A single point sorption test for the routine determination of the phosphorus requirement of low to moderate P-fixing soils

The phosphorus requirement (P0.11) of low to moderate P-fixing soils from the South African tobacco industry and KwaZulu-Natal was interpolated from multiple point P sorption isotherms. The relationship between P0.11 and a single point sorption measurement, consisting of the P concentration left in solution after equilibrating for 24 h additional rates of either 70 or 100 mg P kg−1 soil, over the soils of the tobacco industry was investigated using non-linear regression analysis. The results showed that a high degree of correlation existed between both variables, suggesting that P0.11 could simply and accurately be predicted from a single P sorption measurement instead of a multiple point sorption isotherm. Confirmation of the predictive value of the functions best fitting the data from the soils of the tobacco industry was validated using data of the soils from KwaZulu-Natal. The results showed that values of P0.11 predicted for the soils of KwaZulu-Natal, using the single point P sorption test in conjunction with the regression functions developed for the soils of the tobacco industry, were highly significantly correlated to the P0.11 values interpolated from multiple point P sorption isotherms. The implication of these findings for the routine determination of the phosphorus requirement of soils by soil testing laboratories and P fertiliser recommendations is discussed.