Soil genesis, mineralogy and chemical composition in a steatite outcrop under tropical humid climate in Brazil

Abstract

Steatite is a metamorphic, ultramafic rock composed by talc, serpentine, magnetite and other unique minerals. Soils developed from this rock is typically unproductive due to high Mg and low P contents, high concentrations of heavy metals, and the extent of such limitations in the humid tropics is still poorly known. Here, we aimed to study pedogenesis, morphology and composition of eight soils formed from steatite in Minas Gerais, Brazil. These soils were a) Lithic Udorthents (3 pedons); b) Oxic Dystrudepts (2 pedons); and c) Acrudoxes (3 pedons). All soils presented high particle density but low bulk density due to high packing void porosity associated to granular structure. Soil pH and exchangeable Mg+2 were relatively high, whereas Mehlich-I P and exchangeable Al+3 were very low. X-ray fluorescence data showed that Entisols and Inceptisols have high (94 a 200 g kg−1) MgO contents, whereas Oxisols are highly enriched in Fe2O3, although still bearing MgO contents as high as 30 g kg−1. The Entisols and Inceptisols were mostly composed by talc, but also had Fe oxides and small amounts of kaolinite, whereas the dusky red Oxisols contained mostly hematite. Although present in the parent material, serpentine was not detected in soils. The considerable amounts of MgO in Oxisols can be explained by rock fragments preserved by a dark Fe oxide coating, visible in thin sections. Magnetic susceptibility was high and increased strongly with increasing weathering stage. Local topography strongly controlled soil formation and composition, which allowed for the existence of three climax vegetations: canyon cloud forest, semi-deciduous forest and savanna, suggesting that soil limitations to native plants are lower than expected.