This conclusion presents some closing thoughts on the concepts covered in the preceding chapters of this book. Soils began in geological time with the advent of vascular plants on the Earth’s surface, but some clay minerals preceded soils. The fundamental principle of soil formation, due to Dokuchaev, has soils related to five factors of formation: time, climate, organisms, relief and parent material. Illites in soils mainly originate from the partial transformation of micas. New clay minerals, silicates and oxides, form by the destabilisation of similar materials in rocks that are exposed to surface chemical conditions of altering aqueous solutions that are slightly acidic and saturated with atmospheric oxygen. Extraction of clays involves both the disaggregation of soils and the dispersion of the disaggregated particles. The role of clays in the formation and stabilisation of microaggregates restrains fine materials against erosion and leads to pores for the supply of water to plants and as spaces for the growth of plant roots.