Trends in Soil Test Phosphorus Dynamics Following Long-Term Application of Poultry Litter and Commercial Fertilizers.

Historical application of poultry litter to Delmarva soils resulted in build-up of soil test phosphorus (P) above agronomic optimum levels and increased risk of P loss. We aim to understand P dynamics during build-up of soil test P in agricultural soils receiving long-term applications of manure or inorganic P fertilizers. Selected soil samples (2000-2013) were collected from plots at two field sites receiving no P, poultry litter at moderate and high rates (224 and 897 g m^-2), or triple superphosphate (TSP; annual P₂O₅ rate = 6.73 and 13.5 g m^-2) for 15 years. Soils were analyzed for water extractable P (WEP), Mehlich-3 P (M3P), and Mehlich 3 P saturation ratio (PSR); a sequential P fractionation (H₂O, NaHCO₃, NaOH, and HCl) was also completed. Soils receiving the high manure rate at both sites exhibited flat or increased WEP and M3P concentrations, while these concentrations often decreased in soils receiving lower manure or TSP rates. In most cases, soils receiving the high manure and/or TSP rates had significantly higher WEP (4.42 mg kg^-1), M3P (123 mg kg^-1), and PSR (28.2%) than soils receiving moderate P rates or no P (averaged 0.86 and 71.5 mg kg^-1, and 18.5% for WEP, M3P, and PSR respectively). Repeated application of manure and TSP at rates exceeding crop removal (high rate) increased P concentrations in both the labile (H₂O-P and NaHCO₃-P) and recalcitrant soil P pools (HCl-P). Diligent management of legacy P soils is necessary to control potential dissolved P release.