Illite transformation and potassium release upon changes in composition of the rhizophere soil solution

Release of 'non-exchangeable' potassium (K) from interlayers of illite is diffusion-controlled and has been shown to depend on the solution concentration of K and other cations (Ca2+, Mg2+, NH4 (+)). We analysed changes in soil solution concentrations of K and competing cations in situ at different distances from the root surface over time and related them to the transformation of illite, as revealed by X-ray diffraction, and chemical measures of differently bound K. Within 49 and 98 days, respectively, 6.4 and 14.4 % of the illite's total K was released upon contact with the root system. Mixed layered minerals increased from 33 (0 d) to 35 (49 d) to 40 % (98 d). Release of K from interlayers and the transformation of illite occurred at soil solution K concentrations close to the threshold of 80 mu M suggested earlier. Concentrations of Ca and Mg increased with decreasing distance from the root surface, promoting the release of K. The NaBPh4 method supposed to determine 'non-exchangeable' K extracted only 1/3 of the total K from illite.