Serial Dilution and EDTA Extraction Indicate Probable Phosphorus Minerals in Dairy, Goat, Swine, and Poultry Manure

The probable solid phases controlling phosphorus (P) salts solubility in goat, swine, chicken, and dairy manures were investigated using chemical modeling software, Visual Minteq, coupled with serial dilution and EDTA extraction. In the serial dilution scheme, the manure (wet weight) to water ratios (MWR) used were 1:1, 1:2, 1:5, 1:10, 1:50, and 1:100. The EDTA concentrations used were 1, 5, and 10 mmol L−1 at the 1:50 MWR. The total elemental concentrations in serially diluted samples were measured by ICP-OES, while in the EDTA extracts concentrations were measured by ICP-OES and P was also measured by the molybdate blue-P method. The percentage of total P dissolved from goat, swine, chicken, and dairy manure using serial dilution at 1:10 MWR was 4, 6, 7, and 34% of the total P; while at 1:100 MWR it was 44, 35, 36, and 65%, respectively. Chemical modeling suggested that between 1:1 to 1:10 MWR, Mg-phosphates, primarily struvite, was the probable solid phase controlling P salts solubility in all manures, except dairy. At the 1:50 and 1:100 MWR, the solid phases controlling P solubility shifted from Mg-phosphate to Ca-phosphate minerals in goat, swine, and chicken manures. The use of ICP or molybdate blue for chemical modeling showed the same solid phases in the EDTA extracts. From the EDTA extractions, it was determined that 5 mmol L−1 EDTA lowered Ca and Mg activities that no mineral phases likely remained in goat, swine, and chicken manures. In conclusion, under the conditions of this study, P dissolution from salts present in manure is controlled by the cation concentration in solution.