The Use of Alum to Decrease Phosphorus Losses in Runoff from Grassland Soils

Phosphorus (P) loss from land can impair surface water quality. Aluminum sulfate (alum)-treated, compared with untreated, manure or slurry decreases P loss when 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 showed that alum decreased mean concentrations of filterable reactive P (FRP) by 25 to 70% and total P (TP) by 20 to 40%, depending on soil P, Al, and Fe concentration and alum application rate. Using these factors, we predicted that FRP losses would be significantly less from alum-treated grasslands than from untreated grasslands for 70 to 96 d. A 14-mo field trial compared runoff P losses from plots that received 0, 25, and 50 kg Al ha(-1) applied within a week of grazing by dairy cattle in spring. Runoff-weighted concentrations (and loads) of FRP and TP decreased in alum-treated plots by 47 to 52% and 25 to 34%, respectively. At US$ 157 to US$ 944 kg-1 P mitigated, cost-effectiveness was estimated as medium to low compared with existing strategies for mitigating P loss in dairy farms but could be improved if applied to critical source areas of P loss. However, additional work, such as determining the need for repeat applications, is required before alum can be recommended to decrease P losses from grazed grassland.