Papers

Pedogeomorphological evolution model of Serra da Canastra, MG

Author: Vinicíus Vasconcelos, Eder de Souza Martins, Osmar Abilio de Carvalho Júnior, José Marques Júnior, Diego Silva Siqueira, Antonio Felipe Couto Júnior, Renato Fontes Guimarães, Roberto Arnaldo Trancoso Gomes, Adriana Reatto

Keywords: Pedology; soil spatial variability; gibbsite; iron forms; colluviums.

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Summary

The gibbsite and iron compounds in tropical soils are controlled by landscape position and variation of hydric conditions along the pedosequences. Typically, in the regional scale the gibbsite ratio decrease with increasing elevation and surface age, while in the local scale the ratio decrease from top to base in a pedosequence. The ratio of oxalate iron and dithionite iron (Feo /Fed ) under oxidizing conditions and well-drained tend to be low (<0.05), as in Oxisols. On other hand, higher ratios (between 0.5 and 1) characterized environments with reduction conditions, indicating the dissolution of crystalline iron forms and its precipitation on less stable forms. The ratio between dithionite iron and the total iron (Fed/Fe2O3) is indicative of the stage of the soil weathering. Fed/FeO3 rations >0.8 are observed in Oxisol, generally. The behavior of these parameters in the landscape is the result of the evolutionary process of catenas. This paper aims to describe a pedosequence in Serra da Canastra, characterized by an opposite polarity of gibbsite and iron forms that indicate a reducing environment. This pedosequence has Oxisol in the top and Gleysol on the base. Samples were collected of the soil diagnostic horizon in 10 trenches. The samples were submitted the following procedures: geochemical analyses (major elements), iron extractable with dithionite-citratebicarbonate (Fed ) and ammonium oxalate (Feo ), thermogravimetric analysis for gibbsite and kaolinite direct quantifi cation and magnetic susceptibility. The data set was processed by multivariate analysis: Principal Component Analysis and Cluster Analysis. Soil transformation through upstream water saturation occurs gradually and discontinued in the landscape. The upper limit of colluvial deposits is marked by magnetic susceptibility, texture and color variation. Principal Components Analysis and Cluster Analysis show physical, chemical and mineralogical trends in the pedosequence. The analyses indicate that the pedosequence is derived of successive sinks and colluviums and the advancement of these hydromorphic environments to the hollows portions of the landscape