The soil, this material located underground, in-between the geological layers and the atmosphere, is filled with host of unsuspected micro-organisms. Yet these invisible organisms play a vital role for all species living above the soil, plants and animals alike. This is what contemporary molecular biology methods help to reveal. Bacteria “rotate” the cycles of the elements of matter on which plants feed, and they can directly enter into associations of mutual benefit (symbiosis) with the roots of some of them. Mycorrhizal mushrooms build networks with and between roots, forming symbioses (or mycorrhizae) through which water and mineral salts are transmitted to the plant in exchange for sugars to the mushroom. This microbial universe of the soil ought to be studied, by focusing on understanding how its interactions work.