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Investigations in an Oxisol-Ultisol Toposequence in S.Paulo State, Brazil¹

ABSTRACT

Four soil profiles were sampled along a 550-m toposequence traverse in São Paulo State, Brazil and were characterized with physical, chemical, mineralogical, and micromorphological analyses. The soils were classified and hypotheses were developed concerning their genesis based on soil differences observed.

The soil on the upper part of the traverse has an ochric epipedon over an oxic horizon and is classified as clayey, oxidic, isothermic Typic Haplorthox. Slightly downslope organic matter content increases to form a Typic Umbriorthox of the same family. The soil below these two has an argillic horizon over an oxic horizon and is classified as clayey, oxidic, isothermic Orthoxic Palehumult. The soil in the lowest part has an argillic horizon over decomposed shale and is classified as clayey, Kaolinitic, isothermic Typic Paleudult. The main characteristics distinguishing the soils in the lower part of the toposequence from those in the upper part were the increase in depth and eventual loss of the oxic horizon, decrease in gibbsite content, decrease in chlorite/vermiculite intergradient clay minerals, increase in 2:1 clay minerals, decrease in fine clay to coarse clay ratio, increase in fine silt to clay ratios, increase in illuviated clay, and increase in bulk density.

The genesis of these soils appears to be controlled mostly by the age of the geomorphic surfaces and by the nature of the parent material.

¹ Published with the approval of the Director of the North Carolina Agr. Exp. Sta., Raleigh, N.C. as Paper no. 4188 of the Journal Series. This work supported in part by Fundação de Amparo a Pesquisa do Estado de São Paulo.

² Soil scientist with Instituto Agronômico, Campinas, Brazil, and F.A.P.E.S.P. Scholar at N.C. State University, and Professor of Soil Science, respectively.

Received for publication October 4, 1973. Accepted for publication February 22, 1974.