Modelling cation exchange in columns of disturbed and undisturbed subsoil
B. LUDWIG & A. KÖLBL
Eur. J. Soil Sci., 53:645-653 (2002)
Cation exchange is often studied with disturbed and dried soils, but the applicability of the results to undisturbed soils is not straightforward. We investigated the value of exchange coefficients obtained from standard procedures for predicting cation exchange in soil. Columns of undisturbed and disturbed subsoil of a Luvisol (SBt horizon) were leached under saturated conditions with 0.4, 4, 20, 41, 102 and 205 mM BaCl2 at a Darcy velocity of 1400 mm day-1. The model PHREEQC was used to calculate one-dimensional transport, inorganic complexation and multiple cation exchange. Two model variants were tested: m1, exchangeable cations obtained by percolation with NH4Cl, and, m2, exchangeable cations obtained by shaking the soil with BaCl2. The exchange coefficients (Gaines–Thomas formalism) were calculated from the ion activities in solution and exchangeable cations obtained by NH4Cl percolation (m1) or shaking with BaCl 2 (m2). Variant m1 predicted cation exchange of the disturbed (homogenized) soil for the entire BaCl2 concentration range, whereas variant m2 resulted in a twofold overestimation of desorbed K for all experiments, which was related to large amount of K released from the soil by shaking with BaCl2. In experiments with undisturbed soil, variant m1 predicted the concentrations of Mg, Ca, K, and Na in the solution phase and the sum of cations released from exchange sites. However, variant m2 predicted changes in ion concentrations and exchangeable cations somewhat less well. This study suggests that the amounts of exchangeable cations and exchange coefficients obtained from experiments with homogenized soil by percolation are useful to predict cation concentrations in column experiments with undisturbed soils.