Kinetics of potassium release from illite as influenced by different phosphates

In order to shed light on the effect of different phosphate compounds on alteration of the soil K-reserve and subsequent K release in the immediate vicinity of fertilizer zones, the kinetics of K release from illite (< 2 μm) in 0.5 M Ca(H2PO4)2, 1 M NH4H2PO4, and 1 M (NH4)2HPO4 solutions were investigated. For comparison, the kinetics of K release from the illite in the CaCl2 and NH4Cl solutions with the same cation concentration and pHs as the phosphate solutions were also studied. The results show that the kinetics of K release in the reaction systems included an initial reaction and a slow reaction. The initial reaction, which occurred in a short period (2–72 h), was largely attributed to exchange of cations in solutions with K in the mineral. The phosphate-induced K release was a slow reaction which followed the same rate process in the whole reaction period (2–720 h). The zero-order equation was chosen to calculate the rate coefficients of K release. The rate coefficients of K release for the slow reactions in the Ca(H2PO4)2, NH4H2PO4 and (NH4)2HPO4 solutions at 25 °C were 2.20, 0.26 and 0.10 mg kg− 1 h− 1, respectively; the rate coefficients increased to 9.67, 1.03, and 1.09 mg kg− 1 h− 1, respectively, at 45 °C. The reactions of K release in the Ca(H2PO4)2 and NH4H2PO4 systems had the activation energy values of 59 and 55 kJ mol− 1, respectively. The data indicate that combined effects of H+- and phosphate-induced alteration of illite were the main mechanism of K release in these two systems. In the (NH4)2HPO4 system, the reaction had an activation energy value of 95 kJ mol− 1; phosphate ions apparently played the leading role in inducing K release. The kinetics of the K release from illite was substantially influenced by different phosphate compounds and, thus, merits attention in understanding the K dynamics in the immediate vicinity of fertilizer zones of soils.