LITHOSEQUENCE AND TOPOSEQUENCE OF OXISOLS FROM GOIAS AND MINAS GERAIS STATES, BRAZIL

NILTON CURI, Purdue University

Abstract

A toposequence of soils developed from basalt in the Central Plateau of Brazil (Goias State) and a pedon from the Quadrilatero Ferrifero (Minas Gerais State) were studied. In the toposequence of soils derived from basalt, from upper (redder Oxisols) to lower (yellower Oxisols) slope positions the following trends were observed: (1) Hematite content decreased whereas goethite content increased; (2) The amount of Al substitution into the goethite structure increased; (3) Gibbsite content decreased whereas kaolinite content increased; (4) Moist soil color changed from dark reddish brown (2.5YR 3/4) to dark yellowish brown (10YR 4/4); (5) Magnetic susceptibility of fine clay decreased and thus maghemite content probably decreased; (6) P adsorption increased whereas P desorption decreased; (7) Mo content decreased whereas Cu, Mn, Zn, Cr, Ti, V, Co and Ni showed weak or no trends. Trace elements are conserved in the solum by slow release, especially from magnetite, and by adsorption and/or incorporation into the structures of the secondary Fe oxyhydroxides. The lithosequence study revealed that the itabirite-derived Oxisol has the following properties relative to the basalt-derived Oxisols: (1) Greater hematite content, except in the case of the basalt-derived soil in the summit position (G6), with smaller mean crystallite dimension; (2) Greater goethite content with smaller amount of Al substitution; (3) Much less gibbsite and virtual absence of kaolinite; (4) Redder moist soil color: dusky red (10R 3/4); (5) Much higher magnetic susceptibility; (6) Smaller P adsorption and larger P desorption; (7) Much lower content of all analyzed essential nutrients except Fe and P; (8) Purer magnetite (Fe(,2)('3+)Fe('2+)O(,4)), whereas the magnetite in basalt-derived soils is high in trace elements. Thus, the effect of the parent material was still evident even in highly weathered soils, indicating the need for the separation between itabirite- and basalt-derived Oxisols in a system of soil classification. A breaking point at > 50% Fe(,2)O(,3), < 1.5% TiO(,2) and > 15,000 x 10('-6) cgs of magnetic susceptibility, all of which reflect the original parent material, could be the differentiating characteristics. Particle density, amount of trace elements and capacity to fix phosphorus are accessory properties that are associated with the differentiating characteristics.

Degree

Ph.D.

Subject Area

Agronomy

COinS