Phosphorus losses in runoff from application of manures and biosolids to agricultural land are implicated in the degradation of water quality in the Chesapeake and Delaware Inland Bays. We conducted an incubation study to determine the relative P solubility and bioavailability, referred to as P source coefficients (PSCs), for organic P sources, which are typically land-applied in the Mid-Atlantic USA. Nine organic and one inorganic (KH2PO4) P amendments were applied to an Evesboro loamy sand (mesic, coated Typic Quartzipsamments) at a rate of 60 mg P kg(-1) and incubated for 8 wk with subsamples analyzed at 2 and 8 wk. There was an increase in Mehlich-3 P (M3-P), water-soluble P (WS-P), iron-oxide strip extractable P (FeO-P), and Mehlich-3 P saturation ratio (M3-PSR) with P additions, which varied by P source. The trend of relative extractable WS-P, FeO-P, and M3-P generally followed the pattern: inorganic P > liquid and deep pit manures > manures and biosolids treated with metal salts or composted. We found significant differences in the availability of P from varying organic P sources. The use of PSCs may be beneficial when determining the risk of P losses from land application of manures and other organic P sources and could be used in risk assessments such as a P site index. These PSCs may also be useful for determining P application rates when organic P sources are applied to P deficient soils for use as a fertilizer source.