Assessing the geochemical inherent quality of natural soils in the Douro river basin for grapevine cultivation using data analysis and geostatistics

The main purpose of this study is to measure the spatial variability of a specific soil characteristic, its content of nutrients and toxic metals, and use the established patterns of variability as an indicator of the inherent quality of the natural soils of the Douro river basin for grapevine cultivation. A total of 108 topsoil samples were collect within the basin catchment. All soil samples were analysed for 26 chemical elements by ICP-ES OPTIMA at ACME Anal. Lab. (ISO 9002 Accredited Co.). For this study, we selected only the chemical elements that are relevant for the vineyards, that is, elements that are nutrients or those which are potentially toxic. In our study we used the following methodology: (i) categorisation of quantitative variables and use of multiple correspondence analysis (MCA) to determine similarities among them; (ii) structural analysis through variography to identify spatial patterns of variability; (iii) mapping of MCA factors through ordinary kriging to define, within the basin, areas in which soils can be characterised according to their contents of nutrients or toxic elements. This methodology allows the determination of spatial patterns of variability for groups of similar variables instead of an exhaustive “one-by-one” variable study, since data analysis reduces the amount of information that must be interpreted in order to assess the natural aptness of the soils for grapes plantation.