historical developments in soil chemistry: ion exchange1

Historical Developments in Soil Chemistry: Ion Exchange1

G. W. THOMAS3

ABSTRACTThe early development of ideas on cation exchange from 1850 to

1900 is described. From 1900 to the present, the coverage has beendivided as the work has concentrated on soil acidity, alkali soils, andclassical cation exchange studies. The discovery of crystallinity in soilclays and the rediscovery of the importance of noncrystalline oxidesadsorbed on crystalline clay minerals as sources of pH-dependent acid-ity and anion exchange are reviewed. Some of the more important con-tributions made in understanding ion exchange in soils are covered insome detail. Other contributions are mentioned. It is concluded thatdespite the progress made, our understanding is still far from com-plete.

Additional Index Words: cation exchange, anion exchange pH-dependent acidity.

SOON AFTER I was asked to prepare a paper on historicaldevelopments in soil chemistry, I found that I would

have to limit its scope to fit the space and time of presenta-tion. I chose the subtopic ion exchange because it is a themeof common interest to soil chemists. The selection of some"developments" and the omission of others has been dif-ficult for me and while I plead guilty to some bias, I hopethat it has been moderated by fairness.

CATION EXCHANGE STUDIES IN THE 19THCENTURY

To H. S. Thompson, a Yorkshire farmer (1850), must gothe credit for the first quantitative studies on cation ex-change. He discovered that the addition of (NH4)2SO4 to acolumn of soil resulted in the appearance of CaSO4 solutionfrom the bottom after leaching. Before his experimentswere published (1848), Thompson told a young consultingchemist to the Royal Agricultural Society, J. Thomas Way,of his results. He had chosen an able man to tell.

Immediately, Way began the first comprehensive studiesin cation exchange, which were published in 1850 and1852. As exchangers Way used a variety of soils, pipe clay,and some home-made alumino-silicates. In addition toNH4

+, he studied the exchange of all ;the cations (andanions) usually found in soils. His results can be sum-marized briefly as follows:

1) The common cations (Na, K, NH4), added as saltsof strong acids, are retained by the soil, and equiva-lent quantities of the same salts of Ca are replaced,e.g. 2KC1 + Soil-> CaCL, + K2-soil).

2) Hydroxides and carbonates of cations are adsorbedcompletely with no replacement of Ca or anion.

3) The strong acid salts (C\~, N<V and SO42~) of Ca

are not adsorbed by soils.4) Clay is the material responsible for cation adsorp-

'Contribution from the Soil Science Laboratory, University of Oxford,Oxford, England. Bicentennial address presented before Div. S-2, SoilScience Society of America, 1 Dec. 1976, in Houston, Texas. Received 12Oct. 1976. Approved 31 Jan. 1977.

2Professor of Agronomy, Univ. of Kentucky, on leave at the Univ. ofOxford from 7 Sept. to 18 Nov. 1976; now at the Dep. of Agronomy,Univ. of Kentucky, Lexington, KY 40506.

tion; organic matter and sand are unimportant.5) Heating or acid treatment tend to destroy the ad-

sorption.6) The adsorption is very rapid, almost instantaneous.7) Increasing the concentration of the added salt in-

creases the amount of adsorption.8) Cation adsorption is irreversible.9) Phosphate is held by soils.

The conclusion about organic matter was wrong and sowas the conclusion about irreversibility, which his own data(Way, 1852) disproved. In other respects, the conclusionswere correct, given the Ca-saturation of his original soils.Conclusions 2 and 3 are only apparently correct because theanalysis of leachates gave no clue to the actual reactions.These results were so revolutionary in their implicationsthat the great agricultural chemist, J. von Liebig, withouttrying to duplicate them, commented: "These experimentsare very remarkable and should be opposed with might andmain," (Wiegner, 1931a). Thus, Way, not Liebig, becamethe father of soil chemistry.

Way was a student of the celebrated chemist ThomasGraham (Forrester and Giles, 1971) who was founder andfirst president of the Chemical Society of London, and whohad the reputation of giving "kernels of thought free fromshell," (Smith, 1884). Another notable student of Gra-ham's was J. Henry Gilbert, cofounder of Rothamsted andstill another was F. Stohmann who, on his return to Ger-many, repeated Way's observations and with a new col-

• league (Henneberg and Stohmann, 1858) looked carefullyat the effect of concentration on the quantity of ammoniumadsorbed by soils, and by doing so became the first tomeasure a solute-solid adsorption isotherm. They alsoshowed that there were differences in adsorption of ammo-nium from different salts, in the order H2PO4~ > SO4

2~ >Cl~ = NO3~, an observation that took many years to under-stand.

