The chromatogram of the soil from a holm-oak wood points out that this sample is set at an intermediate level regarding the quality of organic matter. This is a significant fact, since the sample comes from a well-preserved natural environment. Even though this study cannot shed light on the reasons that may explain this finding, from an educational perspective it may be interesting to speculate about the role that the geological structure of soil plays in the formation of humus. In that regard, the subterranean drainage that characterizes the calcareous bedrock of the sampling site constrains the surface water availability which might affect the process of humus formation. This is consistent with the fact that the holm-oak wood,from where the sample was collected, shows a strikingly poor shrub and herbaceous layer.