Solubility of soil phosphorus as influenced by urea

The objective of this study was to investigate possible ways of mobilizing residual fertilizer P as a result of local pH elevation caused by urea hydrolysis. The response of water-soluble P (P_w) and dissolved organic C (DOC) to urea hydrolysis was monitored in three cultivated soils and at two P levels for up to 127–135 d and compared with corresponding changes in soils limed with Ca(OH)₂. Hydrolysis of urea was complete in 8–15d during which soil pH increased by 1–1.5 units at the maximum. Subsequently, the pH decreased to or below the original level owing to nitrification. Mobilization of soil P was enhanced substantially in parallel with the increase in pH, the peak P_w occurring simultaneously with the highest pH value. In all urea-treated soils, P_w remained at an elevated level for at least 60d. As compared to urea, elevation of soil pH with Ca(OH)₂ had only a minor and inconsistent influence on P_w. In mobilization of soil P, the urea-induced increase in pH and a simultaneous production of NH₄⁺ ions proved to be superior to liming with Ca(OH)₂. It was hypothesized that when an acid soil is amended with urea, phosphate is first displaced by OH^? ions, resulting in elevated solution P concentrations. A simultaneous dissolution of organic matter contributes to the persistence of high P concentration by competition for sorption sites on Fe and Al oxides, and thus retards the resorption of P.