O elevado teor de matéria orgânica dos Organossolos confere-lhes características edáficas próprias, que podem ser contrastantes em relação ao observado em solos minerais. Devido à forte interação entre a matéria orgânica e os elementos a ela adsorvidos, os métodos de rotina em fertilidade do solo podem não avaliar de forma adequada a acidez nos Organossolos. Os objetivos deste trabalho foram caracterizar e estabelecer relações entre atributos do complexo sortivo e a acidez em solos com alto teor de carbono. Foram avaliados doze perfis, onze Organossolos e um Cambissolo, de diferentes Estados brasileiros, coletados de 2000 a 2002. Os perfis foram separados em ambientes de várzeas e planícies litorâneas e solos de ambiente altimontano e planalto. Além da caracterização dos perfis, segundo métodos da Embrapa, o pH foi analisado por diferentes métodos e foram determinados os teores de C, usando um analisador elementar (C_CHN), e a matéria orgânica pelo método da mufla (MO_mufla). Pelos resultados, verificase que os métodos de determinação de pH em água, em KCl e o pH SMP podem ser usados para expressar a acidez desses solos. Observou-se alta correlação entre o pH SMP e a acidez potencial. Os teores de C_CHN e de MO_mufla tiveram correlação significativa, porém negativa, com o pH em água e positiva com os valores T e H. O comportamento diferenciado da acidez dos solos, em função do ambiente de formação, indica ser importante esta separação no manejo da acidez e na avaliação da fertilidade em Organossolos.

The Upper Solimões river region, western Amazon, is the homeland of indigenous populations and contains small-scale agricultural systems that are important for biodiversity conservation. Although traditional slash-and-burn agriculture is being practiced over many years, deforestation there is relatively small compared to other Amazon regions. Pastures are restricted to the vicinity of cities and do not spread to the small communities along the river. Inceptisols are the main soil order (>90%) in the area and have unique attributes including high Al content and high cation exchange capacity (CEC) due to the enrichment of the clay fraction with 2:1 secondary aluminosilicates. Despite its importance, few studies have focussed on this soil order when considering land use effects on the fertility of Amazon soils. Thus, the objective of this study was to evaluate changes in soil fertility of representative land use systems (LUSs) in the Upper Solimões region, namely: primary rainforest, old secondary forest, young secondary forest, agroforestry, pasture and agriculture. LUSs were significantly differentiated by the chemical attributes of their topsoil (0-20 cm). Secondary forests presented soil chemical attributes more similar to primary rainforest areas, while pastures exhibited the highest dissimilarity from all the other LUSs. As a whole, soil chemical changes among Inceptisols dominated LUSs showed patterns that were distinct from those reported from other Amazon soils like Oxisols and Ultisols. This is probably related to the presence of high-activity clays enriched in exchangeable aluminum that heavily influenced the soil chemical reactions over the expected importance of organic matter found in most studies conducted over Oxisol and Ultisol.

The objectives of this study were to establish geochemical background values of aluminum (Al), iron (Fe), arsenic (As), barium (Ba), cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), manganese (Mn), nickel (Ni), lead (Pb), vanadium (V), and zinc (Zn) for eastern Amazon soils and to assess the relationship between soil chemical and granulometric attributes using univariate and multivariate procedures. Samples from the topsoil (0–0.2 m) were collected in several areas with minimal or no human intervention. Pseudo total concentrations of potentially toxic elements were extracted via acid digestion and quantified via inductively coupled plasma optical emission spectrometry (ICP-OES). The results revealed that the soils present high acidity, low levels of cations and organic carbon, and highly heterogeneous geomorphological characteristics. The elements Fe (9300 mg kg⁻¹) and Al (8200 mg kg⁻¹) showed the highest mean concentrations in the study, while As (0.8 mg kg⁻¹) and Cd (0.1 mg kg⁻¹) presented the lowest levels. The results of this study indicate that the 90th percentile can be used to formalize quality reference values for the State of Pará and that the 98th percentile can be considered for areas rich in mineral deposits.