For soil there are two different meanings: geographically (also soil or even pedosphere – from the Greek πέδον, pedon, ground, earth and sphere when considered part of the geosphere), is the surface layer that covers the earth’s crust, resulting from the alteration of a rocky substrate, called parent rock, by chemical, physical and biological action exerted by all surface agents and organisms present on it, can include both sediment and regolith.

The second meaning considers the soil from the pedological point of view, that is not only as a surface but as the compound of that extremely variable mixture of organic substance and mineral substance that allows the life of plants and animals, in the absence of these two components we do not speak of soil. In this respect, soil is present exclusively on planet Earth.

Three main soil functions can be identified:

  1. productive function, the ability of soils to maximize the transformation of radiant energy into chemical energy;
  2. protective function, due to the effects of regulation, absorption and distribution of water flows, the function of storage of organic carbon, and also filter and buffer of pollutants;
  3. naturalistic function, the ability to host and maintain biotic reserves and to transmit the signs of ecosystem and human history.

The formation of a soil is conditioned, first of all, by the particular composition of the parent rock on and from which it evolves; other important factors for its genesis are the local topography, regional climatic characteristics, vegetation cover, etc. A factor of fundamental importance is water: an example is provided by the action of rain that, by slow and progressive infiltration, comes to alter the granites or, by runoff of meteoric water, causes the erosion of Mediterranean-type soils, exposing the underlying rock layers, giving rise to the so-called “skeletal soils”. Important causes involved in the alteration of the parent rock are:

  • differences in temperature (e.g., frost) that cause its crumbling and subsequent decomposition;
  • the action of penetration and corrosion due to plant roots;
  • the crumbling of the surface layers by incrusting mosses and lichens.

In the genesis of a soil are also essential the accumulation of organic materials and the migration of chemical elements. The layer of dead leaves, fallen and piled up on the ground (the so-called litter), soaked by the rains, is slowly invaded by numerous hosts of saprobic organisms (Bacteria, Protozoa, Flagellates and Rhizopods, Rotifers, Nematodes, etc..); moreover, the whole mass is soon penetrated by fungal mycelia. In a second time, saprophagous demolition organisms intervene, such as Oligochaetes (earthworms), Myriapods, Gastropod Molluscs (slugs and snails), Mites, Collemboles and various groups of Insects, which attack, fragment and digest (in part) the plant debris. From the slow decomposition of this organic material and from its mixing with inorganic particles and with untouched vegetal fragments, a particular type of soil, humus, derives. Its chemical characteristics depend largely on the botanical species whose leaves have formed the litter.

In fact, a woodland environment made up of deciduous plants (oak, birch, hornbeam, etc.) on an original calcareous soil, leads to the formation of a sweet humus, with a sensibly neutral reaction, called mull. In this case, the accumulating plant material has been extensively processed and digested by earthworms and the humus becomes a complex mixture of clays and organic colloids, characterized by a dark coloration in the upper horizon (coprogenic forest mull).

Partial digestion of plant debris by breakers, especially on poor soil, gives a second type of humus, coprogenic but little altered and with a rather powdery consistency: moder. A third type of humus originates (often on siliceous parent rock) from a litter of resinous or very lignified leaves (for example, Conifer needles or Ericaceae leaves); this is not very suitable for a complete attack by Oligochaetes and Myriapods; however, it is partly eroded by Molluscs, large Collemboles and mites. In this way a brute humus (called mor) is formed, acidic, with an almost felt-like consistency, little decomposed and invaded by mycelia of saprophytic fungi, which slowly digest it.

Through the action of water, which permeates the humus and percolates in the underlying soil, there is the migration of elements (soluble mineral ions, clay particles, etc..) in the various layers of soil. The course of such a transport depends on the regional topographic and climatic situation of the soil itself. In arid climates, it is very common for elements to be transported upwards by evaporation, with the formation of real surface crusts (for example, of sodium chloride or gypsum or ferruginous materials). Particle entrainment can also occur to the deeper layers of the soil, through the action of percolating rainwater (leaching), as occurs in humid climates.

Rainwater can also cause a leaching by dragging humic acids (due to an incomplete mineralization of the litter of needle trees) followed by the release of iron by the clays as a result of the attack of these acids; again due to the action of water, iron migrates in depth in the form of colloidal ferric hydroxide for which it forms an ashy band composed almost only of silica: in this case we speak of podsolization. It should be noted that the activities of microorganisms are particularly intense in the rhizospheres (part of the soil surrounding the roots and underground organs of vascular plants).

In this regard, it is useful to remember the symbiotic, mycorrhizal and saprotrophic activities of many fungi; the enzymatic digestion of cellulose, nitrification, the fixation of free nitrogen by Bacteria and Blue Algae, etc. Taking into account all these factors, and with a careful vertical analysis of the soil, it is possible to define the nature and composition, structure and succession of the different layers, that is, the profile of a soil, which includes a set of levels, which in turn can be divided into several horizons. In practice, it was found that natural soils have a wide range of differentiations, related to different environmental conditions of formation: in relation to this, several classifications have been developed, some of these take into account for example the relationship between soil and parent rock, distinguishing native soils, formed in situ, from allochthonous soils, which do not have a relationship of derivation with the underlying rock, as formed by materials conveyed by surface water.

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