Soil Colloids and the Soil Solution
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SOIL COLLOIDS AND THE SOIL SOLUTION1
BY FRANK K . C A M E R O N ~
To no branch of modern scientific inquiry does there pertain a more confusing literature than to soil chemistry. In this confusion there has been developed a special termin- ology founded on misconceptions and false analogies, but re- tained with an amazing perversity.
For instance, the addition of lime either as calcium hy- drate or as carbonate, to most soils, induces conditions es- pecially favorable to the growth of certain types of plants, such as the clovers, alfalfa, etc. Again, East soils when wetted and brought into close contact with blue litmus paper redden the paper. Hence, i t appears, judging from current litera- ture, that the vast majority of soils are acid, in spite of the facts. ( I ) that some soils to which lime has been added suffi- ciently to induce good growth of clover will yet appear to redden litmus paper more vigorously than other soils which will not support the clover effectively; and (2) that the reddening of the litmus paper is, in most cases, a rather obvious phenome- non of selective absorption; and ( 3 ) that these same acid soils yield aqueous extracts, which. when boiled to expel car- bon dioxide, are more often alkaline than neutral and quite rarely acid. The term acid soil being appropriated then to mean a soil which better supports certain crop plants after b e i q limed, how ma)- one n i th propriety designate that soil occasionally encountered whose aqueous extract shows the actual presence of a soluble acid, i. c . , responds to the test for a hydrogen ion
It has long been the fashion with many writers on soil chemistry to ascribe the power shown by soils in absorbing
potassium from aqueous solutions of potash salts more rapidly relatively than they absorb sodium and certain other bases,
paper prepared for the symposium on colloid chemistry during the
Scientist in Charge, Soil Laboratory Investigations, Bureau of Soils, hlontreal meeting of the American Chemical Society
U S. Department of Agriculture, nashington, D. C.
2 Frank K. Cameron
to the formation and presence of zeolites in the soil. There appears to be geological and petrographical evidence that zeolites are, or may be, deposited a t ordinary temperatures from aqueous solution. But as has been shown elsewhere, the concentration of free alkali in the solution from which such a deposition might conceivably take place, must be far higher than could ever exist in a soil fit for the production of crop plants; and furthermore, no reputable observer has ever yet reported actually seeing a zeolite in a soil, although many hundred soil samples have now been examined by trained microscopists with this special purpose in mind. By some i t has been claimed thnt the soil zeolites are sub-microscopic, and exist in the clay particles only, and since the assumption of colloids in soils has become common nowadays, it is eyen postulated that the soil zeolites are in the colloid condition, which is clearly a mere juggling of terms with a confusion of ideas.
It is to be feared that the term colloid, as commonly used in soil literature, is not entirely free from the same character of objection applying to acid soils and soil zeolites. I-rofessor Remsen once designated basic salts a s a sink of iniquity into which we cast compounds we do not understand. Acid soils, soil zeolites, and soil colloids appear to be such sinks, and the colloid sink to be the deepest of them all.
While, with a large proportion of the writers on soil topics, colloids seem to be nothing more or less than a sufficient if de? $iier 7 e s o i t to explain things m hen they are not ingenious enough to devise some other explanation, the subject has been approached seriously. It will be well, before proceeding, to call attention to some of the arguments which are usually advanced to demonstrate the existence of colloids in soils.
Van Bemmelen, because of the numerous apparent
Proceedings of the Eighth International Congress of Applied Chemistry, New York, 1912, Vol. XV, pps. 43-48.
Landw. Versuchs St., 35, 67-136 (1888); 37, 347-73 (1890); Zeit. anorg. Chem., 2 2 , 313-79 (1899); 23, 321-72 (1900) ; 42, 2 6 5 ~ 3 2 4 (1904); 46, 322-57 (1910).
Soil Colloids aizd the Soil Solaitioia 3
parallelisms between the properties of soils and colloids, con- sidered the only rational explanation to be the presence of colloids in soils. -4s supporting this conclusion, he extracted a number of Dutch soils and some of foreign origin with aqueous solutions of acids of various concentrations. In a similar way, he examined mechanical separates of soils From these data he classified the soils according to the ratio of re- acting weights of alumina to silica or to some of the bases ex- tracted by the acids. Became, in general, these ratios were not whole numbers, he concluded that many of the soil com- ponents, especially in the finer state of division, could not be definite compounds; hence, they must be something else, i. c. , colloids.
