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A few aspects of 50 years of soil chemistry
Geoderma, 12 (1974) 281--297 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
A F E W ASPECTS O F 50 Y E A R S O F SOIL C H E M I S T R Y
A.C. SCHUFFELEN
Laboratory of Soils and Fertilizers, Agricultural University, Wageningen (The Netherlands) (Submitted for publication May 14, 1974; accepted June 26, 1974)
ABSTRACT Schuffelen, A.C.,1974. A few aspects of 50 years ofsoilchemistry. Geoderma, 12: 281--297. Some aspects of the development of soil chemistry during the past 50 years are discussed, special attention being paid to concepts concerning adsorption phenomena and soil phosphate. In the case of adsorption, the nature of the material and the double layer are emphasized. The concept of amorphous colloidal material is shown to be replaced by that of crystalline clay minerals. The empirical adsorption equations made room for considerations based on thermodynamic principles, which in turn were abandoned in favor of older concepts laid down in newly developed models. Attention is paid to the nature of soil acidity which was ascribed originally to the presence on adsorption sites of Al ions, then of H ions, and presently of a combination of H and A1 ions. Potassium fixation as a problem of the specific binding of counter ions also receives attention. In contrast to research on adsorption phenomena, research on soil phosphates has retained its empirical character. Only in the last few years has more fundamentally directed research on this subject been initiated. INTRODUCTION
During the last 50 years, soil chemistry has undergone marked development. Out o f an empirical subject of study in the twenties -- when practically no hypotheses and theories were known -- it has grown into a science based on theoretical knowledge and mathematical formulations sufficiently exact to support prognoses on the expected course of various processes. This change was n o t only due to better classification and correlation of the known facts but more to a more careful observation of data, obtained from well-aimed experiments based on previously set-up hypotheses. It goes without saying that this development was enhanced to a large degree by the progress made by analytical chemistry and later especially by instrumental chemistry during the same period. The developments in analytical techniques, starting from gravimetric analysis, via titrimetric to colorimetric, spectrochemical, X-rayspectrochemical and finally to scanning spectrometry, have made it possible to carry o u t more sensitive and faster analyses without the necessity of making large concessions in the accuracy of the observations. Through correct application of radioisotopes, radiochemistry has also contributed to obtaining
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a better understanding of many processes by opening new possibilities of research. Also the progress made in electrochemistry, especially during recent years, led to the elaboration of a new technique of analysis by the use of selective electrodes. Finally, the developments in electronmicroscopy should be mentioned as a factor which has greatly increased our knowledge of the morphology of soil materials. Of great importance for present views on the relations between the soil solutions and the solid phases of soils were the achievements in physical chemistry which were reached partly b y the soil chemists themselves. Development Of crystallographic studies by means of X-ray analysis have contributed to the foundation of the 7th Commission of the I.S.S.S., which t o o k over a part of the activities of the 2nd Commission. Ancient rules of solubility and insolubility were reintroduced into the chemical studies of the soil when a new leaf was turned in inorganic chemistry, largely due to the revival of thermodynamics in the last decennia. Unfortunately, this field of activity has been neglected t o o long. Less important, unfortunately, has been t h e influence of organic chemistry on soil chemistry studies. Too few scientists with modern schooling in organic chemistry have devoted themselves to the study of humus in the soil to be able to solve the very complicated problems of its nature and functions. However, also in this field, the empiric approach has been abandoned and new conceptions found, based on a synthesis of theoretical considerations. It is of course n o t possible within a short r~sum~ to cover all developments in the field of soil chemistry. Therefore, it seems to be better to choose only a few problems in the whole working area and follow their development in the course of the past 50 years. The choice of these problems is arbitrary and is made by the author who has taken part in their study during the whole of this period and witnessed the changes of interest of scientific workers and of himself as well in different problems involved in this study. The central point of all the studies in soil chemistry has always been the relations existing between the solid and liquid phases of the soil. The phenomena of ion exchange and of solubility come to the fore again and again. Whether these studies concern the genesis of soils, the availability of nutrients for plants, transport of ions to plant roots, or the general phenomena of leaching or fixation, the above-mentioned problems always occupy and mainrain a central position. This is, after all, quite understandable because the chemical studies of soils form one of the foundations upon which soil fertility studies are based. The latter have always been of interest because of their direct importance for the food supply for animals and humans. The scope of this report has been limited further to studies of chemistry of soils per se and n o t to applications in other aspects of soil science. ADSORPTIVE M A T E R I A L IN SOILS
A number of components in the solid phase of the soil show phenomena
283 of adsorption or ion exchange. Such materials were defined early as the fraction < 2~, although the only further c o m m e n t about its composition was that it consisted of aluminium silicates. In the beginning of the twenties the general opinion was that the inorganic part of what is now described as the soil adsorption complex consisted of two components. One of these components was considered to consist of decomposition residues, whereas the other was supposed to be newly formed (Gedroitz). In fact, both components were discerned by the analytical technique based on alternate acid and alkaline extractions as empirically developed by Van Bemmelen. Wiegner supposed that the inorganic adsorption complex was of a colloidal nature which -- in view of the general conceptions of the colloids at that time -- implied that it had to be amorphous. Therefore, his most important empirical studies of the cation-exchange phenomena were carried out using permutites as model substance. This view was shared by Mattson. Mainly after 1930, he described the soil colloids as mixed gels of the negatively charged silica gel and the positively charged gels of sesquioxides (Fe203 naq and A1203 naq). The adsorption capacity of the mixture would be connected with the surplus of charge which in turn would depend on the ratio between the constituent gels. A large number of experimental data were produced by Mattson to make this hypothesis acceptable. At the end of the second decade, experiments by Hendricks and his co-workers showed that crystalline material was also definitely present in the soil fraction < 2~. In colloid chemistry, in which the solid phase was until then considered as being amorphous (e.g. gold sol and silver iodide sol), it was found that sols were crystalline. The opinion that the inorganic adsorption complex is crystalline gradually became accepted. In this field of activity scientists such as Grimm, Bragg, Hoffmann, Edelman, and Marshall have done work of great importance between 1930 and 1950. Nowadays soil material showing ionic-exchange phenomena is designated by the term clay minerals, in which three main groups are discerned, viz. the montmorillonite, the illite and the kaolinite groups. Materials of these groups differ also in their behavior as to the ion-exchange phenomena. The development of views on this matter led to a better insight on the composition of the clay minerals although one should never forget the possibility that clay minerals present in the surface layers of our agricultural lands may show different properties from the model materials used in m a n y studies. For a more detailed discussion on clay minerals, reference may be made to a paper presented for Commission Seven. In contrast to the increase of knowledge of the inorganic adsorption complex, limited progress was made in the study of the organic fraction. This is partly due to the fact that the organic matter is very difficult to isolate, but also to the multiplicity of the organic compounds in the soil. The grouping. of compounds into fulvic acid, hymatomelanic acids, humic acids and humin as elaborated and proposed by Sven Oden about 1920, has remained a conventional m e t h o d which did not increase insight into their nature. The opinion
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that humus was formed mainly through the activity of microorganisms, as put forward by Waksman, was in itself correct but failed to give a clear picture of humus composition. The cellulose hypothesis (Marcusson), the aldehyde amino acid synthesis (Enders), the conception of polyphenolic amino acids (Flaig, Eller) and the regularly recurring lignin hypothesis (Fischer, Flalg) all give only incomplete pictures of the formations, compositions and particularly of the stereo chemistry of humus. The description often used of humus, as an at random polymeric product of polyphenols, amino acids, aldehydes and fragments of liguin (Swaby), certainly gives an impression of its composition and of the many possibilities of variations in the polymerisation, but it still does not give a complete definition of humus. 'This description concurs with the old view because it suggests that humus is a material of varying composition which depends on the nature of its constituents and prevailing conditions (Mattson). THE SOURCE OF THE NEGATIVE CHARGE
The property of the adsorption complex of soils t o adsorb cations found an early explanation in a hypothesis of clay acids. This hypothesis was supported by the shapes of the titration curves of soils which showed a great resemblance with those of weak acids (Bradfield). This analogy with humic acids -- which in fact are true acids -- and can split off hydrogen ions out of COOH- or OH-groups, is only formal. The negative charge, confirmed round the thirties by electrophoresis, is a surface charge which, at least for a considerable part, is not determined by the presence of acid groups. Analogous to developments in colloid chemistry, where the AgI-sol formed an important object of study (Kruyt), it was believed by Wiegner that clay particles carried a double layer. The inner layer, the layer of ions determining the potential, was thought to consist of OH- ions. The 6uter layer, the layer of counterions, consisted of the exchangeable cations and was responsible for the electroneutrality of the clay particle. In the literature before approximately 1940 different models from colloid chemistry were proposed, e.g. the condenser model (Helmholtz), diffuse double layer (Gouy-Chapman) or a combination of the two (Stem). However, earlier studies (Marshall) had already shown that this hypothesis was incorrect. In fact, clay particles do not have a double layer and the surface charge is not a consequence of adsorption of ions determining the potential, but of a surplus of electrons following an isomorphic substitution. During formation of a clay mineral a number of Si 4÷ sites within the crystal lattice are taken by A13+ and in A13÷ sites this ion may have been replaced by Mg~÷ or some other divalent ion. This substitution leads to a surplus negative charge (electrons) expressed as a negative surface charge. The charge resulting from the isomorphic substitution is neutralized by attraction of cations (counterions) and has a constant value (Schofield) independent of the pH. When A13÷ is replaced by Mg 2÷, it mostly takes place towards the centre of the crystal, so that in this case the surplus charge is more diffusely
