Mineralogy and chemistry of three soils along a topossequence from the upper Solimoes Basin, western Amazonia

Little is known about chemical and mineralogical attributes of soils from the upper Amazon, compared with well-studied soils from the middle and lower Amazon. In the well-drained uplands of Upper Amazon, due to the Andes Cordillera proximity, soils are expected to be chemically richer. This work aimed to increase the knowledge of soils from the upper Amazon, by means of studying three soils along a typical topossequence at Benjamin Constant, near the Brazilian-Peruvian Border. These soils range from the uplands, where Al concentration is high (possessing an aluminic character) down to the floodplain, with eutric, richer soils. They were classified as Argissolo Amarelo Ta aluminico abrupto, Plintossolo Argiluvico aluminico abruptico and Neossolo Fluvico Ta eutrofico in the last version of the Brazilian System of Soil Classification. The results showed greater nutrient and primary minerals amounts and youthful development in these soils, compared with well-drained, deeply weathered soils from eastern Amazonia, derived from older, pre-weathered sediments or ancient crystalline rocks. The low amounts of Mn and Fe in the Plintossolo in all fractions analysed, compared with the other soils, indicate that removal is pronounced n this pedo-environment, whereas in the Neossolo Fluvico, new sedimentary additions allow the occurrence of higher Fe and Mn amounts. The maximum phosphorous adsorption capacity values are generally lower in the surface soil horizons, increasing with depth, accompanied by higher clay contents, or plinthite occurrence. Thus, exposure of subsurface layers may be a limiting factor for agriculture in these soils.