POTASSIUM-CALCIUM EXCHANGE EQUILIBRIA IN SOILS: THE LOCATION OF NON-SPECIFIC (GAPON) AND SPECIFIC EXCHANGE SITES

Several studies (e.g. Bolt et al., 1963; Beckett, 1964a) of the exchange equilibria between K and Ca on soil colloids or pure clay minerals have drawn attention to the presence of sites with different affinities for K. There is so far no evidence to indicate the location of sites with the highest affinity, though several workers (e.g. van Schouwenburg and Schuffelen, 1963) have assumed that they lie on the edge-faces of stacks of clay plates, or in the spaces between the expanded leaves of partially weathered stacks of clay plates. Earlier work has resolved the K: Ca exchange isotherms of soils and clays into a curved part at low values of aK/√aCa, attributed to exchange at sites with a high specific affinity for K, and an upper linear part commonly described by the Gapon equation and attributed to non-specific sites. The present work on soil clays and clay minerals shows that Na hexametaphosphate or changes in pH affect the curved but not the linear part; cetyl trimethyl ammonium bromide and changes in the amount or charge of exchangeable Al affect the linear but not the curved part of the isotherms. At low pH, the linear but not the curved part of the kaolinite isotherm obeys Schofield's Ratio Law. Grinding has more effect on the curved than on the linear part. So the specific sites are attributed to the edges or peripheral interstices of stacks of clay plates, and the non-specific sites to their planar surfaces. The specific sites take up K more slowly from solution than the non-specific sites. The isotherms of completely dispersed bentonites have no curved part. The specific sites are attributed to the wedge-shaped interstices opened between clay plates by weathering, from which exchange is diffusion-controlled. Added organic cations reduce the numbers of both kinds of site; peroxide treatment increases them. This paper confirms that the exchange sites with highest affinity for K are indeed associated with the edges, rather than the faces, of stacks of clay plates.     

SODIUM-CALCIUM EXCHANGE EQUILIBRIA IN SALT-AFFECTED AND NORMAL SOILS

Sodium-calcium exchange equilibria in soils with a range of cation exchange capacity (CEC) and surface charge density (SCD), showed calcium preference whether the results were interpreted by a thermodynamic treatment, by an electric diffuse double layer equation, or by an empirical approach. Surface soils preferred Ca more than the corresponding sub-soils, which was mainly attributed to their higher SCDs and organic carbon contents (OC). Significant correlations between OC and (ΔG_o)c_a→_Na, and OC and SCD, suggest the effect of OC on Ca preference to be mainly through increased SCD rather than to specific ion exchange sites. Significant correlations between ΔG_o and SCD, ΔG_o and K_G (the Capon constant), and between corrected SCD and K_G, suggest that all three approaches can be used to describe Na:Ca exchange equilibria in these soils, and that the equilibria are related to intrinsic soil properties.     

SOLUBLE ALUMINIUM AND CALCIUM-ALUMINIUM EXCHANGE IN RELATION TO THE pH OF DILUTE CALCIUM CHLORIDE SUSPENSIONS OF ACID SOILS

Measurements of pH and A1 concentration were made on 10^-2 M CaCl₂, suspensions of a number of acid soils that had been limed to give 3 range of pH values, and exchangeable A1 and Ca+Mg were determined in 1.0 M NH₄Cl extracts. The variation of pH with A1 concentration did not support the theory that pH is controlled by the solubility of Al(OH)₃. For some of the soils, proton release on hydrolysis of A1₃₊ions in solution accounted for the pH values, and explained quantitatively the variation of pH with the Ca:Al balance of the exchange complex, taking account of the selectivity coefficient for exchange, K_ca→A1 Although K_ca→A1 was smaller for soils containing more humus, their pH values were also less than those predicted by the hydrolysis of A1³⁺ in solution, indicating that they contained other sources of protons, presumably the carboxyl groups in humus.     