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distributed over the surface. If Si4+ is replaced by A13÷, it is mostly in the surface layer of the crystal; the charge is then less diffusely distributed and more specific bindings of counterions are possible. Beside this type of electric charge a second one occurs. It is formed by dissociation of H + ions from =Si--OH and possibly also from =A1--OH groups situated at the edges and corners of clay minerals. This type of charge is, of course, dependant on the pH and the measured value increases with increasing alkalinity of the medium. The old conception of clay acids finds its reflection in this observation. This charge is of small magnitude as compared with the charge due to isomorphic substitution. In the case of well-crystallized clay minerals, it seldom amounts to more than 5% of the total exchange capacity. In slightly or non-crystallized clay minerals such as allophanes, it may amount to more than half the value of the total exchange capacity (Fieldes). In fact the modern conceptions of the nature of the adsorption complex include all former ideas on this subject. One part is crystalline b u t the presence of amorphous or incompletely crystallized components is usually assumed. More than one c o m p o n e n t is supposed to take part in ion-exchange reactions. Although much new information has been collected during the period of fifty years, most views held at present may be found in the literature of the twenties, although earlier ideas are usually described in less explicit form than those of today. The exchange capacity of the second c o m p o n e n t of the adsorption complex, humus, has been' described from the very beginning as resulting from a formation of salts of cations with the COOH- and OH-groups of humic acids. In case of humic compounds with small molecules these salts are often water soluble. In c o m p o u n d s with large molecules the salts are non-soluble. Salts of alkali metals and humic compounds with large molecules are highly peptized and therefore very mobile. Compounds of alkali-earth metals have little tendency for peptization. Contrary to the exchange capacity of the clay minerals, the adsorption capacity of humus is very much affected by the pH-value of the medium. In this respect the behavior of humus resembles very much that of a insoluble acid. THE EXCHANGE
CAPACITY
In the thirties many soil scientists paid attention to the exchange capacity of soils. From the point of view of the fertility status of soils, it is an important property. Through lack of a clear insight in the structure of a double layer and its effect on the exchange capacity, the attention was mainly centred on empirical formulae which would represent as much as possible the adsorption isotherm. Wiegner's school especially did much research work to establish the connection between the formulae and experimental data. There are in fact t w o models to be found in literature which appear in many variations. The first model is a logarithmic description showing that the amount of
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adsorbed material ~ is proportional to the logarithm of the concentration of this material in the equilibrium solution (c}: logT=a+blogc This formula originates from Freundlich and was applied without essential alterations to soil studies by Wiegner and others. Sometimes more parameters are added for adjustment of the formula to the result of experiments ( R o t h m u n d and Kornfeld). The second model has its starting point in the assumption that the adsorption isotherm has the shape of a hyperbola. It originates from Langmuir who derived for the adsorption of gases, the relation between the adsorbed quantity 7 with the concentration of the gas in gaseous phase (c). This relation was represented b y the formula: b.c "y = F l+b.c
where F is the maximum amount to be adsorbed and b is a constant depending on the binding energy between the adsorbent and the gas. A largely identical formula for soils was derived by Vageler. This was not, however, based on a theoretical equilibrium between adsorbed matter and an e m p t y surface but used as a starting point the assumption that the adsorption isotherm would have the shape of a hyperbola. It is most remarkable that in all these formulations the fact that the adsorbed ion must replace a counterion was not taken into account. If the adsorption isotherm for ions is treated mathematically in the same way as done by Langmuir for gases, the following formula would appear: bt 01
?=r" blcl +b2e2
where 7 is the a m o u n t adsorbed; F is the maximum amount to be adsorbed; ot and c2 are the respective concentrations of ion 1 and ion 2 in the solution; and bl and b2 are constants, which depend on the binding energy of ions 1 and 2, respectively. In experiments carried o u t between 1930 and 1950 the fact that ion exchange is connected with the sort of the complementary ion to be exchanged finds its expression in the so-called lyotropic series. From many studies in this field of research, a series may be demonstrated as found by Jenny in his experiments with Putnam clay where the difficulty of exchange increased in the following order: Li
Rb< Cs
Ca< Si
La
Why all soils did not have exactly the same sequence within the series became clear when better formulations of the ion-exchange phenomena were made. At present, very little use is made of the above formulation. This is partly due to the fact that a direct determination of the adsorption capacity
287
has been made possible by the analytical techniques of percolation or multiple treatment b y a salt solution. Another reason is that considerations of relations existing between adsorbed quantities and concentration of the equilibrium solution are studied with different models. For some estimations (as, for instance, in column chromatography which are of sufficient accuracy in some cases) one can still successfully use these classic empirical equations. MODERN FORMULATIONS
Gradually, the empirical equations, partly derived from the Guldberg and Waage equation for ionic equilibria, were replaced by conceptions based on thermodynamics. This t o o k place mainly between the years 1950 and 1960 when many scientists adopted this approach following the work of Schofield (Babcock, Low, Coleman, Peech, Bolt, Laudelout). The starting point is the equation of a reaction, as for instance between a monovalent (+) and a bivalent (++) ion: exch(++) + s a l t ( + ) ~ 2 exch(+) + salt(++) that is:
(++)a + 2(+)o ~ 2(+)a + (++)o or
g(++)a + 2~(+)o = 2 p ( + ) a +/~(++)o and there: P =Po + R T l n a
(a+)2 a ~ K
(a'~)a
(a+)2o - -
(a~+)o
where t~ is the chemical potential, a the activity and K an equilibrium constant. The indices a and o indicate, respectively, the adsorbed phase and the solution. Although this formula is correct in its generality, it is -- due to this generality -- difficult to apply in a particular case. The reason is that the ion activities are difficult or impossible to measure. In a solution the activity may be measured with good approximation, or it may be calculated. For the adsorbed phase it is not measurable and cannot be calculated. When the activity in the above formula is substituted by concentration (c) with the corresponding activity coefficient (f), the following formula is obtained: •
(r) o
=K
(c+h "(f++h (a'~)o(t~)o
288 or ff+) o • (f+h----K
(C+)a
(f++)o "(f+)2a
(C+)o
The measured value of K is then not the thermodynamically correct one, but an empirical factor being the product of the real K value together with a number of unknown activity coefficients. Therefore, for practical application, it is better to use models and to introduce certain assumptions. One of the oldest models is the one of the Donnan equilibrium, originally elaborated for a system of t w o compartments separated from each other by a semi-permeable membrane. Mattson, Wiklander, Schuffelen and others have used this model for systems with a double layer. Its formulation reads: (C+)i
(C+)o
V(c+)i V(C+)o where c, the concentrations, and the indices, i and o, indicate respectively the double layer and the solution. In principle this model can be used to describe the effect of the valence of an ion in relation to the concentration of ions in systems with double layers, but it is usually insufficient for quantitative estimations (Schofield). The Donnan system has still the disadvantage that activity coefficients are u n k n o w n factors (Klarenberg). Another model is one in which activities of the ions in the adsorbed phase are supposed to be in the same proportion as the mole fractions. In fact, the double layer is hereby considered as a solid solution. Taking this principle as a starting point, Kerr drew up the following equations for equilibria between ions of the same valence: +
--=K
a n d - - =K - -
and Vanselow did the same for systems with ions of different valence:
--=K
P
3,++
In these formulae ~, represents the adsorbed quantity, c the ion concentration in the solution, and K and p constants. The third model has been derived from the build-up of the double layer as developed b y Schofield, Errickson and Bolt with the diffusion model of Gouy-Chapman as a starting point and an assumption of the Boltzman distribution of ions in the double layer. The final formulation is then:
289
,~,+
C+
=KG" 7 ~
X/C +*
a formula which, in fact, is identical with the equation given by Gapon in the thirties. This equation of Gapon describes quite well the results obtained in the absence of specific forces between the adsorbing material and adsorbed ions or in other words, when the adsorption is caused by Coulomb forces. KG is then a calculable function of the charge density. When the formula of Kerr is used, K values are found which in the lyotropic series range from a b o u t 1 to 5, going from Li to Cs. For divalent ions the selectivity constant ranges between 1 and approximately 2. Also by applications of the Gapon-equation the values for KG ranges between 1 and 2. However, this does n o t hold when specific forces have to be dealt with as it is the case with adsorption of potassium by mite. It is interesting to note that, in spite of the fact that good thermodynamical formulations have now been made available, for models one goes back to those which have been developed in the thirties or even earlier. Two examples of adsorbed ions may be given, which have continuously drawn attention during the period of the last 50 years. These are the pair of hydrogen and aluminium ions and the potassium ion. SOIL ACIDITY
After 1920 a sudden development occurred in the search for the origin of the acidity of soils. Before that time the acid properties were ascribed to humic acids and the alkaline reactions to the occurrence of calcium carbonate. Now aluminium and hydrogen ions entered the picture. The observation, made in the beginning of this century by Veitch and Daikuhara that soils show an acid reaction after addition of salt was confirmed and thoroughly studied, particularly by Kappen. For the first time Kappen mentioned exchangeable A1 ions which when hydrolized gave rise to the formation of acid: Al-clay + 3KC1 ~ K-clay + AIC13 A1C13 + 3 H 2 0 -* AI(OH)3 + HC1 Usually a reasonable agreement could be found between the quantity of A1 and titratable H ions in the KC1 extract, which fact seemed to support the hypothesis. In that period three types of soil acidity were distinguished, viz.: (1) The active acidity represented by the H ions, which could be titrated in a H20 extract and were supposed to originate from free acids and acid salts. The measuring of this acidity was soon replaced by determining the pH value.