As further proof of the existence of colloids in the soils, van Bemmelen advanced the results of experiments on the rate of el-aporation of water from wet soils, which rates he found to be but little different irom the rcte obtaining with a surface of free water
Cushmari' found that on wet grinding of a silicate, the grains were coated with a gel, colloid or ' . pectoid ' ' which could be dyed. Sjollema? considers that practically all the soil components, excepting quartz grains and undecomposed mineral fragments, are colloids because they are colored by organic dyes.
Atterberg considers the physical properties of the soil to be the properties of colloid more or less modified by the presence of coarse particles. He maintains that the colloid properties give the only rational basis for soil classification, and has devoted3 much energy to developing methods for measuring the colloid properties.
Russell, who stands deservedly in the first rank of present- day investigators of soil phenomena, takes very strong ground. He regards the clay fraction of a soil as a colloid because of
Bull No 92, Bureau of Chemistry, U S Dept of Agric , 1905.
See for instance, I1 Agrogeologen Konferenz, Stockholm, 1911, p 5, * Landw l-ersuchs S t , 53, 67 (1905)
e t seq., Kolloid Chem Beih , 6 , 55-89 (1914)
4 Frank K . Cameron
its plasticity, a fact also noted by van Bemmelen and numerous other writers. He also notes that, in some clay suspensions Brownian movements can be observed, and that clay suspen- sions are markedly affected by the addition of small amounts of electrolytes and some other substances, showing floccula- tion and deflocculation phenomena. The wetted soil shows the very similar, if not identical phenomena, of crumbing on the one hand and puddling on the other.
Russell1 holds that the clay particles of the soil form compound particles which are responsible fcr most of the inherent characteristics cf & soil, the properties of these com- pound particles being the properties of colloids. In discussing the medium from which plants deril-e their sustenance, he says i t is a colloidal complex of organic and inorganic com- pounds, usually more or less saturated with water, that en- velopes the mineral particles ; it is, therefore, analogous t a the plate of nutrient jelly used by bacteriologists, while the mineral particles serve mainly to support the medium and control thc supply of air and water and to some extent the temperatures. Russell even goes so far as to divide modern soil chemists into two schools on the basis of a belief in the existence of soil colloids, to wit: (I) the nature of the col- loidal substances in the soil; these are supposed by van Bem- melen and his school to be decomposition products of weathered silicates, and by Whitney to be particles of any composition, provided the size is sufficiently small; (2) the constitution of the soil solution, van Bemmelen supposing it to be in equi- librium with a solid solution or colloidal complex, and, there- fore, to depend as to its concentration on the masses of its constituents present in the complex, while Whitney supposes it to be in equilibrium with definite silicates and to be constant in concentration.
Throughout the literature, even the most recent, there seems to be an implication that colloids are possessed of mys-
Soil Conditions and Plant Growth, by Edward J. Russell, Dr. Sc . , (Lond.), Longmans, Green & Co., London, etc., 1912, pp. 56-9, 75-7, 1 1 0 .
Soil Colloids and the Soil SolutioTz 5
terious, almost uncanny properties. The term colloid in soil work, a t least, has tended to promote a confusion of ideas, make against clear thinking, and all too frequently has been an excuse for mental laziness. The time has now come, and especially because of the prominence given to col- loids in Russells very important monograph, when the situa- tion must be faced frankly.
If Professor Bancrofts definition of a colloid is accepted as simply a phase sufficiently divided there can l x no argument that colloids exist in the soil. A soil contains particles of all mechanical sizes, down to below recognition by the microscope and towards an indeterminate minimum. Thus the amount of surface exposed in the soil is very large, even enormous, as compared with the mass of the components. Surface effects, especially adsorptions, crumbing (possibly flocculation) and puddling (possibly deflocculation) are \-ery pronounced, and the chemistry of the soil, as has been pointed out elsewhere, is to a large extent the chemistry of surface p h e n ~ m e n a . ~ If it be recognized, therefore, that the dis- tinctive properties of colloid are surface properties, then there can be no objection to calling soil chemistry a branch of colloid chemistry. In the soil the amount of the surface exposed is not only very great in the aggregate, but is probably very large for all three types of solids, t i z . , definite compounds, solid solutions, and adsorption complexes.
