Dc. Olk et al., KINETICS OF POTASSIUM FIXATION IN VERMICULITIC SOILS UNDER DIFFERENT MOISTURE REGIMES, Soil Science Society of America journal, 59(2), 1995, pp. 423-429
Fixation of K in vermiculitic soil and the residual benefit from ferti
lizer-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 wettin
g-drying cycles and in long-term aerobic incubations with soil moistur
e held constant at different matric pressures. In all studies, 12 mmol
KCl kg-1 soil was applied before monitoring K fixation. All rates con
formed to first-order kinetics. Each wetting-drying cycle had a rapid
fixation phase lasting 2 d with slower fixation thereafter, and rate c
onstants of fixation decreased in each successive cycle for both phase
s. Depending on the soil, rate constants of the rapid phase in the fir
st 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 0.05
8 d-1 in the first cycle and 0.005 to 0.021 d-1 in the second cycle. I
n 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 co
ntrolled by K+ diffusion. When soil moisture was held constant in a lo
ng-term aerobic incubation, K fixation rate constants increased at low
er soil moisture levels. Rate constants of fixation were 0.00055 d-1 a
t - 0.03 MPa and 0.00114 d-1 at - 0.48 MPa, an increase of 107%. The m
easured fixation rates were sufficient to explain long-term fixation m
easured in a related field study.