Thirty year paired watershed study shows treating poultry litter with alum reduces the legacy effect on phosphorus runoff.

Andrew Sharpley was one of the first scientists to point out that the effects of best management practices (BMPs) on improving water quality are often slower, smaller, and shorter-lived than expected due to legacy phosphorus (P). One BMP for reducing P runoff and ammonia (NH₃) volatilization that has been widely adopted is treating poultry litter with aluminum sulfate (alum). Because of the economic benefits of improved poultry production and reduced propane use, alum is now used to grow >40% of the broiler chickens in the United States. The objective of this study was to determine the legacy effects of alum treatment of poultry litter on P runoff. A 30-year study was conducted using paired watersheds, which were fertilized with untreated or alum-treated litter for 21 years and were not fertilized for the following 9 years. During the fertilization period (1995-2015) when incidental P losses were occurring, the annual P loads from untreated litter (2.0 kg P ha^-1) were 2.4 times higher than alum-treated litter (0.84 kg P ha^-1). Phosphorus losses during the unfertilized period (legacy P) from 2016 to 2024 were much lower, as would be expected; annual P loads in runoff from the watershed that had received untreated litter (0.64 kg P ha^-1) were almost twice as high as that from alum-treated litter (0.34 kg P ha^-1), which was due to lower water-soluble P in the soil. This study demonstrated that treating manure with alum has a long-lasting effect on P runoff from pastures, even after fertilization has ceased, providing further evidence that this is a sustainable BMP.