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.