The soil consists of a mixture of solid particles, water and air. Solid particles can be inorganic or organic. The inorganic parts are mainly minerals: silicates, oxides and hydroxides of iron, aluminium, manganese, etc. that, according to their dimension, are classified in skeleton and fine earth, which are then divided into sand, slime and clay. These derive from the alteration of rocks into smaller and incoherent material, which accumulates to form superficial deposits. The deposit of incoherent material can occur in the same place as the rock was disintegrated.
The degradation processes of vegetal residues (leaves, fruits, dry branches or whole plants) and dead animals lead to the creation of organic fractions of the soil. Organic compounds can keep unaltered for long periods (non-humic compounds) or be subject to deep and fast changes in their original chemical structure (humic compounds or humus).
Water and air occupy the free spaces between solid particles (pores), and form a thick and extended network that allows water to move in the ground.
There are groups of terrestrial animals that eat insects, worms and other invertebrates...
In Sub Arctic Swedish regions, some scientists studied the changes of permafrost...