Serial Dilution and EDTA Extraction Indicate Probable Phosphorus Minerals In Dairy, Goat, Swine and Chicken Manure.

The probable solid phases controlling phosphorus (P) solubility in swine, chicken, goat, and dairy manures were investigated using chemical modeling software, Visual Minteq, coupled with serial dilution and EDTA extraction. In the serial dilution scheme, the water to manure wet weight ratios (WMR) used were 1:1, 2:1, 5:1, 10:1, 50:1, and 100:1. The EDTA concentrations used were 1, 5, and 10 mmol L^-1 at the WMR 50:1. The total mineral concentrations in serially diluted samples were measured by ICP-OES, while in the EDTA extracts concentrations were measured by ICP-OES and also by the molybdate blue-P method. Serial dilution of the manures showed that the amount of dissolved P was linearly related to the amount of water added for WMR of 1:1 to 10:1, but was nonlinear at higher WMR. At the 10:1 WMR, the amount of dissolved P in swine, chicken, goat, and dairy manure was 6, 7, 4, and 34%, respectively, of the total manure P. For the WMR 100:1, the amount of dissolved P was 35, 36, 44, and 65% of total P in swine, chicken, goat, and dairy manure, respectively. Chemical modeling suggested that at low WMR, Mg-phosphates, primarily struvite, were the probable solid phases controlling P solubility in all manures, except dairy. As WMR increased, the solid phases controlling P solubility shifted from Mg-phosphate to Ca-phosphate minerals in chicken, swine, and goat manures. From the EDTA experiment it was determined that the concentrations of EDTA required to dissolve all soluble P was 10 mmol L^-1 in chicken and goat, 5 mmol L^-1 in swine, and 1 mmol L^-1 in dairy manure. Furthermore, the use of ICP-P or molybdate blue-P for chemical modeling showed the same solid phases in the EDTA extracts. This indicates that P concentration determined with ICP can be used for chemical modeling of manure solutions. Overall, this study suggests that the mineral solid phases determine the rate of P solubility from animal manure.