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Secondary Iron and Manganese Distributions and Aquic Conditions in a Mollisol Catena of Central Iowa

Department of Agronomy, Iowa State Univ., Ames, IA 50011

*Corresponding author (fak{at}iastate.edu).

ABSTRACT

Redoximorphic features, saturation, and reduction of Fe are the essential properties that define aquic conditions. Proposed colorimetric tests for reduction of Fe did not respond in sola of a Mollisol catena from central Iowa. The objectives of this study were to determine the distributions of redox-sensitive metals as potential indicators of aquic condition and relate them to soil morphology and water table depth of soils to differentiate aquic from nonaquic conditions. The variabilities of different forms of extractable Fe and Mn throughout the soil profile and their relationships to aquic and nonaquic conditions were investigated for five soils from a Mollisol catena in central Iowa. Selective dissolution methods were used to extract various forms of Fe and Mn. There was a pronounced decrease in total Fe as the duration of saturation increased. With permanent saturation and presence of gray matrices in soil horizons, content of Fe_d and Mn_d (citrate-bicarbonate-dithionite-extractable Fe and Mn) decreased, but Mn_d increased where periodic saturation exists. The ratio of Mn_d/Fe_d with its depth function in conjunction with water table data and soil morphology separated aquic from nonaquic conditions among members of the Mollisol catena studied. Therefore, secondary Fe and Mn distributions within this Mollisol catena are better indicators of aquic and nonaquic environments than the colorimetric tests for Fe²⁺. This ratio needs to be tested in other areas and could be used as an alternate criterion instead of reduction of Fe when a colorimetric test does not differentiate aquic from nonaquic conditions.

Journal paper no. J-15439 of the Iowa Agric. and Home Economics Exp. Stn., Ames, Project no. 2091. Contribution from the Agric. and Home Economics Exp. Stn., Ames, IA.

Received for publication March 11, 1994.

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