Technical Abstract: Composting reduces the available N content of organic materials by immobilizing it and converting it to a slow release form. The effect of composting on P is less clear. Adding compost to soil can increase water extractable soil P by direct addition, dissolution, displacing sorbed, or reducing sorption capacity for P. Organic acids can affect soil P and vary with compost maturity. The objective of this study was to examine the effect of compost maturity on soil P. Turkey litter was mixed with orchard grass at a 3:1 volume ratio, turned frequently, and temperature, carbon dioxide, and oxygen measured regularly. Compost samples were taken at day 0 when the compost was made, and at weeks 2, 4, and 8. Compost samples were added to a loam and clay soil and then incubated for 8 weeks. At 1d, and 2, 4, and 8 weeks, water and Mehlich 1 extractable P were determined. Age of compost did not affect the Mehlich 1 extractable P fraction but did affect water extractable P on day 1 in the loam soil. These data suggest that water extractable P may increase when loam soils are amended with biologically active, immature compost or when the sorption capacity of the soil is not sufficient to offset the effects of the compost addition. Because water extractable P is implicated in runoff events, caution should be exercised in applying immature composts to critical source areas within the watershed which are most vulnerable to P loss in surface runoff and erosion.