Samuel Johnson, a student of Liebig's, but less ob-stinate than his mentor, reviewed the work of Way and cor-rected his two wrong conclusions (Johnson, 1859). The ad-sorption work of Henneberg and Stohmann, Johnsonrecognized, showed that nutrients can go into solution afterthey are adsorbed (exchange is reversible) and thus be takenup by plants. He also found that Way was incorrect in stat-ing that organic matter did not adsorb NH4. Johnson foundit to hold even more NH4 than clay did. Finally, it was John-son who coined the term exchange of bases for Way's ob-servation. Some may know Johnson better as the first direc-tor of the first American experiment station in Connecticut.

Most of the work between the late 1850's and 1900 wasan extension of or elaboration upon Way's original work.Chief among the workers were Eichhorn (1858), whoshowed that crystalline zeolites could be changed easilyfrom one to another merely by replacing the cation present(e.g., Ca to Na); Peters (1860) who did detailed studies onconcentration, anion and cation effects, amplifying Hen-neberg and Stohmann's results; and J. M. van Bemmelen(1877) who showed that cations other than Ca could be

230

THOMAS: DEVELOPMENTS IN SOIL CHEMISTRY: ION EXCHANGE 231

replaced from soils, and who incidentally did the first workon soils with appreciable Na in them.

Of this group, van Bemmelen (1888) made the greatestcontributions. His data show conclusively that cation ex-change is not restricted to Ca in the soil, as Way assumed,but works with other cations as well. He also formulated thefirst mental picture of cation exchange, the descendent ofwhich, still firmly in Dutch hands, is the double-layertheory.

A very complete review by Sullivan (1907) was used forthe period 1858 to 1888, since most of the references are inGerman. It is the same review, by the way, that Kelley andCummins (1921) used to rediscover Way's work as detailedin Kelley (1964). In reading Sullivan's review, one is struckby the straight-forward results of the early investigators. Ihave concluded that it was an accident of place. The soilsused were from northwestern Europe, where young land-scapes and mild weather prevail. As a result, exchange be-tween Ca, Mg, K, and Na ions in the soil and added NH4ions gave results which could be analyzed using the gravi-metric and titrametric methods available. The spread of ionexchange research to eastern Europe, Russia, and theUnited States began to give strange results, unknown toWay. Increasingly, beginning about 1900, cation exchangestudies became studies of problem soils. Because it is im-possible to group these studies under one heading, we willconsider several kinds of research separately from thispoint.

SOIL ACIDITYThe history of research on soil acidity must start with Sir

Humphry Davy's book Elements of Agricultural Chemistry(1813), not because it explained or even described soil acid-ity, but because it contained a method for determiningCaCO3 in soils. Davy's book was read avidly by a youngVirginian, Edmund Ruffin, who was to become a successfulfarmer, scientist, publisher, politician, and rebel, but who,at that time, was a very unsuccessful farmer trying to keepfrom losing his land near Petersburg. Ruffin analyzed forCaCO3 in his soils according to Davy and found none. Thenhe analyzed for CaCO3 in the productive limestone soils ofthe Shenandoah Valley and also found none, whereas Davyhad almost always found large amounts in British soils.Ruffin did find that the limestone-derived soils containedsome acid-soluble Ca, whereas his own did not. Accord-ingly, he proceeded to remedy this problem by applyingpartly decomposed oyster shells to each of his fields, keep-ing careful records of the amounts of shell used and of cropyields before and after "marling". Thus he became the firstman to lime for the right reason: To neutralize soil acidity.He was mistaken, however, in believing that all acidity wasdue to organic acids. Ruffin also discovered overliming,and described perfectly the symptoms of zinc deficiency incorn (Zea mays L.)

His book, An Essay on Calcareous Manures (1832), wasone of the earliest scientific works on soils arid crops. Ruffinseems to have been forgotten as an agricultural scientistafter 1861, perhaps because of his insistence on firing thefirst Confederate cannon at Fort Sumter and his suicide afterAppomattox to avoid living under "the perfidious Yankeerace." We owe Emil Truog (1938) a great debt of gratitude

for rediscovering Ruffin, whom he called "The father ofsoil chemistry in America."

