Phosphorus sorption in Natal soils

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Abstract
Phosphorus (P) sorption poses a severe constraint on the ability of many soils to supply adequate amounts of P to plants. In an effort to quantify the P sorption capacities of soils in Natal, P sorption isotherms of 50 topsoils from a number of localities in the province were established. The amount of P sorbed at a solution P level of 0.2 mg P L−1, defined as the standard P requirement, varied from 5 to 1174 mg kg−1. Standard P requirement was greatest in highly weathered clay soils and lowest in sands. Various soil parameters were correlated with standard P requirement and isotherm slope at 0.2 mg P L−1. Oxalate-extractable aluminium, organic matter and sample density gave the best correlations. Because the method for the determination of sorption isotherms is tedious and does not lend itself to routine use, the feasibility of predicting the isotherm slope by near infra-red reflectance spectroscopy was also investigated. Preliminary results were encouraging and a calibration based on the samples used in this study yielded an r 2 of 0.82 and a residual standard deviation of 184.