EXCHANGE EQUILIBRIA OF POTASSIUM VERSUS CALCIUM AND SODIUM IN SOILS FROM A SEMIARID REGION, INDIA

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Abstract
The K-Ca and K-Na exchange equilibria on 3 soil samples differing in their organic C content (OC) and cation exchange capacity (CEC) were studied. Homoionic (Ca or Na saturated) soils were equilibrated, with the solutions having a large range of K adsorption ratios (PAR) at 50 meq(L)-1 total electrolyte concentration. The data were analyzed on exchange equilibria, using a thermodynamic approach and various selectivity coefficients. The normalized exchange isotherms for K+-Ca2+ systems suggested higher specificity for K+ to Ca2+ in the first 50-75% of K+ saturation. In the K+-Na+ system the specificity for K+ to Na+ was, however, higher for the whole range of exchange isotherms. The soil high in OC (Soil 1) had higher preference for K+ than the soils low in OC (Soils 2 and 3). The values of standard free energy change of the exchange reactions (.DELTA.Gr0) for K+-Ca2+ and K+-Na+ systems were negative for all 3 soils tested. These values were, however, more negative for K+-Na+ than for K+-Ca2+ systems, suggesting higher preference for K+ in the former. A comparison of .DELTA.Gr0 values suggested that the effect of CEC on the K+ specificity of these soil samples was only marginal, though that of OC was quite spectacular. The values of the Gapon selectivity coefficient (KG) decreased first, sharply, and then gradually with the increase in exchangeable K percentage (EPP). On the basis of (KG), the exchange isotherms were divided into 3 zones, representing exchange sites with high, medium and low specificities for K+. The arbitrary limits for the 3 zones were < 5, 5-38 and > 38; < 4, 4-15 and > 15; and < 5, 5-15 and > 15 for Soils 1, 2 and 3, respectively. The exchange selectivity coefficient (KN) and .DELTA.Gr0 for K+-Na+ systems were different only marginally for Soils 1 and 2. However, for K+-Ca2+ systems, the values of different selectivity coefficients (KG, Vanselow selectivity coefficient, and Krishnamoorthy-Davis-Overstreet selectivity coefficient) and .DELTA.Gr0 for Soils 1 were considerably different from those for Soil 2. This suggested a differential role of OC on changing the selectivity of soil for K+ in K+-Ca2+ and K+-Na+ systems.