Two different calcareous soils (15 and 31 % CaCO3 contents) were subjected to application of 0, 2 and 4 % mature P enriched rice straw compost with and without inoculation with Bacillus mega-therium, the phosphate dissolving bacteria (PDB) before planting barley grains in pots containing 4 kg soil under greenhouse conditions. Measure-ments of plant growth and P uptake as well as evaluation of Olsen-P and P- Q / I kinetic parame-ters in cultivated soils were followed up at 3, 6 and 9 weeks from planting. Obtained results confirmed the significant beneficial action of compost and/or PDB applications in increasing growth and P up-take by plants grown on both tested calcareous soils with values were dramatically lower in the 31% than in the 15% CaCO3 soil. Values of meas-ured Olsen –P were affected by the application of compost and PDB in both tested soils in a manner similar to that of P uptake where significant in-creases were recorded particularly as the growth season gets advanced with lower values were ob-tained in the high CaCO3 soil. Application of rice straw compost and/or PDB activated the kinetic parameters describing P availability in both tested calcareous soils. Correlation coefficients between P uptake by whole plants and the different parame-ters expressing P availability in soils revealed high sensitivity of the parameters of phosphorus buffer-ing capacity (PBC) and Olsen-P in predicting soil P status and subsequently controlling P fertilization for long growth season plants grown on the lower calcareous soil (15% CaCO3). The parameter of Q0 proved its significant possibility to be used for very short growth season plants grown on both lower and higher (15 and 31% CaCO3) calcareous soils. Other tested parameters, even the Olsen-P, lost their sensitivity in predicting P status in the higher calcareous soil; the application of rice straw com-post and/or bio fertilizer couldn't meet sufficiently or meet hardly P requirements of plants grown on the high CaCO3 soil. So, additional work seemed to be needed to cover this shortage in controlling P fertilization in the high CaCO3 content soil.