter of 22 inches. When running at
a speed of 850 revolutions per minute about 45 minutes were usually
required for throwing out particles larger than 1 micron in diameter.
This was checked by microscopic observation of the colloidal, fine,
and coarse fractions. The colloidal material passing through the
supercentrifuge in the extractions described on pages 16 and 17
ESTIMATION OF COLLOIDAL MATERIAL IN SOILS. 21
contained, as near as could be judged by observation, very few
particles larger than 0.3 micron. Hence the difference between the
colloidal material obtained in previous extractions and that obtained
in this separation consisted in the fact that in the first case the upper
limit of size of particles was in the neighborhood of 0.3 micron while
in the latter case the upper limit of size was 1 micron.
The finer and coarser soil particles above colloidal size were col-
lected in separate fractions as in the previous separation, but in this
separation a quantitative recovery of these fractions was attempted.
The samples of soil fractionated in this way consisted of 50 grams of
Sharkey soil and 100 grams each of Cecil soil, Huntington soil,
Huntington subsoil and Sassafras subsoil.
Table 4 shows the quantities of colloidal, fine and coarse materials
separated from each soil, together with the adsorptive capacities of
these materials and of the unfractionated soils. Included in this
table, also, are the results obtained with two samples of soil—the
Vega Baja soil and a second sample of Sharkey soil—which were
fractionated according to the method described on pages 16 to 17;
that is, the colloidal material was graded by the supercentrifuge, so
the upper limit of the particles was about 0.3 micron instead of 1
micron.*°
TABLE 4.—Adsorptive capacities of soils and soil fractions.
Adsorption per gram
Part | of material.
Description of soil fractions. of whole |
soil. | | |
| Dye. | H20 | NH.
~ i
| | |
Cecil clay loam, soil: Per cent.| Gram. | Gram. | Gram
TL eRe RE 2) eee re eee 100.0 | 0,0078 0. 0442 © 0. 0034
0 er SE a DS ee 9.4 | . 0840 | . 2439 | . 0165
bare ee see a se Ce I EVE 7.6 | . 0154 | . 0714 | . 0076
ES Ss Ee ee 83.8 | . 0000 | 0028 | . 0010
Cecil clay loam, soil (duplicate determination): |
CUTIES Ts a ee ee Ne ee 100.0 | . 0078 | 2 0442} 22522 22
pits: boon PLE gl Be ae EE ee A ee ee 9. 4 | . 0975 | F284" fh) 2th a
wo i ee ee ee eel Ce 10. 4 | . 0007 | 205091 | sta.
SPE E SPSS Vis ee es ed 2 ee ee ee er ae 78.3 | . 0000 | < OO420F SIE 3 LIES
Huntington loam, soil:
TL Bila oe Rd ea ee 100.0 | 0245 | . 0533 0104
ltl = Jobe SOR? ae Se eee 10.3 | 0949 | . 2221 0270
en eo oR Be ae es Be ae ae 21.9 0288 | . 0476 0098
(PURSE EAT pT? see = a a ee 64.0 | 0114 | 0215 0062
Huntington loam, subsoil: |
ETE pes BN ol ee Die ee ee a ee ee 100.0 | 0182 | . 0792 0083
lL ee Dab. RR Ba ae 13.3 | 0918 | . 2996 0226
a Te a SE ens ae ee 19. 4 | 0143 | =. 0665 0072
