Factors of carbontes formation
In normal conditions, the original material undoubtedly has an influence on the carbonate content in the soil.
In most cases the parent rock constitutes the initial source of carbonates,either because they already existed there, or because although they were not originally present, the carbonates have formed in the soil as a result of the weathering of original minerals rich in calcium (plagioclases, pyroxenes and amphiboles,...)
However, soils with calcic horizons formed from materials in which there neither existed carbonates nor minerals which could generate them by weathering, are by no means exceptional. The origin of carbonates in these cases is explained by eolic and hydric contributions.
In principle, calcic horizons can be found below any kind of relief, but owing to their specific conditions of formation, they tend to accumulate in certain physiographical positions.
The hypodermic circulation in limestone regions makes the carbonates migrate from the higher zones of the hills and concentrate in the lower slopes.
Vegetation plays an important role in the formation of these horizons, as plant roots absorb water, thus producing the precipitation of salts.
Moreover, the vegetation is capable of forming carbonate crystals which provisionally remain accumulated in their tissue, and after humification, the synthesized carbonates are incorporated into the soil.
The synthesis of carbonates has also been observed as a result of the metabolic action of certain bacteria.
Moreover, there are numerous authors who have highlighted the action of organisms in the formation and transformation of calcic horizons.
Climate represents an essential factor in the translocation of the carbonates through the soil.
Thus, in humid climates the representative process is the leaching of carbonates and it is unusual that accumulation occurs, whilst in arid or semi-arid climates, precipitation is generally insufficient to eliminate the carbonates from the profile.
However, as a series of pedological parameters are involved in this process (such as the permeability of the horizons), on certain occasions the role of climate may be obscured.
Moreover, we must always take into account the possibility that the calcic horizon was formed in the past, under climatic conditions which may have been very different from the present.
In any case, carbonates play a very important role in arid regions, to the extent that the process of carbonation may be considered the most representative in these regions.
Apart from the global quantity of precipitation, its distribution decisively affects the behaviour of the carbonates. Thus, the Mediterranean climate, in which rainfall is concentrated in the colder months (maximum dissolution) and with hot, dry summers (intense desiccation and therefore precipitation), provides ideal conditions for the formation of calcic horizons.
In hyperarid climates carbonation and decarbonation processes are not active.
It is universally accepted that the formation process is rapid, but its quantitative evaluation is very difficult, as numerous climatic factors (quantity and distribution of precipitation, evapotranspiration, temperature,...) and pedological ones (availability of Ca, permeability, type and concentration of the soil solution,...) are involved in their development.
References in litterature point to an accumulation rate ranging between 0.025 and 10mm/year.
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