The soil, the surface layer of the earth's crust, is created by the action of atmospheric agents (water, frost, wind, temperature ranges) which decompose the ground, breaking it up and moulding it. Subsequently, it is consolidated by the action of the living animal and vegetable organisms.
The vegetable organisms penetrate it with their roots to obtain nutritional elements and by doing so, modify it chemically. The animal organisms, the worms, insects and rodents, constantly mix it up as they dig their tunnels to feed on the vegetable organisms.
At the end of their life cycle, both vegetable and animal organisms return to the ground the elements which they originally removed, along with those they synthesized or elaborated in their metabolic processes, in the form of cellulose, lignin and proteins, which, as they combine in the soil, modify its chemical and physical make-up.
This is a perfect ecosystem which reaches a calance involving thousands of species of bacteria and fungus, unicellular algae and protozoans, hundreds of species of insects and worms, the biological activity of the roots of the plants and the activity of reptiles and mammals (fieldmice, moles and marmots).
The soil's fertility depends on the balanced presence of three basic elements, nitrogen, phosphorus and potassium, and a dozen other elements indispensable for vegetation such as sulphur, calcium and magnesium. But besides its chemical composition, the physical characteristics of the ground are also very important. A large quantity of sand in the soil makes it easier for roots to penetrate whereas the opposite is true of soil that has a preponderance of clay.
Even the colour of the soil itself must not be underrated when considering its suitability for cultivation. For example, black-coloured soil absorbs solar rays more rapidly and so will heat up quicker in the spring than grey-coloured soil and thus can be used earlier for sowing. Soils can be red, brown, grey and black and, besides, there are also those made up of more than one "horizon", the term used for describing a coloured layer.