Soil Physical Characterization

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1. INTRODUCTION Amazonian Dark Earths (ADE) normally show plaggic, terric or hortic horizons. These horizons are identified by the dark matrix colors of the top layers, and often by the presence of potsherds, lithic artifacts and charcoal pieces (Sombroek, 1966; Ranzani et al. 1970; Kern and Kampf, 1989). The more widely accepted theory about the origin of these epipedons is that they were improved by Amerindian populations in pre-Colombian Indian settlements. Human beings had a significant influence on these sites, as confirmed by the presence of high phosphorus (P) contents (Smith, 1980; Kern and Kampf, 1989; Lima et al., 2002) that are characteristic of anthropogenic horizons (Smith, 1980; McDowell, 1988). Despite the high amounts of P, calcium (Ca), magnesium (Mg) and micronutrients, textures are typically lighter in the top horizon than the deeper horizon and are also lighter relative to the surrounding soils. The top horizons on ADE sites show some differences in soil physical and hydraulic properties compared to the adjacent soils in the region. They have certain chemical characteristics and contain large amounts of charcoal and often ceramic pieces that require specific soil physical methods to characterize them. Their structure is typically granular (Sombroek, 1966; Silva et al., 1970; T. Cunha, unpubl. data) with slightly interlocking edges. The high amounts of soil organic matter (SOM) in ADE strongly influences the soil’s color, structure, and hydraulic properties. Bulk density values are commonly smaller in the top horizons in ADE than in adjacent soils near Belterra (Santarem), and are greater near Manaus. At most investigated sites, ADE displays soil physical properties that are advantageous for agricultural purposes. The workability of ADE is easier, and the drainage is usually very good. Because of this easy workability and ADE’s sustainability in relation to surrounding soils, the local population intensively uses these sites (M. Hiraoka, this issue). ADE seems to be a very resilient soil type, able to keep its good soil physical qualities when submitted to intensive soil management. A depletion of productivity of those highly fertile soils has been mentioned by many farmers and reported by Smith (1980) and German (2001). It is probably related to some soil physical degradation and changes in the fluxes of mass