Statistical analyses of geochemical variables in soils of Ireland

Geochemical variables in soils are affected by multiple factors, and different variables have different responses to these factors, causing complicated multivariate relationships between them. In this study, the relationships between a total of 45 geochemical variables in soils and in particular mineral soils of Ireland were investigated using multivariate analyses following an investigation into their probability features and data transformation based on a total of 1310 surface soil samples. There was strong variation in the values of geochemical variables. Multi-modal features in the histograms and multi-kink features in the normal quantile–quantile (Q–Q) plots were observed for many variables implying the existence of multiple populations. Obvious outliers were identified using the normal Q–Q plots. Most of the variables did not pass a Kolmogorov–Smirnov test for either normal or lognormal distribution, and the Box–Cox power transformation was effective in transposing data to a form suited to further parametric statistical analyses. There were generally good correlations between most metals, but relatively poor correlations between extractable nutrients (available P, K, and Mg) and metals. The extractable nutrients were more affected by agricultural activities that were more spatially variable compared with natural factors such as geology and soil type. Cluster analysis classified the variables into groups of metals of mainly natural origin, mobile elements, and variables that are affected by agricultural activities and pollution. Among the three factors of rock type, soil type and land use investigated, land use was found to be the least important influencing factor for most of the geochemical variables studied.