Past research has demonstrated that animal manure application may increase soil test phosphorus (STP) differently than inorganic fertilizer when applied at the same total P rate. However, the exact mechanisms controlling the increase in STP when manure is applied are still poorly understood. In this study, 42 manures (beef, dairy, goat, sheep, horse, turkey, chicken, and swine) and a fertilizer were applied to 25 soil series at a total P rate of 40 mg   kg^-1 and incubated for 10 weeks along with an untreated control. The manures used contained between 2.8 to 48.7 mg kg^-1 total P of which from 28 to 96% was inorganic P and from 4 to 28% was enzymatically hydrolysable. There was a wide range in soil physical and chemical properties: 14 to 69 mg kg^-1 initial Bray P-1, 5.3 to 8.1 soil pH, 13 to 56 g kg^-1 organic matter, and 31 to 226 g kg^-1 clay. The increase in STP with manure application was related to total inorganic P applied (inorganic P soluble in water+NaHCO₃+NaOH+HCl) for 10 soil series. For another 12 soil series, the increase in STP was related to the total applied inorganic and also to the total hydrolysable P. Analysis of variance showed that the hydrolysis of organic P was related most strongly to soil clay content. Soils with clay content greater than 121 g kg^-1 showed no signs of P hydrolysis and soils with clay content lower than 71 g kg^-1 showed that all hydrolysable P was hydrolyzed. It is hypothesized that increased clay content provided physical protection against organic P hydrolysis through increased total surface area and greater adsorption sites. However, more research needs to be conducted to fully understand the role of clay content on manure organic P hydrolysis.