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The Fractionation of Soil Colloids (1)By

IRVIN C. BROWN, and HORACE G. BYERS, U. S. Departmentof Agriculture, Bureau of Chemistry and Soils.

Introduction

It has been customary for soil chemists to regard the colloidseparated from a dispersion in water as essentially homogeneous'It does not follow, however, that it h*s been considered as a chem-ical unit, though the materiel extracted tes been treated as fairlyrepresentative of the total colloid. Complete removal of the totalcolloid is never obtained. Bradfield (3) points out differencesin the composition of three fine fractions of Putnam silt loam, sep-arated by the centrifuge. Gile et al (6) noted differences in theadsorptive capacities of colloid separated into different fractionsby repeated dispersion and separation by the centrifuge, but con-cluded 'that "on the whole the different fractions were similar inadsorptive capacity to the first sample extracted". Robinson andHolmes (11) also note differences but conclude that there is com-paratively little variation in the composition of colloidal mater-ial extracted from the seme soil. Iwanow (9) separated a very fineportion of colloid from a sodium saturated chernosem soil by meansof Birkfeld filters which, after ignition fand leaching, showedstriking differences in analyses from the other colloid fractions,>>ut the results are largely invalidated by the pretreatment of thefractions. Denison (5) attempted the fractionation of colloidsfrom the C horizon of lateritic soils by the removal with the cen-trifuge of the easily dispersed colloid^, and subsequent removalof the more difficultly dispersible colloid by dispersion and sub-sidence. The results obtained from the very small fractions ob-tained were somewhat inconclusive, but so remarkable os to warrantfurther investigation.

A preliminary fractionation of Davidson clay loam, P^ horizon,by the method used by Robinson an* Holmes (ll) essentially confirmedtheir findings in that no serious segregation of colloid fractionsof varying composition w^s produced. The general conclusion wasreached that more satisfactory results might be obtsined by attempt-ing fractionation on the basis of size distribution, the results ofwhich form the body od this paper.

(l) This paper is an abstract of a bulletin on the same topic tobe published at a later date and which will present a much greaterquantity of data, as well as 0 better discussion of the data and oftheir significance.

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Description of the samples.

The soils selected for this study were chosen in the firstinstance to secure colloids which were presumably mixtures ofmaterials and which might therefore be expected to furnish frac-tions of diverse composition. They were also soils of which thislaboratory possesses complete profile samples upon which consid-erable work hr 1 already beerj done. As the work -Jeveloped, itseemed desirable to include two samples which, on the basis ofinformation et hand, were not to be expected to show widely dif-ferent fractions. The samples chosen and their descriptions aretabulated in Table 1. They represent soils from different partsof the United States now in various stages of progressive weather-ing, from the chernosem to the lateritic soils.

Method of extraction of colloid fraction*

The colloid w-a separated from the soils f.nd fractionatedby the use of two Sharples super-centrifuges. One-half to twokilograms of soil were dispersed in 10 to 15 liters of water inc stirring apparatus described by Holmes and Edgington (8) towhich just sufficient ammonium hydroxide wt;s f.dded to renderthe suspension faintly alkaline. The sands were removed by pour-ing th« suspension through t. 300 mesh sieve and this operationWES repeated until about JiO gallons were obtained. The sepura-tion of the suspension into different colloid fractions, clay and -silt, was carried out by the methods thoroughly described in bul-letins of this Bureau (6, 11), and in stages represented by therote of flow through the centrifuge and necessary speed to obtainthe sizes described in the text and' tables.

Discussion.

The data presented in Table 2, of the colloid fractions, clay,and silt of horizon 2 of the Amarillo silt loam (No. 4575)show astriking similarity in the fine fractions, which is to be expectedof z soil not markedly weathered. This is alse in harmony with theprofile data reported by Anderson and Byers (<;) and is to be ex-pected of soils in s region o f low rainfall. The organic matteris relatively undecomposed and the soil particles less hydratedthen soils exposed to p higher rainfall. The uniformity of thecolloid fractions nsy be taken to indicate the presence of anacid complex of constant composition. Increases in the silica-sesquioxide ratios of the dry ?nd silt are due to the presenceof free quartz (determined petrographically by W. H. Fry, of thisBureau) which probably also contaminates the 0.3 to 1-Xi fraction,as indicated by the somewhat sharp alteration of silica-sesqui-oxide ratio.

The fractions of horizon 1 of the Marshall silt loam (No. 191),a northern prairie soil developed under much the same conditions ee

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tha Amarillo but exposed to sufficient rainfall to prevent the ac-cumulation of P Inyer of calcium carbonate. The small but defi-nite increase of the silica-sesquioxide ratio with increase in par-ticle size indicates that the process of removal of silica hag beenoperative as the process of weathering he.s developed. This mightbe expected in soils of high pH values while the removel of sesqui-oxides raoy be expected ih highly acid soils.

