Sodium‐calcium Exchange Equilibria in Salt‐affected and Normal Soils

Summary 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.