Technical Abstract: The purpose of this laboratory incubation study was to assess the solubility of P in alum-treated poultry litter (ATPL) when applied to three Virginia soils at equivalent P-based rates. Three poultry litter sources; one that had received no alum additions and two that had received alum additions were utilized in the study. These litter sources and KH2PO4 were applied at rates of 66, 132, and 197 mg P kg-1 with a 0-P check treatment included for each soil. Soils were incubated for 1 year and samples were collected at 1, 3, 6, and 12 months after treatment application. Data collected was used to calculate the relative extractability of P applied in the three litter sources. Results indicated that ATPL applications reduced P solubility compared to non-treated poultry litter (NPL); this effect was most significant at 1-month after application. However, at 12-months significant differences in extractable P between NPL and ATPL treatments were limited at the 66 mg P kg-1 rate. This resulted from continuous decline in the extractability of P applied in the NPL, whereas the extractability of P applied in the ATPL source changed little with time and in some cases increased slightly between the 1 and 3 month sampling periods. Calculated values of relative extractability were found to be influenced not only by litter source but also P application rate, soil type, and incubation time. Therefore use of relative extractability values as P source coefficients should be done with caution, because experimental protocol can have profound impacts on their magnitude.