Bioavailability and distribution of phosphorus among inorganic fractions in calcareous soils

Deficiency of phosphorus (P) is widespread in calcareous soils. Phosphorus uptake by plants responding to P application on such soils generally shows poor correlation with their soil test P values. Experiments were conducted under laboratory and greenhouse conditions to elucidate the relationship of various inorganic P fractions in eleven calcareous soils with P uptake by plants, P extraction by electro‐ultrafiltration (EUF), and P extraction by other commonly used methods. Total P ranged 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 EUF followed the power function equation. The intercept values calculated 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 constant) 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 dry weight and total P content in plants on all soils except the Kotli series (Entic Chromustert). The CB‐P significantly (P < 0.05) correlated 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 power function equation together explained 62% variation in NaHCO3 extractable P.