SO SEt DDD SY SE ee - ee 63.3 | 0053 0292 0031
Sassafras silt loam, subsoil: |
Ll: a bee eS ee ee ee ere 100. 0 . 0276 | 0568 . 0074
JOT eb! SE ee ee a ee ee 14.4 . 1398 . 2732 . 0293
WeRr SERRE Tee 2 ee SIS a, ee 20. 6 | . 0222 . 0546 0079
(SUE ED Pri Fe ge ee Be ae a ee ee ee 61.9 0053 0041 0004
Sharkey clay, soil: |
tL Eh s SS ERIE BES cae See ee oy Eee Lot A 100.0 | 1997 1755 0345
SLT 2 es P Se = et 5 ee 42, 4 | 3790 | . 2956 0520
we i i ee ae 53.1 | 0657 | .0772 0172
Sharkey clay, soil (separation by supercentrifuge):
1 ah PUL ee ee ee ye ae ee eee 100. 0 | . 2128 . 1984 . 0358
Ele WE ee Be ee ee ae 31.5 | 3592 | . 3037 . 0514
permamreewee O6e. COLE rer ai, Ty AltoL ia ris: 38. 9 | . 1075 | . 1280 | . 0244
Coarse fraction..-.........- f JUSS eee? PY mer) eee ee 31.8 .0825 | .0834 . 0168
Vega Baja me loam, soil (separation by supercentrifuge):
UC SUES UL 2 ae ee en ee ee 100.0 . 0369 | . 1805 . 0147
Ppemrapiger pe Oa hg re re ae ee hace oe coke 30. 5 | . 0530 . 2974 | . 0187
Reever cteegtemes Pee Ne Ey eT ee) oS ep PALE: 38.6 | . 0492 . 2332 . 0196
ETS Lz ed ee ee en ae 31.8 | . 0094 . 0260 . 0044
10 In the case of the Sharkey soil absolutely all the extractable colloidal material was not isolated, since
the fractionation was undertaken chiefly for the study, to be described later, on the “‘alteration in adsorp-
tive capacity of the colloid produced by extraction.’
57580—24——+4
22 BULLETIN 1193, U. S. DEPARTMENT OF AGRICULTURE.
In the case of all soils, except the Sharkey (separation by super-
centrifuge), the quantities of colloid isolated were all that could be
obtained with the methods employed. The coarse and fine fractions
of all soils except this one, therefore, contain only what we may term
“‘unextractable” colloid. The fine and coarse fractions of the
Sharkey soil (separation by supercentrifuge) on the other hand con-
tain a small amount of colloid capable of isolation in addition to the
unextractable.
The quantity of colloidal material present in these fine and coarse
fractions was estimated with the petrographic microscope. The
details and accuracy of the method have been described by Fry
in another publication (13).
Microscopical determinations of the colloidal material present in
the fine and coarse fractions of Table 4 are reported in Table 5.
Table 5 also shows the quantities of colloidal material in the fine and
coarse fractions which the dye, water, and ammonia adsorption ratios
adsorption per gram of fraction
adsorption per gram of extracted colloid
indicate. The adsorption ratios were calculated from data given in
Table 4.
TaBLE 5.—Percentages of unextracted colloid in fine and coarse soil fractions.
Percentage of colloidal material in the
sample indicated by—
Description of samples. ey Dye Water \Anmunionie
eeooienl adsorp- | adsorp- | adsorp-
C eee tion tion tion
A ratio. ratio. ratio.
Cecil clay loam, soil: | Per cent. | Per cent.| Per cent. | Per cent.