POTASSIUM-ALUMINIUM EXCHANGE IN ACID SOILS I. KINETICS

A kinetic method is described and investigated for determining exchangeable Al in acid soils using M NH₄Cl solutions the pH and leaching rate of which was critical for obtaining accurate and reproducible values. Molar concentrations of ammonium acetate displaced at least part of the Al chelated to functional groups in the soil organic matter. The adsorption kinetics of K and Al from 10^?3 M chloride solutions on the NH₄-saturated forms at the original soil pH of these, under leaching conditions similar to the ‘extraction’ method, showed that, when the atom ratio K/A1 > 1 in the equilibrating solution, initially more K was adsorbed than at equilibrium, although Al did not ‘over-equilibrate’ when K/A1 < 1. At least 24 h were required for attaining equilibrium in K :A1 equilibria work with soils and clays. The kinetics of desorption (extraction method) and adsorption of K and Al obeyed the first-order and parabolic rate respectively. This is attributed to the large difference in anion concentrations in the two methods.     

DIFFUSE DOUBLE LAYER THEORY APPLIED TO Na-Ca EXCHANGE EQUILIBRIA IN TEMPERATE AND SEMI-ARID TROPICAL SOILS

A cation exchange equation based on diffuse double layer (DDL) theory was tested on 26 surface and sub-surface soils from 6 field experiments in temperate and semi-arid tropical regions. Sodification (the fraction of total charge on the surface neutralised by the excess of Na⁺ plus the deficit of Cl^?), or the exchangeable Na percentage, ESP, of these soils in relation to SAR, the molar ratio (Na]/[Ca + Mg] ^1/2, was evaluated from the observed data on Na:Ca exchange in two ways, using the DDL equation: (1) by multiplying the surface charge density of the soil with a mean correction factor f (based on the whole isotherm) assuming no interaction between adjacent clay platelets; this predicted the sodification of the soils satisfactorily between 0–30 ESP, though at the highest SAR values (i.e. > 60), predicted ESP values were significantly smaller than the experimental values for 23 of the 26 soils; (2) by assigning values to the extent of interaction Y_d (directly related to the electric potential Ψ_d midway between adjacent clay platelets) over the whole isotherm; Y_d was found to increase by varying degrees in different soils with SAR and ESP. Also in 12 of the 15 pairs of soils compared, the mean value, _d, over the whole Ca-Na exchange isotherm was appreciably larger for surface soils (which had higher surface charge densities) than for the subsoils from the same sites. The suggestion that the Y_d parameter provides a better criterion than the f parameter for characterising and comparing Na:Ca exchange equilibria in contrasting soils is discussed in relation to the effects of soil components.     

THE OXIDATION OF IRON SULPHIDES IN SOILS IN RELATION TO THE FORMATION OF ACID SULPHATE SOILS, AND OF OCHRE DEPOSITS IN FIELD DRAINS

Aerating pyritic soils causes acidification and the forrnation of acid sulphate soils, or cat-clay. The Oxidation of pyrite in soils is associated with the deposition in tile drains of a form of ochre quite distinct from that formed by the action of filamentous iron bacteria. Pyrite-derived ochre results from the action of Thiobacillus ferrooxidans, which, below pH 3.5–4.0, catalyses the Oxidation of Fe²⁺ and pyrite. In soils less acid than c. pH 4, pyrite oxidizes relatively slowly by chemical reactions to Fe²⁺ and SO²₄^?. Under these conditions iron enters the drains as Fe²⁺ and is there oxidized by T. ferrooicidans and deposited as hydrated ferric oxide. Once the soil becomes acid enough for T. ferrooxidans to multiply, the rate at which pyrite oxidizes increases several-fold, and at c. pH 3 iron appears in the drainage water in the ferric form. Liming seems to decrease the rate of Oxidation.     

THE TIMES REQUIRED FOR THE ATTAINMENT OF AIR-WATER EQUILIBRIUM IN CLAY SOILS

The coefficient of diffusion of air through the pore water of clay soils has been measured directly and has been shown to be markedly less than for free water and to decrease with water content. It has been shown that these low coefficients of diffusion are not accounted for by the sinuosity of the flow channels, or by any important departure from Henry's law of solution at curved air-water interfaces. The main factors appear to be constrictions in the flow channels and the reduced viscosity of the adsorbed water. It is concluded that air-water equilibrium in partly saturated soils will be attained much more slowly than is generally assumed.     

