Twenty‐year Phosphorus Trends in Forage Systems Receiving Aluminum Sulfate‐treated Poultry Litter

Adding aluminum sulfate or alum to poultry litter is a best management practice (BMP) that reduces ammonia emissions and P runoff and leaching, although its long-term effects on forage growth and P uptake are largely unknown. The objective was to determine if reducing soluble P in litter with alum would result in forage P deficiencies or reduce yields. A 20-yr study was conducted to determine effects of four rates of alum-treated litter, untreated litter, and ammonium nitrate (NH4NO3), as well as an unfertilized control on tall fescue (Festuca arundinacea Schreb.) yield and P uptake. Phosphorus uptake increased as litter rates increased for both sources but was greater (P <= .05) for untreated litter, likely owing to high water-extractable P in soil. Over 20 yr, both litter sources had greater P uptake than NH4NO3. Average annual yields by fertilizer source across rates were 6.92, 6.64, 5.10, and 3.38 Mg ha(-1) for alum-treated litter, untreated litter, NH4NO3, and the control, respectively. Forage yields increased with litter application rate, but there was no difference due to litter source, whereas yields were 34% lower with NH4NO3 due to soil acidification and forage P deficiency (<= 0.2% P). Multiple regression indicated tall fescue P uptake was affected by total P application rate and water-extractable phosphorus (WEP), whereas yields were influenced by N rates, Mehlich III P levels, soil pH, and precipitation. These results indicate that amending soils with alum-treated litter does not adversely affect tall fescue yields or P utilization, therefore alum is a sustainable BMP in pasture systems.