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Effect of Flooding on the Eh, pH, and Concentrations of Fe and Mn in Several Manitoba Soils¹

ABSTRACT

The changes in Eh, pH, and concentrations of Fe and Mn in the solutions extracted from six Manitoba soils saturated with water were studied at various times of incubation at room temperature. Gradual reduction of the extracts was observed during the first 3 to 4 weeks of incubation. After this period, the extracts, in many instances, gradually returned to relatively oxidized states with occasional reduction. Somes changes in pH were observed but the magnitude of change was relatively small in soils with a relatively high cation exchange capacity. Both Fe and Mn concentrations generally increased with increased reduction. This trend was more pronounced for Mn than for Fe.

E⁰ values calculated from experimentally measured Eh, pH, and concentrations of Mn appeared to indicate the presence of the Mn₂O₃ – Mn²⁺ redox system. The calculated values of E⁰ based on the assumption that the Fe₂O₃ – Fe²⁺ redox system was operative were far greater than the theoretical value.

Almost all the Fe in soil solution was complexed with organic matter whereas Mn was either not complexed or only weakly complexed. The metal-organic matter complexes were negatively charged. The soil solutions had a greater affinity for Fe than did 10^-4M EDTA. It is suggested that the very high degree of complexing of Fe in soil solution was most probably responsible for failure to obtain the theoretical E⁰ values for the Fe₂O₃ — Fe²⁺ redox system from the experimental data.

¹ Contribution from the Dep. of Soil Science, Univ. of Manitoba, Winnipeg, Canada.

² Formerly Canadian Commonwealth Scholar and Associate Professors of Soil Science, respectively. Present address of senior author: Ministry of Agriculture and Natural Resources, Benin City, Nigeria.

Received for publication August 29, 1972. Accepted for publication December 8, 1972.