Geoadditive models for the analysis of the spatial distribution of soil salinity in a Sardinia coastal area

In the agronomical context the use of kriging to build maps of salinity indicators (sodium absorption ratio, electric conductivity in saturation extract) in the presence of auxiliary variables is quite common. Universal kriging requires auxiliary variables to have a linear effect on the response, but it is well known how this assumption is seldom verified and often violated in a typical agronomic trial. In this paper a geoadditive model is proposed to analyse the spatial distribution of a soil salinity measure, the sodium adsorption ratio and its nonlinear relations with some explanatory variables, soil textural, chemical and hydraulic features as well as the spatial correlation described by a standard spatial autocorrelation function, in order to investigate if natural causes or secondary causes related to human improper soil use played a role on high salinity levels of the soils and ground waters within the Muravera-Villaputzu (Sardinia, Italy) coastal plain.