After Ruffin, it was 70 years until research on soil aciditywas resumed. F. P. Vietch (1902), a chemist with theUSDA, had determined that Ca(OH)2 equilibration withacid soils to a faintly pink phenolphthalein end point was areasonably good lime requirement test for crop growth.Shortly afterwards. C. G. Hopkins et al. (1903) devised alime requirement method using the titration of IN NaCl ex-tract. Vietch (1903) agreed to compare the two methods andone year later (Vietch, 1904) wrote a paper on the results.Vietch's heart seems to have been influenced by our oldfriend Samuel Johnson (1888) who declared that all per-manently productive soils were calcareous, but his resultsshowed that while IN NaCl did not replace all the acidity(as measured by Ca(OH)2 equilibration) it was a good es-timate of the lime requirement in mineral soils. More im-portantly, he discovered that the "acidity" replaced byNaCl was primarily A1C13, not HC1. He had opened adebate which is still going on, but he never again participa-ted in it himself. Indeed, the full credit for his remarkableobservations was not to come for nearly 60 years.

Some of the giants of soil chemistry were soon attractedto the study of soil acidity. Emil (lime spelled backwards)Truog (1916) attacked Cameron and Bell's (1905) idea thatacid soils were artifacts caused by selective adsorption ofcations, leaving the acid anions in solution. Truog main-tained that acid soils were a natural consequence of removalof exchangeable cations by dilute H2CO3 during weather-ing. He further showed that there are different quantities ofacid removed by salts, depending mostly on the anion, notthe cation as would be the case if selective cation adsorptionwere involved. The differences in acidity removed by chlo-ride and acetate salts were in the neighborhood of 1:4 for amineral soil and 1:10 for an organic soil. In addition, Truogargued that the acid (mineral or organic) must be insolubleor it would have leached out.

Richard Bradfield (1923, 1925), who is still very active,argued forcefully for Truog's conception of an insolubleacid in the clay fraction of soils. Titrating clays removedfrom the naturally acid Putnam subsoil, which he in-troduced to the world, he calculated a Ka of 3 x 10~7, aboutthe same as the weak, carbonic acid. This explained theacidity removable by a buffered salt, but not the significantacidity replaceable by a neutral salt. The potentiometric ti-trations gave (and still give) poor end points, but conduc-tometric titrations gave sharp endpoints that coincided withthe potentiometric curves at about pH 8.

At the same time, several workers were busy extractingsoils with neutral salts (Daikuhara, 1914; Mirasol, 1920;Hartwell and Pember, 1918; Conner, 1921, 1926) and relat-ing the quantity of Al found to the growth of plants. Al-though their data were convincing, their explanation of Alas a truly exchangeable cation was countered by Kelley andBrown (1926) and Page (1926), who maintained that the Alwas dissolved by exchangeable H+ during the salt extrac-tion. It is especially interesting that Kelley and Brownshould have argued this point. Their own data clearly showthat the addition of A1C13 to a neutral soil eventually re-sulted in almost complete Al saturation of the soil.

In any event, there was one more shot to be fired by theAl forces before their retirement from the battle. Paver and

232 SOIL SCI. SOC. AM. J . , VOL. 41, 1977

Marshall (1934) considered the contention of Kelley andBrown and Page, that the Al in salt solutions is formed bythe reaction of exchangeable H+ with Al compounds inclays. They decided that exchangeable H did react to releaseAl, but that the reaction was one which had not even beenthought of before: H+ ions dissolve Al from the clay struc-ture causing it to become exchangeable. It is not dissolvedduring the extraction. Next, they investigated the finding(Kelley and Brown, 1926) that Al addition to base-saturatedsoils reduced the cation-exchange capacity (CEC). Theyfound that reduction in CEC was related to both pH and theconcentration of Al present. Their explanation of this lastfact was not entirely clear, but their data were correct as weshall see.

In this monumental contribution, Paver and Marshall an-swered most of the questions about Al in soil acidity, butthe time was not right for such answers. Even Marshallseems to have had his doubts, as evidenced by the fact thathe devoted only two paragraphs to these results in his bookwritten 15 years later (Marshall, 1949).

In addition to the questions of H and Al, there was aquestion of defining what a saturated soil is. To Ramann(1911) and Gedroiz (1918-19), the definition was simple:Soils with a pH of 7 are saturated, soils with a lower pH areunsaturated with bases (Ramann's term). Page (1926) andKelley (1927) agreed with this analysis. Hissink (1924)proposed the determination of "% base saturation" as a cri-terion, but his determination of "saturation" was aBa(OH)2 titration to a point where none of the Ba(OH)2added reacted with the soil (about pH 12). Calcareous soils,using Hissink's scheme, were only 55% base saturated.

Bradfield (1927) noted this discrepancy and Bradfield andAllison (1933) proposed a reasonable solution. Since it iswell known that calcareous soils in equilibrium with CO2 ofthe atmosphere have a maximum pH of 8.3, and since mostagricultural soils are predominately Ca saturated, theysuggested that base-saturated soils be defined as those inequilibrium with CaCO3 at a CO2 partial pressure of 0.0003bars. This view is held almost universally today (theoreti-cally) although a soil in equilibrium with CaCO3 is not prac-tical to achieve in most acid soil areas. Mehlich's (1939)BaCl2-triethanolamine (pH 8.2) method for exchangeableacidity approximates Bradfield's "saturation."

