Effect of degree and duration of water saturation on iron, manganese and exchangeable cations in wetland soils of Maputaland, KwaZulu-Natal, South Africa

Abstract

Soils of the Maputaland Coastal Plain are naturally grey, do not contain any redoximorphic features, and are therefore problematic to interpret during wetland delineation. This study was consequently initiated to determine if these soils were able to reduce in a controlled environment. The study was set up by saturating disturbed soil cores, packed to a density of 1.4 Mg m−3 and then saturating them to 65%, 70%, 75%, 80%, and 85% of porosity. The redox potential (Eh), pH(H2O), iron (Fe), manganese (Mn), and soluble and exchangeable cations were then measured every 3.5 d in the cores. The pH, Fe and Mn increased significantly with increased degree of water saturation, whereas Eh decreased significantly. The concentrations of exchangeable cations were not affected significantly. The optimal threshold degree of water saturation for the onset of reduction was calculated to be between 72% and 78% of porosity and can be used with some confidence to quantitatively differentiate between oxidised and reduced systems during the evaluation of volumetric soil water content data.