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Some part of the phosphorus in soils is bound organically. Organic P is often dominant form in top soils, though largely unavailable for plants. A large propotion of P that is applied to soil as fertilizers rapidly becomes unavailable to plants, accumulating in inorganic P fractions that are fixed by chemical adsorption and precipitation, and organic P fractions that are immobilized in soil organic matter and microbial biomass. The average content of organic phosphorous in cultivated soils ranges from 5-50 percent of total P (Sarapatka, 2003). Fertilized soils contain a significant amount total soil P, which some 50-80% may exist in inorganic form (McLaughlin et al., 1990). The importance of soil organic P as source of plant available P depends on its rate of solubilization and the rate of inorganic P release.