The efficacy of good practice to prevent long-term leaching losses of phosphorus from an irrigated dairy farm

Phosphorus (P) can be leached from intensive land uses, including grazed dairy farming. There is some evidence to suggest that P-leaching can enrich groundwater, especially where fertiliser or farm dairy effluent P (FDE) is applied to soils of low sorption capacity. We measured P fractions in leachate at 70 cm depth from two soils of low-P sorption capacity in an intensively grazed dairy farm, one a free-draining shallow soil and another a deep and moderately well-drained soil. As per industry good practice, the soils were maintained at an agronomic optimum and received P as either fertiliser or a lower rate of fertiliser plus FDE, applied according to regional rules and industry guidelines to avoid the FDE ponding on the soil surface and leaching to depth. Our hypothesis was that rules and guidelines were not sufficient to prevent P losses, especially in the free-draining soil. In response to annual applications of 40 kg P ha(-1) yr(-1) as fertiliser or 30 kg P ha(-1) yr(-1) as fertiliser and 10 kg P ha(-1) yr(-1) as FDE, dissolved and particulate P concentrations increased annually 4 to 7%. Mean total P load over the 14-yr period of measurement (2001-2015) from the FDE-treated, free-draining shallow soil was 1.46 kg ha(-1) yr(-1,) much greater than the same soil without FDE (0.25 kg ha(-1) yr(-1)) or the moderately well-drained soil with or without FDE applied (0.12 kg ha(-1) yr(-1), for both treatments). Leaching losses were attributed to a combination of high hydraulic conductivity enhanced by the presence of macropores and the increasing P saturation of macropore walls. An enrichment in dissolved reactive P was also detected in a well intercepting groundwater at 10-m depth. However, the source of the enrichment was unclear. These data suggest that despite following industry good practice, regional rules and industry guidelines significant P losses may occur when FDE is applied to soil at rates designed to maintain soil Olsen P in an agronomically optimal concentration. It is unclear if applying less FDE at lower rate, would decrease P losses. Therefore, less P must be applied, made less available for loss, or P-rich FDE not applied to this freely draining shallow stony soil (or similar soils) under irrigation.