Phosphorus Sorption in Soils and Clay Fractions Developed from Different Parent Rocks in Limpopo Province, South Africa

Phosphorus (P) sorption dynamics in soils have implications for the environment and soil fertility. Soils and clay fractions that were developed from basalt, granite, arkosic sandstone, and gneiss in Limpopo Province, South Africa were analysed for their P adsorption characteristics and external phosphorus requirements (EPR). The relationship between the P adsorption parameters and EPR of the soils and clay fractions were also assessed. The Langmuir adsorption isotherms for the soils and clay fractions gave a better fit with slightly higher R-square values relative to the Freundlich adsorption isotherms. The Langmuir P sorption maxima were between 285.71 and 833.33 mg/kg and 238.09 and 625.0 mg/kg for the soils and clay fractions, respectively, and the EPR values ranged from 7.78 to 92.91 mgP/kg and 5.13 to 65.85 mgP/kg for the soils and clay fractions, respectively. The variations in the EPR suggest a single, uniform P fertiliser application to the soils could cause under-fertilisation and over-fertilisation problems. The soils that were developed from basalt, relative to the others, showed no risk to the water quality in the region at the current rate of P fertiliser application. The P sorption parameters of the soils and clay fractions showed no statistically significant differences. Hence, the P sorption parameters of the clay fractions could be reliable predictors of the P sorption and buffering in their respective soils.