DISSOLUTION OF PHOSPHATE ROCK DURING THE COMPOSTING OF POULTRY MANURE- AN INCUBATION EXPERIMENT

Dissolution of phosphate rocks (PRs) during composting with poultry ma nure was examined using a radioactive P-32 labelled synthetic francoli te and North Carolina phosphate rock (NCPR) through laboratory incubat ion experiments. Francolite or NCPR was mixed with different poultry m anure composts at a rate equivalent to 5 mg P g(-1) and the dissolutio n was measured after 60 and 120 days incubation by a sequential phosph orus (P) fractionation procedure. The use of P-32 labelled francolite showed that in manure systems, PR dissolution can be measured more acc urately from the increases in NaOH extractable P (Delta NaOH-P) than f rom the decreases in HCl extractable P (Delta HCl-P) in the PR treated manure over the control. The dissolution measurements showed that app roximately 8 to 20% of francolite and 27% of NCPR dissolved during inc ubation with poultry manure composts in the presence of various amendm ents. Addition of elemental sulphur (S degrees) to the compost enhance d the dissolution of PRs. The results provide no evidence for the bene ficial effect of protons (H+), produced during the nitrification of NH 4+ in manure composts, on PR dissolution. The low level of dissolution of PR in poultry manure composts was attributed mainly to the high co ncentration (4.8 x 10(-2) mol L(-1)) of calcium (Ca2+) in manure solut ion.