Characterization of iron in alkaline EDTA and NH ₄ OH extracts of podzols

Summary Mössbauer and ESR spectroscopy have shown that the iron extracted from the Bh horizons of an iron humus podzol and an iron podzol by EDTA at pH 9.1 is predominantly in the form of complexes of polymeric Fe(III) hydroxide and oxide with organic matter (O.M.). Small amounts of monomeric Fe(III)‐O.M. and Fe(III)‐EDTA complexes also occur. In contrast EDTA at pH 7 extracts iron from these podzols predominantly in the form of iron‐EDTA complexes. Some monomeric Fe(III)‐O.M. complex also occurs in a pH 9.1 NH 4 OH extract of these horizons and in a pH 9.1 EDTA extract of the B 3 horizon of a peaty podzol. Dialysis experiments show that the particle dimensions of the polymeric hydroxy Fe(III)‐O.M. complex, which accounts for about 66% of the Fe extracted from the iron humus podzol and about 36% of that from the iron podzol, are greater than 2.4 nm. The thermal behaviour of the Mössbauer peaks indicated that the size of the iron cores was of the order of 5 nm, thus suggesting that the complex probably consists of hydroxyiron cores surrounded by large organic molecules. Results from XRD and IR suggest that these hydroxyiron cores may have structural organizations similar to those of goethite and ferrihydrite. The relationship between these forms of iron in the extracts and those in the soil is briefly discussed.