Charge Distribution and the Interactive Effect of pH and Ionic Strength on Phosphate Adsorption Properties of Two Benchmark Soils from Botswana

The constraint imposed by phosphate (PO4) deficiency on soil fertility in Botswana necessitated the study of ionic strength () and pH effects on electric charge distribution and PO4 adsorption of two representative soils: Typic Pellustert and Typic Haplustalf. Electric charge increased with increasing soil pH, but the effect of mu on electric charge distribution was indistinct. The point-of-zero-salt-effect (PZSE) occurred at pH 4.8 in the Pellustert and at almost 6.0 in the Haplustalf. Ionic strength decreased PO4 adsorption in the Pellustert, but increased it in the Haplustalf. The Scatchard and reciprocal linearization of the Langmuir model adequately described PO4 adsorption of the soils under varying mu and pH x mu conditions. The PO4 adsorption maxima (M) and affinity index (b) decreased with increasing mu in the Pellustert, but not in the Haplustalf. The decreasing M and b with increasing mu in the Pellustert was explained by electrostatic repulsion from the highly negative charge surface, whereas the increasing M and b with increasing mu in the Haplustalf was attributed to enhanced access to the surface sorption sites due to compression of diffuse double layer. Under varying pH x mu conditions, PO4 sorption sites were resolved into low- and high-energy sites, depending on the concentration of added PO4. Because b is related to the ease of PO4 detachment from soil surfaces into solution for plant uptake or leaching, PO4 added to this Pellustert at mu>0.01 would likely leach freely, and hence mu monitoring is imperative for PO4 fertilization strategy of this Pellustert.