Dependence of Different Exchange Selectivity Coefficients on Sodium Saturation of Soils

Exchange equilibria of Na-Ca and Na-Mg were established on soils with different organic matter (O.M.) contents. The data on the composition of the equilibrium solution and the exchange complex were used to calculate selectivity coefficients viz, Gapon (KG), Vanselow (Kv), Krishnamoorthy-Davis-Overstreet (KKDO), KC, KN and Ksu. None of these coefficients was constant at different levels of Na-saturation. KG was the most and Ksu the least dependent on exchangeable sodium percentage (ESP). The soils high in O.M. showed better correlation between selectivity coefficients and ESP than those low in O.M. The effects of organic matter and nature of divalent cation on selectivity coefficients were, however, secondary to that of ESP.