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 t o soil chemistry. In this confusion there has been developed a special terminology founded on misconceptions and false analogies, but retained with an amazing perversity. For instance, the addition of lime either as calcium hydrate or as carbonate, to most soils, induces conditions especially 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 literature, t h a t the vast majority of soils are “acid,” in spite of the facts. ( I ) t h a t some soils to which lime has been added sufficiently 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) t h a t the reddening of the litmus paper is, in most cases, a rather obvious phenomenon of selective absorption; and ( 3 ) that these same “acid” soils yield aqueous extracts, which. when boiled to expel carbon 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 t h propriety designate t h a t 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 hlontreal meeting of the American Chemical Society Scientist in Charge, Soil Laboratory Investigations, Bureau of Soils, U S.Department of Agriculture, n’ashington, D. C.
2
Frank K. Cameron
to the formation and presence of zeolites in the soil. There appears to be geological and petrographical evidence t h a t 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, 1 9 1 2 ,Vol. XV, pps. 43-48. Landw. Versuchs St., 35, 67-136 (1888); 37, 347-73 (1890); Zeit. anorg. Chem., 2 2 , 313-79 ( 1 8 9 9 ) ; 23, 3 2 1 - 7 2 ( 1 9 0 0 ) ; 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, considered 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 reacting weights of alumina to silica or to some of the bases extracted by the acids. Became, in general, these ratios were not whole numbers, he concluded that many of the soil components, 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 presentday investigators of soil phenomena, takes very strong ground. H e regards the clay fraction of a soil as a colloid because of Bull No 92, Bureau of Chemistry, U S Dept of Agric , 1905. l-ersuchs S t , 53, 67 (1905) See for instance, I1 Agrogeologen Konferenz, Stockholm, 1911,p 5, e t seq., Kolloid Chem Beih , 6 , 55-89 (1914)
* Landw
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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 t h a t clay suspensions are markedly affected by the addition of small amounts of electrolytes and some other substances, showing flocculation 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 t h a t the “clay” particles of the soil form “ compound particles which are responsible fcr most of the inherent characteristics cf & soil, the properties of these compound particles being the properties of colloids. I n discussing the medium from which plants deril-e their sustenance, he says i t “is a colloidal complex of organic and inorganic compounds, usually more or less saturated with water, that envelopes 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, t o wit: ’ ‘ (I) the nature of the colloidal substances in the soil; these are supposed by van Bemmelen 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 equilibrium with a solid solution or colloidal complex, and, therefore, 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. S c . , (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 colloids in Russell’s very important monograph, when the situation must be faced frankly. If Professor Bancroft’s 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 distinctive 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 it is possible t o trace the soil particles, definite compounds in the form of the wellJour. 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.
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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 comminution 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 complexes. In fact it is a well sustained generality t h a t potzssium 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.X1Si308does 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
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See Van Hise: U. S. Geol. Surv., Llonograph No. 47, p. 333, 1904, a n d Kahlenberg and Lincoln: Jour. Phys. Chem., 2, 77-90 (1898).
Soil Colloids and the Soil 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 b y 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-evertheless, 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 protectil-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~vup the rate of solvent action on the soil particles. The ordinary ferruginous-humus-clay mass, because it sticks so persistently to the coarser particles, is frequently 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 astonishing amount of misinfornation current. At a recent meeting of a scientific organization, an agricultural chemist of recognized distinction, in discussing a paper which had just been read, made the startling statement that there could be no such thing as an adsorption of potassium by a soil because potassium is not a colloid and it is a well-known fact(?) that See, for instance, Seki LandI$ Versuchs S t , 79-80 (dem Andenken Oskar Kellner gewidmet), 873, and the literature there cited.
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Fraxk K . Cameron
colloids alone can be adsorbed and then only by other colloids. He even went so far as to deny the probability of Patten’s observations on the adsorption of methylene blue by powdered quartz,l although admitting them as possible if the quartz grains had a coating of colloidal silica; their only importance, he claimed, would be in showing methylene blue to be a colloid. Finally, it is to this protective coating of colloid material t h a t most teachers of soil physics ascribe the formation of aggregates of soil particles so important in determining the highly desired “crumb” structure of soils and so important for good tilth, “good heart,” the textural relations, water holding capacity, and adaptability to types of crops and crop rotations; in fine, to soil physics and soil management in general. Unfortunately, however, there is no positive evidence one way or the other for the existence of these gel coatings. The phenomena of crumbing and of puddling, while very similar to, if not identical with, the phenomena of flocculation and deflocculation, the phenomena of plasticity, etc., seem to be accounted for readily enough by the presence of the fine clay particles, and there is no obvious necessity for assuming the presence of any gel, much less that it is a protective coating. Practically, there is a serious dificulty in applying observations on ordinary suspensions, gels and colloid solutions to the interpretation of soil phenomena. In a soil containing the optimum, or somewhat less than the optimum water content for plant growth, it appears that practically all the water (soil solution) is spread out ol-er the grains or crumbs, in films.2 This film water is held to the soil under stresses of such magnitude as may considerably affect the solvent power of the water, although direct proof t h a t such is the case is wanting. It is very probable that the liquid-gas surface Trans. Am. Electrochem Soc., I O , 67-73 (1906). Cameron and Gallagher: Bull. KO. 50, Bureau of Soils, U. S. Dept. Agric., 1908. 50 mechanical means has yet been devised for extracting this soil solution or separating it from the solid phases, hence we do not yet know what its composition actually is and can only make inferences from indirect methods of experimentation.