290 (2) The exchange acidity, which is the a m o u n t of A1 ions exchanged by NaC1 and titratable after hydrolysis. (3) The hydrolytic acidity, or the titratable H ions in a Na-acetate extract. The release of those H ions was ascribed to an adsorption of NaOH by soil particles. All these "acidities" were determined by titrations with an alkali and thus are potential acidities, which only gradually replaced the determination of the active acidity described nowadays as the pH. The introduction of determinations of the degree of saturation (V) falls in about the same period. It is calculated as the quotient of the sum of the exchangeable bases (S) and the total adsorption capacity (T). The sum of exchangeable bases was determined by an extraction of the soil with a dilute acid followed by a back titration of the a m o u n t of neutralized acid (S). T was determined as S + H, where H was determined by a conductometric titration with Ba(OH)2 (Hissink). In this determination the Al ion was n o t taken into account. The H ion is mentioned as such and is put beside all other cations (S). A co-worker of Hissink, Van der Spek, also described soil acidity as a result of H ions: H-clay + KC1 -* K-clay + HC1 This leads to a discussion of whether it is the H ion or the AI ion which causes the acid reaction of the KC1 extract and also of the soil. Kappen and Trenel are the great advocates of the A1 hypothesis. Page, Bradfield and Wiegner support the H hypothesis. In 1931 Kappen accepts the hydrogen hypothesis. The support for the H hypothesis arose partly from the so-called suspension effect, a phenomenon described by Bradfield and Pallman. Their investigations showed that the pH value of a soil suspension depends on the concentration of the suspension. In short, the measured concentration of the H ions appeared to be proportional to the concentration of the suspension. On the one hand, this indicates a minor role of aluminium and, on the other hand, it showed t h a t the H ion concentration in the double layer had an effect on the measured pH. Wiegner described it also as an effect of the double layer. Others ascribed it to a kind of Donnan equilibrium, but all the explanations were rather indefinite. The situation changed when the statement was put forward that, from the point of view of thermodynamics, there could be no suspension effect in the sense of the suggested effects of the double layer (Du Rietz, Rabinowitch, Kargin). When considered thermodynamically in an equilibrium situation, the activity of the H ions in the double layer must be equal to that in the equilibrium solution and, as the pH is measured by a reversible H electrode, no difference in activity can be measured. This could be easily confirmed by changing the position of a reversible H electrode and of a non-reversible KC1 bridge to a calomel electrode within the suspension and the supernatant solution. These experiments showed that the suspension
291 effect originated from the KC1 bridge and n o t from the H electrode. This had already been suggested in 1942 by Loosjes b u t his explanation of this effect as being a capillary effect could n o t be maintained. The suspension effect is caused by a diffuse potential arising between the KC1 and the suspension. This potential has been treated theoretically by Overbeek. In the fifties and sixties much attention was paid to this p h e n o m e n o n by Jenny, Coleman, Bolt, Babcock, Davis, Overstreet, Peech and others. It is quite certain that the last word on this question has not y e t been spoken, because the suspension effect was also demonstrated b y Pallmann through measuring the H ion concentration by the rate of inversion of sugar. The concentration of H ions calculated from the rate of inversion appeared to be directly proportional to the concentration of the suspension. Improvements of pH measurement--- starting from the slowly acting hydrogen gas electrode, via the faster quinhydrone electrode to the quickly working glass electrode -- made it possible to measure the H ion activities on a large scale and to switch over from the titrimetric acidity determination to electrometric measurements. This development again diverted attention from the A1 ion, although it was known that the addition of KC1 to the soil led to the so-called salt effect forming A1 and Fe salts. This p h e n o m e n o n was explained by the assumption that the addition of salt would lead to such a high concentration of H ions within the double layer that the clay mineral would be attacked under liberation of A1 and Fe (Hudig). In the forties the opinion that AI ions could also be adsorbed was again put forward. This differs from the former opinion, where i t w a s assumed that adsorption of A1 ions would lead to an extension of the crystal lattice, analogous to the growth of the AgI crystal; according to this conception A1 could not take the place of a counterion. Paver, Marshall, Schofield and Chernov had in the forties taken the view that acid clays could contain exchangeable H ions and A1 ions as well. Russell's b o o k has again drawn attention to this problem by his treatment of the Schofield hypothesis. Here it is not only the A13÷ ion b u t also the hydroxy-ions AI(OH) 2÷ and AI(OH)~ which can be exchanged and so are introduced as counterions. Mainly b y interpretation of titration curves of acid clay minerals, Coleman and Low had come in the sixties to the conclusion that acid soils contained aluminium in the double layer in one of its ionic forms. This concept comes to a development where it is assumed that exchangeable H ions in artificial H clays are gradually replaced by A1 ions. The kinetics of this reaction have been formulated b y Kerr, Laudelout and co-workers. Jenny draws attention to the fact that when AI(OH) complexes can be adsorbed a step is made again in the direction of Mattson's hypothesis of mixed gels. On the other side, aluminium adsorption may be considered as a sort of crystal growth which again leads to the hypothesis that A1 ions should n o t be seen as real counterions b u t as ions which form part of the crystal. Here again, it appears that old conceptions tend to reappear often in a somewhat modified form. This is often the case, n o t only in the sphere of soil chemistry.
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P O T A S S I U M FIXATION
The next ion getting much attention is the potassium ion because of its exceptional position in exchange phenomena. Some softs have a tendency to bind potassium in such a specific way that their exchange for other ions becomes difficult or even impossible. In the thirties this phenomenon was designated as potash fixation, a name which it has kept until now. Originally this phenomenon was observed in the field. Some soils, known as being poor in potash, did not respond to potash dressings. When potassium was added to these softs in the form of a solution of a potassium salt, it appeared that it could be fixed in a not directly exchangeable form, especially after drying (Volk). Since then, different explanations of this phenomenon have been given in the course of the years. About 1940 it was established that fixation of potassium was not caused by organic constituents of the soil (Joffe, Kolodny, Bakhulin, Hauser) but bound to the inorganic part. Different hypotheses were put forward to explain this phenomenon. Gorbunov supposed that during drying the K ions are so far dehydrated that they become, as it were, stuck within the Helmholtz-layer. He also assumed a sort of sticking of soil particles during drying. Both phenomena would lead to an irreversible fixation of potassium ions. Volk assumed a formation of new muscovite-type minerals formed out of the AI(OH)3 and SiO2 present in the soil together with potassium ions. It was also suggested that small micaceous fragments poor in potassium, would grow into larger particles in t h e presence of potassium ions, which then would be taken up in the crystal lattices (Schuffelen), Later investigations tried to find a connection between potassium fixation on the one side and the presence of certain clay minerals on the other. Efforts have often been made to distinguish wet fixation, occurring in soil particles with a high fixing capacity, and dry fixation, as found in nearly all soils (Hauser, Van der Marel). For montmoriUonite the fixation depends on the specific type of clay mineral, the beidellite form showing the highest degree of fixation (Drosdorff, Truog and Jones, Jackson and Hellman). mite has a very strong fixing capacity (De Turk', Stanford, Wear, Wiklander, Van der Marel). Also vermiculite may show a strong fixation of potassium (Gruner, Barshad, Walker, Coleman). Other minerals such as biotite (Barshad), glauconite (Chaminade) and phosphates can fix potassium (Joffe). A very systematic investigation on this subject was carried out by Van der Marel, who under standard conditions, examined the capacity for potassium fixation of many minerals. Fixation is connected with the amount of the fraction < 2p and is usually enhanced by drying, by increased concentration of the potash salt solution and by increase of the pH. A number of hypotheses are then put forward to explain this phenomenon. It is stated that there is a connection between the different forms in which potassium is present in the soft:
293
K in solution ~- K exchangeable ~- K in fixed position ~ K within the mineral. The reaction between K in solution and K in exchangeable form is reversible and comes quickly to an equilibrium; of both the other equilibria the third takes place quickly to the right side b u t slowly to the left side. The last equilibrium moves only slowly in either direction, although plant roots are able to take K out of the minerals (Schachtschabel, Reitemeier). According to Chaminade no potassium fixation occurs when 4.5% of the total exchange capacity is covered by exchangeable potassium. During the fifties several types of potassium fixation were discerned. In one t y p e potassium ions penetrating between the lattice plates are trapped after contraction of the plates, and the potassium ions then become nonexchangeable. This form occurs in the case of illite, some types of montmorillonite and vermiculite. Another type of fixation is found in nontronite, beidellite and some intermediates. Here the K ions are strongly b o u n d by sites in the lattice which have a strong negative charge caused by A1--Si substitution and where K ions fit very well spatially. The third t y p e is found in minerals which have become poor in potassium by weathering and show " o p e n K sites" which are then taken by K ions. This t y p e of fixation may be found in soils of Norway and Finland. The fourth t y p e of fixation is found in some zeolites and permutites in which narrow pores cause very slow cation exchange. These observations have been compiled by Schuffelen and Van der Marel. During the years since 1960, a discussion has started of mechanisms of potassium fixation n o t based on experimentally established data b u t on the build-up of minerals. The electron-microscope proved to be a very valuable aid in checking some of the theoretical deductions. New viewpoints have been opened by the compilation research of Reitemeier, Arnold, Jackson and others. Many points have been clarified by the investigations of Rich and Graf yon Reifenbach. The problem of potassium fixation has also been approached from quite a different point of view. Determinations of selectivity constants (KG) showed that when the K ion and, for instance, illite were considered, the " c o n s t a n t " was n o t constant at all, b u t showed a strong variation depending on the amount of adsorbed potassium. When large amounts of exchangeable K were present, the value of KG amounted to about 2, a value of the same order as found for other ions, b u t with only a little K in exchangeable form, the value of K could increase to a b o u t 40 (Van Schouwenburg, Schuffelen, Schwertmann). By careful analysis the connection between the a m o u n t of exchangeable K and the selectivity constant could be elucidated. If three different sites are assumed on the particle, where potassium may be adsorbed -- viz. the planar surface, the corners of the crystal and between the layers of the lattice -three values of K G could be calculated, each being independent of the number of sites taken by the potassium ions. The values of KG amounted respectively to 2, 100 and ~ for the planar surfaces, the edges and the interlattice sites of an illite. This hypothesis fits well with ideas on the structure of this
