Interrelation of Inorganic Phosphorus Fractions and Sorghum-Available Phosphorus in Calcareous Soils of Southern Khorasan

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 Ca10-P Al-P Ca8-P Ca2-P Oc-P Fe-P. The results also indicate Olsen P correlates positively and significantly with Ca2-P and Fe-P fractions and positively but not significantly with the Al-P fraction.