THE CONSTITUTION OF THE INORGANIC SOIL COLLOIDSby

HORACE G. BUYERS (1)

In opening a round-table discussion of the consti-tution of inorganic soil colloids, I wish to present certainpostulates (in the sense given by Webster: "To postulateis to lay down or enunciate an assumption without proof)and to present, in very brief outline, certain bases forthe assumptions made, after which I hope those whodiscuss the matter will confine themselves to the citationof experimental or theoretical grounds for assenting toor dissenting from the postulates. This procedure shouldthen result in the segregation of those points upon whichthere is general agreement and those for which the evi-dence is adverse. There should also result from the dis-cussion suggestions for investigations which will produceevidence for or against those assumptions which are notadequately supported. It is not too much to hope that thenet result should be more systematic research on soilsand a clearer mutual understanding by each of theother's problems. Some of the postulates may be regard-ed as almost axiomatic. Probably some may be regardedas generally accepted now, and possibly some may beregarded as quixotic.

1. All soil colloids of the inorganic type may beregarded as derived from igneous rocks or theirsecondary products by the process of weather-ing.

For the purpose of this discussion we may consideras colloid any soil particle smaller than 1 micron in itsgreatest dimension.

2. The process of weathering may be consideredas of two types:(a) Fragmentation, or physical weathering;(b) Hydrolysis followed by possible transloca-tion through true solution or colloidal suspen-sion.

With the processes of fragmentation we need notconcern ourselves in this connection, since whatever thedegree of comminution, even to colloidal dimensions, theresulting products can not be soil—at least in the senseof the word as used by the Soil Survey.

Hydrolysis, on the other hand, is the fundamentalprocess which produces soil colloid. This process—adouble decomposition in which water is one of the re-acting components — is of necessity occurring at alltimes when hydrolyzable minerals are in contact withwater. It proceeds at a rate dependent upon the char-acter of the mineral, the surface exposed, the tempera-ture, and the rate of water movement. In assigning tothis process the primary role in soil formation it is notnecessary to either deny or minimize the parts played insoil formation by carbon dioxide and other products ofanimal or plant katabolism, since these products simplyalter the rate of hydrolysis by altering the hydrogen ionconcentration and by producing a variety of effects upon

the hydrolytic products. Some of these effects will findlater mention.

3. The inorganic soil colloid may be regarded asessentially the product of hydrolysis of feld-spars.

This postulate is presented not only because of therelatively enormous quantity of feldspars in igneousrocks, but because, as will be shown below, the othercomplex silicates, when hydrolyzed, should product ma-terials of like type. Some igneous rock materials do notsuffer hydrolysis to any notable degree.

4. The hydrolysis of minerals takes place in defin-ite steps, giving rise to definite compounds, andthe compounds are the colloid complexes fromwhich the colloids themselves are produced.

In making, this assumption the writer is quite awarethat it is contrary to the view expressed by van Bem-melen and by Stremme, and held by many others, whoregard the inorganic soil colloid as a mixture of silicaand sesquioxides to which the other components are at-tached in a loose and varied relation to which the funda-mental laws of chemistry do not apply.

On the other hand, it is in accord with the olderviews of Way, and perhaps of all who regard clay as theultimate product of feldspathic weathering, as well asmany more recent soil students, among whom Kelley andMissink may be mentioned specifically.

If, therefore, we can trace the compounds resultingfrom successive degrees of hydrolysis which ought to beexpected in different soils, we should find the colloidsof these soils to possess properties corresponding to thoseto be expected in such substances. Conversely, if weproperly classify soil colloids with reference to diversetypes, we should be able to distinguish the kind of com-pounds which ought to possess the properties found. Bothmethods of approach should be employed and both shouldlead to the same conclusions. These indirect methods ofidentifying the colloid components are employed by rea-son of the fact that isolation of the individual soil com-ponents in pure condition is impossible. The fundamentalmeans of isolation can not be used since the compoundsare neither soluble in appreciable amounts or volatilewithout decomposition.