The colloid fractions of both the Amarillo and the Marshallare fairly uniform in composition, indicating the srme type of acidcomplex in both soils. The' finer fractions hrve a smaller quantityof organic matter and en increased combined water content, indicat-ing greater hydration of the finer particles and lesser decomposi-tion of organic matter.

The Beckett silt loam, B horizon, is a part of a profile samplereported by Anderson and Byers ('<,) and wrs chosen for this study asrepresenting a more nearly normal P horizon of the podsol type. Theanalyses of the colloid fractions in Table 3 shows striking contrastswhen compared with those of the Amarillo and Marshall. The finerfractions markedly decrease in percentrge of siller, whereas .theAmarillo and the Marshall colloid fractions are nearly constrnt.The wide difference in the character of the colloid is indicatedby the silica-sesquioxide ratio. The pH of this soil horizon is4.1 and the exchangeable base saturation is hut !6«8/£, as found byAnderson and Byers (2). The concentration of the orgrnic matter inthe finer portions is in contrast with the behavior of the saturatedcolloids, and is the reverse of the behavior of the organic matterin the saturated soil profile, i. e., the organic matter decreasese< s the depth increases (loc. cit.)- This behrvior in the podsolindicates that in the podsol profile it w.r s carried down in inti-mate association, possibly chemical combination, with the ironoxide from the A to the B horizon. This is also borne out by thehigh combined water of the finest fraction, approaching the valuesobtained for the lateritic soils and indicating the complete hy-drolysis of this finest portions

In the lateritic Davidson soil (No. 4440) we find a third typeof behavior of the fractions of the B horizon. The sample is a por-tion of the profile sample reported by Middleton (10). The profileanalyses show a distinct transfer of iron from the upper to the lowerhorizon but to o lesser degree than in the Beckett podsol in whichpodsolization h^s markedly increased not only the iron content butalso the alumina content (2).

In accord with this we find the perfectly distinct concentrationof iron oxide ih the finer particles, also emphasized by the ironoxide alumina ratio. When these facts are considered in the light ofcorresponding data for the. Cecil clay loam from North Carolina(No. 6278, ?able 4) and for the profile PS reported by Dennison (5),

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it seems difficult to avoid the conclusion th»t the iron oxideis carried from the upper to the lower horizon in a very finestate of subdivision, or possibly in true solution with organicacids of the humus type. Just r s in the podsol there is en ac-cumulation of organic matter with the finer particles and alsoa higher value for the combined water.

The Cecil clay loam (No. 6478, Table 4) is a more highlylaterized soil then the Davidson, and the soil material whichis disintegrated is much deeper. The E^ horizon is a portionof the ssme profile but not the s' me sample reported by Dennison.The profile analyses of the colloids show close similarity tothe extensive investigation of Cecil colloids reported by Holmesand Edgington (8).. In accordance with these properties of thecolloid of the Cecil profile we find distinct alteration of col-loidal properties with colloidal size (See Teble 7). In thiscolloid no marked alteration °f *he organic content or of thecombined water occurs* r:nd in this respefit it resembles the be-havior of the saturated colloids like the Amarillo and Marshall.It is probable th<3t 'this fr-ct is due to the extremely deep andporous character of the profile which h-s permitted the finelydivided material to penetrate to greater depths before deposition.

The Durham sandy loam, Cg horizon, from Stone Mountain,Georgia (No. 6ii63, Table 4) is derived from granite. The pro-file is the saoe *>s th^-t upon which data are reported by Denni-son (5), but the sample is not identical. The value of the silica-alumina ratio indicates a mature soil, *.a does slao the extensiveremoval of bases. On the other hrnd, the- rapid increase of thepotassium content of the coarser particles indicates the presenceof much undecomposed feldspathic material. This, With the smallconcentration of iron oxide in the finer particles, indicates im-maturity of the colloid in this horizon. The relatively largeamount of organic matter pt so grtet a depth and its failure tobe present in increasing quantity ir, the finer fractions indicatesconsiderable transfer of material from the upper soil strata. Therelatively high combined water rilso indicates the presence of hy-drated oxides. These facts indicate that this material may be inpart new colloid formed in place, c-nd in part completely laterizedmaterial curried down from upper strata* This is in harmony withthe detr presented by Dennison on c. different sample of the samesoil, thouch the silica-sesquioxide ratio is much greater in thepresent case.

