Binary and Ternary Exchange of Potassium on Calcareous Montmorillonitic Soils

Simultaneous K exchange with Ca, Mg, and Na occurs in soils, but most of the published information is obtained from binary systems. The theoretical question of whether selectivity coefficients obtained in binary systems are valid for ternary systems has not yet been clarified; however, no significant difference has been found between selectivity coefficients in binary and ternary systems. Potassium exchange with Mg plus Ca was studied in two calcareous soils with variable concentrations of Na, using batch and miscible‐displacement methods. Preference for K over the divalent cations was found in both soils, regardless of Na concentration and the experimental methods. Modified Gapon ( k G ) and Vanselow ( k V ) selectivity coefficients were calculated for each cation pair in binary and ternary systems. In both soils, the k v and the k G decreased with increasing K fraction in the solution, but the k G value was less affected than the k v . A simple mathematical solution of the Gapon binary equations, K − (Mg + Ca) and Na − (Mg + Ca), in combination with an assumption of constant cation‐exchange capacity (CEC), was used to predict the amount of exchangeable K as a function of solution composition in a ternary system. The high correlation of predicted exchangeable K with the directly measured values shows that binary selectivity coefficients can be used in multicationic systems on calcareous montmorillonitic soils.