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

Abstract

The constraint imposed by phosphate (PO₄) deficiency on soil fertility in Botswana necessitated the study of ionic strength (μ) 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 µ 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 µ and pH × µ conditions. The PO₄ adsorption maxima (M) and affinity index (b) decreased with increasing µ in the Pellustert, but not in the Haplustalf. The decreasing M and b with increasing µ in the Pellustert was explained by electrostatic repulsion from the highly negative charge surface, whereas the increasing M and b with increasing µ in the Haplustalf was attributed to enhanced access to the surface sorption sites due to compression of diffuse double layer. Under varying pH × µ 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 µ > 0.01 would likely leach freely, and hence µ monitoring is imperative for PO₄ fertilization strategy of this Pellustert.

Keywords:

• Adsorption maximum
• affinity index
• electric charge
• point-of-zero-salt effect

Acknowledgements

We thank Botswana College of Agriculture for allowing us to use laboratory facilities and S. M. Machacha for providing the soil samples for this study.