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Research was conducted on nitrogen (N) surface run-off losses 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 cattle slurry, farmyard manure and inorganic N and phosphorus (P) fertiliser were compared, over a 4-year period (1993-97). P losses from the same studies are reported in a separate paper. The application of cattle slurries to the silty clay loam soil increased the loss of solids and NH4(+)-N in surface water flow compared to control plots receiving inorganic fertiliser only, or no treatment, but had little effect on NO3(-)-N losses by this route. Results were consistent with other observations that rainfall events immediately after manure applications are particularly likely to be associated with nutrient run-off losses. Losses via subsurface flow (30 cm interflow) were consistently much lower than via surface water movement and were generally unaffected by treatment. Increasing slurry application rate and, in particular, slurry solids loading, increased solids and NH4(-)-N losses via surface run-off. The threshold, above which the risk of losses via surface run-off appeared to be greatly increased, was ca. 2.5-3.0 t/ha slurry solids, which approximates to the 50 m3/ha limit suggested for slurry within UK 'good agricultural practice'. Sealing of the soil surface by slurry solids appears to be a possible mechanism by which polluting surface run-off may occur following slurry application on susceptible soils. Total losses of NH4(+)-N and NO3(-)-N during the 4-year monitoring period were insignificant in agronomic terms, but average soluble N concentrations (NH4(+)-N + NO3(-)-N) in run-off, ranging from ca. 2.0 mg/l, up to 14.0 mg/l for the higher rate slurry treatments. Peak concentrations of NH4(+)-N > 30 mg/l, are such as to be of concern in sensitive catchments, in terms of the potential for contribution to accelerated eutrophication and adverse effects on freshwater biota.

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Losses of total and molybdate‐reactive phosphate (MRP) were measured for 4 yr in water flowing from interconnecting mole and pipe drains under 0.24 ha plots of a heavy cracking clay soil. The molybdate‐unreactive phosphate (MUP) obtained by difference appeared from the results to be mainly phosphate carried on suspended soil material. The losses of all three categories of phosphate were closely related to the cumulative drainflow, with the same relations covering three of the four years, designated the synoptic years. Losses were much greater in the other year, probably because phosphate was applied after, rather than before, plowing and to very wet soil. The ceiling to annual total phosphate loss, assuming 500 mm drainage, was 0.4 kg ha−1 in the synoptic years, with 76% of the loss occurring as MUP. It was about 1 kg ha−1 in the high‐loss year, with 88% as MUP, implying that MUP comprised 96% of the extra loss that year. Halving the phosphate application lessened the loss of total phosphate but not conclusively, and the decrease in MRP loss was not significant. Restricting and thereby delaying drainage lessened losses of MUP, probably because suspended material carrying it was allowed to settle. Increasing the spacing between mole channels from 2 m to 4 m increased the losses of MUP but not MRP. This probably happened because to meet a flow pathway connected to a mole channel, rainwater had to travel further horizontally and collected more phosphate‐containing soil material.