The relationship between the phosphorus requirement and some phosphorus characteristics of selected soils of the South African tobacco industry

Abstract

Estimates of the phosphorus requirement (P0.11) of 20 selected soils of the South African tobacco industry were interpolated from phosphorus sorption isotherms. The relationships between P0.11 and various soil phosphorus characteristics, including Bray 2 P content and various indices of P-fixation, were investigated using regression analysis. The results showed that P0.11 varied widely and is influenced by both the level of Bray 2 P content and the P-fixation capacity of the soil. Measurements of the P-fixation capacity of the soil included the more formal buffer power indices derived from multiple point P sorption isotherms and both the phosphorus desorption index (PDI) and the sorption point index (SI), each a single point test easily determined routinely. SI was shown to relate closely to the buffer power indices and to be a quick and useful index of the soil P-fixation capacity. PDI, on the other hand, was found to be of limited value to predict the phosphorus fixation of the soils. An equation, allowing for P0.11 to be routinely predicted from Bray 2 P content and SI is proposed. Its predictive value was validated from data obtained from five additional soils that originated from the tobacco industry. The implications of this equation for P-fertiliser recommendations and future field calibration programmes are discussed.