Soils from Brazilian Amazonia

Abstract

The Brazilian Amazonia region can be conveniently separated into 11 sectors, which represent large pedoenvironments at a continental scale. In a global panorama of the region, from this simplified and useful division, there is a high pedodiversity in the Amazon, despite the predominantly monotonous landforms, at macroscale. Soils of the Sedimentary Basins vary according to strong geological-structural control, coincident with the division of the sub-basins. Close to the Andes fold belt, soils of the Acre Basin, above the Iquitos Arch, have an Andean influence and are mostly young (Cambissolos, Luvissolos, and Argissolos), eutrophic, and high-activity clay. However, the aluminic character is very common. Between the Iquitos and Purus Archs, in the Solimões or Upper Amazonas basin, soils have Plinthite to varying degrees (Plintossolos and Argissolos), but mostly dystrophic. Downstream the Purus Arch to the Monte Alegre Arch, the mid-Amazon basin is strongly associated with Latossolos or Argissolos (always dystrophic), usually yellowish, derived from the pre-weathered Alter do Chão or Belterra Formations. At the low Amazon and Marajó island, under the influence of strong marine tides and by the gigantic sedimentary and hydrological load of the great Amazon river, extensive floodplains have Neossolos Flúvicos, Gleissolos, Plintossolos, and Planossolos. In the crystalline basement rocks of the Amazon Craton, gently dissected landforms reveal dominance of Argissolos or Latossolos (yellow and red-yellow), and generally dystrophic, except where mafic rocks occur. The floodplain soils of the tributaries are almost always dystrophic. The presence of petroplinthite in shallow soils under wet climates suggests that they formed under past climates much drier than the present ones. Anthropogenic soils (Indian Black Earths), with high levels of SOM, Available P, and CEC, occur frequently not only on the bluffs above the floodplain on well-drained lands of the Amazon region, but also on the Várzea floodplain, as buried paleosols. The Roraima and Rondônia Highlands have varying shallow or deep, dystrophic or eutrophic soils, depending on landforms and lithology. A few high-fertility soils, derived from mafic rocks, occur in both highland regions and are usually intensively cultivated. Sandy soils resulting from the extreme podzolization, with the formation of deep, acidic and chemically poor Espodossolos, are characteristic of the Rio Negro Basin. They occur in extensive flat and low-lying plains, under Campinarana vegetation. These extensive Tropical Podzols were formed by the clay destruction of a previous Latossolos mantle under super humid climates. Also, Savanna islands (cerrado, campos) occur throughout Amazonia, and are mostly associated with poorly drained or imperfectly drained soils, but may also occur in higher ground (yellow Latossolos), impermeable parent materials (Monte Alegre) or sandy soils (Alter from the ground). They are floristically much poorer, compared to the core savannas of the Central Plateau. The Amazon/Solimões River floodplains, together with the Purus and Juruá rivers, constitute the largest eutrophic alluvial space in Brazil, and one of the most extensive worldwide, making the use and sustainable cultivation virtually continuous since pre-Columbian times.