294 mineral, ideas formed on the basis of mineralogically oriented research. Beside potassium, NH4 ÷ ions, Rb ÷ ions and Cs ions are fixed by the soil. NH4 fixation in the field is only temporary because of subsequent nitrification. Fixation of Cs is important because this ion appears as a product of fall-out and can be withdrawn by fixation from the nutritional chain (Scott-Russel). EXCLUSION OF ANIONS As soil particles show a net negative charge, they will tend to exclude anions from the surface. When moving in the diffuse double layer from the solution towards the wall of the soil particle, there is a steady decrease of room for the anions. This is called anion exclusion or negative adsorption. As early as the twenties it was known from measurements that this exclusion must exist; it was then indicated by the term "salt-free layer" (Trofimov). This image fitted in very well with the Donnan conception where abrupt changes are considered rather than diffuse transitions. Also Alten and Vageler mention the salt-free water layer. A more quantitative treatment of the anion exclusion, taking as a starting point the double layer of Gouy-Chapman, has been given in the fifties by Schofield for a system with only monovalent ions. Later this work has been extended by Bolt, Warkentin and De Haan for systems containing ions of different valences. As there are no specific forces acting in the anion exclusion, this phenomenon can be applied successfully to determining the specific surface of the adsorber. It has often been used for this purpose. A particular case of anion exclusion exists for phosphate. This ion may also be adsorbed by the positively charged sites of the clay, so that measurements represent a net result of exclusion and adsorption. Careful theory and experimentation have enabled De Haan to unravel both the phenomena. PHOSPHATE Contrary to the studies of the adsorption of cations and exclusion of anions, which were strongly directed towards a better knowledge of the content and development of physico-chemical hypotheses, the studies of phosphate remained for a long time in the empirical stage. The reason is that the phosphate status of softs usually formed the main object in these studies. Efforts have always been made to find new and better methods of extraction in order to determine the phosphate-fertility status of soils. Emphasis was laid on establishing correlations between the determined quantity of phosphate and yield, mainly as a basis for phosphate-fertilizer application. The extraction media ranged from water, via weak acids or buffer solutions to strong acids (Dirks, Scheffer, De Vries, Schachtschabel, Bray, Olsen, Truog, Van der Pauw, Zinzadze). All these extractions remain conventional means for determining the phos-
295 phate content and their merit depends solely on the degree of correlation with results of field and pot experiments. They give only little information about the nature of phosphate compounds present in the soil. About 1930 the first compilation of a research into the relation between the solubility of phosphate and soil acidity was put forward (Gaarder). This led to recognition of the difference between the A1 and Fe phosphates occurring under acid conditions and of Ca and Mg phosphates present in alkaline media. This early research may be considered as pilot studies for the selective extraction introduced later by Chang and Jackson. A number of different forms of phosphate present in the soil could be discerned by this method, viz., the easily soluble phosphate, the calcium phosphates, the aluminium phosphates, the iron phosphates, and the occluded phosphates. This method has shown that aluminium phosphates show the largest decrease in concentration when the soil is covered by vegetation. This was a surprising result because it was mostly assumed that calcium phosphates were mainly taken up by the plant under the influence of the acid-producing root. Another, but also analytical approach came into use when the radioactive isotope( 32P) became available. By using the system of isotopic dilution, it was possible to determine the phosphate pool of soils (Fried, Dean, Larsen, Scott-Russel). But as the radioactive phosphate and a number of phosphates forming part of that in the soil come very slowly to an equilibrium, this method also has to be qualified as a conventional determination of a value called labile phosphate. Neither has the method, where the rate of isotopic exchange is measured, given more insight in the behavior of phosphate in the soil (Uhlrich, Scheffer, Sissingh). All this is obviously connected with the many-fold forms of phosphate which collectively form the soil phosphate. Not only organic phosphorus but also adsorption of phosphate by sesquioxides and clay minerals, precipitation of unsoluble compounds, and inorganic phosphates make the whole problem very difficult to unravel {Williams). Actually, only specialized investigations of the properties of its components may at present be qualified as exact research. Little research has been done on organic phosphate, although it amounts to about one half of the total quantity of phosphate present in the soil. The main constituents are nucleic acids showing a fast turnover, phosphatides and the phytin or inositol hexa-phosphate with very little turnover (Peech). Inorganic phosphates were investigated rather intensively in the sixties. Although a total of about 40 different compounds have been found in the soil," properties of only the three most important ones have been studied. After preliminary experiments in the fifties in the study of the relations between calcium, iron and aluminium phosphatesand their transitions (Rathje), this work was extended in the sixties and, based on the phase rule, it is making good progress. Taking as starting point the solubility products of different phosphates and hydroxides of iron and aluminium, one can represent in clear diagrams
296
the theoretical relations between the concentration of phosphate in solution, the nature of the solid phase and the pH (Lindsay and co-workers). The experimentally obtained data for soils appear to be either in good or bad agreement (Peech). This is caused by the slow rate at which the equilibrium conditions are reached. Research into the rate of dissolution has been started but has n o t as y e t been sufficiently elaborated. Here the gap existing in the research carried out so far comes clearly to the fore. It is the lack of data of rate constants which form a bottle-neck for more sophisticated prognoses. These rate constants do not play a role of great importance for practical application of exchange equilibria because of their magnitude. In the case of potash fixation.and potash release, they are much smaller and are n o t sufficiently investigated. Our knowledge of the constants in the case of phosphates is far from being complete and it is this lack of knowledge which makes it impossible as y e t to apply the theory quantitatively for practical purposes. In the last period of soil chemistry research t w o quite new aspects have come forward. These follow from the observation that a number of ions in the soil are b o u n d to humus in the form of chelates (Scheffer, Lindsay). This means that the availability of these ions for plant roots is not dependent anymore on the classic ionic equilibria alone. The total effect of adsorption, precipitation and chelating are points to be raised for further research. The fact that transport of ions to the plant r o o t and into the deeper layers can be described by a double process (viz. flux and diffusion) has drawn attention during the last years (Barber, Quirk, Nye, Tinker, Frissel). The consequence of this is as y e t quite incalculable. Here as well the lack o f rate constants is clear. The processes which are often described statically have to be treated kinetically. A survey of 50 years of research cannot be complete unless it is written by a professional historian. Selection of high-lights by a researcher is always arbitrary because findings that made the most impression on him and all those in which he has himself participated come to the fore automatically. I hope that, bearing this in mind, the reader will still obtain some picture of the research in soil chemistry during the last half century. SELECTED REFERENCES Bear, F.E., 1955. Chemistry of the Soil. New York. Bolt, G.H., 1967. Cation exchange equations used in soil science -- A review. Neth. J. Agric. Sci., 1 5 : 8 1 Bradfield, R., 1923. The chemical nature of a colloidal clay. Univ. Mo., Agr. Exp. Station Res. Bull., 60 Chernov, V.A., 1947. The Nature of Soil Acidity. Academy of Sciences of the U.S.S.R., Moscow Flaig, W., 1967. Ver~nderungen am Lignin und Anlagerung yon Stickstoff-haltigen Verbindungen zu Beginn der Bildung und bei der Nutzung yon Torf. Landbauforsch. VOlkenrode, 1 7 : 1
297
Gapon, E.N., 1933. The theory of exchange adsorption in soils. U.S.S.R.J. Gen. Chem., 3 : 1 4 4 (in Russian) Gedroitz, K.K., 1929. Die Lehre vom Adsorptionsverm0gen der Boden. Dresden und Leipzig Hissink, D.J., 1928. Beitrage zur Frage der Bodenadsorption. Soil Res. (Suppl. Proc. Int. Soil Sci. Soc.), 1 : 4 Jenny, H., 1961. Reflection on the soil Acidity Merry-Go-Round. Proc. Soil Sci. Soc. Am., 25:428 Marshall, C.E., 1949. The Colloid Chemistry of the Silicate Minerals. New York Mattson, S., 1931. The laws of soil colloidal behavior, VI. Soil Sci., 3 2 : 3 4 3 Oddn, S., 1922. Die Humins~uren. Dresden und Leipzig Schofield, R.K., 1947. A ratio law governing the equilibrium of cations in solution. Proc. Int. Congr. Pure and Applied Chem., 11 th, London, 3 : 2 5 Schuffelen, A.C. and Van der Marel, H.W., 1956. Potassium fixation in soils. Potassium Congress, Rome, 1955, 157 Wiegner, G., 1929. Boden und Bodenbildung. Dresden und Leipzig
A F E W ASPECTS O F 50 Y E A R S O F SOIL C H E M I S T R Y
A.C. SCHUFFELEN
Laboratory of Soils and Fertilizers, Agricultural University, Wageningen (The Netherlands) (Submitted for publication May 14, 1974; accepted June 26, 1974)