Both the Cecil profiles of which colloids from the C horizonore presented in this paper (Table 5) are characterized by numerousfolded strata of different materials in the C horizon, which ore ofvarying width r nd color. The first of these is from the C horizon.(No. 6274) from near Stone Mountain, Georgia, and is a composite ofthese numerous strata of .disintegrated mica schist. The uniformityof the silica-sesquioxide ratio in the fractions points to the lack

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of violent weathering nt this depth. T,hi9 also, through f rac-tionation, explains the irregulari ty of the iron oxide-alumina,ratio. The silica base ratio marked ly increases as the particlesize decreases, emphasizing the loss of the soluble bases withprogressive Weathering. The second is from two or three layersof the Co horizon of the Cecil clay loam (No. 6<i8l) and -is fromthe same soil profile from N o r t h Carolina reported by Denison (.5)but not the same sample. The colloidal material of this sampleis much poorer in silica o.nd richer in alumina than Dennison 'ssample. It is aIso much lower in organic matter and in phosphates.However, the two are similar in character, and differences nay beescribed to local accumulation. The silica sesquioxide ratios arevery low, approaching that of a true laterite. In soils like thesewhose pH values are less then 5, there is a strong inclination toassume that colloid accumulation at this depth was formed in place/By this assumption the feldspathic material produces aluminium hy-droxide simultoneously with, or very shortly following, the pri-mary hydrolysis. But due to the high content of organic matter,the writers prefer to assume tha t both the alumina and organicmatter hpvc- been deposited at this depth, either from colloidalsuspension or from solution due to the surface alkalinity of thefeldspathic material to which at tent ion has been strongly directedby Cushtman (4) and others. Under these conditions e. certain amountof fractionation hes taken place whereby the iron oxide hPs beendeposited nt higher levels and the alumina carried to a greaterdepth.

In Tables 6 and 7 are presented data on water vapor adsorp-tion over 3$ sulphuric acid and over 30$ sulphuric acid, and theheat of wetting of the fract ions of the eight colloids examined.There are also given the ratio of the amount of water adsorbed over3% acid to that over 30%.

The heat of wetting in calories per gram closely parallelsand is nearly the s£'.se as the percentage of water adsorbed over 30$sulphuric acid, a fact which h?s been mentioned by Anderson (l). Inevery case the values for all thre- measurements decrease with in-creasing particle size.

The moisture adsorption over 3$ varies greatly with the parti-cle size, which strongly emphasizes the fact that the method of Robin-son (ll) and Gile (6) for the estimation of the quantity of colloidin the soil is, at best, a rough approximation, if course, the vari-ation from the approximate nenn value of the' weighted fractions is notso divergent but it is clear that with a. fine colloidal soil the val-ues hfive not the same significance <38 in the coarser colloid. Com-position of the colloid and surface exposed as well ts the treatmentthe colloid h's previously received doubtless hna a great influenceon its subsequent beb.rjvp.or, though the tr^tinent be identical for£.11 kinds of colloid. The composition hr,s P. most marked effect, PS

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is indicated by the ratio of the moisture adsorbed over 3% sul-phuric acid and over 30% acid. It is to be noted that in eachcolloid the ratio is fairly constant, though the actual quanti-ties of water adsorbed vary widely. The marked difference inthe adsorption ratio is between the group of soils representedby the Amarillo, Marshall, and Beekett on one hand, and the grouprepresented by the Durham, Davidson and Cecil types on the other.Water adsorption of colloids is most seriously effected by changein humidity if the bases of the colloids are low in proportion tosilica.

There is a possibility that the adsorption ratio may be cor-related with the existence in the colloid of alumino-silicic acids,which are hydrated readily Pnd retain their water by hydrationwhen the humidity is low if the acid complexes are true clays ofthe Montmorrillonite type, and which are easily dehydrated if ofthe Halloysite type (7). It h*s been shown by Henricks and Fry(7) th?t the Amarillo, Marshall, and Beckett are colloids of theformer type and the Durham, Davidson and Cecil of the latter.The theoretical considerations for these soil acid complexes willbe more fully discussed in a department bulletin dealing with thesedate.

Summary

Sight colloids have been extracted from seven different soilprofiles and fractionated into three portions, according to size.These fractions End the corresponding clay «<nd fine silt have beenanalyzed by the fusion method ?.nd the organic matter, water vaporadsorption over 3$ f ind 30% sulphuric acid, and the heat of wettinghave also been determined. Various ratios derived from these datahove been calculated.

The chemical composition of the Amarillo colloid is quite uni-form, as is also that of the Marshall colloid, and in both there isshown evidence of quartz particles of colloidal size. In the colloidfractions of the Beckett silt loam there is shown the most markedsegregation of material in di f ferent fractions.

In the colloids of the lateritic soils there is less markedbut still distinct evidence of fractionation.

