Pedogenesis on the uplands of the Diamantina Plateau, Minas Gerais, Brazil: a chemical and micropedological study

Little is known about the soil distribution on quartzitic uplands of Brazil. The Diamantina Plateau, where deeply weathered oxisols (latosols) and shallow sandy soils occur side by side, represents part of the higher and oldest planation on the quartzitic uplands of Minas Gerais State. We studied representative soils from the Diamantina Plateau to help understand the pedogenesis and landscape evolution of this part of Minas Gerais. We found evidence of a lithological and tectonic control on the distribution of soils in the landscape. Polygenetic oxisols (Acrustox) occur at the top of the landscape at altitudes of 1300 m or more, resting uncomformably on quartzitic saprocks (Xanthic types) or mafic rocks (Rhodic types). They change downslope to in situ Quartzpsamments and Haplorthods. In the B horizon of the Xanthic Acrustox, quartz grains smaller than 0.3 mm. are surrounded by gibbsitic plasma, forming subspherical microaggregates. Microaggregates in the Bw horizon of all upland oxisols display inherited features of intense biological activity regardless of the parent material or texture. The nutrient-poor catena of Xanthic Acrustox-Ustic Quartzpsamments-Oxyaquic Haplorthods represents a sequence of increasing hydromorphic conditions downwards, in which clay minerals are destroyed in low pH and ferrolysis. In the poorly drained soils downslope, low pH and eH induce the movement of reduced Fe2+ and organic matter (OM)-bound Al and Fe both per descendum and through lateral flow downslope. In the bottom, micromorphological features and the chemistry of organic matter indicated that some Bsh horizons represent buried A horizons subjected to post-burial podzolization, corroborating morphological field data. This indicates the pertinence of the study of humic fractions in defining the Bhs (spodic) horizons of Brazilian spodosols. (C) 2002 Elsevier Science B.V. All rights reserved.