Kinetics of Soil Potassium Sorption–Desorption and Fixation

Abstract

Though the potassium (K) status of the soil often relies only on its exchangeable-K content, release from nonexchangeable K also contributes to plant nutrition. This process, as well as the opposite one, K fixation, are diffusion controlled and thus depend on time. This study was conducted to (i) assess the influence of time (2 h, 1 d, and 8 d) on soil K dynamics; and (ii) compare the soil ability for K release and fixation (β) estimated from sorption–desorption experiments with the soil fixation capacity (FC) estimated from fixation experiments at two different K doses. The studied soil samples came from a long-term fertilization experiment established in 1976 by the Swiss Research Station Agroscope ACW in a Gleyic Cambisol (FAO classification system). They correspond to two extreme K treatments: (i) no K fertilizer applied but with P fertilizer applied in quantities equivalent to the uptake by the crops and (ii) K and phosphorus (P) fertilizers applied in quantities equivalent to the uptake by the crops with an additional 166 kg K ha⁻¹ y⁻¹ and 26.2 kg P ha⁻¹ y⁻¹. Estimated equilibrium K concentrations decreased and, conversely, the soils’ K buffer power (i.e., the soil capacity to resist a change in soil solution K concentration following removal or input of K to the soil–plant system), β, and FC increased when the soil K content decreased. Time significantly influenced only the sorption–desorption curve and the release–fixation of the low-K-status soil. Here, K buffer power, β, and FC increased with time. A first-order model described the time series of exchangeable-K levels during the K-fixation-capacity experiments. However, a parabolic diffusion model was superior at explaining the variance. FC and β were linearly related (r²  = 0.92; P < 0.01), but FC systematically overestimated β. This shows that FC did not appear as a perfect surrogate of β.

Keywords:

• Exchangeable K
• K release
• nonexchangeable K
• soil K kinetics
• soil K status

Acknowledgments

This work was funded by Swiss Research Station Agroscope ACW and National Institute for Agriculture Research (INRA, France).