Water soluble phosphorus released by poultry litter: effect of extraction and time after application

The normally alkaline pH of poultry litter limits the solubility of P forms, especially the inorganic ones. Poultry litter acidification after field applications could result in increased P solubilization, so the use of water-soluble P (WSP) concentrations measured at the original litter pH might lead to an underestimation of the risk of P contamination of runoff water. In the laboratory, we studied the influence of pH (original and target pH 6 and 7) and shaking time (0.5, 1.0, 2.0, and 4.0 h) on the amounts of Total Dissolved P (TDP) and Molybdate Reactive P (MRP) extracted from two broiler and one breeder litters. Additionally we measured pH, MRP, and TDP evolution in the thatch and top 1 cm of soil during 115 days after application of broiler litter to a Bermudagrass pasture. Acidification of litter suspensions increased TDP by 34 to 72% and MRP by 24 to 69%. In the field, broiler litter pH decreased from 8.1 to 6.7 within 30 days after the application. The following evidence suggests that the WSP measured at the original litter pH might have been considerably less than that released in the field: (a) Based on adsorption isotherm data, the 97 g P g^–1 of soil applied as MRP would have been insufficient to result in an increased concentration of 16 g P g^–1 of soil as MRP; (b) The total increase in Dissolved Unreactive P (DUP) observed in soil (38 g cm^–2) was twice the amount measured in the litter at the original pH; and (c) The increase in MRP measured in soil 59 days after litter application could be linked to additional amounts of DUP not accounted for in the analysis at the original pH. These results highlight the importance of measuring WSP under conditions similar to those encountered by the litter after application.