Plant availability of soil organic P may be characterized by estimating the activity of indigenous enzymes which convert organic P to inorganic P with an excess addition of substrate. This study will estimate the mineralization of organic P to inorganic P on soils that have been co-applied with biosolids and water treatment residuals (WTR) by assaying the following enzymes: phosphomonoesterase, phosphodiesterase, pyrophosphatase, and phytase. In 1991 the test plots were co-applied with a single rate of 10 Mg ha^-1 biosolids and 5, 10, or 21 Mg ha^-1 WTR. In 2001 the plots were split with one half receiving a second co-application. Soil samples were taken in 2003 and 2004. The Phytase enzyme assay has rarely been used in soils before and thus required some procedural development. Results of all enzyme assays will be presented.