Bioavailability of inorganic phosphorus fractions in calcareous soils estimated by Neubauer technique, iron-impregnated filter paper, and chemical tests

Plants commonly suffer from phosphorus (P) deficiency in calcareous soils. Plant responses to P application on such soils mostly show poor correlation with their soil test P values. Experiments were conducted on 24 different soil samples under laboratory and greenhouse conditions to illustrate the relationship of various inorganic P fractions in different calcareous soils with P uptake by plants, P extraction by iron-impregnated filter paper, and P soil test values estimated by 0.5M NaHCO3 and ammonium bicarbonate diethylene triamine penta-acetic acid. Total P in the 24 soils ranged from 652 to 1245 mg kg(-1) with a mean of 922 mg kg(-1). A major proportion (98%) of inorganic P was in HCl-P (Ca-bound) form. The HCl-P (Ca-bound) ranged from 296 to 729 with a mean of 480 mg kg(-1). The iron (Fe) and aluminum (Al)-P (NaOH-P) ranged from 0.92 to 12 mg kg(-1) with a mean of 1.57 mg kg(-1). The Fe-P (citrate-dithionite bicarbonate) ranged from 0.22 to 4.40 mg kg(-1) with a mean of 5.99 mg kg(-1). Data regarding P release from the soil matrix obtained by desorption with iron-impregnated filter paper was best described by the Elovich equation. Range of slope and intercept values were found to be 5.48 to 17.3 and 17.23 to 56.27 mg kg(-1) respectively. Intercept values calculated for the Elovich equation may be related to labile P initially available for plant uptake in soils. Intercept values calculated for the Elovich equation correlated (r = 0.77) significantly (p < 0.01) with NaHCO3 extractable (Olsen-P)P. Significant correlation (p < 0.05) of intercept with CDB-P (r = 0.44) and of slope with HCl-P (0.43) suggested that the initially available P, regulated through CDB-P, is replenished by HCl-P [calcium (Ca) bound].