BIOAVAILABILITY AND DISTRIBUTION OF PHOSPHORUS AMONG INORGANIC FRACTIONS IN CALCAREOUS SOILS

Deficiency of phosphorous (P) is widespread in calcareous soils. Phosp horus uptake by plants responding to P application on such soils gener ally shows poor correlation with their soil test P values. Experiments were conducted under laboratory and greenhouse conditions to elucidat e the relationship of various inorganic P fractions in eleven calcareo us soils with P uptake by plants, P extraction by electro-ultrafiltrat ion (EUF), and P extraction by other commonly used methods. Total P ra nged from 92.5 to 862.5 mg P kg-1 soil. A major proportion (88-99%) of inorganic P was in HCl-P (Ca bound) form. Phosphorus extraction by EU F followed the power function equation. The intercept values calculate d ranged from 0.13 to 0.94 mg P kg-1 soil. The NaOH-P (nonoccluded Fe and Al-bound P) and CB-P (citrate-bicarbonate extractable P) together explained 76% variation in the intercept values. The slope (rate const ant) correlated (r = 0.58) with the HCl-P (Ca bound) form. Application of 90 mg P kg-1 soil significantly (P < 0.01) increased plant shoot d ry weight and total P content in plants on all soils except the Kotli series (Entic Chromustert). The CB-P significantly (P < 0.05) correlat ed with plant dry matter (r = 0.73) and total uptake of P by plants (r = 0.71). Corresponding values for NaHCO3-P correlated significantly ( P < 0.05) with plant dry matter yield (r = 0.62) and total uptake of P by plants (r = 0.68). The NaOH-P and CB-P together accounted for 76% variation in NaHCO3-P. Slope and intercept values calculated for the p ower function equation together explained 62% variation in NaHCO3 extr actable P.