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 (AG,)c~+N~, 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 AGO and SCD, AGO and KG (the Gapon constant), and between corrected SCD and KG, 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.

Exchange behaviour of potassium versus magnesium was studied on surface soil samples of 3 Ustochrepts from a semiarid tropical region in relation to different levels of cattle farmyard manure (FYM, 0,2.5,5 and 10 %). Magnesium saturated soil samples were equilibrated with KCl + MgClz solutions having a range of equivalent ion fraction of K from 0 to 1 in the equilibrium solutions. The experimental results were analysed and interpreted, using different exchange selectivity quotients and thermodynamic parameters.Application of FYM caused a small but consistent increase in the K-preference over Mg as depicted from the normalized exchange isotherms. Standard free energy of K-Mg exchange were strongly negative (AGO -6.97 to-9.47 kJ eq-' by Gaines and Thomas approach; AG'o -10.85 to-14.15 kJ M-I by Babcock and Duckart approach), suggesting a strong thermodynamic preference for K over Mg in these soils. For comparable treatments, AGO were about -0.84 to -1.25 kJ eq-' more negative for K-Mg, compared to K-Ca system reported earlier. AGO for 10 % FYM treatments became more negative over the controls by 2.34, 1.40 and 0.53 kJ eq-' for Hissar, Panipat and Pehowa soils, respectively.All the selectivity quotients scrutinized in this investigation, viz., Gapon (KO), Vanselow (Kv)

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