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Morphology, Mineralogy, and Genesis of a Hydrosequence of Oxisols in Brazil¹

ABSTRACT

Research was conducted to establish the genetic relationship between soil color patterns and natural drainage characteristics in a hydrosequence of Oxisols formed in Tertiary/Quaternary sediments of the Cerrado (Savanna) Region of the Central Plateau of Brazil. The soils classify as Acrustox and Plinthaquox in Soil Taxonomy and as Dark-Red Latosols, Red-Yellow Latosols, and Hydromorphic Laterite in the Brazilian classification scheme. In addition to standard soil characterization analyses, x-ray diffraction and sodium citrate-bicarbonate-dithionite extractions were used to determine soil mineralogy. Precipitation was recorded and weekly positions of the water table at sampling sites were measured in observation wells for a period of 1 yr. Kaolinite, gibbsite, quartz, and Fe oxides are the dominant minerals in all sites. Relative amounts of these minerals show similar desilication trends with depth at all sites in the hydrosequence. Well-drained soils with water tables >3 m had reddish hues (2.5YR or 5YR) indicating an oxidizing environment and a codominant hematite and goethite Fe mineralogy. The upper organic rich solum at sites with seasonally high water tables (<2 m) had yellowish hues (10YR or 7.5YR) and a dominantly goethitic Fe mineralogy. In the deeper perennially saturated zones of all profiles the matrix color has reddish hues (2.5YR or 10R) and a dominantly hematitic Fe mineralogy. Stratigraphic, geomorphic, and pedogenic evidence supports the hypothesis that soils having reddish (2.5YR or 10R) hues and codominant hematite and goethite Fe mineralogy formed in a prior environment, then underwent changes in Fe oxide mineralogy in response to changes in hydrology brought about by landscape evolution and climate change. The organic rich upper sola of soils having a seasonally high water table show color adjustment to yellowish (10YR or 7.5YR) hues and dominantly goethitic Fe mineralogy.

¹ Contribution from the Dep. of Agronomy, Cornell Univ., Ithaca, NY 14853. Supported in part by the TropSoil Program.

² Soil Scientist, EMBRAPA-CPAC, Planaltina, DF, 73300, Brazil, now Graduate Research Assistant, and Assistant Professor, Dep. of Agronomy, Cornell Univ., Ithaca, NY 14853, respectively.

Received for publication July 3, 1986.