Selectivity coefficients for calcium-magnesium-sodium-potassium exchange in eight soils

Summary

The six selectivity coefficients for simultaneous Ca-Mg-Na-K exchange were calculated from soil extract cation activities and exchangeable cation concentrations for eight salt affected soils. These values were compared with selectivity coefficients calculated from solution cation concentrations and the exchangeable cations for the same soil samples. The lyotropic series for these soils in order of replaceability ease was Na≧ Mg > Ca > K, whereas the generally accepted series is Na > K > Mg > Ca. The selectivity coefficient values varied between soils, but did not vary with depth in each soil. Potassium exchange selectivity coefficients have not previously been available for use in exchange models and are reported here for eight soils With these data, models can include K exchange in high K soils and soils irrigated with high K waters. Exchangeable sodium percentage (ESP) can be calculated on a programmable hand-held calculator for a soil using the chemical data and these selectivity coefficients. All ionic strength and ion pair corrections and selectivity calculations for this study were carried out on a programmable hand-held calculator.