Many factors

A mature soil, from the point of view of pedogenesis, can be defined as a layer of sedimentary rock that is inhabited by living organisms. For pedogenesis (a process that is truly never ending because all soils are slowly and continually changing) purposes, the soil is transformed into a tank, with a practically unlimited capacity, containing organisms. The consequent biodiversity has a very high ecologic value that is also very useful for Man. The characteristics of the biodiversity of the microhabitats in the soil are defined directly by a number of factors, but mainly by the variations in the availability of water and air and the temperature.
Water
The spaces (pores) that form between the solid particles play a decisive role as far as the presence of water in the soil is concerned, and consequently the presence of organisms. The water content is subdivided into the various chemical/physical forms that water can be found in: vapour, gravitational water, capillary water, hygroscopic water, crystallization water. Gravitational water accumulates in the larger cavities and tends to precipitate to the deeper layers due to the simple effect of weight. It is probably the most immediate source of water supply for the hypogeal biological community. Capillary water collects in microscopic cavities and spaces where it is held with a certain energy. Hygroscopic water binds to the various substances that are present in the soil with an even greater energy, thus its biological availability is even more limited. Crystallization water is not available for the organisms. Therefore water is held by the soil with a certain energy that varies depending on its particular form. Water tends to move in the soil following this principle, obviously interfering with the capacity of plants and other organisms to reach and maintain the right moisture level. All this is of primary importance because it not only conditions the presence/absence of hypogeal organisms, but also their seasonal or circadian (daily) migrations.
Air
In principle, observation of hypogeal atmosphere shows a “qualitative” composition that is very similar to the epigeal atmosphere, with some significant “quantitative” differences, i.e. with regard to the dosage of the single gases. For example in the soil, CO2 is present in a quantity that is about ten times greater than in the air of the epigeal atmosphere, while O2 is present in minor quantities. Furthermore, the hypogeal atmosphere is often saturated with water vapour. The lack of oxygen in the ground can support the breathing requirements of the resident biological communities for no more than a few days. However in normal conditions, it is very improbable that O2 should represent a limiting factor, because sufficient air is stored in the pores to guarantee an abundant supply (bearing in mind that in air, the diffusion of O2 is 300,000 times greater than in water).
Temperature
The temperature in the microhabitats in the ground is directly proportional to the atmospheric temperature and the solar radiation that reaches the surface layers. Furthermore, it is in turn influenced by biological factors, such as the presence of vegetation. Temperature range follows circadian and seasonal rhythms, and on the surface, the recorded temperatures range from a few degrees below zero to +60°C (depending on the latitude and altitude).
However at deeper and deeper levels, the temperature range becomes much less substantial. The high temperatures of the ground are often accompanied by arid conditions that interact in a rather complex manner with the hypogeal organisms, usually with negative results specially on their breathing systems. However, in relation to the thermal conditions in which underground life takes place, the most important factor that has been established is that rapid and very extensive excursions provoke effects that are much more harmful than extreme conditions characterized by a constant trend.

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Facts