As the clay mineral concept gained ground, the interest insoil acidity seemed to decline until Mehlich (1942, 1943)made some remarkable observations associated (as hethought) with clay minerals. Mehlich noted that the % basesaturation of North Carolina soils varied with soil type atgiven pH values; or to use Bradfield's approach, the ioniza-tion constants of the clay acids varied from one soil to an-other. Pierre and Scarseth (1931) had observed precisely thesame thing 11 years earlier, but Mehlich had samples ofpure clay minerals which he compared with his soils. Fromsimilarity of the curves, he deduced the apparent mineral-ogy of the soils. He also used organic matter as an analogueof surface soils. Mehlich correctly observed that because ofthe marked differences in the pH vs % base saturationcurves, the % base saturation required for practical limingwould vary between 20 and 80%.

But it was two of Mehlich's students, N. T. Coleman andM. E. Harward, who were destined to change the outlookon soil acidity even more markedly. As a post-doctoral as-

sistant in Berkeley in 1948-49, Coleman discovered a trans-lation of the Russian Chernov's (1947) outstanding work onAl in acid soils. He also saw a demonstration of changingNa-bentonite to H-bentonite by merely pouring a suspen-sion of the Na clay through a H-resin, by that ever cleverexperimenter, Perry Stout. On his return to North Carolina,Coleman and Harward (1953) discovered that resin-treatedclays or clays leached rapidly with IN HC1 had entirely dif-ferent properties than clays prepared by electrodialysis ordilute acid leaching. It should be pointed out that the INHC1 treatment was possible because they used a granulated(Utah) bentonite that permitted rapid leaching whereasWyoming bentonite did not.

As Paver and Marshall (1934), Chernov (1947), andSchofield (Russell, 1950) had pointed out, electrodialyzedand naturally acid clays (and those prepared from diluteHC1) were primarily Al saturated. Truly H-saturated clays,on the other hand, were strongly acid, and had the physicalproperties of Li or Na clay. The results published in 1953and Harward and Coleman (1954) reduced much of the ear-lier work to a shambles. Here was the answer to Wiegner's(1931b) and Jenny's (1932) questions on the anamalousposition of "H" in the lyotropic series. In truly H-saturatedclay, conductometric titration curves showed the H+ to behighly dissociated. This was shown independently by P. F.Low (1955) using H-clays prepared from Ag clays andsubsequently titrated conductometrically. Incidentally,Jenny (1961) in his excellent review mentioned that thosediscoveries were made using the same old methods preva-lent in the 1920's. What he failed to observe was that thenew discovery was of a H-clay.

In the wake of these exciting discoveries, the work of athen obscure soil mineralogist in nearby Virginia went un-noticed. C. I. Rich (Rich and Obenshain, 1955) found thatin soils formed from mica schist, there was not onlyexchangeable Al, but also large quantities of non-exchangeable Al, which blocked exchange sites and pre-vented the collapse of the vermiculite-like clay. Rich foundfurther that most soils in Virginia contained some of this"interlayer hydroxy Al". This corresponded to the Alwhich blocked the exchange sites in Paver and Marshall's(1934) work as Rich at once realized. Rich's view wasstrengthened by a remarkable piece of work by one of hisstudents (Hsu and Rich, 1960) using an organic exchangeresin, where added Al could be determined quantitativelyand showing that hydrolysis and subsequent fixation ofexchangeable Al was the rule, rather than the exception.Chernov's (1947) view had been the same, but, unfortu-nately, he was not to figure in the outcome of the debatebecause the published translation of his work did not comeout until 1964.

Coleman's response to Rich's findings was as follows:Acid soils have exchangeable Al. The difference betweenKCl-exchangeable Al and total "Bradfield" acidity is dueto pH-dependent (Schofield, 1949) charge on clays. In dif-ferent minerals the proportions of permanent charge andpH-dependent charge vary (Coleman et al., 1959). Thisview was essentially an evolution of Mehlich's (1942)view, using Schofield's terminology and in the light of therediscovery of the importance of A13+.

I was caught between these two views because I had beena student of Coleman's and was now an employee of Rich. I


tried to reconcile both of their results (Thomas, 1960a) byshowing that montmorillonite did not become covered withhydroxy-Al under the same conditions that resin and acidsoils do. By 1963 (after a number of experiments had beenconducted), Coleman accepted the idea that Rich was morenearly correct than he had been. In other words, in mostmineral soils, the pH-dependent charge is largely a result ofpolymerized hydroxy-Al, which is nonexchangeable byKC1, but which can ba titrated with a base (Coleman andThomas, 1967).

