Surface complexation model of cation adsorption on humic soils: 2. K⁺-ca²⁺exchange equilibria

We evaluated the ability of a new surface complexation model “multi-layer model” of Nakahara and Wada (1995: Soil Sci. Plant Nutr., 41, 659-669) to predict the K⁺-Ca²⁺exchange equilibria on a humic Andisol. Under the assumption that the relative permittivity of the interfacial region is constant at any points, we could calculate the K⁺-Ca²⁺exchange equilibrium conditions using the four parameters previously obtained from K⁺and Ca²⁺adsorption data. The model predicted the variation of the exchange selectivity coefficient of Van-selow (K _V) and the pH of the bathing solutions as a function of the equivalent fraction of adsorbed K⁺satisfactory. We concluded that our model can be used for the calculation of the chemical equilibria of soil solutions of humic soils.