Technical Abstract: Animal manures and organic sources of nutrients such as crop residues are major sources of organic phosphorus (P) applied to soils. They are attracting a great deal of research attention and regulatory interest because animal digestive systems are markedly inefficient at retaining dietary phosphorus, and much of the feed phosphorus that occurs as myo-inositol hexakis dihydrogenphosphate or phytic acid is found intact in the animal feces. As manure organic P is environmentally bioactive, a novel in situ enzyme hydrolysis assay was developed to identify water soluble- and labile complexed P pools, and to determine mechanisms controlling P solubilization and hydrolysis to replenish the environmentally sensitive water-soluble P fraction. The overlapping in situ extraction and hydrolysis method provides a relative scale of chemical and biological stability of the released P. The phytase-hydrolysable P assay provides more information about on-farm P management than just knowing total P content of the manures. The assay¿s simplicity and robustness over the wide range of manure characteristics may increase routine evaluation of whole-farm accumulation of environmentally sensitive P forms. As extracellular phosphohydrolases are ubiquitous in the environment, biological and biochemical mechanisms more adequately reflect the availability of manure and soil P pools to soil microorganisms and plants over time. Moreover, these mechanisms can reveal the underlying potential for the timed release of potentially bioactive P and protracted impairment of aquatic ecosystems by agricultural P released to the environment.