One other major riddle was cleared up at about the sametime: The role of organic matter in soil acidity. The work ofGillam (1940) and of Broadbent and Bradford (1952) in-dicated that carboxylic acids in organic matter were respon-sible for most of the acidity in the acid range. There was oneproblem with this view, however. Carboxylic acids have Kavalues of about 10~4 whereas titration curves of organicmatter showed values of about 10~6. Using the titrationapproach of Bradfield (1925) and the Henderson-Hassel-balch equation, Martin and Reeve (1958) in Australiashowed that the abnormal weakness of soil clay acids wascaused by Al just as the abnormal weakness of soil clayacids was. Removal of Al from organic matter gave an ap-parent Ka of 10~4. Schnitzer and Skinner (1963a, 1963b) inCanada have carried this work further showing that Al andFe are hydroxylated in organic matter. It appears that, inmineral and organic acids, A13+ and hydroxy Al of anaverage composition [A1(OH)2]6

6+ are the cations which de-termine "acid strengths" in most soils. As Jackson (1963)suggested, Al is a "unifying concept", especially in soilacidity.

STUDIES ON ALKALI SOILSEugene Hilgard (1906), in California, seems to have been

the first to have investigated alkali soils in a scientific way.He concluded that their properties were determined by thepresence of Na2CO3, formed by the reaction

CaCO3 + Na2SO4^ Na2CO3 + CaSO4.Despite his perception in other matters, Hilgard apparentlywas entirely ignorant of the cation exchange work done dur-ing the 1800's and his idea of how Na2CO3 was formed wasshown to be erroneous soon after it was proposed.

It is striking that substantially the same discoveries aboutthe role of cation exchange in alkali soils should have beenmade by four men, independently, working in very differentenvironments. The four were Kelley of California, Hissinkof Holland, de' Sigmond of Hungary, and Gedroiz of Rus-sia. Kelley stumbled on to his discovery while investigatingthe effect of NaNO3 fetilizer on soil physical properties;Hissink, during reclamation of former ocean sediments; de'Sigmond investigating poor soils near the Danube; andGedroiz, while doing a repetition of Way's original, fun-damental experiments.

During Kelley's investigation (about 1914) he found thatwhile the cations were exchanged by soils, the anions werenot. As Kelley (1964) said

At the time this experiment was initiated, I had no idea thatcation exchange would take place. The results . . . suggesteda search of the literature, which soon revealed that J. T. Wayof England had made similar experiments in 1850 with prac-tically identical results.

A summary of his literature review and his findings waspublished (Kelley and Cummins, 1921) showing that alkalisoils were formed by an exchange of cations so that Nabecame an exchangeable cation of importance. When thisoccurred, Na, in the presence of H2CO3 could be exchangedfrom the soil and form Na2CO3: Na2 Soil -I- H2CO3 —» H2-Soil + Na2CO3.

Hissink (1907) found that soils reclaimed from oceansediments contained much more Na than normal soils andthat their physical properties did not allow good drainage,de' Sigmond (1927), who began work in 1901 found thatalklai soils are scattered in spots among good land becauseof poor drainage and wondered why they became worse asthe soluble salts were removed by draining. Experimentingwith artificial zeolites, he found that they became morehydrated with Na saturation. Repeating the experimentswith soils gave the same results.

Gedroiz (1912) came to almost the same conclusions asKelley about the formation of Na2CO3. After treating aChernozem with NaCl and washing it, Gedroiz discoveredthat it had the properties of a Solonetz (alkali soil) and thatthe Na2CO3 was formed from the exchangeable Na. As de'Sigmond wrote (1926):

If the fact is considered, that three investigators like Gedroiz,Kelley, and the author, very far distant from one another, andunder very different conditions, quite independently from oneanother, and starting from different evidences, concluded infull agreement on the same point; that in alkali soils a consid-erable part of the exchangeable cations is represented by Naand this combined Na may be responsible for the bad physicalproperties of the alkali soil, it is evident that. . . the theory ofHilgard again needs some correction, and reclamation ofalkali soils is not simply a'soil washing process.

After about 10 years of work (Kelley and Brown, 1925)gave a reasonably modern view of alkali soils, dividingthem into three classes:

1) Soils with a high concentration of soluble Na saltsand much replacement of Ca and Mg from ex-change sites; Na2CO3 present.

2) Soils with a mixture of Na and some Ca salts andCa and Mg replaced to some degree.

3) Soils with a high concentration of both Na and Casalts and no replacement of Ca and Mg from theclay.