5. Orthoclase, KAlSi3O8 may be assumed to havethe structure:

O = Si — OK

o

(1) Chief. Division Soil Chemistry, U. S. Bureau of Chemistryand Soils.

I O /O /I oI /

O = SiIt is recognized that this is but one of some eight

constitutional formulas which have been suggested forPage 47

this mineral. None of these has, or can have in thepresent state of our knowledge, adequate proof. It istherefore proper scientific procedure to assume thesimplest possible structure which accords with all avail-able facts. The above is assumed to be such.

6. Hydrolysis of a compound of this type shouldproceed by two practically simultaneous steps,which may be represented by the equation:

If this component, or its salts, exists in soil, itshould be found in immature soils and in mature soilsformed under limited rainfall, or in humid areas wherethe mean temperature is low. Under no possible circum-stances should it ever be expected to be found in purecondition for reasons some of which are obvious andsome of which will appear as the topic is developed.

Such a compound should be amphoteric and moremarkedly acidic than basic. It should also be subject tofurther hydrolysis.

That at least one step of this hydrolysis occurs isscarcely hypothetical since it was experimentally demon-strated by Cushman that when orthoclase, and similarminerals, are finely ground and treated with water, asuspension is produced having a pH value of 8.4. Re-cently, L. T. Alexander, in our laboratories, by grindingorthoclase to colloidal dimensions, secured a suspensionwith a pH value of 9.4 from which, by electrodialysis,almost 30 per cent of the original potassium could beextracted as KOH.

In U.S.D.A. Technical Bulletin 319 it is suggestedthat this hypothetical acid be called montmorrilloniticacid, since the nearest approach to a compound havingthe required composition is the montmorillonite reportedby Ross and Shannon (Ross, C. S. and Shannon, E. V.1926. The minerals of bentonite and related clays andtheir physical properties. J. Am. Ceramic Soc. 9, 77-96).This compound, if pure, should have a silica-alumina ra-tio of six. That a soil colloid of this ratio should be foundis not to be expected, as will be shown by consideringthe further sequence of the process.

7. When hydrolysis of montmorrillonitic acid occursit may take place at either or both of the weaklinks and results in the production of silicicacid. If the hydrolysis occurs at both links, itmay be represented as follows:

contrary, the percolating waters should remove silicaand ultimately leave a substance which has the silica-alumina ratio of two and differs from ordinary kaolinchiefly in its water content. This is presumably the ma-terial which is commonly known as clay. (It is to bekept in mind that the soil term "clay," as used in ex-pressing the texture of soils, is not limited to the clay ofthe mineralogist.)

The relation between this material and halloysite isvery close indeed and hence the proposal to call it halloy-sitic acid. It is to be noted, however, that if this materialwere to exist wholly deprived of bases, it would be veryunstable, so far as its water content is concerned. Itmight be expected to lose water readily, and reverttowards or even below the water content of kaolin. Thebasis of this expectation is the analogy between the com-pounds of silica in the second member of the fourthperiodic group and the similar compounds of carbon, thefirst member of the group. On this analogy the halloy-sitic acid, when free, should behave as do hydroxyl com-pounds of carbon when more than one hydroxyl groupis attached to one carbon atom, e. g.,

It should also follow that the halloysitic acid has alower base-exchange capacity, despite the fact that bothare tribasic acids.

It should be apparent that between these two acidsthere may exist a "hemimontmorillonitic acid", represent-ed by the formula:

This substance has the same empirical composition aspyrophyllite, one of the clay minerals, H2O. Al2O3. 4SiO2.It would be proper, therefore, to call the correspondingacid pyrophyllic acid. There is some reason for believing'

Pane 48

If this acid exists and should suffer dehydration,such as is assumed to occur in the formation of Kaolinite(see page 8) there should be produced a compound ofthe formula

pyrophyllic acid

carbonic acid oxalic acid

This is the behavior of Si(OH)4 and SiO(OH)2. which, ondrying, give silicon dioxide.

On similar grounds it should be expected that thehalloysitic acid is a weaker acid than montmorrilloniticacid, just as carbonic acid is weaker, as well as more un-stable, than oxalic acid.

