A new approach to quantify the utilization of non-exchangeable soil potassium by plants

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To predict the contribution of soil K fractions of different mobility to K supply of plants, the kinetics of K release from soil was related to the kinetics of K uptake of young sugar beet and wheat plants. For this purpose K release rates from soil were measured by continuously percolating samples of a luvisol with 0.01 M CaCl2 solution and effective diffusion coefficients, De, were determined. Two soil K fractions of different mobility were obtained. De values of the more mobile ‘exchangeable K’ and the less mobile ‘non-exchangeable’ K fraction were found to be 58.9 × 10−9 and 8.2 × 10−9 cm2 s−1, respectively. In a pot experiment, sugar beet and wheat plants were grown, for 15 days and both root growth and K uptake were measured. K uptake kinetics of both crops was determined in a separate experiment using flowing solution culture. To integrate these data quantitatively, the simulation model of Claassen et al. (1986) was applied. Results show that calculated total K uptake agreed closely with real K uptake of the plants. On this basis, 64 and 79% of the K taken up by wheat and sugar beet plants was derived from the rapidly released ‘exchangeable’ and 21–36% from the less mobile ‘non-exchangeable’ soil K fraction.