Aluminum geochemical characterization in agricultural soils: sequential chemical extraction combined with mineralogical analysis of the fine fraction

Abstract

Understanding both the concentration and distribution of aluminum (Al) in agricultural soils is essential to provide a benchmark on the productivity of such soils. In this study, a combination of sequential chemical extraction and mineralogical investigation of the fine fraction was used in order to evaluate Al pools in agricultural soils in Galicia (NW Spain), an Atlantic European region where the wet climate favors leaching of soil basic cations. Determination of Al (soluble/exchangeable/specifically adsorbed, bound to manganese oxides, associated with amorphous compounds, bound to oxidizable organic matter, associated with crystalline iron oxides, and residual fraction) in the topsoil samples was performed to evaluate its potential environmental availability in the study area. Aluminum contents were determined by flame atomic absorption spectrophotometry and the mineralogy of the fine fraction of these soils was studied by X-ray diffraction (XRD). The results showed that Al is mainly in the residual phase, which represents Al incorporated in the lattice minerals, and consequently the threat of toxicity to plants and the environment is reduced. The XRD results showed the presence of hydroxy-Al interlayered vermiculites and kaolinite, accompanied by small amounts of goethite, mica, and quartz.