GHTO MTA CHLOM Ae 2.0 Shek BAS 2 asta ae ee cian cB ee eee sce eee 74 18.3 29.3 46.1
Wosrselraetlon 5) ess. Psa AS- Se lsto ja. 5 =o oes a he aye Sei ee | 4 .0 ila Gu
Cecil clay loam, soil (duplicate determination): |
AMIGO MTACUIONE Fotis 25,2 2s Mee dating BANGS ots Sein ws URNS © Soe ee 55 oat TSAO a aoc pees
COFTSOMFAGH ON 65 AP on Roe HES: «ook athe cle cae ce Sede 4 .6 1 Pst ya a pe em oh
Huntington loam, soil:
INI CMU CUEON - 2:2 = cde MS 2k 2 = oi Rtbore ciclateites cee © eile ee aaienas 48 30.3 21.4 36.3
WOATSCHTACHION -)-¢ ERE 2 =<. atten. oho s cea aeien aortic ercerece 8 12.0 9.7 23. 0
Huntington loam, subsoil:
ir O CRACHHOM £2. 4 Se 5, 8 ooo eA. os coos oh oak Aare ee 25 foro) 22.2 31.9
(CoarsenractiOn <).'2 isis e% SoUs SYS AP mer oe cee ae ee 10 5.8 9.7 IB AV
Sassafras silt loam, subsoil:
ITM OMNACHTONS.\. co caltes. sc. Ds OM «nc cmos pac. ste Rees oot = 38 15.9 20.0 | 26.9
COSTSOMTACIION =o ge sc 21 LA eee ee a eS eee 7 ek aS 2 3.8 | aie 1.4
Sharkey clay, soil: |
Wine fraction. =. : =. sks.) .2. 2 S252---- Se ee Bis, pols peed 42 LW MB) 26.1 | Son
Sharkey clay, soil (separation by supercentrifuge):
SEI TIOANAGHEGINs= He AereE So, to eee om oe eee cain cles ee aes coe eeee 52 29.9 42.1 | 47.5
COarTSONTACLLONM {050 <2. So Sach aes ice See ee ee dee eee eee eee 25 23. 0 27.5 OZ. d.
Vega Baja clay loam, soil (separation by supercentrifuge):
HINGMMAGHON=. = Bresso. ee bane JERE Deas ste san see ee = 97 92.8 78.4 | 104.3
WOATSOMTAC ELON! Coes cc's Sa Mo cic adie sree aise aotemee me ersys 14 MC 8.7 | 23. 4
Both microscopical determinations and adsorption ratios indicate
that a considerable quantity of colloidal material is left in the frac-
tions of finer soil residues in spite of the repeated washing and rubbing
to which this material was subjected. In the coarser soil residues,
however, very little colloidal material is present as a rule. One
11 Jt may well be that there is no reai distinction between the colloid which was extracted and that which
was not.
ESTIMATION OF COLLOIDAL MATERIAL IN SOILS. a3
would expect this colloidal material to differ in adsorptive capacity
- from the material which was isolated, from the mere fact that it was
not brought into suspension by the same treatment.
As already pointed out, evidence as to whether the unextracted
colloid differs in adsorptive capacity from the extracted colloid can be
obtained by comparing the quantities of colloid determined micro-
scopically with the quantities indicated by the adsorption ratios. If
the adsorption ratios indicate the same percentages of colloidal
material as the microscopical observations the colloid in the residues
presumably has the same adsorptive capacity as the extracted colloid.
If the adsorption ratios are lower than the microscopical determina-
tions, it is to be assumed that the unextracted colloid in the fractions
has a lower adsorptive capacity per gram than the colloid extracted,
and if the adsorption ratio is higher than the microscopical determin-
ation the unextracted colloid doubtless is more adsorptive than the
extracted colloid.
Comparisons of the microscopical and adsorption determinations
given in Table 5 indicate that the unextracted colloidal material
usually has a considerably lower adsorptive capacity for dye and
water than the extracted colloid, but the adsorptive capacities of the
two kinds of colloid for ammonia is as a rule approximately alike.
It should be borne in mind that these estimations of the relative
adsorptive capacities of the extractable and unextractable colloids
apply on the one hand to material in the condition in which it is after
extraction and on the other hand to material in the unextracted
condition. If it is true that the process of extraction does not alter
the adsorptive capacity of either the extracted or unextracted col-
loid, it would follow that the two kinds of colloidal material—if they
are distinct kinds—have different adsorptive capacities as they
exist in the untreated soil. However, data presented in the following
ages indicate that the colloidal material after extraction probably
as a somewhat different adsorptive capacity from what it had in
the untreated soil. In Table 9 the adsorptive capacities of the
extractable colloid before and after extraction are given, and the
probable relation between the adsorptive capacities of the colloid in
the two conditions is expressed in factors given in columns 8 to 10 of
that table.
