How could rocks be transformed into loose materials that can support rich vegetation? What are the specificities of soils compared to rocks and unaltered formations studied by geology? This article will insist on their organization in aggregates and the presence of a multiform porosity favourable to an intense biological activity as well as to the circulation and storage of water. The formation of soils is the consequence of alterations of various geological materials (the “parent materials” formerly called “mother rocks”), under the action of precipitations and living beings, over a long period of time. A second article will detail the factors that determine their formation, their appearance, their properties and their functioning.