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Soil-landscape-parent material relationship and pedogenesis of some "Mar de Morros" soils

Little is known about the effect of mafic parent materials on soil genesis in the humid "Zona da Mata", Minas Gerais, although there are many detailed studies on soils developed from felsic to mesocratic gneiss, notably Latosols and terrace Podzolic soils. To fill this gap, soils in two topolitosequences, comprising A and B or A and C horizons, were sampled and studied. In the first topolitosequence, at Serra de Guiricema, a Red-Yellow Latosol on the top, a Argilluvic Chernosol, a Vertisol and a Red Nitosol on the mid-slope, were studied. In the second topolitosequence, at Ponte Nova Depression, on the Rio Doce Valley, a Red-Yellow Latosol, two Redish Nitosols, an Argilluvic Chernosol, a Red-Yellow Argisol, a Gleisol and a Fluvic Neosol were studied. In addition, a Bw horizon of a Red-Yellow Latosol associated with amphibolite intrusion, unrelated to the two topolitosequences above, was studied as contrast. The results indicated that soils developed from mafic rocks tend to be eutrophic and kaolinitic, with some participation of 2:1 clays, such as illite, smectite and smectite/illite interstratified, when less weathered. The lower Feo/Fed ratio in the Latosols indicates the dominance of well-crystalline Fe-oxides. In the Reddish Nitosols, however, the Feo/Fed ratio was constrained by the preferential attack on illite and maghemite in the oxalate extraction, compared with DCB extraction. The Latosol sampled on mafic intrusion showed chemical characteristics similar to those developed on gneiss, suggesting a presence of overlaying reworked material. The biotite and amphiboles present in the gneiss are the main source of Fe, contributing to Fe2O3 levels higher than 11 dag kg-1 in the regional Latosols. Soils with high activity clays have their genesis and 2:1 mineral stability attributed to ferromagnesian richness in the parent material, concentrating pedoenvironment and, in some cases, to the northern slope exposure, favoring high evapotranspiration. In the Gleisol, the local relief and ill drainage seem to be the main causes of the 2:1 expansive clays occurrence. The Red-Yellow Argisol, though located in flat terraces, lack 2:1 clays, even with high Fe2O3 contents, confirming results obtained by other studies in the same region.

crystalline rocks; mafic rocks; clay mineralogy; tropical soils; Latosols; chemical characterization

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