ABSTRACT Schuffelen, A.C.,1974. A few aspects of 50 years ofsoilchemistry. Geoderma, 12: 281--297. Some aspects of the development of soil chemistry during the past 50 years are discussed, special attention being paid to concepts concerning adsorption phenomena and soil phosphate. In the case of adsorption, the nature of the material and the double layer are emphasized. The concept of amorphous colloidal material is shown to be replaced by that of crystalline clay minerals. The empirical adsorption equations made room for considerations based on thermodynamic principles, which in turn were abandoned in favor of older concepts laid down in newly developed models. Attention is paid to the nature of soil acidity which was ascribed originally to the presence on adsorption sites of Al ions, then of H ions, and presently of a combination of H and A1 ions. Potassium fixation as a problem of the specific binding of counter ions also receives attention. In contrast to research on adsorption phenomena, research on soil phosphates has retained its empirical character. Only in the last few years has more fundamentally directed research on this subject been initiated. INTRODUCTION
During the last 50 years, soil chemistry has undergone marked development. Out o f an empirical subject of study in the twenties -- when practically no hypotheses and theories were known -- it has grown into a science based on theoretical knowledge and mathematical formulations sufficiently exact to support prognoses on the expected course of various processes. This change was n o t only due to better classification and correlation of the known facts but more to a more careful observation of data, obtained from well-aimed experiments based on previously set-up hypotheses. It goes without saying that this development was enhanced to a large degree by the progress made by analytical chemistry and later especially by instrumental chemistry during the same period. The developments in analytical techniques, starting from gravimetric analysis, via titrimetric to colorimetric, spectrochemical, X-rayspectrochemical and finally to scanning spectrometry, have made it possible to carry o u t more sensitive and faster analyses without the necessity of making large concessions in the accuracy of the observations. Through correct application of radioisotopes, radiochemistry has also contributed to obtaining
282
a better understanding of many processes by opening new possibilities of research. Also the progress made in electrochemistry, especially during recent years, led to the elaboration of a new technique of analysis by the use of selective electrodes. Finally, the developments in electronmicroscopy should be mentioned as a factor which has greatly increased our knowledge of the morphology of soil materials. Of great importance for present views on the relations between the soil solutions and the solid phases of soils were the achievements in physical chemistry which were reached partly b y the soil chemists themselves. Development Of crystallographic studies by means of X-ray analysis have contributed to the foundation of the 7th Commission of the I.S.S.S., which t o o k over a part of the activities of the 2nd Commission. Ancient rules of solubility and insolubility were reintroduced into the chemical studies of the soil when a new leaf was turned in inorganic chemistry, largely due to the revival of thermodynamics in the last decennia. Unfortunately, this field of activity has been neglected t o o long. Less important, unfortunately, has been t h e influence of organic chemistry on soil chemistry studies. Too few scientists with modern schooling in organic chemistry have devoted themselves to the study of humus in the soil to be able to solve the very complicated problems of its nature and functions. However, also in this field, the empiric approach has been abandoned and new conceptions found, based on a synthesis of theoretical considerations. It is of course n o t possible within a short r~sum~ to cover all developments in the field of soil chemistry. Therefore, it seems to be better to choose only a few problems in the whole working area and follow their development in the course of the past 50 years. The choice of these problems is arbitrary and is made by the author who has taken part in their study during the whole of this period and witnessed the changes of interest of scientific workers and of himself as well in different problems involved in this study. The central point of all the studies in soil chemistry has always been the relations existing between the solid and liquid phases of the soil. The phenomena of ion exchange and of solubility come to the fore again and again. Whether these studies concern the genesis of soils, the availability of nutrients for plants, transport of ions to plant roots, or the general phenomena of leaching or fixation, the above-mentioned problems always occupy and mainrain a central position. This is, after all, quite understandable because the chemical studies of soils form one of the foundations upon which soil fertility studies are based. The latter have always been of interest because of their direct importance for the food supply for animals and humans. The scope of this report has been limited further to studies of chemistry of soils per se and n o t to applications in other aspects of soil science. ADSORPTIVE M A T E R I A L IN SOILS
A number of components in the solid phase of the soil show phenomena
283 of adsorption or ion exchange. Such materials were defined early as the fraction < 2~, although the only further c o m m e n t about its composition was that it consisted of aluminium silicates. In the beginning of the twenties the general opinion was that the inorganic part of what is now described as the soil adsorption complex consisted of two components. One of these components was considered to consist of decomposition residues, whereas the other was supposed to be newly formed (Gedroitz). In fact, both components were discerned by the analytical technique based on alternate acid and alkaline extractions as empirically developed by Van Bemmelen. Wiegner supposed that the inorganic adsorption complex was of a colloidal nature which -- in view of the general conceptions of the colloids at that time -- implied that it had to be amorphous. Therefore, his most important empirical studies of the cation-exchange phenomena were carried out using permutites as model substance. This view was shared by Mattson. Mainly after 1930, he described the soil colloids as mixed gels of the negatively charged silica gel and the positively charged gels of sesquioxides (Fe203 naq and A1203 naq). The adsorption capacity of the mixture would be connected with the surplus of charge which in turn would depend on the ratio between the constituent gels. A large number of experimental data were produced by Mattson to make this hypothesis acceptable. At the end of the second decade, experiments by Hendricks and his co-workers showed that crystalline material was also definitely present in the soil fraction < 2~. In colloid chemistry, in which the solid phase was until then considered as being amorphous (e.g. gold sol and silver iodide sol), it was found that sols were crystalline. The opinion that the inorganic adsorption complex is crystalline gradually became accepted. In this field of activity scientists such as Grimm, Bragg, Hoffmann, Edelman, and Marshall have done work of great importance between 1930 and 1950. Nowadays soil material showing ionic-exchange phenomena is designated by the term clay minerals, in which three main groups are discerned, viz. the montmorillonite, the illite and the kaolinite groups. Materials of these groups differ also in their behavior as to the ion-exchange phenomena. The development of views on this matter led to a better insight on the composition of the clay minerals although one should never forget the possibility that clay minerals present in the surface layers of our agricultural lands may show different properties from the model materials used in m a n y studies. For a more detailed discussion on clay minerals, reference may be made to a paper presented for Commission Seven. In contrast to the increase of knowledge of the inorganic adsorption complex, limited progress was made in the study of the organic fraction. This is partly due to the fact that the organic matter is very difficult to isolate, but also to the multiplicity of the organic compounds in the soil. The grouping. of compounds into fulvic acid, hymatomelanic acids, humic acids and humin as elaborated and proposed by Sven Oden about 1920, has remained a conventional m e t h o d which did not increase insight into their nature. The opinion
284
that humus was formed mainly through the activity of microorganisms, as put forward by Waksman, was in itself correct but failed to give a clear picture of humus composition. The cellulose hypothesis (Marcusson), the aldehyde amino acid synthesis (Enders), the conception of polyphenolic amino acids (Flaig, Eller) and the regularly recurring lignin hypothesis (Fischer, Flalg) all give only incomplete pictures of the formations, compositions and particularly of the stereo chemistry of humus. The description often used of humus, as an at random polymeric product of polyphenols, amino acids, aldehydes and fragments of liguin (Swaby), certainly gives an impression of its composition and of the many possibilities of variations in the polymerisation, but it still does not give a complete definition of humus. 'This description concurs with the old view because it suggests that humus is a material of varying composition which depends on the nature of its constituents and prevailing conditions (Mattson). THE SOURCE OF THE NEGATIVE CHARGE
The property of the adsorption complex of soils t o adsorb cations found an early explanation in a hypothesis of clay acids. This hypothesis was supported by the shapes of the titration curves of soils which showed a great resemblance with those of weak acids (Bradfield). This analogy with humic acids -- which in fact are true acids -- and can split off hydrogen ions out of COOH- or OH-groups, is only formal. The negative charge, confirmed round the thirties by electrophoresis, is a surface charge which, at least for a considerable part, is not determined by the presence of acid groups. Analogous to developments in colloid chemistry, where the AgI-sol formed an important object of study (Kruyt), it was believed by Wiegner that clay particles carried a double layer. The inner layer, the layer of ions determining the potential, was thought to consist of OH- ions. The 6uter layer, the layer of counterions, consisted of the exchangeable cations and was responsible for the electroneutrality of the clay particle. In the literature before approximately 1940 different models from colloid chemistry were proposed, e.g. the condenser model (Helmholtz), diffuse double layer (Gouy-Chapman) or a combination of the two (Stem). However, earlier studies (Marshall) had already shown that this hypothesis was incorrect. In fact, clay particles do not have a double layer and the surface charge is not a consequence of adsorption of ions determining the potential, but of a surplus of electrons following an isomorphic substitution. During formation of a clay mineral a number of Si 4÷ sites within the crystal lattice are taken by A13+ and in A13÷ sites this ion may have been replaced by Mg~÷ or some other divalent ion. This substitution leads to a surplus negative charge (electrons) expressed as a negative surface charge. The charge resulting from the isomorphic substitution is neutralized by attraction of cations (counterions) and has a constant value (Schofield) independent of the pH. When A13÷ is replaced by Mg 2÷, it mostly takes place towards the centre of the crystal, so that in this case the surplus charge is more diffusely
285