According to Kelley and Brown, class 1 required Ca in ad-dition to leaching, class 2 was questionable, and class 3required only leaching and drainage.

It is difficult to think of another soil problem that hasyielded results so quickly, so independently repeatable, andso completely answering the questions involved. Of course,further modifications were made by these men and others asthe years followed, de' Sigmond (1927) showed, for exam-ple, that some Na-affected soils which become acid, stillshow the structural effects of Na. Kelley (1951) later sum-marized the work on alkali soils in an excellent little book.In reading it, one senses the comradeship that existed be-tween the four workers.

In 1954, the views on saline and alkali soils (USDA,1954) became largely institutionalized, but further researchgoes on, Notable contributions have been made by C. A.Bower and his coworkers (Bower and. Goertzen, 1955;

234 SOIL SCI. SOC. AM. J . , VOL. 41, 1977

Bower et al., 1957) which have led to the present under-standing of Na-affected soils.

CATION EXCHANGE STUDIES IN THE 20THCENTURY FROM 1910 TO 1930

As I have noted previously, attention was drawn awayfrom classical cation exchange studies soon after 1900 tostudy problems in soil acidity and alkali soils. However,during the same period, considerable progress was made inunderstanding cation exchange itself.

Gedroiz (1918, 1919) and Hissink (1924) both worked onthe speed of cation exchange equilibria, finding that Na-Caexchange was almost instantaneous, but that NH4-Ca ex-change was somewhat slower. Both workers used concen-trated (IN) solutions. The experiments must have been con-vincing, for very little of this sort of research has been donesince. Truog (1916), Kelley and Brown (1927), and Brad-field (1925) generally maintained that cation exchange wasa chemical process, since it was reversible and generallystoichiometric. In this, they agreed with Way. Wiegner(I931a), Gedroiz (1922), and Hissink (1924) described ex-change as a colloidal phenomenon, caused by a concentra-tion of cations at the colloid-solution interface. Specifically,Gedroiz thought the reaction to be due to surface tension,Hissink to a negatively-charged clay which had cations sur-rounding it (Helmholtz double layer), and Wiegner thoughtthat there was an inert core surrounded by an adsorbed layerof Si-O and OH surrounded by a swarm of cations. Theseideas were variations on van Bemmelen's theme.

Truog (1916) sarcastically remarked that "various prop-erties . . . are ascribed to colloids in order that certain phe-nomena may be explained without going to the trouble offinding the real cause." Kelley later (1948) noted with someglee that even Wiegner used the ratio SiO2/Al2O3 to explainthe variation in CEC "suggests that cation exchange is anordinary chemical reaction." Bradfield (1923) showed thatmixtures of SiO2, A12O3 and Fe2O3 did not have the ex-change properties of a soil clay with the same analysis.Kelley and Brown (1927) seemed puzzled by CEC dif-ferences of different clays and ascribed them to geologicaldifferences and/or partial decomposition of clays, whichwere not bad guesses.

Robinson and Holmes (1924) published a detailed studyof clay samples taken from the USDA Bureau of Soilsmapping program. Their results showed a relation (althoughnot a very good one) between SiO2/Al2O3 + Fe2O3 and CECof the clays. This paper had a great influence on twoworkers who were about to change the course of soil chem-istry and an even greater effect on another worker who didall he could to resist that change. This was Sante Mattson,who has just celebrated his 90th birthday.

Mattson was, and is, a Swede who worked in the USDAand then at Rutgers for a total of about 8 years. His earlywork was very original and can be summarized as an at-tempt to relate all soil properties to the ratio SiO2//?2O3 =A12O3 + Fe2O3). Using clays taken from Robinson andHolmes' study, he conducted cataphoretic studies (migra-tion rate of colloids in an electric field) (Mattson, I926a),developed electrodialysis for removing cations and anionsfrom clays (Mattson, 1926b), and studied the adsorption ofboth cations and anions by clays (Mattson, 1927). Mattson

believed that soil colloids were chemical mixtures of SiO2,A12O3, and Fe2O3. Since SiO2 is acidic, he believed it to bethe primary source of CEC. The oxides of Al and Fe, on theother hand, were amphoteric, that is, they were positivelycharged at low pH and negatively charged at high pH.Using this system as the source of both positive and nega-tive charges on clays (depending on ratios and pH), Mattsonwrote a series of more than 25 papers on ion exchange,swelling, and other properties of soils.

