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Pedogenesis, or soil formation, is the process of geological alteration of source rock that gives rise to soils. Every rocky substrate tends, over time, to achieve a relatively stable equilibrium with the external environment, and in particular with the atmosphere, through very gradual, but profound, chemical-physical modifications.
Soil formation occurs through various stages that begin with the physical-mechanical breakdown effected by the weather, by means of significant thermal fluctuations that fissure the rock and thus open it up to rain water. Rocks, composed of layers of various minerals, tend to break apart starting at the surface, which has greater exposure to the forces of freezing and thawing. To this must be added chemical decomposition of the rock through the solvent properties of water: water’s carbonic acid content enables it to solubilise the calcium carbonate in minerals. When the rocks are thus de-calcified, they are weakened and begin to release the mineral elements that are indispensable to the first colonizers of the soil, the lower-order plants. The first organisms to establish themselves are mosses and lichens; by producing certain enzymes they are able to biologically degrade the minerals in order to extract the elements they need to build their own biomass. The natural biological cycle of such plants leads to the formation of a thin organic stratum that in turn allows the soil to host plants with more demanding nutritional requirements. Thus, through the combined action of all these described phenomena, there occurs a gradual but profound alteration of the original minerals and the formation of an organic component that represents the premise and the basis for each soil’s degree of fertility.
Over the course of millennia, and in long cycles of the subtlest changes, various elements in the soil shift downwards from the surface, and the soil thus gradually diversifies into different strata and horizons.
Simplifying a bit, we can distinguish three separate horizons in a soil profile:
The first, and closest to the surface, is called the eluvial horizon; it is the richest in organic humus substances and displays greater biological activity. Darker in colour, it undergoes the leaching effect of water, with calcium carbonates being thus deposited into lower strata. Any clays that occur, formed from the breakdown of silicates, are mixed with a gravely schist that can amount to 50% of soil volume.
The second horizon, called the illuvial zone, is where the carbonates accumulate, as well as certain cations such as iron, alluminium, and other minor elements that made up the source rock.
The deepest and last layer, called the pedogenetic substratum, is composed of un-decomposed rock, the same material present on the surface predating the degradation process.
The current profile of a soil does not, of course, represent the endpoint of that soil’s evolution, inasmuch as the process of change will continue, affecting ever deeper strata, forming soils with ever higher clay content; of darker colour, given the accumulation of organic materials; and with ever finer texture containing only hard, more permanent mineral complexes, such as granites and quartzes.
This slow evolution of soil, called ferritization, will go on for thousands and thousands of years until new orogenetic movements will modify the earth’s crust or until new glaciers will cover the area with new layers of rocky materials coming down from the Alps.