For a comparison, then, of the adsorptive capacities of extractable
and unextractable colloids as they exist in the untreated soil, it is
necessary to correct the adsorption ratios given in Table 5 for pos-
sible alteration produced in the extracted colloid, upon which the
ratios are based. Factors given in Table 9 were used for this purpose.
The corrected adsorption ratios and microscopical determinations
on samples containing an appreciable amount of colloidal material
are given in Table 6.’
Data in Table 6 show that even in the untreated soil the extractable
and unextractable colloids usually differ considerably in adsorptive
capacities. The difference in adsorptive capacities of the two kinds
of colloid is not at all constant; it is large in the case of the Cecil soil
and practically negligible in the case of the Vega Baja soil. In most
12 Samples containing a small quantity of colloidal material had such low adsorptive capacities that the
adsorption ratios could not be determined accurately. The comparisons of the adsorption ratios and
microscopical determinations of colloid were, therefore, not significant in the case of these samples (Table 5),
and they were omitted from Table 6.
24 BULLETIN 1193, U. S. DEPARTMENT OF AGRICULTURE.
cases the unextractable colloid is more nearly equal to the extractable
colloid in adsorptive capacity for ammonia than in adsorptive capac-
ity for water or dye. ‘This is apparent also on averaging the results
of the ten samples. The adsorptive capacity of the unextracted
colloid averages 64 per cent that of the extracted colloid for dye, 58
per cent for water and 83 per cent for ammonia.
TABLE 6.—Relative adsorptive capacities of extractable and unextractable colloidal material,
as indicated by the relative percentages of colloid shown by microscopical and by cor-
rected adsorption ratio determinations.
] ]
| _ Adsorptive capacity of
| ' unextractable colloid
for dye, HO, and
Colloid present by corrected— NH, expressed rela-
| Colloid | tive to adsorptive ca-
| present | _ pacity of the extract-
Soil fraction in which colloid present. | by micro- | able colloid taken as
| scopical | 100 in each case.
| count. | ns :
| Dyead-{| Water lsucattontal
sorption | adsorp- | adsorp- | Dye. | H.O. | NH.
ratio. {tion ratio.|tion ratio. |
5 za | iz
Cecil clay loam soil: | Per cent. | Per cent. | Per cent. | Per cent. |
IN GnTAChlON. 2 xh.5 225255 .~ 465 74 | | 18 36 | 28 23 49
Fine fraction (duplicate).......... 55 | 1 | 14 [PE 2320523 2 2512 9.b 23
Huntington loam: | ; H
Boll ane (rachidd cis. c oes oe 48 | 30 | 18 26 | 63 38 54
Subsoil, fine fraction:............. 25 | 17 |} 20 21 | 68 80 | 84
Sassafras silt loam:
Subsoil, fine fraction.............. 38 17 | 19 22 45 50. 58
Sharkey clay soil:
HINGMLACHLON ane ce cicc ce hee eet 42 18 | 26 31 | 43 62 71
Separation by supercentrifuge— | '
BRine frachione:. 222 occu saa2 | 52, 23 32 35 | ‘4 62 67
Coarse fraction................ 25a 17 21 24 | 68 84 96
Vega Baja clay loam, soil (separation }
by supercentrifuge):
Mine fraction. >. 22>. .02-L-tiesv34- 97 | 98 85 101, = 101 88 | 104
Coarse fractions £2 hose ec ccec soe 14 | 19 9 23 | 136 64 | 164
Conclusions regarding representativeness of a sample of colloidal
material.—The data obtained on the adsorptive capacities of suc-
cessive lots of colloidal material isolated from the same soil and on
the relative adsorptive capacities of the extractable and unextract-
able colloid show that a small sample of colloidal material is not
exactly representative of all the colloid present in many soils. Part,
therefore, of the disagreements between the quantities of colloid
indicated by the dye, water, and ammonia adsorption ratios in Table
2, may be ascribed to failure to obtain an exactly representative
sample of the colloidal material. Sampling of the colloidal material,
however, does not appear to be the only cause of disagreement in the
adsorption ratios.
