Disputable issues in interpreting the results of chemical extraction of iron compounds from soils

Abstract

In Russia, iron is chemically fractionated according to a parallel scheme. Pyrophosphate-soluble iron (Fepyr) is considered to participate in organomineral complexes, oxalate-soluble iron (Feox) is believed to enter amorphous + poorly crystallized compounds, and dithionite-soluble iron (Fedit) is meant to represent the free (nonsilicate) compounds. However, the investigations prove that the commonly used subtraction operations (Feox − Fepyr) and (Fedit − Feox) are invalid because of the nonadditive action of the reagents in the parallel scheme of extraction. The low selectivity of reagents requires a new interpretation of chemically extracted iron compounds. In automorphic soils, the content of oxalate-soluble iron should be interpreted as the amount of Fe(III) capable of complexing with organic ligands; in hydromorphic soils with a stagnant moisture regime, it should be interpreted as the amount of iron (III) capable of being reduced in a short time. The content of dithionite-soluble compounds should be regarded as the amount of iron (III) within both (hydr)oxides and silicates potentially prone to reduction.