Phosphorus reactions and leaching potential in soils amended with sewage sludge

The objective of this study was to evaluate the teaching potential of P from a range of sewage sludge- or P fertilizer (monocalcium phosphate)-amended topsoils that had different P contents. Leaching trials (70 d) were conducted in 30-cm-long PVC columns; P-related parameters were then determined at different depths in the leached soils. In most cases, more (P < 0.01) P was released from the P-fertilized than sludge-treated soils. Increasing native Olsen P from 15 to 62 mg kg(-1) had negligible effect on P release; however, there was a sharp increase in P leached between 92 and 134 (maximum) mg Olsen P kg(-1). This increase was much greater in the P-fertilized than sludge-treated soils. At all depths in all leached soils, resin P decreased in the order: P fertilized > sludge > control. At the lowest depth (16-20 cm) of the leached soil, the degree of P sorption saturation decreased in the same order and was related curvilinearly (P < 0.001) to cumulative P leached (at 70 d), This relationship demon strates the importance of P sorption saturation in controlling P leaching in sludge- and fertilizer-treated soils. The more rapid rate of P saturation and P leaching in P-fertilized than sludge-treated soils emphasizes the role of the P source in adsorption-desorption processes.