We studied the potassium-calcium and potassium-sodium exchange equilibria on three soil samples differing in their organic carbon content (OC) and cation exchange capacity (CEC). Homoionic (Ca or Na saturated) soils were equilibrated, with the solutions having a large range of potassium adsorption ratios (PARs) at 50 meq L-1 total electrolyte concentration. We analyzed the data 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 to 75 percent 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 (ΔGr°) for K+-Ca2+ and K+-Na+ systems were negative for all the three 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 ΔGr° 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 potassium percentage (EPP). On the basis of (KG), the exchange isotherms were divided into three zones, representing exchange sites with high, medium, and low specificities for K+. The arbitrary limits for the three zones were < 5, 5 to 38, and >38; > 4, 4 to 15, and >15; and > 5, 5 to 15, and > 15 for Soils 1, 2, and 3, respectively. The exchange selectivity coefficient (KN) and AGr° for K+-Na+ systems were different only marginally for Soils 1 and 2. However, for K+-Ca2+ systems, the values of different selectivity coefficients (viz., KG, Vanselow selectivity coefficient, and Krishnamoorthy-Davis-Overstreet selectivity coefficient) and AGr° 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.
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