Soil formation in a toposequence of oxisols from Patos de Minas region, Minas Gerais State, Brazil.

Tuffite-derived soils, widespread in the Patos de Minas Region, are among the most productive soils of Minas Gerais State, Brazil, and have potential for greater agricultural development. To understand how properties of these Ustox relate to landscape features, a topossequence with 40 m relief in 500 m, was sampled on the flatter stretches of a valley slope. From summit to footslope positions the following trends in properties of the B horizon (about I m depth) were observed: 1. moist soil color graded progressively from dusky red to strong brown with mottles; 2. mica content increased; 3. hydroxy interlayered vermiculite, present in very small amounts, showed no clear trends; 4. kaolinite content decreased in an irregular fashion; 5. gibbsite and anatase decreased; 6. hematite decreased; 7. goethite decreased slightly; 8. maghemite was absent in the footslope positions and present in small amounts in backslope and summit positions; and 9. magnetic susceptibility decreased steadily. Downslope movement of S1 or preferential removal of S1 from upper slope positions controls the weathering and formation of clay minerals in the landscape. In the lower shope positions, which are wetter than upper positions, goethite formation apparently is favored relative to hematite nad maghemite. Goethite was found to have a smaller crystallite size and more alsubstitution than hematite. Differential X-ray diffraction analyis (DXRD) combined with multipleregression analyis provided a quantitative estimate of the different (...)

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