distributed over the surface. If Si4+ is replaced by A13÷, it is mostly in the surface layer of the crystal; the charge is then less diffusely distributed and more specific bindings of counterions are possible. Beside this type of electric charge a second one occurs. It is formed by dissociation of H + ions from =Si--OH and possibly also from =A1--OH groups situated at the edges and corners of clay minerals. This type of charge is, of course, dependant on the pH and the measured value increases with increasing alkalinity of the medium. The old conception of clay acids finds its reflection in this observation. This charge is of small magnitude as compared with the charge due to isomorphic substitution. In the case of well-crystallized clay minerals, it seldom amounts to more than 5% of the total exchange capacity. In slightly or non-crystallized clay minerals such as allophanes, it may amount to more than half the value of the total exchange capacity (Fieldes). In fact the modern conceptions of the nature of the adsorption complex include all former ideas on this subject. One part is crystalline b u t the presence of amorphous or incompletely crystallized components is usually assumed. More than one c o m p o n e n t is supposed to take part in ion-exchange reactions. Although much new information has been collected during the period of fifty years, most views held at present may be found in the literature of the twenties, although earlier ideas are usually described in less explicit form than those of today. The exchange capacity of the second c o m p o n e n t of the adsorption complex, humus, has been' described from the very beginning as resulting from a formation of salts of cations with the COOH- and OH-groups of humic acids. In case of humic compounds with small molecules these salts are often water soluble. In c o m p o u n d s with large molecules the salts are non-soluble. Salts of alkali metals and humic compounds with large molecules are highly peptized and therefore very mobile. Compounds of alkali-earth metals have little tendency for peptization. Contrary to the exchange capacity of the clay minerals, the adsorption capacity of humus is very much affected by the pH-value of the medium. In this respect the behavior of humus resembles very much that of a insoluble acid. THE EXCHANGE
CAPACITY
In the thirties many soil scientists paid attention to the exchange capacity of soils. From the point of view of the fertility status of soils, it is an important property. Through lack of a clear insight in the structure of a double layer and its effect on the exchange capacity, the attention was mainly centred on empirical formulae which would represent as much as possible the adsorption isotherm. Wiegner's school especially did much research work to establish the connection between the formulae and experimental data. There are in fact t w o models to be found in literature which appear in many variations. The first model is a logarithmic description showing that the amount of
286
adsorbed material ~ is proportional to the logarithm of the concentration of this material in the equilibrium solution (c}: logT=a+blogc This formula originates from Freundlich and was applied without essential alterations to soil studies by Wiegner and others. Sometimes more parameters are added for adjustment of the formula to the result of experiments ( R o t h m u n d and Kornfeld). The second model has its starting point in the assumption that the adsorption isotherm has the shape of a hyperbola. It originates from Langmuir who derived for the adsorption of gases, the relation between the adsorbed quantity 7 with the concentration of the gas in gaseous phase (c). This relation was represented b y the formula: b.c "y = F l+b.c
where F is the maximum amount to be adsorbed and b is a constant depending on the binding energy between the adsorbent and the gas. A largely identical formula for soils was derived by Vageler. This was not, however, based on a theoretical equilibrium between adsorbed matter and an e m p t y surface but used as a starting point the assumption that the adsorption isotherm would have the shape of a hyperbola. It is most remarkable that in all these formulations the fact that the adsorbed ion must replace a counterion was not taken into account. If the adsorption isotherm for ions is treated mathematically in the same way as done by Langmuir for gases, the following formula would appear: bt 01
?=r" blcl +b2e2
where 7 is the a m o u n t adsorbed; F is the maximum amount to be adsorbed; ot and c2 are the respective concentrations of ion 1 and ion 2 in the solution; and bl and b2 are constants, which depend on the binding energy of ions 1 and 2, respectively. In experiments carried o u t between 1930 and 1950 the fact that ion exchange is connected with the sort of the complementary ion to be exchanged finds its expression in the so-called lyotropic series. From many studies in this field of research, a series may be demonstrated as found by Jenny in his experiments with Putnam clay where the difficulty of exchange increased in the following order: Li
Rb< Cs
Ca< Si
La
Why all soils did not have exactly the same sequence within the series became clear when better formulations of the ion-exchange phenomena were made. At present, very little use is made of the above formulation. This is partly due to the fact that a direct determination of the adsorption capacity
287
has been made possible by the analytical techniques of percolation or multiple treatment b y a salt solution. Another reason is that considerations of relations existing between adsorbed quantities and concentration of the equilibrium solution are studied with different models. For some estimations (as, for instance, in column chromatography which are of sufficient accuracy in some cases) one can still successfully use these classic empirical equations. MODERN FORMULATIONS
Gradually, the empirical equations, partly derived from the Guldberg and Waage equation for ionic equilibria, were replaced by conceptions based on thermodynamics. This t o o k place mainly between the years 1950 and 1960 when many scientists adopted this approach following the work of Schofield (Babcock, Low, Coleman, Peech, Bolt, Laudelout). The starting point is the equation of a reaction, as for instance between a monovalent (+) and a bivalent (++) ion: exch(++) + s a l t ( + ) ~ 2 exch(+) + salt(++) that is:
(++)a + 2(+)o ~ 2(+)a + (++)o or
g(++)a + 2~(+)o = 2 p ( + ) a +/~(++)o and there: P =Po + R T l n a
(a+)2 a ~ K
(a'~)a
(a+)2o - -
(a~+)o
where t~ is the chemical potential, a the activity and K an equilibrium constant. The indices a and o indicate, respectively, the adsorbed phase and the solution. Although this formula is correct in its generality, it is -- due to this generality -- difficult to apply in a particular case. The reason is that the ion activities are difficult or impossible to measure. In a solution the activity may be measured with good approximation, or it may be calculated. For the adsorbed phase it is not measurable and cannot be calculated. When the activity in the above formula is substituted by concentration (c) with the corresponding activity coefficient (f), the following formula is obtained: •
(r) o
=K
(c+h "(f++h (a'~)o(t~)o
288 or ff+) o • (f+h----K
(C+)a
(f++)o "(f+)2a
(C+)o
The measured value of K is then not the thermodynamically correct one, but an empirical factor being the product of the real K value together with a number of unknown activity coefficients. Therefore, for practical application, it is better to use models and to introduce certain assumptions. One of the oldest models is the one of the Donnan equilibrium, originally elaborated for a system of t w o compartments separated from each other by a semi-permeable membrane. Mattson, Wiklander, Schuffelen and others have used this model for systems with a double layer. Its formulation reads: (C+)i
(C+)o
V(c+)i V(C+)o where c, the concentrations, and the indices, i and o, indicate respectively the double layer and the solution. In principle this model can be used to describe the effect of the valence of an ion in relation to the concentration of ions in systems with double layers, but it is usually insufficient for quantitative estimations (Schofield). The Donnan system has still the disadvantage that activity coefficients are u n k n o w n factors (Klarenberg). Another model is one in which activities of the ions in the adsorbed phase are supposed to be in the same proportion as the mole fractions. In fact, the double layer is hereby considered as a solid solution. Taking this principle as a starting point, Kerr drew up the following equations for equilibria between ions of the same valence: +
--=K
a n d - - =K - -
and Vanselow did the same for systems with ions of different valence:
--=K
P
3,++
In these formulae ~, represents the adsorbed quantity, c the ion concentration in the solution, and K and p constants. The third model has been derived from the build-up of the double layer as developed b y Schofield, Errickson and Bolt with the diffusion model of Gouy-Chapman as a starting point and an assumption of the Boltzman distribution of ions in the double layer. The final formulation is then:
289
,~,+
C+
=KG" 7 ~
X/C +*
a formula which, in fact, is identical with the equation given by Gapon in the thirties. This equation of Gapon describes quite well the results obtained in the absence of specific forces between the adsorbing material and adsorbed ions or in other words, when the adsorption is caused by Coulomb forces. KG is then a calculable function of the charge density. When the formula of Kerr is used, K values are found which in the lyotropic series range from a b o u t 1 to 5, going from Li to Cs. For divalent ions the selectivity constant ranges between 1 and approximately 2. Also by applications of the Gapon-equation the values for KG ranges between 1 and 2. However, this does n o t hold when specific forces have to be dealt with as it is the case with adsorption of potassium by mite. It is interesting to note that, in spite of the fact that good thermodynamical formulations have now been made available, for models one goes back to those which have been developed in the thirties or even earlier. Two examples of adsorbed ions may be given, which have continuously drawn attention during the period of the last 50 years. These are the pair of hydrogen and aluminium ions and the potassium ion. SOIL ACIDITY
After 1920 a sudden development occurred in the search for the origin of the acidity of soils. Before that time the acid properties were ascribed to humic acids and the alkaline reactions to the occurrence of calcium carbonate. Now aluminium and hydrogen ions entered the picture. The observation, made in the beginning of this century by Veitch and Daikuhara that soils show an acid reaction after addition of salt was confirmed and thoroughly studied, particularly by Kappen. For the first time Kappen mentioned exchangeable A1 ions which when hydrolized gave rise to the formation of acid: Al-clay + 3KC1 ~ K-clay + AIC13 A1C13 + 3 H 2 0 -* AI(OH)3 + HC1 Usually a reasonable agreement could be found between the quantity of A1 and titratable H ions in the KC1 extract, which fact seemed to support the hypothesis. In that period three types of soil acidity were distinguished, viz.: (1) The active acidity represented by the H ions, which could be titrated in a H20 extract and were supposed to originate from free acids and acid salts. The measuring of this acidity was soon replaced by determining the pH value.