As far down the gradient of size as i t is possible to trace the soil particles, definite compounds in the form of the well-
Jour. Phys. Chem., 18, j+9 i1914). For convenience, the mechanical separates of soils are usually obtained
between arbitrary limits and are designated as sands, silts, and clays. Thus the term silt does not necessarily mean, in soil literature, that the material has been deposited from a water suspension, neither does the term clay imply the composition of the material is that of kaolin or kaolinite, nor even that these definite compounds are present. Fletcher and Bryan: Bull. No. 84, Bureau of Soils, U. S. Dept. Agric., 1912.
Cameron and Bell: Bull. No. 30, Bureau of Soils, U. S. Dept. Agric., 1905; The Soil Solution, by Frank K. Cameron, The Chemical Publishing Co., Easton, Pa., 1911, pp, 67-9.
6 Fraak K . Cameron
known rock and soil forming minerals have been recognized. It seems reasonable to suppose that they still persist as such beyond the limits of positive identification, even perhaps to extreme attenuation. Together with this mineral mixture of definite chemical individuals. the results of degradation agencies, there is always present a more or less indefinite mass of indeterminate compounds, in extreme state of com- minution in which a p p a , e ! i t l j oxides of altlminum, of iron (ferric) and organic residues f humus) predominate. There is very good reason to believe that some of the components of this clay are solid solutions-for example. the so-called basic phosphates of iron and elumina. And there are equally good reasons for believing in the presence of adsorption com- plexes. In fact it is a well sustained generality that potzs- sium and certain other normal constituents of the soil tend to segregate in the clay separates, which admit G f no other explanation than that of selecti\-e adsorption.
The minerals of the soil are continually reacting with water, by hydrolysis, and 1-ery often with secondary reactions whicfi yield products in the colloid condition. For example, consider the comparatively simple case of orthoclase, assuming for simplicity in presentation, that it has actually the theoretical formula K.A41Si108. Then K.A%lSilOs + WOH KOH + H.A1Si308. So far as is known the acid H.X1Si308 does not exist; but as fast as formed, there is a splitting of[ of silica SiOz, perhaps progressively, with the formation of H .A41Si206 (pyrophylite), H.AlSi0, (kaolinite), and H.A102 (diaspore).
It seems probable that the silica and alumina thus formed, a t least for a time, and possibly even for a time also, the other products of hydrolysis, persist in the colloid condition, either as true gels or colloidal solutions. Similarly ferrugjnous gels, and even perhaps gels of magnesium hydroxides are formed and more or less temporarily affect the constitution of the soil complex. But while admitting the possibility or
See Van Hise: U. S. Geol. Surv., Llonograph No. 47, p. 333, 1904, and Kahlenberg and Lincoln: Jour. Phys. Chem., 2, 77-90 (1898).
Soil Colloids and the Soi l Solution
even probability of such gels being formed from a p ~ i o r i considerations as well as laboratory manufacture, it must also be admitted that there is no satisfactory direct evidence of their presence in a soil under field conditions. The principal evidence for their presence cited in the current soil literature, are the results obtained in the selective absorption of dyes by soils. But no case has been brought forward which cannot be satisfactorily explained by the fact merely that the soil particles present a large surface for absorbent action.] h-ever- theless, the importance of this gel formation is considered, among soil in\-estigations usually) to be very important as affording a protective coating to the soil grains. This pro- tectil-e coating is generally assumed to prevent solvent action of soil water upon the coated particles, obviously an incorrect assumption, since diffusion must necessarily proceed through the gel as through water. Of course, such a coating might mechanicall\- slo~v up the rate of solvent action on the soil particles. The ordinary ferruginous-humus-clay mass, be- cause it sticks so persistently to the coarser particles, is fre- quently supposed to prel-ent solvent action. This material can, however, be separated quite effectively from the coarser particles, merely by shaking in water to which a little ammonia has been added to deflocculate the soil aggregates. Here again there seems to be possible nothing more than a mechanical slowing up of the rate of solvent action.
Another role popularly assigned by soil investigators to the supposed gel formed on the surface of soil particles is the making possible of adsorption effects. Here there is an as- tonishing amount of misinfornation current. At a recent meeting of a scientific organization, an agr...