Identification, characterization, and genesis of "plintossolos" developed from the Itapecuru Formation in Maranhao State, Brazil
Abstract
Soils with plinthite represent 13.5% (44,000 Km$\sp2$) of Maranhao State, Brazil, but little is known about their characteristics and genesis. They formed from a Lower Cretaceous-age ferruginous sandstone, the Itapecuru Formation, or sediments derived thereof, in a tropical sub-humid climate with strong seasonal dry/wet cycles. The mean annual air temperature is 26$\sp\circ$C, and the annual rainfall is 1800 mm. The objectives of this study were to determine the morphological, chemical, physical, and mineralogical properties of the soils, classify them in the Brazilian and Soil Taxonomy systems, and explain the processes by which they formed. Six soils were sampled, two on backslopes, two on shoulder landscape positions, and two on footslopes. The soils on the backslopes (Ustic Dystropept and Typic Ustorthent) lacked plinthite and had the least soil formation, highest cation exchange capacity, and most 2:1 expansible silicate clay minerals. The other four soils had horizons with hard ironstone above horizons with plinthite. Plinthic horizons were relatively low in total Si, high in crystalline Fe oxides (goethite and lepidocrocite), but low in amorphous Fe compounds. These soils contained more 2:1 clay minerals and had higher CEC than many plinthic soils in other parts of the world. Two of the plinthic soils (Inceptic Plinthustult and Plinthic Dystropept) were on shoulder positions and moderately well drained, and two (Typic Plinthaquult and Plinthic Paleaquult) were on footslopes and poorly drained. Within a plinthic horizon, reddish plinthite bodies contained five times more Fe than the yellowish non-plinthic soil matrix. Plinthite formed in horizons that were seasonally saturated with water and thus experienced reducing conditions. In the soils on the should positions, Fe was probably reduced and mobilized in upper horizons and translocated downward to form plinthic horizons. In the footslope positions, however, Fe was most likely mobilized in upslope soils and carried by throughflow to the plinthic horizons. Modifications in both the Brazilian and Soil Taxonomy classification systems were proposed to accommodate the plinthic soils.
Degree
Ph.D.
Advisors
Franzmeier, Purdue University.
Subject Area
Agronomy|Mineralogy
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