VARIABLE ATMOSPHERE DTA IN IDENTIFICATION AND DETERMINATION OF ANHYDROUS CARBONATE MINERALS IN SOILS

The value of modem commercial DTA equipment in the identification and determination of anhydrous carbonate minerals can be greatly improved by incorporating a controlled-atmosphere (carbon dioxide) technique. The unproved peak definition and increased height enables the detection of carbonate minerals at concentrations of the order of 0.25 per cent by weight in a 50 mg sample. Further, the related changes in temperature of the decomposition reaction peaks provide additional information for the identification of carbonate mineral reactions. The method has particular application in mineralogical and pedological investigations.     

THE EFFECT OF THE CONCENTRATION AND MOVEMENT OF SOLUTIONS ON THE SWELLING, DISPERSION, AND MOVEMENT OF CLAY IN SALINE AND ALKALI SOILS

Changes in permeability of a soil containing montmorillonite, and the swelling of orientated aggregates of extracted clay have been measured for a range of exchangeable sodium percentages and electrolyte concentrations. The concentrations at which clay moved out of the soil in the percolates have been found and compared with the concentrations at which aggregates dispersed in stationary solutions, and the concentrations at which soil suspensions dispersed when shaken. The results indicate that: (a) permeability begins to decrease at the same concentration as the clay begins to swell, (b) the changes in permeability are directly controlled by the swelling of clay until clay dispersion and movement begins, (c) the concentration at which clay disperses depends on the mechanical stress applied, (d) when small mechanical stresses are applied, the proportion of the clay which swells and disperses depends directly on the exchangeable sodium percentage, and (e) large mechanical stresses may disperse most of the clay even at low exchangeable sodium percentages.     

ADSORPTION OF CADMIUM AND ITS EXCHANGE CHARACTERISTICS IN SOME ISRAELI SOILS

Cd adsorption isotherms were measured on five Israeli soils ranging in specific surface area from 65 to 315m²/g. Retention capacity of Cd ranged from 4.7 × 10³ to 10 × 10³μg/g soil (8.4 to 17.9 mequiv/100 g soil) and was correlated with the specific surface area. Almost all the adsorbed Cd was exchangeable with 0.5 M CaCl₂ and the remaining fraction was extractable in 0.5 M HCl. Plots of K_d, the distribution coefficient of Cd between the solid and solution phases, vs. Γ, the surface density of adsorbed Cd, showed that all soils behave similarly, K_d sharply diminishing with Γ. It was suggested, based on comparison with literature data on Cd adsorption on montmorillonite, that below γ s 0. 5 × 10¹⁷ ions/m², specific adsorption mechanisms prevail whereas above it the adsorption is mainly due to electrostatic interactions and ion exchange.     

THE MINERALOGY OF THE CLAY FRACTION FROM BASALTIC SOILS IN THE GALILEE, ISRAEL

The mineralogy of the clay fraction from basaltic soils in the Galilee, Israel, has been studied by X-ray and differential thermal analysis techniques, supplemented by electron-micrographs and chemical determinations. The mineralogical composition of the clay was greatly influenced by the amount of rainfall. In SE Galilee, where rainfall is 400–550 mm/annum, the dominant clay mineral is montmorillonite, with kaolinite as the second most important component. With increasing rainfall the amount of montmorillonite in the clay falls, and the amount of amorphous oxides of Fe and Al increases. In N and NE Galilee, rainfall is 550–700 mm/annum and the major part of the clay is composed of kaolinite or halloysite, quartz, and amorphous oxides of Fe and Al. The decrease in the amount of montmorillonite with increasing rainfall is explained by decomposition to kaolinite and amorphous oxides.     

THERMODYNAMICS OF K–Ca ION EXCHANGE IN SOILS1

Cation exchange characteristics of the K:Ca saturated forms of five soils were measured at 25°C and 50°C. The rates of isotopic exchange of ⁴²K and ⁴⁵Ca were too fast to be measured except that of ⁴²K in the K:Ca Harwell soil at 25°C. The slower isotopic exchange of K in this soil was attributed to the presence of a zeolite, clinoptilolite. The intra-particle diffusion coefficient, D_i, of K in this soil increased with K-saturation to a maximum at about 40 per cent K, probably because of the ‘blocking’ action of the larger hydrated Ca ions at small K-saturations in clinoptilolite. The CEC, measured by isotopic exchange along the K:Ca adsorption isotherm, decreased with increasing temperature probably because some interlayer spaces collapsed. The standard free energy, enthalpy, and entropy changes were negative for the reaction Ca-soil+2K⁺? 2K-soil+Ca⁺⁺. These results seem to show that K is more strongly bound than Ca by the soil and that the Ca-preference shown by the isotherm at small external electrolyte concentration is caused by entropy changes in solution. Calculated activity coefficients of the exchangeable ions changed with K-saturation similarly at both temperatures but values at 50°C were smaller than at 25°C.     

