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Effect of Parent Rocks on Chemical and Mineralogical Properties of Some Oxisols in Brazil¹

ABSTRACT

A toposequence of soils developed primarily from basalt in the Central Plateau of Brazil and a pedon developed primarily from itabirite (a hematite-quartz metamorphic rock) were studied in the Quadrilatero Ferrifero. The itabirite-derived Oxisol consists almost entirely—900 to 950 g kg^–1 (90-95%)—of Fe oxides and oxyhydroxides, compared with 250 to 300 g kg^–1 in the basalt-derived Oxisol. The soil on itabirite also has coarser particle size, higher particle density, and somewhat lower surface area. It lacks kaolinite and gibbsite but contains more goethite and has greater magnetic susceptibility. It also is much lower in all essential plant nutrients (except Fe and P), and much lower in several non-nutrient minor elements as well. Magnetite grains in basalt are relatively abundant in minor elements which could be released slowly during mineral weathering. The effect of the parent material is 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. In itabirite-derived soils the Fe₂O₃/TiO₂ ratio (H₂SO₄ digestion) is >16 and the magnetic susceptibility of the whole soil is > 189 x 10^–6 m³ kg^–1 (15 000 x 10^–6 cgs). These limits of soil properties reflect the original parent material and could be the differentiating characteristics in soil classification systems. Particle density, amount of trace elements, and capacity to fix phosphorus are accessory properties associated with the differentiating characteristics.

¹ Contribution from the Agron. Dep., Purdue Univ. Agric. Exp. Stn., West Lafayette, IN 47907. Journal Paper no. 10,632.

² Former Graduate Fellow on leave from Departamento de Ciencia do Solo, Escola Superior de Agricultura de Lavras, Minas Gerais State, Brazil (MEC/CAPES program); and Professor, respectively.

Received for publication February 21, 1986.