Organic materials and manures generally contain a combination of organic and inorganic P compounds. Organic P compounds are not directly digestible by many animals without the aid of naturally occurring or added enzymes. As organic P compounds are excreted and added to soil, soil phosphatase enzymes begin the conversion of organic P compounds to inorganic phosphate, which can be utilized by plants.

A number of studies have been conducted to compare the plant nutrient value of manures and composts with soluble P fertilizers. Some research has found that manure P is effective as soluble P fertilizer for meeting the nutrient demand of crops, while other research has shown that manure P is initially less effective than soluble P fertilizer. Since a significant fraction of manure P may be present in organic compounds that require hydrolysis prior to plant uptake, microbial activity can play an important role in the short-term release of plant-available P. The physical and chemical properties of the organic substrate will also influence the short-term inorganic P release rate. However, the long-term value of organic P sources can make an important contribution to plant nutrient requirements.

The application of manures and composts commonly results in gradual accumulation of P in the soil when they are applied to meet the N requirement of the growing crop. The N:P ratio in manures is generally lower than in the portion of the crops removed from the manured field. Continued overapplication of P may initially result in elimination of P deficiencies, but excessive soil P concentrations may lead to water quality problems due to runoff or leaching. Similar to fertilizers, attention must be given to placement, rate, timing, and form to use this resource effectively.