Soil Colloids a?id the Soil Solutiofi
9
tension affects appreciably the liquid-solid surface tension, although in what manner and to what extent it is yet impossible t o say. Consequently, great reserve must be exercised in attempting to apply to soil phenomena the reasoning developed from observations on gels and ordinary suspensions. Certainly the water films in the soil are continually changing in thickness; there must be a consequent change in the distribution of solutes between absorbent and solvent, and probably two changes of surface tension at least. Just how to correlate these several factors and especially in view of the fact that they are each and all continually undergoing change, no man yet knows. Until a clearer vision of the interrelation of these factors is gained, it is vain to look for any correlation of the agricultural importance of a particular soil with arbitrary measurements of its supposed colloidal properties. Of the necessary consequences of the colloidal constitution of the soil as described in the foregoing paragraphs very much the most important, theoretically and practically, is the concentration of the soil solution, a t least with respect t o those constituents derived from the soil minerals. From what has been said above, it will be correct and convenient to classify the soil components as follows: I . Definite compounds, or chemical individuals. 2 . Indefinite complexes, but homogeneous, i. e., solid solutions. 3. Indefinite complexes, but not homogeneous, i. e., adsorption complexes. For any given temperature, a definite compound has a definite solubility, which is altogether independent of the relative masses of the solid and the liquid. This solubility may be affected by the presence of a, third substance, especially if the solutes are electrolytes. In soil solutions the concentrations are so small that mutual solubility effects can be considered negligible, usually, excepting only those due to dissolved carbon dioxide. There are, however, two modifying influences which are probably effective in the case of soil
Frank K . Cameron
IO
minerals. It has been shown by Ostwald’ and by Hulett2 t h a t a very finely comminuted substance may have quite a different solubility than the same substance when in coarse particles. For instance, the solubility of gypsum may be increased a fifth by grinding the solid to very fine particles. The extremely fine mineral particles in the clay portion of the soil may well, therefore, have different solubilities than those minerals in the fine sands, and intermediate solubilities may pertain to the finer silt particles. Again, many if not most of the soil minerals are salts of strong bases with weak acids. On dissolving, they are more or less completely hydrolyzed, and one of the products of hydrolysis, the acid usually, is so very slightly soluble t h a t its active mass can be considered negligible. Consequently a simple application of the mass law to the reaction: Mineral water unhydrolyzed salt base acid
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shows that the concentration of the base must become very large, relatively, before equilibrium is attained. It is quite improbable that equilibrium is ever actually attained in the soils of humid areas. And it is equally improbable that anp of the ordinary rock forming minerals can be synthetized a t the concentrations and temperatures ordinarily realized in soils.3 From these considerations, therefore, it appears t h a t the major part of the definite compounds which dissolve in the soil water, under humid conditions, are hydrolyzed, and that the soil solution never reaches the concentration of the more soluble constituents required for definite equilibrium or saturation.” Practically, therefore, so far as regards Zeit. phys. Chern., 34, 496 (1900). Ibid., 37, 385-406 (1901). 3 m’hile highly improbable that zeolites are formed under soil conditions by “building up” processes, there is to be admitted the possibility of their formation by some “building down” mechanism n o t yet explained. Certainly there is geological evidence that zeolites are sometime alteration products of feldspar. On the other hand there is a respectable body of cumulative evidence in geelogical literature suggesting that concentrations may occur in adsorption films sufficient to induce “building up” processes. Obviously, however, plant roots do not come into contact with solutions of such concentrations.