In arid or semi-arid regions no change in the analy-tical results would follow, but changes in physical andchemical behavior should result. In humid areas, on the

this to be the dominant acid of the less weathered col-loids.

If, then, these stages of hydrolysis are possible, weshould find in soils colloids ranging in silica-alumina (orsesquioxide: vide infra) ratio from nearly six to two,depending upon the temperature, rainfall and durationof their development. The properties of these colloidsshould have a general relation to their silica-sesquioxideratios, but it does not follow that the correlation shouldbe perfect.

A further corollary of the above hypothesis is thatif the colloid were derived from feldspars of the oligi-clase, labradorite, or anorthite classes, in which the ini-tial ratio of silicate alumina is two, the montmorrilloniticacid can not be formed. If the feldspathic material weremixed (and this seems to be the usual condition: see Danasystem of Mineralogy, pp. 314, 325) then there shouldbe produced a mixture of the two acids. If the soil col-oids are derived from mica, the primary product may bethe same as from the disilicates. If we assume for micathe structure given below, then by hydrolysis:

There is no direct evidence that this hydrolysis act-ually pursues this route. There is a little indirect evi-dence that it does not. The suggestion is therefore madeas a possibility, but not as a probability.

8. Under drastic hydrolysis, i. e., at high tempera-ture and heavy rainfall, or at high hydrogen-ionconcentration, further hydrolysis of the halloy-sitic acid may be assumed to occur.

silicicacid

Since silicic acid is more soluble than alumina, con-tinued eluviation should ultimately produce an alumin-ous laterite, though the formation of secondary quartzis also possible. That such hydrolysis is slow and difficultis evident from the enormous masses of clay of the kaolintype, which exist in nature; and that it may be causedto occur is made evident by the behavior of clay as aresult of successive treatment with acid and alkali.

The colloids resulting from this stage of hydrolysisare those present in laterites. In lateritic soils we mayassume a mixture of three colloids; aluminum hydroxide,

silicic acid, and halloysitic acid, or their salt-like deriva-tives. In these soils it should be expected that both basecontent and base exchange capacity should be low.

9. If halloysitic acid exists, it is possible for part-ial, or complete, dehydration to occur, and ifcrystallization of the product should occur, thechange would be irreversible and give rise to adefinite compound.

halloysitic acid kaolinite

This type of behavior is characteristic of the hy-droxyl compounds of carbon and the behavior of silicicacid itself. The relative rarity of kaolinite and the abund-ance of kaolin-like clay are quite in harmony with thisassumption. Indeed, it is also in harmony with the fact,emphasized by W. O. Robinson, that white soil colloidsso frequently approach the composition of kaolin.

10. If the previous assumptions are valid, it willfollow that these aluminous derivatives of thefeldspars will have the following analytical ra-tios:Montmorillonitic acid 3H2O . A1203 . 6 SiO2Pyrophyllic acid 3H20 . A1203 . 4 SiO2Halloysitic acid 3H20 . A1203 . 2 SiO-2Aluminum hydroxide 3H20 . Al2O3

It will follow, therefore, that the more closely the felds-pathic original material approximates the trisilicate andthe smaller the extent of weathering the higher the silica-sesquioxide ratio and as the hydrolysis proceeds the lowerthe ratio, provided leaching is sufficient to remove thesilica as rapidly as it is liberated. The ultimate silica-alumina ratio is O. All the forms are tribasic as acidsand all are amphoteric. This seems to be the case thoughconflicting evidence appears to be available.

The water content of the four colloidal materials, ifpure and free from bases, should be 10.6%, 13.6 %,19.6% and 34.6%, respectively. Insofar as bases andacid radicals are present, these percentages should belowered by the water equivalent of these substances.

It is a fact that the higher the silica-sesquioxideratio of the soil colloids the lower is their loss on ignition.This fact is, however, not a safe basis on which to rest aconclusion, since it is not to be expected that the twoalumino-silicic acids would not dissociate at all at 100to 105o C.

11. The behavior of the silicates of iron, with re-spect to hydrolysis, may be expected to be sim-ilar to that of the alumino-silicates, with suchdifferences as are to be expected from thepositions of iron and aluminum in the periodicsystem.

