Effect of inorganic P fertilization on the re-sponse of barely growth and P uptake as well as different parameters expressing P availability in-cluding those of P- quantity- intensity relation ship was evaluated for a high CaCO3 content calcare-ous soil (31 %) subjected to application of 0, 2 and 4 % mature rice straw compost with and without inoculation with Bacillus megatherium, the phos-phate dissolving bacteria (PDB). Measurements of plant growth and P uptake as well as evaluation of Olsen-P and P- Q / I parameters in the cultivated soil were followed up at 3, 6 and 9 weeks from cultivation. Obtained results indicated a significant positive action for P fertilization in increasing the significant activation of growth and P status in grown barley plants resulted from application of used rice straw compost and /or bio-fertilizer. Inor-ganic P fertilization also increased values of the different parameters describing P availability in the concerned soil and alleviated significantly the de-pressive action of high existence of CaCO3 in P availability. This is true if the response was regard-ed to P fertilization or to the activating the roles of the application of both rice straw compost and used bio fertilizer. Equations calculated for re-gression of P uptake by different plant parts over the different parameters of P availability in the tested soil and as responded to P fertilization showed that the parameter of P capacity (Q0) is more suitable, if compared to Olsen-P and other kinetic parameters such as equilibrium P potential (EPP) and P buffering capacity (PBC) and its sen-sitivity increased with inorganic P fertilization to this calcareous soil having high CaCO3 content (31 %).