APPARENT CATION-EXCHANGE EQUILIBRIA IN PODZOLIC FOREST SOILS

The Gaines-Thomas selectivity coefficient, K, was used to express the relation between the cations in solution and the cations in exchange s ites in podzolic forest soils. Soil solution was obtained by centrifug ing a fresh bulked soil sample. Exchangeable cations HX, AlX, CaX, MgX and KX and effective cation-exchange capacity, CECe, were determined with 0.1 M BaCl2. Apparent values of K indicated a preference of Ca2over Mg2+ and over Al3+ in O, A and B horizons (log KAl-Ca < 0 and log KMg-Ca ( 0), whereas log KK-Ca and log KH-Ca exceeded zero. The horiz ons were similar with respect to log KH-ca, and the differences in log KMg-Ca were small. Log KK-Ca and log KAl-Ca increased in the horizons in the order O < A < B. Log KAl-Ca was not significantly correlated w ith the fraction AlX/CECe. Log KMg-Ca was positively correlated with t he fractions HX/CECe and AlX/CECe, and negatively correlated with log (CaX/MgX). The selectivity coefficient of binary cation exchange seeme d to be applicable to in situ soil solutions. However, the fraction of each cation on exchange sites should be based on the CECe rather than on the sum of the two cations. The latter, also, seemed to be accepta ble in cases of exchangeable cations with a large relative content in soil, e.g. in Al3+-Ca2+ exchange in A and B horizons, and in H+-Ca2+ e xchange in O and A horizons.