There is a debate concerning the optimum time of phosphorus application under Egyptian soil conditions. For this purpose, a laboratory incubation experiment was carried out at Soils Department, Faculty of Agriculture, Mansoura University to determine the inorganic phosphorus forms (Olsen-P, Ca-P, Oc-P, Al-P and Fe-P) in calcareous and non-calcareous  (alluvial ) soil types after different times of incubation. Phosphorus was applied at rates of 10, 20 and 30 mg / kg soil. Inorganic phosphorus forms was determined in both soils at the beginning of the experiment (at zero time of phosphorus application), and after one, two, three, four and five weeks from application. Results indicated that inorganic phosphorus forms increased with increasing phosphorus application rate. Available phosphorus form recorded the highest value after the immediate phosphorus application. Available phosphorus concentration decreased gradually over the time to reach the lowest value after the fifth week of application. On the other hand, the other precipitated phosphorus forms were increased over the time of application. Available P form was converted to unavailable P forms in the order Ca-P> Oc-P>Al-P>Fe-P. Results also showed that the fixation power of the calcareous soil was higher than the non-calcareous soil. Based on the obtained results of our experiment, it is recommended to add phosphorus fertilizers at the critical stage of phosphorus needed  by plants.