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Effect of Redox on the Solubility and Availability of Iron¹

ABSTRACT

The solubility and availability of Fe to plants were measured in redox-controlled soil suspensions to compare theoretical relationships with experimental observations. A laboratory device was constructed to control the redox potential of a soil suspension to ± 10 mV. After 4 weeks of incubation at a given redox, the solution phase was separated from the solid phases and analyzed. The activity of Fe²⁺ was calculated from solution analyses by correcting for hydrolysis, complexation, and activity coefficients. Graphs of activity vs. redox revealed that below pe + pH 8.5, Fe²⁺ was controlled by FeCO₃ (siderite). Above pe + pH 8.5, Fe₃(OH)₈ (ferrosic hydroxide) controlled Fe²⁺ activity. Ferrous ion activity was calculated from published solubility data and plotted as a function of redox. Ferrosic hydroxide was shown to be the iron-controlling solid phase in many of these studies. Under conditions of low pH, lepidocrocite(-FeOOH) or goethite(-FeOOH) was the controlling solid phase of Fe. Under controlled redox conditions, it was demonstrated that Fe uptake by rice (Oryza sativa cv. Starbonnet) was related to the concentration of Fe²⁺ in solution.

¹ Contribution from the Dep. of Agronomy, Colorado State University, Fort Collins, CO 80523. Published as Scientific Series no. 2708. Supported in part by the Division of Agricultural Relations, Agricultural Research Branch, Tennessee Valley Authority, Muscle Shoals, Ala., under agreement no. Colo. 1131-101.

² Graduate Research Assistant and Professor, Dep. of Agronomy, Colorado State University, Fort Collins, CO 80523.

Received for publication December 17, 1981. Accepted for publication September 14, 1982.

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