Potassium-calcium exchange in five Ap soils from paddy fields and its effect on potassium concentration in soil solution

We measured the selectivity coefficient (K _V) for K-Ca exchange equilibrium as a function of K-saturation on two smectitic soils, two halloysitic soils and a soil dominated by noncrystalline aluminum silicates. The selectivity coefficients for all the soils decreased as K-saturation increased. Among the five soils, the halloysitic soils exhibited higher K _Vvalues than the smectitic soils, while the K _Vvalues for the soils dominated by noncrystalline aluminum silicates were comparable with those of the halloysitic soils at low K-saturation but the K _Vvalues were even lower than those for the smectitic soils at higher K-saturation. Numerical simulations showed that the K concentration in the soil solution after addition of a K-salt differed significantly among the soils, depending on the exchangeable K status and K-selectivity of the soils.