Introduction
Soil
The soil is thin layer covering the Earth’s crust, in contact with the atmosphere: its nature and composition depend on an equilibrium between environmental factors of a chemical, physical and biological nature. It represents an important natural resource because it allows the growth of spontaneous vegetation and as a consequence an environment that is rich and varied, where many forms of life live, mainly micro-organisms and insects. Here, thanks to the presence of microflora and microfauna, the nitrogen, carbon and phosphorus cycles are completed. These are fundamental for all living beings. The soil is also a source of nourishment because it allows the growth of agricultural crops and animal farming, furthermore it provides wood that is used by a part of the world population as fuel.
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