Weathering sequence of soils along a basalt-sandstone toposequence in the Brazilian Cerrado

Despite a multitude of studies on fertility and physics of tropical soils, the effect of soil parent material on their genesis, chemical and mineralogical composition is not yet fully understood. Here, we studied a toposequence of four deep, well-developed soils along a basalt-sandstone transition, comprising two Ferralsols and one Luvisol on the summit and midslopes of a basalt plateau, and one Cambisol at a lower sandstone plateau. Basalt soils had high (42-77%) clay contents, and were mostly comprised by gibbsite, Fe oxides, and poorly crystalline kaolinite, whereas the sandstone soil had < 20% clay and kaolinitic clay. The studied Ferralsols were moderately to highly fertile, presented pH values 6.4-7.2 and blocky instead of the common granular structure, contrasting with most gibbsitic Ferralsols in tropical Brazil. The Luvisol showed very high base nutrient contents but lower pH (5.5-6.0), and the sandy Cambisol was also relatively fertile. All soils had no exchangeable Al3+, which was also unusual for Cerrado soils, which are mostly very acidic and nutrient-poor. Most likely these unusual features result from the formation of an interstratified clay mineral complex between kaolinite and mica, which occurred as a major phase in the Luvisol and as traces in Ferralsols. Micromorphological analysis showed that celadonite, a green-colored inclusion probably comprised by muscovite/paragonite micas, was a major component of the basalt and the last mineral to weather within the Luvisol. Thus, the toposequence ranged from the less developed, high-activity clay Luvisol to extremely weathered, gibbsitic Ferralsols, with the sandstone Cambisol in an intermediate stage of development.