The objectives of this research were to determine inorganic phosphorus (P) fractions in calcareous soils of southern Khorasan and find their relationship with sorghum-available P. Eighty soil samples were obtained and analyzed for some physical and chemical characteristics, among them 24 samples that varied in plant-available P and soil properties were used for soil testing. From 24, 8 samples were selected for P fractionation as well. Five extraction procedures were used for soil testing. Results indicate that the extracted plant-available P by the Olsen et al. (1954) and Paauw (1971) methods show the greatest correlation coefficients with plant P uptake and sorghum dry matter. The sequential inorganic P extraction analyzing indicated that the abundance of various inorganic fractions was in the order Ca 10 -P . Al-P . Ca 8 -P . Ca 2 -P . Oc-P . Fe-P. The results also indicate Olsen P correlates positively and significantly with Ca 2 -P and Fe-P fractions and positively but not significantly with the Al-P fraction.

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