The geomorphic surface concept allows interrelationship among various branches of soil sciences, such as geology, geomorphology and pedology. The purpose of this study was to investigate soil-geomorphic surface relations along an upland-downland toposequence in the region of Humaitá, Amazonas State, Brazil. A transect of 4,500 m was studied from the water divide downstream to the flood plain of the Madeira river. The geomorphic surfaces were identified based on the slope rupture, stratigraphic criteria and on other field observations. Trenches were dug in the identified slope segments, the soils were morphologically characterized and samples collected from each horizon. The physical properties particle size distribution, water dispersible clay and flocculation degree, soil and particle density, total porosity, and saturated hydraulic conductivity were determined. Chemical analysis involved pH in water and KCl; exchangeable Ca, Mg, K, Na, and Al; available P, Al + H and organic C; total SiO2, Al2O3 and Fe2O3 by sulfuric acid digestion; free iron oxides extracted with dithionite-citrate-bicarbonate and poor crystalline iron oxides with ammonium acid oxalate. The values of ∆pH, silt/clay, Ki and Fe d/Fe t ratio indicated more weathered soils at the top of the landscape, than along the infiltration slope, convex slope and alluvial footslope, coinciding with the older age of the soils of the geomorphic surface I than of II and III. The soil variations in the studied transect were related to the surface age, parent material and slope gradient. The understanding of the geomorphic events was important to explain the variation of soil properties in the toposequence.
geomorphic surfaces; Amazon region; pedogenesis; soil-landscape relations