The ferric iron occurring as replacement of ironfor aluminum in the complex silicates may be expectedto play the same role as the aluminum. The existence ofthe mineral nontronite (2H2O . Fe203 . 2SiO2) is usuallyassumed to justify this assumption, though the rarityof this mineral is such as to indicate that this it not thenormal fate of the iron content of the hydrolyzed ironsilicates. There is also some room for doubt as to non-tronite being a true ferric analogue of ordinary clay.

Page 49

The ferrous hydroxide so produced is a fairly strongbase and may react as such (vide inira) or it may bycontact with air and water pass rapidly into ferric hy-droxide. Ferric hydroxide is very readily converted atrelatively low temperature to intermediate hydrates andultimately to hematite. This alteration is known to occureven in the presence of water. While it is possible, asmay be inferred from Mattson's recently published work,that free silicic acid and ferric hydroxide may produce,at favorable pH values, a secondary clay complex ofeither the montmorillonitic or halloysitic type, neverthe-less it appears probable that by tar the greater portionof the iron-oxide content of soil colloids exists as moreor less dehydrated ferric hydroxide.

12. It must be assumed that the bases of the col-loids are present as acid, or neutral, salts andthe acid radicals as neutral or basic salts of theamphoteric soil complex.

So far as the bases are concerned, this is essentiallythe view of Way (about 1850) in his discussion of whathas since become known as base exchange. It also seemsto be in harmony with the conclusions of Mattson, ar-rived at in a vastly different manner, as expressed inhis Laws of Soil Colloidal Behavior, IX (Soil Science,Sept. 1932).

The extent to which the bases and acids are helddepends primarily on the character of the colloid com-plex, but also upon the bases originally present in themineral from which the complex was developed, the con-tent in soluble salts of leaching waters, and the extentof leaching.

The metallic salts, if they exist, should manifest theproperties of salts of strong bases with weak acids. Theyshould be strongly basic when completely neutralizedand at a pH value of 7 there should be large quantitiesof "exchangeable hydrogen" present. The behaviorshould be analogous to that of carbonates, borates, sul-fides, etc. modified by the marked insolubility of boththe salts and the free acids and acid ions. Were it notfor the much higher ionization constants of phosphoricacid, their behavior should closely resemble that of thephosphates.

That the general behavior of these salts is as indi-cated is too well known to require much discussion, butit may be well to refer to the recent work of Kelley, ofBaver and Scarseth, of Truog, and particularly to thatof Puri. Puri demonstrates to his own satisfaction, atleast, that the soil colloid acids are tribasic. The saltsof these acids are so extremely insoluble that it may beassumed that their soluble portions are completely ioniz-ed, and consequently their metathetical behavior (baseexchange) is immediate.

That this is the case was pointed out by Way andhas recently been shown in detail by Kelley and his co-workers. It should follow that the most acidic of the soil

acids (Montmorillonitic acid) should be less basic thanthe weakest acid (aluminum hydroxide) and in conse-quence have less capacity for retention of acid ions. Thatthis is actually the case has been recently demonstratedby Gile in his unpublished study of the effect of colloidson the availability ot phosphate. At least his results maybe so interpreted.

Conversely, the base content of high silica-aluminaratio colloids and their exchangeable base capacity shouldbe greater than those of lower ratio. This is the case ingeneral, also the exchangeable base content should followroughly the silica-sesquioxide ratio except when moditiedby excessive leaching.

13. That all the bases are not immediately remov-able from soil colloids by treatment with strongacids is to be attributed to the combined opera-tion of two properties of the colloids: theirgreat insolubility and the semi-permeability oftheir sols.

That the colloid complex is insoluble requires no specialcomment.

