KINETICS OF POTASSIUM FIXATION IN VERMICULITIC SOILS UNDER DIFFERENT MOISTURE REGIMES

Fixation of K in vermiculitic soil and the residual benefit from fertilizer-K addition are influenced by soil moisture. The purpose of this study was to determine K fixation rates under simulated field moisture regimes. Fixation rates were measured during three consecutive wetting-drying cycles and in long-term aerobic incubations with soil moisture held constant at different matric pressures. In all studies, 12 mmol KCl kg-1 soil was applied before monitoring K fixation. All rates conformed to first-order kinetics. Each wetting-drying cycle had a rapid fixation phase lasting 2 d with slower fixation thereafter, and rate constants of fixation decreased in each successive cycle for both phases. Depending on the soil, rate constants of the rapid phase in the first cycle ranged from 0.064 to 0.143 d-1 and were 0.026 to 0.061 d-1 in the second cycle. Slow-phase rate constants ranged from 0.030 to 0.058 d-1 in the first cycle and 0.005 to 0.021 d-1 in the second cycle. In samples maintained as saturated pastes, the rapid phase was extended to 5 d but the rate constant was smaller in magnitude than when soil was drying. The rapid phase represented an abiotic process that was controlled by K+ diffusion. When soil moisture was held constant in a long-term aerobic incubation, K fixation rate constants increased at lower soil moisture levels. Rate constants of fixation were 0.00055 d-1 at - 0.03 MPa and 0.00114 d-1 at - 0.48 MPa, an increase of 107%. The measured fixation rates were sufficient to explain long-term fixation measured in a related field study.