Soils with brown or yellowish brown hues are dominant in the high altitude areas of southern Brazil. This feature is a consequence of the cool weather and high humidity conditions, which contribute for the formation of goethite and complete soil xantization. However, in some sites, soils present brown and yellowish brown hues in surface and transitional horizons, yet preserving red colors in subsurface horizons what may represent a relic of a drier and hotter climate condition from the past. In this work, structural characteristics of the iron oxides goethite and hematite were determined in reddish and brown/yellowish brown horizons to help understand the genesis of these soil features. Two profiles of Red Nitosols (Dark Red Podzolic soils) with brown/yellowish brown surface horizon were described and sampled in the Planalto de Lages, Santa Catarina, Brazil. Morphological, physical and chemical methods were employed to characterize these profiles. X-ray diffraction analysis was used for the iron oxide identification from soil horizons, laterite crust, pedotubules, and gibbsite nodules. Structural parameters of goethite in the brown/yellowish brown horizons were different from those in the red subsurface horizons, mainly with respect to isomorphic replacement of iron by aluminum which was higher in the former; hematite showed similar characteristics in all horizons. These differences and similarities, respectively, suggest that hematite and possibly part of the goethite with less substitution of iron for aluminum substitution were preferentialy dissolved in the surface horizons. Thus, surface horizon xantization may be a result from residual accumulation of goethite population with high replacement of iron by aluminum, and from new goethite formed by iron reprecipitation in the present environmental conditions.
soil yellowishing; goethite; hematite; climatic changes