Kinetics of monoammonium phosphate-induced potassium release from selected soils

Abstract

When monoammonium phosphate fertilizer is applied to soils, the phosphate concentration is very high and the pH is quite low in the immediate vicinity of the fertilizer granule. In this study, the release of potassium (K) by 1 M NH4H2PO4 solution at pH 4.0 from three Chinese soils, Oxisol, Alfisol and Entisol, was investigated. The results showed that NH4H2PO4 promoted K release from these soils. The NH4H2PO4-induced K release obeyed the zero-order kinetics; the release-rate coefficients from these soils at 25 °C ranged from 72 to 170 μg K kg−1 h−1, much higher than observed (4–37 μg kg−1 h−1) in 1 M NH4Cl at pH 4.0. The combined effect of phosphate and proton on the alteration of K-bearing minerals was evidently the major mechanism of the NH4H2PO4-induced K release from the soils. The data indicated that the application of NH4H2PO4 fertilizer may enhance the rate of K supply in soils and lead to the conversion of some of N and P in the fertilizer to a slowly available form in soils because of the formation of a crystalline reaction product, ammonium taranakite. Key words: Potassium dynamics, phosphate application, ammonium taranakite