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
