SOIL FORMATION IN A TOPOSEQUENCE OF OXISOLS FROM PATOS DE MINAS REGION, MINAS GERAIS STATE, BRAZIL
Abstract
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 soils relate to leandscape features, a toposequence 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 1 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) geothite decreased slightly; (8) maghemite was absent in the footslope positons and present in small amounts in backslope and summit positions; and (9) magnetic susceptibility decreased steadily. Downslope movement of Si or preferential removal of Si from upper slope positons controls the weathering and formation of clay minerals in the landscape. In the lower slope positions, which are wetter than upper positions, goethite formation apparently is favored relative to hematitie and maghemite. Goethite was found to have a smaller crystallite size and more Alsubstitution than hematite. Differential X-ray Diffraction Analysis (DXRD) combined with multipleregression analysis provided a quantitative estimate of the different iron oxide minerals present in the sample. A hand magnet attached to a small spring balance provided a quick and reliable quantitative estimate of soil magnetic susceptibility in the field. A simple redness index was calculated for mottled horizons. Also, a new redness index that emphasizes the role of hue in soil color relationships was used.
Degree
Ph.D.
Subject Area
Agronomy
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