Characterisation of soil potassium supply as derived from sorption–desorption experiments

Plant K uptake depends on soil K supply and this can be modelled using the K concentration in the soil solution (C K), the soil K buffer power (S K) and the effective K diffusion coefficient. With the appropriate sorption–desorption curve, the parameters C K and S K can be estimated from the equilibrium K concentration (C K,0) and from the slope of the curve at C K,0 (S K,0). However, buffer power is frequently estimated by the ratio of the soil exchangeable K content (E K) to C K. Up to now, S K,0 had not been compared with this ratio, nor had C K,0 been compared with any C K estimate from soil solution extract measurements. To address this question, we collected 45 soil samples from 15 K fertilisation trials in France. The soils differed widely in their physicochemical characteristics, soil solution K concentration and buffer power. For each soil sample, a sorption–desorption curve was established from 16-h experiments performed at the estimated Ca concentration of the soil solution. The C K,0 estimates were compared with the K concentrations measured in the soil solution obtained either by direct centrifugation (C C K) or by centrifugation with 1,1,2-trichlorotrifluoroethane (TFE C K). On average, C C K values were 16% higher than TFE C K values, whereas C K,0 values were intermediate between the two. The K buffer power increased when CEC increased and when C K decreased. Multiple linear regressions using either CEC at the soil pH and TFE C K^–1 or E K and TFE C K ^–1 as independent variables accounted for more than 98 or 95% of the variability of S K,0. The buffer power estimated by the ratio of E K to TFE C K overestimated by 100% the S K,0 value obtained from the sorption–desorption curves.