Soil is the thin and fragile layer forming Earth's epidermis. It is a dynamic, loose and porous body of variable thickness (between a few centimetres and a few metres). Soil is formed by continuous transformations of the initial subsurface material (rock or deposit) under the influence of physical, physicochemical, chemical and biological processes. It is, with the oceans, one of the 2 main components of Earth in which life is particularly active. Soil is the source and the place of development of almost all human activities, and human life greatly depends on it.

Soil is at the hearth of terrestrial ecology. It is part of a multitude of landscapes and ecosystems (forested, agricultural, wet lands, anthropogenic) where it evolves while interacting with the other constituents (water, air, vegetation, fauna). The active processes in soil strongly influence terrestrial ecosystems and make it one of the most important resources on Earth.

The 5 key roles of soil show its importance for human kind: 1. Soil contributes to plant growth by acting as a support and providing core nutrients as well as a favourable environment for root development. Soil properties have an impact on the type of vegetation, such as calcareous soil versus chalk plant (a plant that grows well in this type of soil), and on the number and type of animal species that can settle in a given environment. 2. Soil cleanses the water. It acts as a living filter for drinking water. 3. Soil is a recycling champion, especially of the biosphere carbon that it stores in great part. 4. Soil provides a habitat for a multitude of living organisms (bacteria, worms, rodents). 5. Soil acts as building material or support for a number of infrastructures (buildings, roads, airports). Knowledge of soil properties is of most importance in all engineering work.

Societies have used and continue to use soil in their own ways for agriculture, ranching, leisure, forest exploitation, landspreading wastes, as building materials, for house, building or road foundations, as metals (iron, aluminium, gold), as a source of energy (biofuels), and so on. The biomass supported by the soil makes possible the production of biofuels such as bioethanol and biodiesel.

As opposed to the widespread belief, soil is not a renewable resource, at least at the human scale. In fact, the progressive degradation of its qualities, caused by the abusive use of lands or erosion, is alarming. This degradation progresses very fast and exceeds by far regeneration by the natural alteration of rocks.

The current world context (demographic growth, the growing scarcity of natural and food resources, the overexploitation of ecosystems, the degradation of the environment, technological changes such as genetically modified organisms [OGMs] and cloning) and the challenges that humankind will face to survive in the 21st century make it necessary to find innovative ways to protect the environment while producing enough food and biomass. Soil will be one of the most solicited resources to meet humankind's endless needs.

See also Soil Science; Soil Conservation; Soil Classification; Agricultural Soil Practices.