At the south of Amazonas state there are about 560,000 hectares of cerrado grassland which are mainly distributed in Humaitá, Lábrea and Canutama counties. The region is not covered by continuous grasslands, but by various isolated units intermixed by forests. The primitive vegetation phases indicate peculiar environments, such as the hydric regimen, natural fertility and aeration of soil. So, this work has as objective to stablish some reasons for the environmental distinction between forest and native grassland at Humaitá ( Amazonas state ) region. For that, 10 soil profiles were morphologically observed and sampled, being 5 profiles under forest and 5 profiles under native cerrado grassland. The samples were collected at 0-20, 20-40, and 60-80 cm depth, through pits. The soils under native grassland and under forest have similar chemical and identical mineralogical attributes. The occurrence of soils with greater effective depth, better dreinage and higher volume of water storage, in association with a higher inclination of the plinthic horizon in the direction of the "igarapé" (narrow riverbank between two islands or between an island and the main land), increasing the water flow in this direction, favor the forest vegetation, while opposite conditions to these ones favor the native grassland vegetation.
Amazon's soils; physical properties; mineralogy; plinthic horizon
