Phosphate sorption and its relation to soil properties

Abstract

In order to study the phosphorus sorption characteristics of six soils from Botswana, 2g of soil were equilibrated at 20°C in 0.01M CaCl2 containing various amounts of dissolved phosphate (0-900 pg P/g soil for two days). The phosphorus disappearing from the solution was considered adsorbed. At low solution concentrations (generally < 10 pg P/ml), the adsorption behaviour could satisfactorily be described the Langmuir isotherm equation. The capacity to adsorb added phosphorus varied appreciably among the soils. The slope of the adsorption isotherm (phosphorus buffer capacity) at a supernatant solution concentration of 0.2 ppm was proved to be a suitable reference index to characterize the phosphate sorbing properties of these soils. The phosphorus buffer capacity (PBC) was found to be closely correlated with a single sorption index “PSI" and to the Langmuir sorption maximum. Using these three sorption indices, phosphorus adsorption was found to be significantly correlated with sesquioxides as well as the % clay content and specific surface area but not with pH, exchangeable A1 and Fe forms or organic matter content.