Treatment of pasture topsoil with alum to decrease phosphorus losses in subsurface drainage

Phosphorus loss from land can impair surface water quality. Losses via subsurface flow can be substantial, but most strategies to mitigate P losses focus on surface runoff. Aluminium sulphate (alum) was applied at 25 and 50 kg Al / ha to a flat, podzol soil under pasture regularly grazed by dairy cattle. Over a year, losses of filtered (<0.45 mm) reactive P (FRP) and total filtered P (TFP) intercepted at 35-cm depth by Teflon suction cups were c. 0.6 and 1.0 kg P / ha, respectively for the control treatment. The 50 kg Al / ha treatment decreased FRP and TFP by 26 and 27%, respectively: no significant difference to the control was noted for alum applied at 25 kg Al / ha. The cost-effectiveness was estimated at 190–952 USD / kg P mitigated. While more cost-effective strategies should be practised first, surface applying alum may provide an option where sub-surface P losses must be lowered further especially if applied to a small area of high P loss.