Dissolution of phosphate rock during the composting of poultry manure: an incubation experiment

Dissolution of phosphate rocks (PRs) during composting with poultry manure was examined using a radioactive³²P labelled synthetic francolite and North Carolina phosphate rock (NCPR) through laboratory incubation experiments. Francolite or NCPR was mixed with different poultry manure composts at a rate equivalent to 5 mg P g^-1 and the dissolution was measured after 60 and 120 days incubation by a sequential phosphorus (P) fractionation procedure. The use of³²P labelled francolite showed that in manure systems, PR dissolution can be measured more accurately from the increases in NaOH extractable P (ΔNaOH-P) than from the decreases in HCl extractable P (ΔHCl-P) in the PR treated manure over the control. The dissolution measurements showed that approximately 8 to 20% of francolite and 27% of NCPR dissolved during incubation with poultry manure composts in the presence of various amendments. Addition of elemental sulphur (S°) to the compost enhanced the dissolution of PRs. The results provide no evidence for the beneficial effect of protons (H⁺), produced during the nitrification of NH₄⁺ in manure composts, on PR dissolution. The low level of dissolution of PR in poultry manure composts was attributed mainly to the high concentration (4.8 × 10^-2 mol L^-1) of calcium (Ca²⁺) in manure solution.