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Transformation of Iron in a Waterlogged Soil as Influenced by Redox Potential and pH¹

ABSTRACT

The distribution of different forms of iron in a waterlogged soil was studied over a wide range of closely controlled redox potential and pH conditions. Increases in water soluble and exchangeable iron were favored by a decrease in both redox potential and pH. The critical redox potentials for iron reduction and consequent dissolution was between +300 mV and +100 mV at pH 6 and 7, and –100 mV at pH 8, while at pH 5 appreciable reduction occurred at +300 mV. The distribution between water soluble and exchangeable iron fractions was highly pH dependent with a decrease in pH at a given redox potential increasing the relative amount of ferrous iron in the soil solution at the expense of that on the exchange complex. A thermodynamic approach to the equilibria between solid phase ferric oxyhydroxide and a water soluble species of iron (Fe²⁺) indicated that it was largely governed by the Fe²⁺ – Fe(OH)₃ system in which the ferric oxyhydroxide was a mixture of goethite and amorphous material.

¹ Contribution from the Dep. of Agronomy, Louisiana State Univ., Baton Rouge, La. 70803.

² Research Chemist on leave from Kyushu Agr. Exp. Sta., Section of Soils and Plant Nutrition, Ministry of Agriculture and Forestry of Japan, Chikugo, Fukuoka Prefecture, Japan and Professor of Agronomy, respectively.

Received for publication April 30, 1973. Accepted for publication September 26, 1973.