290 (2) The exchange acidity, which is the a m o u n t of A1 ions exchanged by NaC1 and titratable after hydrolysis. (3) The hydrolytic acidity, or the titratable H ions in a Na-acetate extract. The release of those H ions was ascribed to an adsorption of NaOH by soil particles. All these "acidities" were determined by titrations with an alkali and thus are potential acidities, which only gradually replaced the determination of the active acidity described nowadays as the pH. The introduction of determinations of the degree of saturation (V) falls in about the same period. It is calculated as the quotient of the sum of the exchangeable bases (S) and the total adsorption capacity (T). The sum of exchangeable bases was determined by an extraction of the soil with a dilute acid followed by a back titration of the a m o u n t of neutralized acid (S). T was determined as S + H, where H was determined by a conductometric titration with Ba(OH)2 (Hissink). In this determination the Al ion was n o t taken into account. The H ion is mentioned as such and is put beside all other cations (S). A co-worker of Hissink, Van der Spek, also described soil acidity as a result of H ions: H-clay + KC1 -* K-clay + HC1 This leads to a discussion of whether it is the H ion or the AI ion which causes the acid reaction of the KC1 extract and also of the soil. Kappen and Trenel are the great advocates of the A1 hypothesis. Page, Bradfield and Wiegner support the H hypothesis. In 1931 Kappen accepts the hydrogen hypothesis. The support for the H hypothesis arose partly from the so-called suspension effect, a phenomenon described by Bradfield and Pallman. Their investigations showed that the pH value of a soil suspension depends on the concentration of the suspension. In short, the measured concentration of the H ions appeared to be proportional to the concentration of the suspension. On the one hand, this indicates a minor role of aluminium and, on the other hand, it showed t h a t the H ion concentration in the double layer had an effect on the measured pH. Wiegner described it also as an effect of the double layer. Others ascribed it to a kind of Donnan equilibrium, but all the explanations were rather indefinite. The situation changed when the statement was put forward that, from the point of view of thermodynamics, there could be no suspension effect in the sense of the suggested effects of the double layer (Du Rietz, Rabinowitch, Kargin). When considered thermodynamically in an equilibrium situation, the activity of the H ions in the double layer must be equal to that in the equilibrium solution and, as the pH is measured by a reversible H electrode, no difference in activity can be measured. This could be easily confirmed by changing the position of a reversible H electrode and of a non-reversible KC1 bridge to a calomel electrode within the suspension and the supernatant solution. These experiments showed that the suspension
291 effect originated from the KC1 bridge and n o t from the H electrode. This had already been suggested in 1942 by Loosjes b u t his explanation of this effect as being a capillary effect could n o t be maintained. The suspension effect is caused by a diffuse potential arising between the KC1 and the suspension. This potential has been treated theoretically by Overbeek. In the fifties and sixties much attention was paid to this p h e n o m e n o n by Jenny, Coleman, Bolt, Babcock, Davis, Overstreet, Peech and others. It is quite certain that the last word on this question has not y e t been spoken, because the suspension effect was also demonstrated b y Pallmann through measuring the H ion concentration by the rate of inversion of sugar. The concentration of H ions calculated from the rate of inversion appeared to be directly proportional to the concentration of the suspension. Improvements of pH measurement--- starting from the slowly acting hydrogen gas electrode, via the faster quinhydrone electrode to the quickly working glass electrode -- made it possible to measure the H ion activities on a large scale and to switch over from the titrimetric acidity determination to electrometric measurements. This development again diverted attention from the A1 ion, although it was known that the addition of KC1 to the soil led to the so-called salt effect forming A1 and Fe salts. This p h e n o m e n o n was explained by the assumption that the addition of salt would lead to such a high concentration of H ions within the double layer that the clay mineral would be attacked under liberation of A1 and Fe (Hudig). In the forties the opinion that AI ions could also be adsorbed was again put forward. This differs from the former opinion, where i t w a s assumed that adsorption of A1 ions would lead to an extension of the crystal lattice, analogous to the growth of the AgI crystal; according to this conception A1 could not take the place of a counterion. Paver, Marshall, Schofield and Chernov had in the forties taken the view that acid clays could contain exchangeable H ions and A1 ions as well. Russell's b o o k has again drawn attention to this problem by his treatment of the Schofield hypothesis. Here it is not only the A13÷ ion b u t also the hydroxy-ions AI(OH) 2÷ and AI(OH)~ which can be exchanged and so are introduced as counterions. Mainly b y interpretation of titration curves of acid clay minerals, Coleman and Low had come in the sixties to the conclusion that acid soils contained aluminium in the double layer in one of its ionic forms. This concept comes to a development where it is assumed that exchangeable H ions in artificial H clays are gradually replaced by A1 ions. The kinetics of this reaction have been formulated b y Kerr, Laudelout and co-workers. Jenny draws attention to the fact that when AI(OH) complexes can be adsorbed a step is made again in the direction of Mattson's hypothesis of mixed gels. On the other side, aluminium adsorption may be considered as a sort of crystal growth which again leads to the hypothesis that A1 ions should n o t be seen as real counterions b u t as ions which form part of the crystal. Here again, it appears that old conceptions tend to reappear often in a somewhat modified form. This is often the case, n o t only in the sphere of soil chemistry.
292
P O T A S S I U M FIXATION
The next ion getting much attention is the potassium ion because of its exceptional position in exchange phenomena. Some softs have a tendency to bind potassium in such a specific way that their exchange for other ions becomes difficult or even impossible. In the thirties this phenomenon was designated as potash fixation, a name which it has kept until now. Originally this phenomenon was observed in the field. Some soils, known as being poor in potash, did not respond to potash dressings. When potassium was added to these softs in the form of a solution of a potassium salt, it appeared that it could be fixed in a not directly exchangeable form, especially after drying (Volk). Since then, different explanations of this phenomenon have been given in the course of the years. About 1940 it was established that fixation of potassium was not caused by organic constituents of the soil (Joffe, Kolodny, Bakhulin, Hauser) but bound to the inorganic part. Different hypotheses were put forward to explain this phenomenon. Gorbunov supposed that during drying the K ions are so far dehydrated that they become, as it were, stuck within the Helmholtz-layer. He also assumed a sort of sticking of soil particles during drying. Both phenomena would lead to an irreversible fixation of potassium ions. Volk assumed a formation of new muscovite-type minerals formed out of the AI(OH)3 and SiO2 present in the soil together with potassium ions. It was also suggested that small micaceous fragments poor in potassium, would grow into larger particles in t h e presence of potassium ions, which then would be taken up in the crystal lattices (Schuffelen), Later investigations tried to find a connection between potassium fixation on the one side and the presence of certain clay minerals on the other. Efforts have often been made to distinguish wet fixation, occurring in soil particles with a high fixing capacity, and dry fixation, as found in nearly all soils (Hauser, Van der Marel). For montmoriUonite the fixation depends on the specific type of clay mineral, the beidellite form showing the highest degree of fixation (Drosdorff, Truog and Jones, Jackson and Hellman). mite has a very strong fixing capacity (De Turk', Stanford, Wear, Wiklander, Van der Marel). Also vermiculite may show a strong fixation of potassium (Gruner, Barshad, Walker, Coleman). Other minerals such as biotite (Barshad), glauconite (Chaminade) and phosphates can fix potassium (Joffe). A very systematic investigation on this subject was carried out by Van der Marel, who under standard conditions, examined the capacity for potassium fixation of many minerals. Fixation is connected with the amount of the fraction < 2p and is usually enhanced by drying, by increased concentration of the potash salt solution and by increase of the pH. A number of hypotheses are then put forward to explain this phenomenon. It is stated that there is a connection between the different forms in which potassium is present in the soft:
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K in solution ~- K exchangeable ~- K in fixed position ~ K within the mineral. The reaction between K in solution and K in exchangeable form is reversible and comes quickly to an equilibrium; of both the other equilibria the third takes place quickly to the right side b u t slowly to the left side. The last equilibrium moves only slowly in either direction, although plant roots are able to take K out of the minerals (Schachtschabel, Reitemeier). According to Chaminade no potassium fixation occurs when 4.5% of the total exchange capacity is covered by exchangeable potassium. During the fifties several types of potassium fixation were discerned. In one t y p e potassium ions penetrating between the lattice plates are trapped after contraction of the plates, and the potassium ions then become nonexchangeable. This form occurs in the case of illite, some types of montmorillonite and vermiculite. Another type of fixation is found in nontronite, beidellite and some intermediates. Here the K ions are strongly b o u n d by sites in the lattice which have a strong negative charge caused by A1--Si substitution and where K ions fit very well spatially. The third t y p e is found in minerals which have become poor in potassium by weathering and show " o p e n K sites" which are then taken by K ions. This t y p e of fixation may be found in soils of Norway and Finland. The fourth t y p e of fixation is found in some zeolites and permutites in which narrow pores cause very slow cation exchange. These observations have been compiled by Schuffelen and Van der Marel. During the years since 1960, a discussion has started of mechanisms of potassium fixation n o t based on experimentally established data b u t on the build-up of minerals. The electron-microscope proved to be a very valuable aid in checking some of the theoretical deductions. New viewpoints have been opened by the compilation research of Reitemeier, Arnold, Jackson and others. Many points have been clarified by the investigations of Rich and Graf yon Reifenbach. The problem of potassium fixation has also been approached from quite a different point of view. Determinations of selectivity constants (KG) showed that when the K ion and, for instance, illite were considered, the " c o n s t a n t " was n o t constant at all, b u t showed a strong variation depending on the amount of adsorbed potassium. When large amounts of exchangeable K were present, the value of KG amounted to about 2, a value of the same order as found for other ions, b u t with only a little K in exchangeable form, the value of K could increase to a b o u t 40 (Van Schouwenburg, Schuffelen, Schwertmann). By careful analysis the connection between the a m o u n t of exchangeable K and the selectivity constant could be elucidated. If three different sites are assumed on the particle, where potassium may be adsorbed -- viz. the planar surface, the corners of the crystal and between the layers of the lattice -three values of K G could be calculated, each being independent of the number of sites taken by the potassium ions. The values of KG amounted respectively to 2, 100 and ~ for the planar surfaces, the edges and the interlattice sites of an illite. This hypothesis fits well with ideas on the structure of this
