How are soils formed?

We tread on it all day long, it is the matrix of our cultures and the support of all terrestrial life: the soil is an essential component of the geosphere and represents the outermost layer of the continental crust. His training is intimately linked to complex interactions between the rocks of the earth’s crust, the atmosphere, the hydrosphere but also the biosphere.

The soil, a living layer

One of the peculiarities of the soil, compared to other geological layers, is to be “living”. The presence of a wide variety of organisms is significant and actively participates in the genesis of soils. It is also the receptacle of a great quantity of matter organic, making it the main carbon sink on the planet. Soils therefore have a crucial role in the fight against global warming, and ensuring their good health is essential today. However, scientists are warning of the constant deterioration of their quality, which severely impairs their functions.

Soils have a very variable composition and structure, which will depend on the nature of the substrate (the bedrock), but also the climatic conditions and the organic processes that take place there. They can form by two different mechanisms. A distinction is thus made between residual soils formed by the alteration of the substrate, and transported soils formed by the addition of constituent elements.

Transported soils

The soils transported can be formed by scree or mudslides, by the alluvium of rivers (alluvial soils), by glacial deposits, by the sedimentary filling of lakes (soils lake) or by the accumulation of sand or dust transported by the wind (wind soils).

Residual soils

Residual soils are mainly associated with weathering processes. The alteration, mechanical and chemical, will thus gradually degrade the substrate progressing downwards, to form different horizons. Four horizons are usually defined: the O horizon, which corresponds to the layer of litter fresh, composed of organic matter undergoing transformation; horizon A, or horizon of leaching ; the B horizon, or average accumulation horizon; then the C horizon, or lower horizon, containing mainly the slightly altered parent rock.

Description and formation of the different soil horizons

The O horizon is therefore the seat of intense biological activity (animal and plant), which will degrade the accumulated organic matter and transform it into humus. Horizon A, located below, consists of a mixture of humus and elements minerals resulting from the intensive alteration of rocks, in particular by rainwater and temperature. The thickness of this level will thus be more or less developed depending on the climatic conditions. In cold or dry countries, this level but also the following ones will thus be of less importance. We will then speak of little or no evolved soils.

The further down you go, the greater the mineral fraction and the lower the presence of living organisms. Thus, the B horizon is characterized by the presence of fine elements, such as clays. These elements, resulting from surface alteration, gradually penetrate in depth until they are stopped by a mechanical barrier (reduction of porosity) or by a modification of the chemical balances (modification of the pH for example).

At the base, the C horizon corresponds to the very little altered level, which contains the source rock.

Different types of floors

Depending on the nature of the minerals, the amount of organic matter and the degree of weathering, several types of soil are defined. Here are some examples.

The soils of tundra are poorly evolved soils, in particular because of the presence of permafrosta continuously frozen level, which limits weathering processes.

Brown soils are advanced soils where the humus is very well mineralized. These are typically the forest soils found in temperate countries, such as France.

Chernozems, or isohumic soils, are characterized by the presence of a large quantity of well-mineralized humus. These very fertile soils are generally present in the steppes semi-arid to Continental climate, like in UkraineRomania, China or Argentina.

The podzolsor soils at raw humusare formed in cold and humid climates, above a substrate with a very acid. The organic matter there is not very mineralized. This attacks and destroys the clays, whose aluminas and the iron will be trained in depth. The surface of the podzols is thus purely siliceous and leached. The B horizon is rich in humic elements and iron. These are very infertile soils.

Oxide soils, like Mediterranean red soils, are characteristic of regions with high rainfall and high temperature. In this type of environment, theoxidation is very important and gives a color ground feature. The absence of surface vegetation, as in the case of the deforestation, causes hardening of the surface. We speak of lateritic crust.