A small sample of the extractable colloidal material which is
dispersable into particles less than 0.3 micron in size appears to be
about 90 per cent representative of all this class of material. The
chief error in sampling appears to be due to the unextractable
collodial material in many, but not all, soils having a different
adsorptive capacity from that of the extractable colloidal material.
A small sample of the colloidal material seems to represent more
nearly all the colloidal material in its adsorptive capacity for ammonia
than in its adsorptive capacity for water or dye.
Pr ge ae
ESTIMATION OF COLLOIDAL MATERIAL IN SOILS. 25
The difference in the adsorptive capacities of the extracted and
unextracted colloids may of course be due to a difference in the
chemical composition of the two kinds of colloids or due merely to
a difference in the physical state, as fineness of subdivision or degree
of induration of the colloidal material. Certain indications point to
the latter possibility. Pressure alone, however, does not appear to
alter the adsorptive capacity of the air-dried colloid. , Dr. L. H.
Adams, of the Geophysical Laboratory of the Carnegie Institution,
very kindly subjected a sample of air-dried Sharkey colloid to pres-
sures up to 15,000 atmospheres. The colloidal material, which con-
tained 11 per cent of moisture when under pressure, was not changed
in adsorptive capacity. This is evident from the results given in
Table 7.
TABLE 7.—Effect of pressure on adsorptive capacity of the Sharkey soil colloid.
H:0 ad- Dye ad-
sorbed per | sorbed per
| gram of gram of
material. | material.
Treatment of sample.
| Gram | Gram.
sna IEEE Ut SOA RE a ho 8 = coo ae neces eneccaencess 0. 3103 | 0. 4365
aise 8 2 00 sams phenes pressure. - uss. a eo eee esc ews . 3082 | . 4395
Pease LO 7 HO atmospheres Pressure - -. . 2... a ne eo taaie snes ale bWslen'e seisent . 3078 | - 4395
epesee 1005, 000atmosphores pressure . .. 2255. -. =... 2-2 ese e nce eee ween ee == eee . 2932 . 4220
ALTERATION IN ADSORPTIVE CAPACITY OF THE COLLOID PRODUCED BY EXTRACTION,
From data presented in Table 4, evidence can be obtained as to
whether the adsorptive capacity of the colloidal material is affected
by extraction. Table 4 gives the adsorptive capacity of the un-
treated soil and the adsorptive capacities, as well as percentages, of
the colloidal material and other fractions into which the soil was
separated. If isolation of the colloidal material does not affect the
adsorptive capacity of the colloidal material, the adsorptions of the
separates into which the soil is divided (colloidal material, fine and
coarse fractions) should amount to just what the untreated soil
adsorbs. If, on the other hand, the quantity of dye, water, or
ammonia adsorbed by the combined cottoidal: fine and coarse frac-
tions amounts to more or less than the quantity of these substances
adsorbed by the untreated soil, it is apparent that the adsorptive
capacities of the colloidal material must be altered by the process
of fractionation.
Table 8 gives the quantities of dye, water, and ammonia adsorbed
by one gram of the untreated soil and the quantities of these sub-
stances that would be adsorbed by 1 gram of the soil separates
combined according to the percentages of each isolated from the soil.
It is apparent from the data in Table 4 that the quantities of colloidal
material, fine and coarse fractions isolated from a soil, do not exactly
equal the amount of soil taken for fractionation, the quantities of
separates recovered ranging from 95.5 per cent to 102.2 per cent of
the original samples. The losses of soil material were probably
chiefly mechanical, although with the large volumes of water used
there must have been some losses occasioned by solubility. re
- 4 oe: 5
Ts ~ 08
% x Rs re. 7)
f