294 mineral, ideas formed on the basis of mineralogically oriented research. Beside potassium, NH4 ÷ ions, Rb ÷ ions and Cs ions are fixed by the soil. NH4 fixation in the field is only temporary because of subsequent nitrification. Fixation of Cs is important because this ion appears as a product of fall-out and can be withdrawn by fixation from the nutritional chain (Scott-Russel). EXCLUSION OF ANIONS As soil particles show a net negative charge, they will tend to exclude anions from the surface. When moving in the diffuse double layer from the solution towards the wall of the soil particle, there is a steady decrease of room for the anions. This is called anion exclusion or negative adsorption. As early as the twenties it was known from measurements that this exclusion must exist; it was then indicated by the term "salt-free layer" (Trofimov). This image fitted in very well with the Donnan conception where abrupt changes are considered rather than diffuse transitions. Also Alten and Vageler mention the salt-free water layer. A more quantitative treatment of the anion exclusion, taking as a starting point the double layer of Gouy-Chapman, has been given in the fifties by Schofield for a system with only monovalent ions. Later this work has been extended by Bolt, Warkentin and De Haan for systems containing ions of different valences. As there are no specific forces acting in the anion exclusion, this phenomenon can be applied successfully to determining the specific surface of the adsorber. It has often been used for this purpose. A particular case of anion exclusion exists for phosphate. This ion may also be adsorbed by the positively charged sites of the clay, so that measurements represent a net result of exclusion and adsorption. Careful theory and experimentation have enabled De Haan to unravel both the phenomena. PHOSPHATE Contrary to the studies of the adsorption of cations and exclusion of anions, which were strongly directed towards a better knowledge of the content and development of physico-chemical hypotheses, the studies of phosphate remained for a long time in the empirical stage. The reason is that the phosphate status of softs usually formed the main object in these studies. Efforts have always been made to find new and better methods of extraction in order to determine the phosphate-fertility status of soils. Emphasis was laid on establishing correlations between the determined quantity of phosphate and yield, mainly as a basis for phosphate-fertilizer application. The extraction media ranged from water, via weak acids or buffer solutions to strong acids (Dirks, Scheffer, De Vries, Schachtschabel, Bray, Olsen, Truog, Van der Pauw, Zinzadze). All these extractions remain conventional means for determining the phos-
295 phate content and their merit depends solely on the degree of correlation with results of field and pot experiments. They give only little information about the nature of phosphate compounds present in the soil. About 1930 the first compilation of a research into the relation between the solubility of phosphate and soil acidity was put forward (Gaarder). This led to recognition of the difference between the A1 and Fe phosphates occurring under acid conditions and of Ca and Mg phosphates present in alkaline media. This early research may be considered as pilot studies for the selective extraction introduced later by Chang and Jackson. A number of different forms of phosphate present in the soil could be discerned by this method, viz., the easily soluble phosphate, the calcium phosphates, the aluminium phosphates, the iron phosphates, and the occluded phosphates. This method has shown that aluminium phosphates show the largest decrease in concentration when the soil is covered by vegetation. This was a surprising result because it was mostly assumed that calcium phosphates were mainly taken up by the plant under the influence of the acid-producing root. Another, but also analytical approach came into use when the radioactive isotope( 32P) became available. By using the system of isotopic dilution, it was possible to determine the phosphate pool of soils (Fried, Dean, Larsen, Scott-Russel). But as the radioactive phosphate and a number of phosphates forming part of that in the soil come very slowly to an equilibrium, this method also has to be qualified as a conventional determination of a value called labile phosphate. Neither has the method, where the rate of isotopic exchange is measured, given more insight in the behavior of phosphate in the soil (Uhlrich, Scheffer, Sissingh). All this is obviously connected with the many-fold forms of phosphate which collectively form the soil phosphate. Not only organic phosphorus but also adsorption of phosphate by sesquioxides and clay minerals, precipitation of unsoluble compounds, and inorganic phosphates make the whole problem very difficult to unravel {Williams). Actually, only specialized investigations of the properties of its components may at present be qualified as exact research. Little research has been done on organic phosphate, although it amounts to about one half of the total quantity of phosphate present in the soil. The main constituents are nucleic acids showing a fast turnover, phosphatides and the phytin or inositol hexa-phosphate with very little turnover (Peech). Inorganic phosphates were investigated rather intensively in the sixties. Although a total of about 40 different compounds have been found in the soil," properties of only the three most important ones have been studied. After preliminary experiments in the fifties in the study of the relations between calcium, iron and aluminium phosphatesand their transitions (Rathje), this work was extended in the sixties and, based on the phase rule, it is making good progress. Taking as starting point the solubility products of different phosphates and hydroxides of iron and aluminium, one can represent in clear diagrams
296
the theoretical relations between the concentration of phosphate in solution, the nature of the solid phase and the pH (Lindsay and co-workers). The experimentally obtained data for soils appear to be either in good or bad agreement (Peech). This is caused by the slow rate at which the equilibrium conditions are reached. Research into the rate of dissolution has been started but has n o t as y e t been sufficiently elaborated. Here the gap existing in the research carried out so far comes clearly to the fore. It is the lack of data of rate constants which form a bottle-neck for more sophisticated prognoses. These rate constants do not play a role of great importance for practical application of exchange equilibria because of their magnitude. In the case of potash fixation.and potash release, they are much smaller and are n o t sufficiently investigated. Our knowledge of the constants in the case of phosphates is far from being complete and it is this lack of knowledge which makes it impossible as y e t to apply the theory quantitatively for practical purposes. In the last period of soil chemistry research t w o quite new aspects have come forward. These follow from the observation that a number of ions in the soil are b o u n d to humus in the form of chelates (Scheffer, Lindsay). This means that the availability of these ions for plant roots is not dependent anymore on the classic ionic equilibria alone. The total effect of adsorption, precipitation and chelating are points to be raised for further research. The fact that transport of ions to the plant r o o t and into the deeper layers can be described by a double process (viz. flux and diffusion) has drawn attention during the last years (Barber, Quirk, Nye, Tinker, Frissel). The consequence of this is as y e t quite incalculable. Here as well the lack o f rate constants is clear. The processes which are often described statically have to be treated kinetically. A survey of 50 years of research cannot be complete unless it is written by a professional historian. Selection of high-lights by a researcher is always arbitrary because findings that made the most impression on him and all those in which he has himself participated come to the fore automatically. I hope that, bearing this in mind, the reader will still obtain some picture of the research in soil chemistry during the last half century. SELECTED REFERENCES Bear, F.E., 1955. Chemistry of the Soil. New York. Bolt, G.H., 1967. Cation exchange equations used in soil science -- A review. Neth. J. Agric. Sci., 1 5 : 8 1 Bradfield, R., 1923. The chemical nature of a colloidal clay. Univ. Mo., Agr. Exp. Station Res. Bull., 60 Chernov, V.A., 1947. The Nature of Soil Acidity. Academy of Sciences of the U.S.S.R., Moscow Flaig, W., 1967. Ver~nderungen am Lignin und Anlagerung yon Stickstoff-haltigen Verbindungen zu Beginn der Bildung und bei der Nutzung yon Torf. Landbauforsch. VOlkenrode, 1 7 : 1
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Gapon, E.N., 1933. The theory of exchange adsorption in soils. U.S.S.R.J. Gen. Chem., 3 : 1 4 4 (in Russian) Gedroitz, K.K., 1929. Die Lehre vom Adsorptionsverm0gen der Boden. Dresden und Leipzig Hissink, D.J., 1928. Beitrage zur Frage der Bodenadsorption. Soil Res. (Suppl. Proc. Int. Soil Sci. Soc.), 1 : 4 Jenny, H., 1961. Reflection on the soil Acidity Merry-Go-Round. Proc. Soil Sci. Soc. Am., 25:428 Marshall, C.E., 1949. The Colloid Chemistry of the Silicate Minerals. New York Mattson, S., 1931. The laws of soil colloidal behavior, VI. Soil Sci., 3 2 : 3 4 3 Oddn, S., 1922. Die Humins~uren. Dresden und Leipzig Schofield, R.K., 1947. A ratio law governing the equilibrium of cations in solution. Proc. Int. Congr. Pure and Applied Chem., 11 th, London, 3 : 2 5 Schuffelen, A.C. and Van der Marel, H.W., 1956. Potassium fixation in soils. Potassium Congress, Rome, 1955, 157 Wiegner, G., 1929. Boden und Bodenbildung. Dresden und Leipzig