Soil Colloids alzd the Soil Solution
I1
the definite compounds in the soil, the concentration of the soil solution is not independent of the ratio of the mass of solid to the mass of liquid, although theoretically such a condition should be obtained if the time element, surface effects and like distributing factors did not enter. Different considerations obtain when the liquid solution is in contact with a solid solution containing a common component. The concentrations of the common component in both liquid and solid phase is dependent upon the relative masses of the phases. Increasing the concentration in one increases (or sometimes decreases the concentration in the others. There is a distribution of the common component represented by the equation C = KC1, or C = KCif1, or perhaps by some more complicated expression. Usuallj-, the exponent 11 is positix-e: but that it may have a negative value is suggested by the case where lime is added to lime phosphate in contact with water. ;1practical example of a solid solution in the soil is where a soluble phosphate has been added, the phosphoric acid being more or less promptly precipitated as a basic ( 7 ) phosphate, i. c., solid solution, and it has been shown that it would require the addition of enormous and impracticable amounts of the phosphate to so increase the concentration of the ferro-phosphoric acid alumino-phosphoric acid and lime-phosphoric acid solid solutions to the point where there would be an appreciable increase in the concentration of the liquid (soil) solution. The third class of soil solids comprises those cases where there is a mineral complex in which a more soluble substance (solute) is condensed or adsorbed on the surface of a much less soluble substance (adsorbent). Here again is a case of distribution and, obviously, again the concentration of the liquid solution is dependent upon the relative masses of liquid and solid; or, more correctly, upon the volume of the liquid and the absorbing surface of the solid. As a case in illustration, probCameron and Bell. Bull. No 41, 1907;Schreiner and Failyer: Bull. No. 32, S.Dept of Agric.
1906, Bureau of Soils, E.
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Frank K . Cameron
ably a large part of the potassium in a soil is adsorbed on the surface of the clay particles. Attempts to study experimentally the distribution of a solute between the solid soil particles and the free water in contact with the soil have shown that the equation is of a quite complex character, owing probably to changing aggregation (flocculation or deflocculation) of the soil particles.' But whatever the form of the equation, it is beyond doubt that the concentration in the soil solution changes coincidentally with and in the same direction as the concentration in the absorbent. As the effective surface of the soil is continually changing, it follows necessarily that the concentration of the soil solution is continually changing. But with such dilute aqueous solutions as are soil solutions, it requires a relative large increase in the adsorbed solute t o produce an appreciable change in the concentration of the liquid phase. The fact has been shown a number of times experimentally with soils and with other absorbents, both by shaking the absorbent with aqueous solutions of the solute and by percolation experiments. It is clear, therefore, that the concentration of the soil solution, resulting from contact with definite compounds, solid solutions and adsorbent films, i. e . , from contact with soil colloids, is determined by the relative masses of the liquid phase and of the solid phase, in so far as the masses of the solid phases determine their effective surface to solvent action. Since the moisture content of a soil is continually changing, the soil either drying out or getting wetted, and since the effective surface of the soil particles is continually changing, the concentration of the soil solution is continually changing or tending to change. But as the foregoing analysis of the mechanism of the soil solution develops, and from the inherent nature of the soil colloids, small changes in concentration in the liquid phase (soil solution) are induced by relatively large changes in the common constituents in the solid phases (soil colloids), a t least with respect to those mineral constituents of recognized importance for plant growth and Cameron and Patten: Jour. Phys. Chem.,
11, j81-92
(1907)
S o i l Colloids aiid the Soil Solutio71
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fertilizer practice. Consequently, under any given climatic conditions, the concentration of the soil solution, with respect to those constituents derived from soil minerals, varies within narrow limits, no matter what may be the relative masses of those minerals in the soi1.l Obviously, other constituents, . organic or inorganic may sometimes varj- quite widelq- in concentration although in all cases the solubility principles just set forth will apply. So much being granted, it follows logically that in the function whateyer its form may be, expressing the relation between crop production and the various natural and artificial factors affecting it, fertilizers as well as each and every other factor are dependent variables, and that furthermore, all soil phenomena affecting crop production are dynamic, contrasted with static, in character: that is, they are all involved in continual changes of some kind or other; and, in fine, each soil must be considered as an individual, with its own inherent characteristics, including crop producing powers. Arguments in detail having already been set forth elsewhere, it is not necessary to repeat them here.2 In this paper it has been shown: I . That soil chemistry can be considered a branch of colloid chemistry, provided a colloid is defined as a phase sufficiently divided where surf ace phenomena are predominant. 2. T h a t the relation of the gas-liquid surface tension to the solid-liquid surface tension is a most important problem requiring investigation for a clear purview of the functions of soil colloids. 3. That it is a necessary consequence of the colloid constitution of the soil, that very small changes in the concentration of the soil solution correspond t o relatively large changes in the composition of the solid phases. respecting those constituents derived from the minerals of the soil. T h a t difference in climate may make marked differences in the concentration of the soil solution is, of course, well recognized. See for instance, hlooney : “The Bahama Islands,” The Geological Society of Baltimore, Johns Hopkiiis Press, 1905, pp. 153-74. * J o u r . Ind. Eng. Chem., I , 806-10 (1909); 3, 188 ( I ~ I I ) , “The So:! Solution.”
