MICROAGGREGATES FROM OXISOLS AND INCEPTISOLS - DISPERSION THROUGH SELECTIVE DISSOLUTIONS AND PHYSICOCHEMICAL TREATMENTS

In the present work, aggregation of soils containing iron oxides was i nvestigated through dispersion treatments. Microaggregates (0.1-0.2 an d 0.2-0.5 mm) of six Oxisols from Brazil and Cameroon and three Incept isols from Mid-Europe were treated with eleven different dispersing ag ents and the resulting pal-tide size distribution was measured. Dithio nite (DCB), which selectively removes Fe-oxides promoted the highest d ispersion. This indicated that Fe oxides participate in aggregation. T reatments with specifically adsorbing anions (citrate, oxalate, bicarb onate, phosphate) disaggregated the samples to different degrees. Citr ate alone was almost as effective as DCB although only small amounts o f Fe were dissolved. This behaviour was related to an anion specific a dsorption on Fe oxides leading to aggregate dispersion. A highly signi ficant correlation was found between degree of dispersion after oxalat e treatment and the ratio Si-0/Fe-0. Dispersion by dissolution of surf ace-active oxides of Fe and Al, by surface charge modification through specific adsorption of anions and disruption of Si-O-Fe bonds are dis cussed. The dispersive effect of various anions followed approximately its order in adsorption affinity for Fe oxides.