Abstract
Abstract. The tropical forests of the Amazon Basin occur on a wide variety of different soil types reflecting a rich diversity of geologic origins and geomorphic processes. We here review the existing literature about the main soil groups of Amazonia, describing their genesis, geographical patterns and principal chemical, physical and morphologic characteristics. Original data is also presented, with profiles of exchangeable cations, carbon and particle size fraction illustrated for the principal soil types; also emphasizing the high diversity existing within the main soil groups when possible. Maps of geographic distribution of soils occurring under forest vegetation are also introduced, and to contextualize soils into an evolutionary framework, a scheme of soil development is presented having as its basis a chemical weathering index. We identify a continuum of soil evolution in Amazonia with soil properties varying predictably along this pedogenetic gradient.
Highlights
Tropical soils can arise from a wide variety of parent materials, climatic conditions, biotic interactions, landforms, geomorphic elements and soil age
In the early days of soil science in this region, Marbut and Manifold (1926) observed at least six different groups of soils occurring commonly in the region, suggesting that many soils occurring in the tropics had little or no morphological difference to those observed in the temperate zone
By that time Sanchez and Buol (1975) had already found that soils previously mapped as Ferralsols in the Peruvian Amazon were Ultisols, Alfisols and Inceptsols (Acrisols, Luvisols/Lixisols and Cambisols in the World Reference Base soil classification system: IUSS Working Group WRB, 2006), suggesting that ancient Ferralsols may be confined to areas of the Guyana and Brazilian shields
Summary
Tropical soils can arise from a wide variety of parent materials, climatic conditions, biotic interactions, landforms, geomorphic elements and soil age. Of Fittkau’s Central Amazonian sub-region IV (east of Manaus) are mainly derived from rocks and sediments from the middle Tertiary with a high probability of belonging to the end of the Cretaceous and likely to have experienced more or less continuous weathering for more than 20 million years (Irion, 1978) Included in this sub-region, the socalled Barreiras formation was originated from CretaceousTertiary sediments, derived from the erosion of the Guyana and Brazilian shields (Herrera et al, 1978). Encompassing Fittkau’s regions I and II respectively, the ancient, pre-Cambrian Guyana and Brazilian shields, with their series of igneous and metamorphic rocks, are placed to the north and south of the lower Amazon River These are the oldest surfaces exposed in South America, with their maximum geological age ranging from 1500 to 3600 million years (Fig. 1b). As well as drawing on previous work of others, original data is presented to help demonstrate the diversity of Amazonian soils, both within and between the various WRB soil groups
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