SELECTIVE EXTRACTION OF AMORPHOUS IRON OXIDES BY EDTA FROM SOILS FROM DENMARK AND TANZANIA

Soils from Denmark and Tanzania were extracted with EDTA solutions of different concentrations and pH. After extraction for 3 months there was no significant (95% level) further increase in amounts of iron (and aluminium, calcium, and magnesium) during longer extraction periods. X-ray diffraction showed no change of the crystalline minerals caused by the extraction, which is believed to be specific for amorphous iron oxides. The EDTA method may thus serve as a reference method for the determination of amorphous iron oxides in soils. Although the difference between EDTA-extractable iron and that extracted during 2 h by ammonium oxalate at pH 3.0 in the dark may be high, the ammonium oxalate method is considered to give a fast and often fair estimate of amorphous iron oxides.