Plants use alternative strategies to utilize nonexchangeable potassium in minerals

Plant species differ in their capacity to use nonexchangeable potassium (NEK) in soils. In this study two typical plants with high K use efficiency, ryegrass and grain amaranth, were compared with regard to their capacity to use K from five K-bearing minerals. Biomass relative yield and K uptake data indicated that ryegrass was much more efficient than grain amaranth at using NEK in minerals. Root exudates of grain amaranth collected under hydroponic culture contained considerable amounts of oxalic and citric acids, while these acids were not detected in ryegrass root exudates. Compared with grain amaranth, the kinetic parameters of K uptake by ryegrass roots were characterized by a significantly higher K uptake rate (V-max) and a significantly lower C-min, the minimum external K concentration at which K is taken up. The dynamic release of NEK from minerals in various solutions showed that the release rate of NEK was largely K-concentration dependent and some thresholds of K concentration prevented further NEK release from minerals. The K thresholds were related to mineral type and increased in the presence of Ca2+ or Na+ in solutions. The positive effect of H+ (20 mmol L-1) on NEK release was also mainly attributed to elevating the thresholds of K concentration, rather than to the effects of weathering. The results indicated that the main mechanism by which plant species efficiently use NEK in minerals was to the capacity of plants to absorb K at low concentrations. The lower the C-min for the root K uptake, the higher the expected NEK use efficiency of the plant.