Gilvan Coimbra Martins

Soil Fertility and Production Potential

Despite important regional differences, some general properties with respect to fertility are common to most Amazonian Dark Earths: high total P and Ca contents and availability as well as often low N and K availability. The information compiled in this chapter explains the extent and mechanisms of the high fertility of Amazonian Dark Earths. However, the authors do not intend to advocate an exploitation of the high fertility of Amazonian Dark Earths, but rather to document the fertility potential of these special soils — highly weathered yet fertile — as well as to understand the properties of such soils with enhanced fertility. Contemporary efforts to develop sustainable landuse technologies should consider evidence provided by ADE research, regardless of whether ADE were intentionally made or not. Management to create fertile soils in the humid tropics has to consider the importance of organically bound nutrients that show low leaching combined with high availability similar to the properties shown here for Amazonian Dark Earths.

Soil Physical Characterization

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

Campos nativos e matas adjacentes da região de Humaitá (AM): atributos diferenciais dos solos

Existem no sul do Amazonas aproximadamente 560 mil hectares de campos de cerrados, distribuidos principalmente, nos municipios de Humaita, Labrea e Canutama. A regiao nao e coberta por campos continuos, mas por varias unidades isoladas entremeadas por matas. As formas de vegetacao primitiva indicam ambientes peculiares, como o regime hidrico, fertilidade natural e aeracao do solo. Assim, objetivou-se no presente trabalho estabelecer algumas razoes da estratificacao ambiental entre mata e campo nativo na regiao de Humaita (AM). Para tanto, foram observados morfologicamente e amostrados 10 perfis de solos, sendo 5 perfis sob mata e 5 perfis sob campo cerrado nativo. As amostras foram coletadas nas profundidades de 0-20, 20-40 e 60-80 cm, em trincheiras. Os solos sob vegetacao de campo nativo e sob mata possuem atributos quimicos semelhantes e mineralogicos identicos. A ocorrencia de solos com maior profundidade efetiva, com melhor drenagem e maior volume de armazenamento de agua, em associacao a maior inclinacao do horizonte plintico no sentido do igarape, aumentando o fluxo de agua nesta direcao, favorecem o aparecimento da vegetacao de mata, enquanto que condicoes opostas a estas favorecem o aparecimento da vegetacao de campo nativo.

Avaliação da condutividade hidraulica do solo saturada utilizando dois métodos de laboratório numa topossequência com diferentes coberturas vegetais no Baixo Amazonas

The objective of this work was to evaluate two different laboratory methods for determining the saturated hydraulic conductivity (Ko), namely, the constant head permeameter method (PCC) and the falling decreasing head permeameter method (PCD) and their applicability and variability to Amazon soils. 125 undisturbed soil samples were collected with an Uhland soil sampler using volumetric rings of 0,072 m height and 0,069 m in diameter. Soil porosity was also estimated by volumetric ring samples collected at the same spots where Ko was evaluated. Disturbed soil samples were also collected for chemical and particle size analysis at the same spots. The results showed that the PCC method was more appropriate for the studied soils Oxisols, leading to the lowest coefficient of variation and standard deviation throughout the topographic sequence. The Ko values were distributed among P1(2,65 to 3,34 cm day-1), P2(2,85 to 3,38 cm day-1), P3(2,86 to 3,63 cm day-1), P4(2,75 to 3,49 cm day-1), P5(2,38 to 3,83 cm day-1) and P6 (2,47 to 3,52 cm day-1); having a tendency to show higher Ko values at soil surface. The use of Ko as a parameter for hydraulic analysis in soils with high porosity in the surface layer and high clay content in the subsoil, as is the case in the Amazon, must be undertaken with caution to avoid compacting the sample and porous discontinuities. Throughout the studied topographic sequence, changes of saturated hydraulic conductivity were more related to changes in soil physical properties than to changes in vegetation cover throughout the studied topographic sequence.

Pedogenetic processes in anthrosols with pretic horizon (Amazonian Dark Earth) in Central Amazon, Brazil

Anthrosols known as Amazonian Dark Earth (ADE) have borne witness to the intensification of sedentary patterns and the demographic increase in Central Amazon. As a result, a recurring pattern has been observed of mounds with ADE arising from domestic activities and the disposal of waste. The objective of this research was to demonstrate the relationship of these anthropic activities with pedogenetic formation processes of ADE in the municipality of Iranduba, Brazil. Disturbed and undisturbed soil samples were taken from two areas of ADE (pretic horizon) and from a non-anthropic pedon. Physical, chemical, micromorphological and SEM-EDS analyses were performed. The coarse material of the pretic horizons consisted predominantly of quartz, iron nodules, ceramics and charcoal fragments, and the fine material is organo-mineral. There was a direct relationship between the color of pretic horizons and the number of charcoal fragments. The thickness of the ADE results from the redistribution of charcoal at depth through bioturbation, transforming subsurface horizons into anthropic horizons. ADE presents granular microaggregates of geochemical and zoogenetic origin. Degradation of iron nodules is intensified in pretic horizons, promoting a reverse pedogenic process contributing to the xanthization process. Surprisingly the anthropic activities also favor clay dispersion and argilluviation; clay coatings on the ceramic fragments and in the pores demonstrate that this is a current process. Processes identified as contributing to ADE genesis included: i) addition of organic residues and ceramic artifacts (cumulization) with the use of fire; ii) mechanical action of humans, roots and macrofauna (bioturbation); iii) melanization of deeper horizons as a result of bioturbation; iv) argilluviation and degradation of iron nodules. This study offers new support to archaeological research in respect to ADE formation processes in Central Amazon and confirmed the hypothesis that ancient anthropic activities may trigger and/or accelerate pedogenetic processes previously credited only to natural causes.