Abstract
General conditions under which the activity ratio of potassium ion to those of calcium plus magnesium ions (AR^K) are kept constant during diluting soil solutions without disturbing cation exchange equilibria were derived based on the thermodynamics of ion exchange. They were i) that the selectivity coefficients of K-Ca and K-Mg exchange reactions depend only on the exchangeable cation composition and ii) that Ca and Mg ions behave identically in the cation exchange reactions. Experimental data from the literature showed that neither of these conditions is met strictly in actual soil systems but the deviation is not large. Practically, however, dilution of soil solutions disturbs the cation exchange equilibria. Numerical simulations on the variance of the AR^K during dilution showed that the cation exchange capacity had the largest effect among the factors such as the exchangeable potassium content, selectivity for potassium, and initial ionic concentration in the soil solution. The change in AR^K caused by forty-fold dilution of a soil solution was calculated to be less than 25% for soils having a CEC of 30 meq/100 g while it was more than 150% if the CEC was 1 meq/100 g.
