Darwin’s tangled bank is the biosphere in poetic microcosm, and the “damp earth” he refers to is soil. Soil is arguably the most complex of all geological materials, a combination of mineral and organic constituents in solid, aqueous and gaseous forms, organized into a loose, porous, horizonated, plant-bearing material, that is constantly changing. It forms as a result of a complex series of interactions and feedbacks between lithosphere, hydrosphere, atmosphere and, biosphere. As the natural geological cover of most of the land surface of the earth, it is the focus of this encyclopedia.