Addition of organic manure not only provides additional sources of nutrients, but improves the soil physical and chemical conditions and may increase the efficiency of added P fertilizers. Information on the availability of P following chemical fertilizer and compost application to soil may improve the management of P fertilization. Studies on P reactions over time and the role of organic matter in calcareous soils are important for developing P fertilizers and manure management practices. The present research work was conducted to study the combined effect of various levels and types of organic and inorganic P fertilizers on P availability in a calcareous soil through an incubation experiment.

Treatments included four rates of P (20, 40, 80 and 160 mg P₂O₅ kg^-1 soil) and a control.  Phosphorus was from one inorganic source (KH₂PO₄) and from two organic sources (cattle manure and sludge compost). The soil was incubated at 25^oC and was maintained at 80% water holding capacity. Change in the amount of available P was measured during 16 weeks. By increasing the time of incubation, P availability in soil significantly decreased for both organic and inorganic fertilizers. It was concluded that the most critical time for incubation was the first week. During this period, the soil lost about 50% of the added P. During the first week, inorganic P fertilizer yielded more extractable P compared with the two organic sources. During the rest of incubation period, the amount of P available from the cattle manure compost was the highest.