Nutrient losses by surface run-off following the application of organic manures to arable land. 2. Phosphorus

Phosphorus (P) surface run-off losses were studied following organic manure applications to land, utilising a purpose-built facility on a sloping site in Herefordshire under arable tillage. Different rates and timing of cattl e slurry, farm yard manure (FYM) and inorganic nitrogen (N) and P fertilise r were compared, over a 4-year period (1993-97). N losses from the same stu dies are reported in a separate paper. The application of cattle FYM and, e specially slurry, to the silty clay loam soil increased both particulate an d soluble P loss in surface water flow. Losses via subsurface flow (30 cm i nterflow) were consistently much lower than via surface water movement and were generally unaffected by treatment. Increased application of slurry sol ids increased all forms of P loss via surface run-off the results suggested that a threshold for greatly increased risk of P losses via this route, as for N, occurred at ca. 2.5-3.0 t/ha solids loading. This approximates to t he 50 m(3)/ha application rate limit suggested for slurry within UK 'good a gricultural practice'. The studies also provided circumstantial evidence of the sealing of the soil surface by slurry solids as the major mechanism by which polluting surface run-off may occur following slurry application on susceptible soils. Losses of total and soluble P, recorded for each of the 4 years of experiments, reached a maximum of only up to 2 kg/ha total P (TP ), even after slurry applications initiating run-off. Whilst these losses a re insignificant in agronomic terms, peak concentrations of P (up to 30,000 mug/l TP) in surface water during a run-off event, could be of considerabl e concern in sensitive catchments. Losses of slurry P via surface run-off c ould make a significant contribution to accelerated eutrophication on entry to enclosed waters, particularly when combined with high concentrations of NO3--N. Restricting slurry application rates to those consistent with good agronomic practice, and within the limits specified in existing guidelines on good agricultural practice, offers the simplest and most effective cont rol measure against this potentially important source of diffuse pollution. (C) 2001 Published by Elsevier Science Ltd.