Long-Term Trends in Phosphorus Leaching Following Phosphorus Fertilization Drawdown.

The risk of phosphorus (P) leaching from the topsoil has been demonstrated  to increase with increasing P content measured as ammonium lactate extractable P (P-AL). Few mitigation strategies are effective for soils with high soil test P and high leaching losses. Extraction of soil P by crop harvesting without P fertilization has been suggested as a method to reduce the risk of P leaching.  According to Swedish recommendations, only a limited number of crops should be fertilized with P when grown on soils with high P-AL values (>120 mg kg^-1), and then at moderate rates.

The aim of this study was to investigate if absence of P fertilization in combination with crop harvesting, could reduce P-AL over time, and thereby reduce P leaching. Lysimeters (1.18 m deep) representing four different soil types (loamy sand, sandy loam, silty clay loam and clay), with initial P-AL values between 77-431 mg kg^-1, were used. The loamy sand, which had the highest P-AL value, received no P during 11 years and the others during 9 years. Phosphorus removal with harvested products declined during the study, although P-AL remained high and nitrogen and potassium were applied to the soil columns during the last years of the study. The reductions in P-AL were 11-37% between start and end of the experiment, with the smallest percentage change in the soil with highest P-AL and longest time without P fertilization. A decreasing trend in DRP concentration in leachate was found for only one soil, the clay soil with the lowest initial P-AL value of 77 mg kg^-1.

Mining P reserves in the soil by crop harvesting and absence of P fertilization, to reduce P leaching losses, seems to be a slow process under Swedish conditions. The results highlight the importance of not over-fertilizing, to avoid high P-AL levels.