The rPtio of the water vapor adsorption over. 3y£ sulphuric acidto that over 30/£ sulphuric acid is about twice as great in the lat-eritic soils ns in the podsol prairie and chernosem samples. It isinferred that this fact and the d i f fe rences in silica-sesquioxideratio indicate the existence of two distinct alumino-silicic acids inthese colloids.

The colloid of the deep soil material of the Cecil soil samplestogether with the organic matter content leads to the assumption of

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its origin in the higher strata. It is inferred that podsolizationis accompanied by a natural process of fractionation.

Much additional data will be presented in the bulletin ofwhich this is an abstract.

It may be of interest to note that we now have under construc-tion a centrifuge by means of which we hope to be able' to furtherfractionate the O.l/i colloid.

LITERATURE CITED.

(1) ANDERSON, ,.. S. 1924. Heat of Wetting of Soil Colloids. Jour.Agr. Research, v. 28.9, p 927-36.

(2) ANDERSON, M. S. and BYERS . H. G. 1931. Character of the Col-loidal Materials in the Profiles of Certain Major Soil Groups.U. S. Dept, of Agri. Tech. Bul. 228.

(3) BRADFIELD, RICHARD. 1923. The Chemical Nature of ColloidalClay. Missouri Agr. Expt. Station Bul. 60, page 18, also 55.

(4) CHSHMAN, ALLERTON S. 1905. Effect of Water on Rock Powders.U. S. Dept, of Agr. Bureau of Chemistry Bulletin 92.

(5) DENISON. I. A. 1930. The Chemical Composition of Colloidal Ma-terial Isolated from the Horizons of Various Soil Profiles.Jour. Agr. Research 40« 469-483.

(6) GILE, P. L. , MIDDLETON, H. E., ROBINSON, W. O. FRY, W. H., andANDERSON, M. S., 1924. Estimation of Colloidal Material inSoils by Adsorption. U. S. Dept. Agriculture, Bul. 1193,p^ 23.

(7) HENDRICKS, S. B. and FRY, W. H. 1930. The Results of X-Raysand Microscopical Examinations of Soil Colloids. Soil Sci-ence ^9, No. 6, p. 469.

(8) HOLMES, R. S., and EDGINGTON, Glenn. 1930. Variations of theColloidal Material Extracted from the Soils of the Miami,Chester and Cecil Series. U. S. Dept, of Agr. Tech. Bul.No. ki

(9) IWANOW, D. W. 1926. Der Absorbiende Komplex des Tschernosembodens. Jour. f. landw. Wiss. Jahoganz. No. 4, p. 268(Russian; summary in German).

(10) MIDDLETON, H. E. 1930. Properties of Soils which InfluenceSoil Erosion. U. S. Dept. of Agr. Tech. Bul. No. 178.

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(11) ROBINSON, W. O., and HOLMES, R. S. 1924. The ChemicalComposition of Soil Colloids. U. S. Dept, of Agr. Bul. 1311.

Table 1

Soil type

Amarillo siltyclay loam

Marshall siltloam

Beckett siltloam

Durham sandy-loam

Davidson clayloam

Cecil clayloamCecil sandyclay loam

Cecil clayloam

Location

Nash. Texas

Maynard, Neb.

Washington,Mass.

Stone Mt.Georgia.

Greensboro,N. C.

Rutherford-t o n , N. c.

Rich brown colorwith faint tint ofred

Highplainsdeposits

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Chemical composition of fractions of soil colloids, clay and silt,from various soils-

Fractionof whole :soil in sper cent:

4575 - Southern Chernozem: Amarillo silty clay loam, Horizon 2,10-20 inches depth.

191 - Nor thern Prairie Soil: Marshall silt loam, Horizon 1,:' 0-14 inches depth.

Table 3.

Chemical compositions of fractions of soil colloids, clay and silt, fromvarious soils. :

Podsol: Beckett silt loam, B horizon, 13-24 inches depth.£t

Lateritic soil: Davidson clay loam, B1, horizon, 9-36 in. depth.

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Table 4.

Chemical compositions of fractions of soil colloids, clay and silt,from various soils-

Lateritic soil* Cecil clay loam, B horizon, 36-72. inches depth. * :

Lateritic soil : Durham sandy loam, C horizon,90-102 inches depth-

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Table 5.

Chemical compositions of fractions of soil colloids, clay and silt,various soils.

Lateritic soil: Cecil sandy clay loam, C4 horizon, 180-196inches depth.

Lateritic soil: Cecil clay loam, C3 hor izon , -112 inches depth.* . * . O •

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Physical propert ies of f rac t ions of colloids f r o m various soils«

Amarillo silty clay loam Beckett silt loam

Marshall silt loam Davidson clay loam.

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Table 7.

Physical properties of fractions of colloids from various soils-

Cecil clay loam. Cecil sandy clay loam.

Durham sandy loam. Cecil clay loam.