SECONDARY IRON AND MANGANESE DISTRIBUTIONS AND AQUIC CONDITIONS IN A MOLLISOL CATENA OF CENTRAL IOWA

Redoximorphic features, saturation, and reduction of Fe are the essent ial properties that define aquic conditions, Proposed colorimetric tes ts for reduction of Fe did not respond in sola of a Mollisol catena fr om central Iowa. The objectives of this study were to determine the di stributions 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 variabilit ies of different forms of extractable Fe and Mn throughout the soil pr ofile and their relationships to aquic and nonaquic conditions were in vestigated for five soils from a Mollisol catena in central Iowa, Sele ctive dissolution methods were used to extract various forms of Fe and Mn, There was a pronounced decrease in total Fe as the duration of sa turation increased. With permanent saturation and presence of gray mat rices 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 func tion in conjunction with water table data and soil morphology separate d aquic from nonaquic conditions among members of the Mollisol catena studied. Therefore, secondary Fe and Mn distributions within this Moll isol catena are better indicators of aquic and nonaquic environments t han the colorimetric tests for Fe2+. This ratio needs to be tested in other areas and could be used as an alternate criterion instead of red uction of Fe when a colorimetric test does not differentiate aquic fro m nonaquic conditions.