Relating the onset of reduction to degree of soil water saturation

Abstract

Literature does not indicate the degree of water saturation at which reduction is expected to occur. This study therefore aimed to determine the degree of water saturation (S) at which reduction is initiated. Soil cores were packed to a bulk density of 1.6 Mg m⁻³ and individually saturated to 60%, 70%, 80% and 90%. A set of cores was analysed every 3.5 d for three months and then once a week for one month. The experiment was terminated after 121 d. Reduction was defined as a decrease in redox potential (Eh) followed by an increase in the soluble Fe²⁺ concentration. The Eh, pH, Fe²⁺ and Mn²⁺ were all significantly affected by degree of water saturation. It was concluded from a laboratory experiment that Fe³⁺ reduction was initiated at a soil oxidisability (pe) of between 6.04 and 7.02 and at degrees of water saturation between S_0.72 and S_0.78. These findings were corroborated with a field experiment done where all in situ soil water measurements in the Weatherley catchment indicated that all the water extracted was reduced and that water extraction was primarily possible at S > 0.69. These determinations are a first confirmation of the hypothesis that reduction sets in at 70% water saturation.