Dairy Manure Impacts on Phosphorus Adsorption Rates and Speciation on Calcite.

            In calcareous soils, phosphorus (P) adsorption and mineral precipitation with calcium (Ca) is an important reaction process that controls P plant availability and leaching potential. Past research has shown that the presence of organic matter can lower phosphorus adsorption, and alter the Ca-P minerals that form. To measure the effects of cow manure on Ca-P mineral precipitation rate and speciation, two calcite-phosphorus adsorption incubations were run to compare the amount of phosphorus adsorbed to calcite in a 1% and 10% manure solution versus deionized water as a function of time. Results show a significant reduction in the percent phosphorus adsorbed to calcite when manure is present compared to samples in deionized water. Using XANES K-edge spectroscopy, hydroxyapatite was observed to be the dominant P species formed in all samples but those incubated in the 10% manure solution. In the 10% manure solution, the P K-edge XANES were different than the XANES from hydroxyapatite and other Ca-P minerals, suggesting that the organic acids in the manure extract inhibited the formation of Ca-P minerals. Results indicate that manure amendment to calcareous soils alters the amount, speciation and overall solubility of Ca-P minerals in the soils, which will affect plant uptake and surface water quality.