Reducing phosphorus runoffand leaching from poultry litter with alum: Twenty-year small plot and paired-watershed studies

Treating poultry litter with alum has been shown to lower ammonia (NH₃) emissions and phosphorus (P) runofflosses. Two long-term studies were conducted to assess the effects of alum-treated poultry litter on P availability, leaching, and runoffunder pasture conditions. From 1995 to 2015, litter was applied annually in a paired watershed study comparing alum-treated and untreated litter and in a small plot study comparing 13 treatments (an unfertilized control, four rates of alum-treated litter, four rates of untreated litter, and four rates of NH₄NO₃). In the paired watershed study, total P loads in runoffwere 231% higher from pasture receiving untreated litter (1.96 kg P ha^-1) than from that receiving alum-treated litter (0.85 kg P ha^-1). In both studies, alum-treated litter resulted in significantly higher Mehlich III P (M3-P) and lower water-extractable P at the soil surface, reflecting greater retention of applied P and lesser availability of that P to runoffor leaching. In soils fertilized with alum-treated litter, M3-P was much higher when analyzed by inductively coupled argon plasma emission spectrometry than by colorimetry, possibly due to the formation of aluminum phytate. Indeed, alum-treated poultry litter leached less P over the 20-yr study: M3-P at 10 to 50 cm was 266% greater in plots fertilized with untreated litter (331 kg M3-P ha^-1) than with alum-treated litter (124 kg M3-P ha^-1). This research provides compelling evidence that treating poultry litter with alum provides short-term and long-term benefits to P conservation and water quality.