Review Article

Two-step approach to determining some useful phosphorus characteristics of South African soils: a review of work done at the ARC-Institute for Soil, Climate and Water


Work carried out at the ARC-Institute for Soil, Climate and Water on soil phosphorus characteristics was reviewed. Regression functions were established in order to test the correlations between P parameters, single point indices and soil properties. The parameters investigated were selected indices of P sorption, including the Langmuir adsorption maximum (Lm), as well as indices of the P buffer power of the soil, including the slope of each of the Langmuir (Lbp), Freundlich (Fbp) and Temkin (Bt) P sorption isotherms. Other parameters included an index of the P requirement of the soil (P0.11), the phosphorus requirement factor in terms of the Ambic 2 extraction procedure (PRFAmbic2) and an index of the slow reaction of P sorption with soils (dx/dt)24. The results showed that to obtain a measurement of these parameters, cumbersome multiple point procedures could mostly be substituted by single point indices more suited for routine determination purposes. Lm, Lbp, Fbp and Bt could readily be estimated from measuring the sorption index (SI) as determined by the National Soil Survey in modal soil profiles. P0.11 could be estimated by combining, in multiple regression equations, SI with a measure of the native P content in terms of either the Bray 2 or Ambic 1 extraction procedures. Alternatively P0.11 could be estimated from measures of either S1 or S2, the concentration of P in soil solution remaining after incubating the soil for 24 h with additional rates of 70 or 100 mg P kg−1 soil respectively. PRFAmbic2 was found to relate to the Ambic 2 phosphorus desorption index. The rate of the slow reaction, (dx/dt)24, was found to relate to both the clay and the citrate bicarbonate dithionite-Fe contents of the soil. The implication of these findings for the routine determination of soil phosphorus characteristics by soil testing laboratories involved in crop nutrition or environmental advisory work is discussed.

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