Geochemical differences in an Oxisol-Spodosol toposequence of Amazonia, Brazil

Abstract

The morphology and geochemical characteristics of the soils of a toposequence near Manaus, Brazil, have been determined. Such toposequences occur on slopes leading down into valleys cut in a nearly level plateau. Oxisols form the upper and Spodosols the lower members of toposequences. The transition from the Oxisols down the slope through a band of Ultisols to the Spodosols is very gradual. Lowest members of the toposequences, Spodosols do not occupy the valley floors. Great contrasts as well as shared characteristics occur among members of the toposequence chosen for study. Proportions of clay in the Oxisols are 80–90% as contrasted to 2–5% in the Spodosols. Changes in amounts of clay are gradual, continuing down the slope. The clay fraction is kaolinitic. All soils are strongly acid and have very low base saturation. The characteristics of the soils imply that several processes of weathering and pedogenesis have been operative. The small amount of SiO 2 in the Oxisols is attributed to dissolution of quartz without corresponding hydrolysis of kaolinite. The decreases in clay contents and increases in quartz in the soils from the upper to the lower end of the toposequence are attributed to a combination of hydrolytic weathering and either or both of clay eluviation and selective erosion of that fraction. The sequence of soils within the toposequences further suggests that a podzolization process begins when a critical stage in impoverishment is reached. Destruction of clay minerals continues but the migration of organo-mineral complexes within profiles becomes important. A part of those complexes moves out of the Spodosols to form black waters in the streams.