IMOGOLITE AND PROTO‐IMOGOLITE ALLOPHANE IN SPODIC HORIZONS: EVIDENCE FOR A MOBILE ALUMINIUM SILICATE COMPLEX IN PODZOL FORMATION

Abstract

Summary Examination by infrared spectroscopy and electron microscopy of the fine clays (<0.5 μm) dispersed at pH 3.5 from H 2 O 2 ‐treated soil indicates that imogolite and proto‐imogolite allophanes are concentrated in podzolic B 2 and B 3 horizons, and make up at least 6 percent of one B 2 horizon soil, which contains virtually no layer silicate clays. It is argued here that imogolite‐type components are the principal source of extractable aluminium and silicon in such horizons, that they may act as cementing agents in indurated horizons, and that proto‐imogolite, a soluble aluminium‐silicate complex, is the predominant mobile form in which aluminium is transported to B 2 and lower horizons of podzols. Comparison of the amounts of aluminium extracted by acetic acid with those extracted by EDTA indicates that extractable aluminium in Bhg, Bh, and organic‐rich A 2 horizons is present principally in organic complexes. It is proposed that the aluminium fulvates concentrated in these horizons are formed in situ .