THE INFLUENCE OF MAGNESIUM IN SALINE AND SODIC SOILS: A SPECIFIC EFFECT OR A PROBLEM OF CATION EXCHANGE?

Na-Mg and Na-Ca exchange isotherms were determined at electrolyte concentrations of 500, 50, 5 and 1 meq per litre for illite, vermiculite and montmorillonite and for three soils containing illite and montmorillonite. This enabled comparisons to be made of clay swelling, dispersion and soil hydraulic conductivity changes between the Na-Mg and Na-Ca systems at known SAR, ESP and electrolyte concentration. Na-Mg montmoriUonite and a montmorillonitic soil behaved identically to the Na-Ca systems at the same ESP and electrolyte concentration: there was no specific effect. At the same SAR, the higher ESP in the Na-Mg system caused greater changes than in the Na-Ca systems. The Na-Mg vermiculite, illite, illitic soil and mixed illite-montmorillonitic soil showed greater changes than the Na-Ca systems at the same ESP, and there was a specific effect. At the same SAR, the higher ESP in all the Na-Mg systems apart from vermiculite increased the differences, but for vermiculite with a lower ESP, the differences were reduced. The lack of a specific effect for montmorillonite is probably related to the demixing of cations with the divalent ions concentrated on the non-swelling (internal) surfaces of the crystals.     

STUDIES OF SOME CRACKING CLAY SOILS IN THE LAKE CHAD BASIN OF NORTH EAST NIGERIA

The pedological characteristics, and physical, chemical and mineralogical properties are investigated for soils developed in a 1 m to 1.5 m layer of lacustrine clay material deposited on a former sand plain in the Lake Chad basin in north east Nigeria. (These soils are being developed for intensive production of rice, wheat and other crops under irrigation.) The surface layer of these soils has about 60 per cent clay, a pH of about 7.9, and a coarse, strong prismatic structure. The sub-soil is often notably higher in clay content, pH and exch. Na. The smectite content of the clay fraction (surface soil) is 35 to 45 per cent and the CEC 350 meq kg^-1. The levels of montmorillonite and CEC, and the size of the cracks in dry soil, although the latter are appreciable, are all notably less than the values reported for the clay soils of the Sudan Gezira. The Lake Chad basin clay soils are not considered to be vertisols according to the FAO Soil Map of the World, on grounds of profile morphology, though it seems the French pedologists would regard them as paravertisols.     

THE MEASUREMENT AND MECHANISM OF ION DIFFUSION IN SOILS

Diffusion coefficients of soil phosphate calculated from flux to chloride form of anion-exchange resin-paper, assuming total depletion of the labile pool at the boundary, were too small and unrealistic. Thus not all the phosphate in the labile pool contributed to the diffusion process while being desorbed at constant pH in the presence of an indifferent anion—Cl—from the resin-paper. Desorption relationships under these conditions, using CaCl₂ at atmospheric CO₂ pressure, were markedly different from the relationship between the long-term ³²P-exchangeable phosphate and the corresponding pore-solution concentrations. In the same soil containing different amounts of labile phosphate, a different desorption relationship was found for each phosphate level. The constant proportionality between amount of phosphate diffusing into resin-paper and square root of time was indicative of a constant concentration at the boundary. The resin-paper: soil system was therefore considered as an infinite composite medium in which diffusion through both resin-paper and soil were rate-limiting. The constant boundary concentrations were estimated by the use of the diffusion coefficients in the resin-paper, the phosphate adsorption isotherm for the resin-paper and the desorption relationship for each phosphate level in the soil. Diffusion coefficients, calculated using the boundary concentration appropriate to each phosphate level, were related to the slopes of the corresponding desorption relationships, resulting in values of the impedance factor similar to those found for K diffusion under similar conditions. The resin-paper method, however, does not provide an accurate enough measure of the diffusion coefficient of soil phosphate to be of any practical use. Until better and simpler methods are found, the diffusion coefficient may be calculated using the slopes of the desorption relationship and the separately determined value of the impedance factor.