Solos derivados de rochas ultrabasicas no ambiente subtropical do Rio Grande do Sul

Argiudolls and Hapludolls derived from ultrabasic rocks (peridotites, pyroxenites, hornblendites, and serpentinites) show high levels of pH, CEC and base saturation, as well as high iron supply held in primary minerals, indicating very young soils for prevailing humid subtropical (thermic, udic) conditions. Low exchangeable Ca/Mg ratios and high levels of Cr, Ni and Co in peridotite and serpentinite derived soils demonstrate the remarkable influence of parent materials on soil composition. The mineralogical composition is highly immature, with anfiboles and chlorite in the sand and silt fractions, and chlorite, chlorite-smectite, and smectite in the clay fraction. These are the least weathered soils found to date in the Rio Grande do Sul State. This small development stage is possibly due to recent exposure, landscape renewal by surface addition of fresh material, and soil solution buffering by ions released from the weathering of the coarse fractions.