The use of alum to decrease phosphorus losses in runoff from grassland soils

Phosphorus (P) loss from land can impair surface water quality. Aluminium sulphate (alum) treatment of manure or slurry has been shown to decrease P loss when waste is applied to land; our hypothesis was that alum may also decrease P loss when directly applied to grassland grazed by dairy cows. A rainfall simulation generated runoff from five soils 2, 4, 7, 14, 38 and 118 d after they were treated with or without dung and alum (0, 20, 40 and 80 kg Al ha-1). For soils with and without dung applied, alum decreased mean concentrations of filterable reactive P (FRP) by 25-70% and total P (TP) by 20-40%, depending on soil P, Al and Fe concentration and alum application rate. An equation (R2 = 0.98) was generated to predict how long after application the effect would be significant. A 14 month field trial compared runoff P losses from plots that received 25 and 50 kg Al ha-1 applied within a week of grazing by dairy cattle in spring (4 months after the trial started). Alum application decreased runoff-weighted concentrations (and loads) of FRP by 47-52% and TP by 25-34%. At 157-944 $ kg-1 P mitigated, cost-effectiveness was estimated as medium compared to exiting strategies for mitigating P loss in dairy farms. Cost-effectiveness would be improved by the spatial and temporal targeting of alum application to areas of a farm or catchment that lose the most P.