Mattson (1927) showed that soil clays high in Fe2O3 4-A12O3 moved towards the cathode in catophoresis experi-ments when the pH was low, but changed direction whenthe pH was high. Clays with a high SiO2//?2O3 ratio alwaysmoved towards the anode. When he added HC1, H2SO4, orH3PO4 to the clays, he found increasing adsorption ofanions in that order. Neutralization of the suspension dras-tically reduced anion adsorption. Mattson concluded thatthe reaction was R(OH)3 + A -» R(OH)2A + OH~. In acidsuspensions the OH~ ions were neutralized, driving the re-action towards the right. The experiments of Joffe andMcLean (1927) using Al solutions with the anions H2PO4,SO4, Cl, and NO3 point to the same conclusion.

THE CLAY MINERAL CONCEPTJust as Mattson's star was rising, an ominous event oc-

curred. X-ray diffraction, pioneered by Bragg, was turnedto clay minerals. This seems to have had no effect on Matt-son, but it stimulated the other two workers, Walter Kelleyand a new face, Sterling Hendricks.

Within a short time, Hendricks and Fry (1930) and Kel-ley et al. (1931), independently, had established that soilclays were primarily crystalline. (Their early X-ray pho-tographs had so much beam scatter that the lines greaterthan 7A were obliterated and both groups repeated theirwork (Kelley et al., 1939; Alexander et al., 1939). Never-theless, within a very short time kaolinite (Ross and Kerr,1931; montmorillonite, Hofmann et al., 1933; Ross andHenricks, 1945; and vermiculite, Gruner, 1934) had beenthoroughly examined and structures developed for them.Marshall (1935) using the structure of Hofmann et al.(1933) for montmorillonite showed that a small amount ofisomorphous substitution (already known in micas) couldaccount for the CEC. This was confirmed by Ross andHendricks (1945) and Kelley (1945), for montmorillonite,but not until the 1950's for kaolinite (Schofield and Sam-son, 1953; Robertson et al., 1954).

Because Mattson pictured the clay fraction of soils as acomplex mixture of SiO2, A12O3, and Fe2O3, his work fellinto disrepute as clear ideas on isomorphous substitution inclays emerged (Marshall, 1935). It seemed apparent that if aclay possessed an inherent charge, the idea of an isoelectricpH, below which clays become positively charged, was notpossible. However, Mattson's data, especially on ca-taphoresis, looked convincing. The solution, it appeared,was to ignore Mattson and hope he would go away, whichhe did.

Schofield (1939) published a little paper showing disturb-ingly Mattson-like tendencies in a soil from South Africaand Mehlich (1952) showed similar properties for someNorth Carolina soils. Dean and Rubins (1947) showed largedifferences in the abilities of soils to adsorb several anions.


The pendulum had begun to swing back towards Mattson'sviews. It was given another hard push by Schofield andSamson (1953) who showed that kaolinite could adsorb Clon its edges while remaining negatively-charged on itsfaces. In fact, kaolinite could even adsorb itself (by edge-face attraction). In such systems, maximum flocculationwas achieved at low pH with zero salt. This work made theearlier studies of Lutz (1936) and Peele (1937) on red soilsin the Carolinas more understandable. Both had found thatwhen the pH of the soils was raised, the structure becameworse. Schofield and Samson showed why.

In addition, to Schofield and Samson's work with pureclays, it was becoming increasingly clear, as mineralogicalmethods improved, that many soil clays had crystalline"backbones" heavily covered with Fe and Al oxides. Inthese systems the cation exchange capacity was partiallyblocked by positively-charged polymers which also couldreact with anions.

Quite recently, there has been a renewed interest in theadsorption of anions by Fe and Al oxides. Kingston et al.(1967, 1968) in Australia have shown that specifically ad-sorbed anions (e.g., SiO3 and H2PO4) give a negativecharge to the oxide surface and that competition betweenspecifically adsorbed anions is determined by the anion abil-ity to give the oxide a negative charge. Their work is con-tinuing, and its most important effect may be a reawakeningof interest in reactions of ions with materials other than clayminerals in the soil.

The practical importance of these observations has notbeen appreciated by soil chemists who work with mont-morillonites or with "young" soil clays. Leaving P aside(since it will always be taken out of solution one way or an-other), SO4 (Ensminger, 1954), Cl, and NO3 (Thomas,1960b) will be held wherever soils are high in R2O3 and thepH is low, just as Mattson showed 50 years ago. Since elec-troneutrality must be preserved, the retention of anions inacid soils means that the whole salt can be removed fromsolution with important consequences on both plant nutri-tion and leaching.

