Impact of liming on extractable phosphorus, exchangeable base cations, and effective cation exchange capacity of eastern Free State soils

Abstract

Soil acidity can adversely affect wheat production in the eastern Free State, South Africa. Liming of the soils is therefore essential. The aim of this study was to establish whether extractable phosphorus (P), exchangeable calcium (Ca), magnesium (Mg) and potassium (K) concentrations and cation ratios remained within recommended ranges after liming. Incubation and field studies were done on soils of wheat fields at Kestell, Fouriesburg, Prinsloo and Melrose in the eastern Free State, South Africa. Lime was applied at different rates to induce soil acidity ranges for determination of Bray 1 extractable P and ammonium acetate (NH₄OAc) exchangeable Ca, Mg and K concentrations. Cation ratios (Ca:Mg, Mg:K and (Ca + Mg):K) and effective cation exchange capacity (ECEC) were also calculated. Apart from the Prinsloo soil, liming decreased extractable P concentration only in the incubation study. In the incubation and field studies exchangeable Ca and Mg concentrations increased while exchangeable K concentration remained unchanged. Despite liming of the soils, exchangeable Ca and Mg concentrations at Kestell and Fouriesburg were still less than the required thresholds. The Ca:Mg ratio decreased at both sites while the Mg:K and (Ca + Mg):K ratios increased with liming. Liming increased ECEC of all soils at all four sites. Our results show that liming does not always result in the recommended cation levels required for soil fertility.

Keywords:

• cation ratios
• effective cation exchange capacity
• exchangeable cations
• extractable phosphorus

Acknowledgements

We are thankful for the financial support of ARC-Small Grain, Winter Cereal Research Trust and Omnia Fertilizer. The contribution of ARC-Small Grain staff, especially Willem Kilian and Willem Otto, and the collaboration of farmers was very valuable. Inputs by Marie Smith from the ARC-Biometric Unit for statistical design and analyses are also appreciated. We are also thankful to Adri Moffat for the linguistic editing of the manuscript.