Soils form the skin of the Earth. Their thickness varies from a few millimetres – there where the soil is very young or scraped-off by external forces (e.g. water, wind, human activity) – to several metres – there where they occur in protected or stable places. They comprise of layers or soil horizons, each with their own characteristics. Soil material consists of a variable and often complex mixture of organic matter, sand, silt and clay particles, or is composed of dominantly organic debris.
Soils are formed under the influence of climate, the material they occur on, the flora and fauna (including human activity), the topography of the terrain, and time. These five factors are commonly known as the Soil forming factors.