That soils show the properties of semi-permeablemembranes was demonstrated in 1912 et seq. by Lynde.In this laboratory numerous experiments have been car-ried out which show that soil colloids are effective,though imperfect, semi-permeable membranes. The high-er the silica-sesquioxide ratio the more effective is themembrane action of the colloid. No publication of theseexperiments has been made because we have not beenable to secure quantitatively reproducible data. We be-lieve the difficulty to be wholly mechanical. If the col-loid micellae are semi-permeable it accounts satisfact-orily not only for the failure to remove the bases com-pletely by base exchange methods but also for the "comeback" of electrodialyzed colloids. It also accounts forthe slow but continued extraction of bases by leachingand electrodialysis. Also for a part of the experimentaldifficulty in determining acidity, lime requirement, etc.,by titration methods.

Since the laterites are very imperfect membranesit is not surprising that they are largely debased byweathering.

It is not pertinent to our present purpose to discussthe organic colloid but because of the character of cer-tain, podzol soils the publication of some recent observa-tions by Dr. M. S. Anderson may be anticipated.

The B horizons of many podzols are highly ferrug-inous and also high in organic matter. This is especiallymarked in the case of the Becket silt loam from Wash-ington County, Massachusetts, reported upon in U.S.D.A. Technical Bulletin 228.

In the course of a series of determinations on or-ganic colloids, Doctor Anderson had occasion to treatthese colloids, after electrodialysis, with ammonia and,after evaporation to dryness, to determine their am-monium content. The ammonium content was quite high.The same colloid treated with a suspension of electro-dialyzed aluminum hydroxide and then with ammoniagave, after evaporation, a very low ammonia content.The obvious explanation is that "aluminum humate" isformed and is not decomposed by ammonia.

Page 50

Iron most frequently occurs in the igneous rocks inthe ferrous condition and without serious error we mayregard it as though it were ferrous silicate as in horn-blende. The hydrolysis of this should take place as in-dicated.

A suspension of the same "humic acid" treated withan electrodialyzed ferric hydroxide sol and these withammonia gives, on evaporation to dryness, practicallyas great an ammonium content as the pure humate. Ifthe previous inference is correct, then it follows eitherthat ferric hydroxide does not combine with humic acidunder these conditions or that the combination is de-stroyed by ammonia.

To throw more light upon this question it was sug-gested that perhaps ferrous hydroxide (a stronger andmore soluble base) might behave as does aluminum hy-droxide. Dr. Anderson therefore carried out the extreme-ly laborious and experimentally difficult task of pre-paring, electrodialyzing and dispersing ferrous hydroxideand mixing it with humic acid. All these operations beingperformed in an atmosphere of hydrogen. This suspen-sion being treated with ammonia and evaporated to dry-ness gave a product which contains no ammonia. When,however, the mixed sols were subjected to oxidation witha stream of air and then evaporated, th2 ammoniumcontent of the dried residue became again that of the

humic acid. This series of experiments, together withother data, lead us to the final assumption which will bepresented at this time.

14. In the course of the hydrolysis of ferrous sili-cates under high humidity and low pH values,the ferrous hydroxide produced reacts with thehumic acid (the cause of the low pH value) toform a salt which is carried downward by thepercolating water. This salt, when subjected tooxidation and precipitation, gives rise to amixture of ferric hydroxide and humic acidcharacteristic of the B horizon of the Becketand similar podzol profiles.

In making this assumption it is immaterial whetherthis oxidation process occurs during the eluviation pro-cess alone, or whether it be supplemented by upwardpercolation during dry periods. The result, a ferrugin-ous B horizon, should be the same in either case. Itwould seem to follow that this layer should be nearlyor quite free from aluminum hydroxide, though thepresence of exchangeable aluminum is not impossible.

B Horizon—Podzol soils

Soil

Becket 1.Superior f.s.l.CefnybrynGoldstoneCaernavonshire

Location

Mass.Mich.WalesWalesWales

SilicaDepth

Inches13-2412-30

7-1219-23 ——14-20 34.7

21.5631.6043.0

Variations in silica/sesquioxide—Becket:

13.0213.4721.7

22.3

A12O3

20.0625.1430.0

Silica

sesqui-oxide

1.281.591.671.481 39

Silica

alum-ina

1.832.132.441.56

Silica

Totalbases

6.257.36

Com-binedwater

9.4413.39

3.3%30%

1.77

ColloidPer-

centages

5.54.9

8.8

0-6", 2.16; 6-11", 0.86; 13-24", 1.24; 24-36", 1.67Variations in silica/sesquioxide—Superior: 0-3", 3.76; 3-8", 4.01; 12-30", 1.59; 30-40", 2.16

B Horizon—Chernozem soils

Filmore s.l.Crete s.l.Holdredge s.l.Keith s.l.Colby s.c.l.1Amarillo s.c.l.Barnes s.l.

