Nonpoint source P pollution is a major concern in the Mid-Atlantic region due to the land application of animal manures and biosolids, which usually provides P in excess of crop needs. Biosolids and manures have been shown to vary widely in their P solubility and will have different relative risks for P loss to surface waters when applied to the soil. In-situ x-ray absorption near edge structure (XANES) spectroscopy and sequential chemical fractionation were used to determine the primary species of P in ten manure and biosolids samples. Speciation results indicated that the predominant forms of P include P sorbed on ferrihydrite and beta tricalcium phosphate in Fe-treated biosolids; hydroxyapatite, P sorbed to Al oxyhydroxides, phytic acid, and soluble P in lime stabilized biosolids, dairy manures, and poultry litters; and hydroxyapatite, P sorbed on ferrihydrite, and phytic acid in lime and Fe-treated biosolids. Chemical speciation results were then used to better explain the changes in P solubility when the same organic P sources were incorporated into Mid-Atlantic soils and incubated under oxidized and reduced conditions. Sorbed phases of P remained relatively stable following incorporation into acid soil, resulting in minimal changes in soil water soluble P (WSP). In addition, incorporation of organic sources containing high concentrations of Ca-phosphates was expected to result in a slow-release of P. However, concentrations of soil WSP were shown to decrease for all organic P amended soils within 180 d. This was likely a result of sorption of released Ca-P by soil oxides. Under reducing conditions, dissolution of P species was not affected by the speciation of organic P sources. In all cases, soil and organic P source derived Fe and Al oxide materials were effective at sorbing any solubilized P before it could enter the water column. Results of these studies suggest that addition of metal salts to biosolids and manures has the potential to prevent dissolved P loss from amended soils for long periods of time.