Potassium release from mineral structures by H+ ion resin

It is hypothesized that the release of interlayer K+ from phyllosilicates is initiated by a very low K+ concentration in ambient soil solutions and near mineral surfaces. Under natural conditions, one of the factors resulting in this low K+ concentration is K+ uptake by plant roots. If this is true then K+ trapped by a ion exchanger should also induce the K+ release from interlayers. Accordingly, a diffusion cell was constructed in which the soil was separated by a semipermeable membrane from a H+ saturated resin. In this study, sand, silt and clay fractions of five different soils from Pakistan were examined for differences in their interlayer K+ release. The particle size fractions were also used to grow a sequence of crop species (wheat, elephant grass, maize and barley) to test whether K+ uptake by the crops was related to K+ extracted by the ion exchanger. The amounts of K+ sorbed by the resin were in the same order of magnitude as the crop K+ uptake, and were correlated significantly (r=0.79, p<0.01). The K+ release kinetics closely followed the Elovich function of which a- (intercept) and b-values (slope) were correlated significantly with crop K+ uptake. The results revealed that K+ release from the interlayers of K+ bearing minerals is initiated by a low K+ concentration near the mineral surfaces. The K+ release kinetics corresponding to the Elovich function has relevance for the K+ uptake of crops. On average, quantities of K+ released from clay and silt fractions were comparable and twice as high as from sand fractions. Interlayer K+ released from silt and clay fractions, found important for plant K nutrition on the studied soils, should be considered in K fertilizer management.