Lime and cow slurry application temporarily increases organic phosphorus mobility in an acid soil

Summary Phosphorus loss from agricultural soils to water is recognized as a major contributor to eutrophication of surface water bodies. There is much evidence to suggest that liming, a common agricultural practice, may decrease the risk of P loss by decreasing P solubility. An unsaturated leaching column experiment, with treatments of control and two lime rates, was carried out to investigate the effects of liming on P mobility in a low-P acid Irish soil, which was sieved and then packed in columns. Phosphorus was applied at the soil surface in the form of KH2PO4 in solution or as cow slurry. Soil solution was sampled at time intervals over depth and analysed for P fractions. Organic P (OP) was the dominant form of P mobile in soil solution. Liming increased OP mobility, probably through increased dispersion of OP with increased pH. Slurry application also increased OP mobility. Results indicated the potential for OP loss following heavy (100 m–3 ha–1) cow slurry application, even from low-P soils, and suggested that liming may increase this risk. Reactive P (RP) was sorbed strongly and rapidly by the soil and did not move substantially below 5 cm depth. As a result, Olsen-P values in the top 2 cm were greatly increased, which indicates an increased risk of RP loss in overland flow. Lime showed little potential as a soil amendment to reduce the risk of P loss.