Phosphorus losses to surface waters following organic manure applications to a drained clay soil

The effects of annual applications of farm manures (pig slurry (PS), broiler litter (BL) and cattle farmyard manure (FYM)), liquid digested sewage sludge (LDS) and inorganic phosphorus (P) fertiliser on P concentrations and losses in tile drain flow from a calcareous clay soil were studied over four winter drainage seasons. The site was under arable cropping in South Eastern UK and had been intensively underdrained in autumn 1994. The tile drainage system comprised of plastic pipes covered with gravel backfill and supplemented by 2 m spaced mole channels. Application of PS in November 1994, 4-6 weeks before the onset of winter drainage, resulted in high concentrations of dissolved P (up to 10 mg l^-1) and total P (TP) in drain flow (up to 75 mg l^-1). TP losses following application of PS over this first drainage season (1155 g ha^-1) were increased four-fold compared to a control receiving no P (277 g P  ha^-1). The majority of the increased loss occurred in the first drainage event due to the rapid transport of the PS through the macropores created in the soil following the installation of tile and mole drains 1 month before the slurry was applied. Application of BL, FYM, LDS and inorganic P fertiliser at maximum recommended rates did not significantly increase P losses in any drainage season, nor did the PS in subsequent years. This study supports current guidelines on good agricultural practice, which recommends that liquid farm manures should not be applied to recently drained clay soils to avoid direct contamination and P enrichment of the drainage water.