CATION EXCHANGE EQUATIONSWiegner (193 la) had long used Freundlich's equation to

describe adsorption, and Vageler and Woltersdorf (1930)had used Langmuir's equation. Kerr (1928) (working underTruog) predictably suggested that a chemical mass actionequation would describe exchange. Vanselow (1932) modi-fied Kerr's equation with a mole fraction term to obtaingreater constancy in heterovalent exchange. Jenny (1936)developed a kinetic equation for homovalent exchangewhich reduced to the mass action equation of Kerr. AfterJenny's work, there was virtually no interest in homovalentexchange in soils, possibly because the issue seemed settledand certainly because it is not of importance in soils.

Beginning with Vanselow (1932) the work on het-erovalent exchange (usually Ca-Na, Ca-K, or Ca-NH4)revolved around the proper equation to reduce variability ofthe constant. Gapon (1933) developed an equation usuallywritten in the square root form which aided constancy con-siderably. Davis (1945) endorsed this equation because itwas less variable. Krishnamoorthy et al. (1948), using sta-tistical thermodynamics, developed an equation similar to

Vanselow's except that Ca in the mole fraction term wasmultiplied by 1.5 for mono-divalent exchange. Eriksson(1952) developed two equations, one from the Donnanequilibrium, which was similar to Vanselow's, and onefrom the Gouy theory which reduced to Gapon's.

Empirically, the U.S. Salinity Laboratory (USDA, 1954)used the same form as the Gapon equation to plot theirresults. Then, Gaines and Thomas (1953) published theirthermodynamic equation in which the mole fraction ofVanselow was replaced by an equivalent fraction.

The work of Schofield (1947a) and Gaines and Thomas(1953) introduced the idea of using the equations as mea-sures of the affinities of cations for clays over a range of cat-ion ratios, rather than to obtain constants. Thereafter, theterm coefficient replaced constant which quietly disap-peared. In retrospect, much of the frantic work to obtainconstants seems to have been misdirected.

Most of the work since the 1950's has used the Gaponequation, probably because it is most familiar to people, tomeasure affinities in the Ca-K and Ca-Na systems. The lit-erature has been flooded with adaptations of Schofield's"ratio law," mostly to describe K availability (Woodruff,1955; Beckett, 1964). The theoretical Gouy equation hasbeen used by Bolt (1955), but needs corrections when K isused (Lagerwerff and Bolt, 1959).

THE PRESENT SITUATIONIn many respects the use of pure clay minerals to repre-

sent soil clays has been a failure. Most of the progress madein the past 20 years has resulted from the realization thatsubtle differences between clay minerals and differences inthe quantities and forms of noncrystalline components giverise to marked changes in cation exchange behavior. Cer-tainly an appreciation of the importance of the series mica -vermiculite in soil reactions (Rich, 1960; Barshad, 1951)has resulted in a better understanding of K, NH4, and Al be-havior. The importance of noncrystalline Al and Fe com-pounds in determining cation (an anion) exchange behaviorin soils almost completely vindicates Mattson. Although hewas wrong in rejecting the clay mineral concept, his earlyobservations on the amphoteric behavior of soils have beenshown to be correct in many cases.

The question raised over 100 years ago, as to whethercation exchange is a chemical or an adsorption reaction, isstill not answered. As Way observed, it resembles the dou-ble decomposition of a salt. On the other hand, the ad-herents of the Gouy and Donnan theories of ion distributionhave developed evidence to support that view. Earlier ideasthat attempted to "prove" the effect of cation size (Wieg-ner, 193la), atomic weight (Gedroiz, 1918, 1919) andvalence (Hendricks, 1945) all have to be modified when aspecific cation exchanger is used.

The Gouy theory as applied by Schofield (1947b),Eriksson (1952), and Bolt (1955) presents an attractivemental picture. Two limitations on the theory (which in itspurity) depends only on the charge density of the clay, theambient soil solution concentration, and the valences of theions present in a free swelling system are obvious: Mostclay systems do not swell freely and cations of the samevalence are not held with the same affinity. Therefore, all

calculations other than those with Li-montmorillonite areincorrect to some extent.

CONCLUSIONThe work reviewed in this article has given us our present

picture of how ion exchange operates in soils. The overallreactions in cation exchange, the clay minerals involved,and the role of cation exchange in both acid and alkali soilsis reasonably well understood as the result of this work.Thomas Way would be proud of his successors.

Still, it is fair to say that our understanding of ion ex-change in soils, where the exchangers are complex mixturesof clay minerals, oxides and organic matter, has not keptpace with the work on purer exchange material. Further, thedetails of how and the rates at which the ion exchange reac-tions occur are not understood even in ideal systems. Theprogress made in the past has depended on solid experi-mental data and it is evident that additional advances willnot occur until we obtain such data on the soils themselves.

238 SOIL sci. soc. AM. J., VOL. 41, 1977

historical developments in soil chemistry: ion exchange1

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