Gênese e composição de solos de diferentes fertilidades desenvolvidos a partir de itabirito em Minas Gerais

Abstract

In Brazil, iron ore deposits are commonly metasedimentary rocks rich in quartz and iron oxides, resulting in soils that are typically stony, coarse-textured, and of low fertility. However, some forest soils developed on itabirites (banded iron formations) under humid tropical conditions in Minas Gerais exhibit unexpectedly high levels of basic cations, a phenomenon that remains poorly explained in the literature. To elucidate the genesis of these anomalous soils, three toposequences (comprising upper, middle, and lower slope pedons) were sampled under native vegetation. Comprehensive physical, chemical, mineralogical, and micromorphological analyses were performed on both soils and fragments of the parent rock. The soils contained abundant coarse fragments (>2 mm) and had clay contents ranging from 11 to 71%, with high particle density due to elevated total Fe₂O₃ contents (23–69%), whereas Al₂O₃ levels were generally below 13%. Despite their inherently acidic reaction, exchangeable Al was absent or very low, and the availability of basic cations ranged from low (upper slopes) to high or very high (lower slopes). Quartz and hematite predominated in soils, but several horizons contained trace amounts of carbonates (likely Fe, Mg, and Mn), while all clay fractions included mixtures of micas, vermiculite, kaolinite, and Fe/Al oxides in variable proportions. Some samples also contained pyrophyllite, stilpnomelane (a rare biotite-like mineral), and apatite. The genesis of more fertile itabirite-derived soils appears to result from a combination of factors: 1) the presence of nutrient-bearing minerals (Mg, Ca) in the parent material; 2) clays with high cation exchange capacity, notably vermiculite; 3) downward transport of soluble nutrients and organic matter; and 4) efficient nutrient cycling and retention under forest cover. These soils have underexplored potential for low-intensity agriculture due to their relatively high natural fertility compared with adjacent soils. Regardless of use, their association with Atlantic Forest remnants and unique pedogenesis make them valuable for both agricultural and conservation purposes.