Phosphorus (P) is present in waterways throughout the United States at concentrations that impair water quality. Agriculture, particularly livestock production, has been identified as a major cause of this impairment. Excess manure P applied to croplands has increased P losses in runoff, leading to surface water eutrophication. We conducted a long-term (36-week) incubation with poultry and dairy manures applied to a silt loam soil to elucidate mechanisms controlling manure P loss to water. Manures were applied to supply the same total P rate to soils with different antecedent plant-available P concentrations (soil test P). There was a strong synergistic effect between dairy manure and soil test P on water extractable P, while soil test P did not affect P loss from poultry manureamended soils. Using scanning electron microscopy and energy dispersive X-ray spectroscopy, we found that poultry manure contained sparingly soluble calcium and magnesium phosphate minerals that controlled soil solution P concentrations, while dairy manure did not. These minerals resemble other biogenic phosphate minerals. Our findings refute current assumptions that all manure P behaves similarly in soils and that organic forms control manure-soil P loss to water.
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