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Regional patterns of well drained upland soil differentiation in the middle Caquetá basin of Colombian Amazonia

1996; Elsevier BV; Volume: 72; Issue: 3-4 Linguagem: Inglês

10.1016/0016-7061(96)00027-4

1872-6259

J.M. Lips, Joost F. Duivenvoorden,

Soil and Unsaturated Flow

As part of a land-ecological survey of the middle Caquetá basin in Colombian Amazonia, a total of 41 well drained upland soil profiles under primary rain forest was studied. Two soil groups were recognised on the basis of principal components analysis (PCA). Soils pertaining to the first group cover about 45–60% of the well drained uplands and classify as Afsols and Acrisols. They are characterised by fine textured, reddish B horizons, and are low in exchangeable bases, but high in exchangeable Al. Furthermore, they contain relatively high total nutrient reserves. Soils pertaining to the second group cover about 35–50% of the well drained uplands and classify as Acrisols and Ferralsols. These soils are characterised by medium textured, yellowish B horizons within 100 cm depth, are low in exchangeable bases and Al, and show very low total nutrient reserves. The soils of the Ali-Acrisol group are mostly developed in Andean-origin deposits of the low terraces of the Caqueta River and the Tertiary Pebas formation. The soils of the Acri-Ferralsol group are mostly developed on shield-derived medium to coarse textured Tertiary deposits and are also found on Palaeozoic sandstone outcrops. The differences between the soils of the two groups are, therefore, principally related to variation in the parent materials and reflect the principal geological structure of the middle Caqueta area. A similar kind of well drained upland soil differentiation is found in the Colombian Amazon area as a whole, including footslope areas near the Andes, and probably also in adjacent parts of Peru and Brazil. The presence of the soils of the Acri-Ferralsol group in the middle Caqueta area and in NW Amazonia is in conflict with general statements that well drained upland soils in upper Amazonia would be less weathered than those from central or eastern Amazonia. The reddish or yellowish colour of the B horizon within a depth of 100 cm provides a practical field characteristic to distinguish the two groups of soils. The colour of the B horizons seems mainly related to dynamics of the soil organic matter. In sandy profiles of the soils from the Acri-Ferralsol group a slight degree of podzolisation may occur in the topsoil. Current international soil classification systems do not adequately separate the two well drained upland soil groups in the middle Caquetá area beacuse total nutrient reserves are not sufficiently incorporated and too much emphasis is laind on th presence of a clay increase.