Abstract
The variety of soils in the State of Acre is wide and their chemical profiles are still not fully understood. The nature of the material of origin of these soils is indicated by the high aluminium (Al) content, commonly associated with high calcium (Ca) and magnesium (Mg) contents. The study objective was to use different methods to quantify Al in soils from toposequences formed from material of a sedimentary nature originating from the Solimões Formation, in Acre, Brazil. Trenches were opened at three distinct points in the landscape: shoulder, backslope and footslope positions. Soil samples were collected for physical, chemical, mineralogical analyses. The Al content was quantified using different methods. High Al contents were found in most of these horizons, associated with high Ca and Mg levels, representing the predominant cations in the sum of exchangeable bases. The mineralogy indicates that the soils are still in a low weathering phase, with the presence of significant quantities of 2:1 minerals. Similar Al contents were determined by the methods of NaOH titration, xylenol orange spectrometry and inductively coupled plasma optical emission spectrometry. However, no consistent data were obtained by the pyrocatechol violet method. Extraction with KCl overestimated the exchangeable Al content due to its ability to extract the non-exchangeable Al present in the smectite interlayers. It was observed that high Al contents are related to the instability of the hydroxyl-Al smectite interlayers.
Highlights
The wide variety of soils in the State of Acre is the result of geological processes related to tectonic reactivations, historic climatic changes induced by variations of the relative sea level and climate (Rossetti & Toledo, 2007), as for example in the arid period in the last ice age (Mayle & Power, 2008)
It can be observed that the water-soluble Al contents are very low, demonstrating that practically all Al determinations by the different methods used in this study was based on KCl extraction
The results indicate that the higher the content of clay and the lower the values for clay cation exchange capacity (CEC), ki and kr, the higher the Al content
Summary
The variety of soils in the State of Acre is wide and their chemical profiles are still not fully understood. The study objective was to use different methods to quantify Al in soils from toposequences formed from material of a sedimentary nature originating from the Solimões Formation, in Acre, Brazil. A natureza do material de origem desses solos pode ser observada a partir do teor de alumínio elevado (Al), normalmente associada com níveis elevados de cálcio (Ca) e magnésio (Mg). O objetivo deste estudo foi utilizar diferentes métodos para quantificar o alumínio em solos de topossequências formadas, a partir de um material de natureza sedimentar proveniente da Formação Solimões, Estado do Acre. A extração com KCl superestimou os teores de Al trocável, em razão da sua capacidade de extrair o Al presente em formas não trocáveis nas intercamadas das esmectitas. Observou-se que o teor de Al elevado está relacionado com a instabilidade das intercamadas das esmectitas hidroxil-Al. Termos de indexação: alumínio trocável, material sedimentar, esmectita, Amazônia ocidental
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