(1) A carbonate

Neb.Neb.Neb.Neb.Kans.Tex.S. D.

layer.

Marshall s.l.Shelby s.l.Palouse s.l.Carrington 1.Wabash s.l.

IowaMo.Wash.IowaNeb.

16-3820-3818-3014-2120-3310-2014-48

53.8052.2052.7553.0048.6051.3246.84

9.287.797.607.226.968.61

11.74

21.0022.1421.4520.4919.2322.1118.35

3.523.263.393.583.473.093.09

B Horizon—Prairie Soils

4.344.004.164.384.293.824.32

7.057.046.825.386.267.056.46

7.377.768.116.297.567.369.06

13-2412-2020-3315-3615-36

47.6947. 9845.7348.0450.55

9.4211.4912.85

8.809.57

22.1023.0323.4325.1920.87

2.882.602.452.643.33

3.663.283.313.234.09

7.767.767.02

10.47.86

9.479.449.569.236.74

1.932.012.011.93

1.85

2.30

50.246.2

3.074.28

39.445.433.829.432.4

B Horizon—Podzolitic soils

Miami s.l. Mich. 18-30 48.00Miami s.l. Ind. 14-18 48.45Vernon f.s.l. Okla. 10-27 45.10Clarksville s.l. Ky. 10-36 43.68Iredell 1. N. C. 10-20 40.173Ontario 1. N. Y. 12-22 42.40Sassafras s.l. Md. 8-22 41.14Leonardtown s.l.1 Md. 8-16 43.14

(1) Mean of 7 profiles.(2) Combined water from another profile.

11.1611.7610.5311.5115.4515.2712.7313.60

23.2925.1726.0027.7026.9424.7129.2627.63

2.592.512.342.101.882.081.892.01

3.533.262.942.692.592.902.382.74

5.949.047.48

10.9613.07

6.6211.9412.29

7.969.319.67

10.5113.582

7.2212.4911.66

2.59

25.218.123.825.563.911.418.924.4

Page 51

B Horizon—Lateritic soili

Page 52

Soil Location Depth SilicaSilica Silica Silicasesqui- alum- Totaloxide ina bases

Com-binedwater

ColloidPer-

centages

Norfolk f.s.l.Orangeburg f.s.l.Kirvin f.s.l.Chester 1.Chester 1.Davidson c.l.Cecil s.c.l.Cecil s.l.2Nacogdoches c.

N. C.Miss.TexasN. J.Va.N. C.N .C.Mis. to Md.Texas

Inches12-3610-3612-2410-2012-189-366-32

8-18

41.6040.3539.9540.0938.1936.3453.3535.6231.85

11.2810.0814.3711.8316.3015.8212.2011.9826.71

31.1033.2729.7832.7650.1731.6836.4436.7226.97

1.841.721.741.681.591.471.281.371.22

2.252.112.282.052.141.951.691.611.96

21.9717.6317.0318.4311.3033.029.039.028.0

11.5010.6311.2013.1410.6712.4813.9813.9110.96

4.38

6.176.281

20.521.559.425.0

54.051.2

H2SO4

(1) Water absorption ratio from another profile.(2) Mean of 17 profiles.

Laterites—Various layers

Nipe clayBauxiteAragon

Cecil c.l.

CubaArk.CostaRicaN. C.

40-60156-214

162-204204-210

5.5523.83

15.8617.67

66.597.56

22.671.42

11.4943.29

34.3853.84

0.170.84

0.550.48

0.820.93

0.780.56

36.9121.

13.836.6

13.5119.70

15.6324.5 4.35

56.317.8

8.0