Formation of soils with plinthite on a toposequence in Maranha˜o State, Brazil

Abstract

Soils with plinthite and ironstone concretions are extensive in Maranha˜o State, northeastern Brazil. They formed from the Fe-rich Itapecuru sandstone in a tropical subhumid climate with distinct wet and dry seasons. Most plinthic soils in Brazil have properties associated with low-activity clays, but these soils were reported to have properties associated with high-activity clays. The main objectives of this study were to characterize the chemical and mineralogical properties of the soils and to deduce the processes of their formation in relation to hillslope position. six soils were sampled, two each on summit, shoulder, and footslope positions. They were analyzed by standard characterization methods and by X-ray diffraction and infrared spectroscopy. The parent sandstone bedrock contained mainly quartz, feldspars, muscovite, biotite, and resistant minerals in the sand fraction, and smectite, mica and kaolinite in the clay fraction. The main Fe-oxide mineral was hematite. Most soil horizons had lower Fe concentrations than the sandstone parent material. Kaolinite, mica and quartz occurred in the coarse clay fraction of all pedons. Soils on the summit positions (Ustic Dystropept and Typic Ustorthent) lacked redoximorphic features, lacked plinthite, had the highest cation exchange capacity, and showed the weakest development. They were rich in smectite, probably inherited from the parent material, and goethite. soils on shoulder positions (Inceptic Plinthustult and Plinthic Dystropept) had the largest concentration of total Fe, mostly as goethite. Those on footslopes (Typic Plinthaquult and Plinthic Paleaquult) exhibited the most redoximorphic features and had horizons in which the Fe was strongly concentrated in ironstone and plinthite, mostly as lepidocrocite and goethite. The crystallinity of these minerals increased from non-plinthic material to plinthite to ironstone concretions. They formed as the hematite in the sandstone weathered. These soils are less weathered than most plinthic soils of Brazil. Diagnostic criteria for some subgroups of the Plinthustult and Dystropept great groups of soil Taxonomy and some new classes for the Brazilian soil Classification system were proposed.