Ground (pedology)

Soil organic and medium of life

Plants and animals benefit from the disintegration of the rocks of the earth's crust and they contribute to it, Co-producing the ground, and drawing there water and rock salt, and sometimes finding there shelters, support or medium essential to their life. The ground was formerly regarded as an abiotic factor, resulting from factors such as geology, the climate, topography… The whole of these abiotic factors, mobilized by the Alive one, and in particular by the micro-organisms, which also recycle nécromasse it and the excrements in the ground constitute the base of the terrestrial ecosystems. Except virus, it is from 100 to 100 million micro-organisms which live in one gram of ground. The ground shelters also the Rhizosphère, complex interface between air vegetable world and mineral world. The processes of exchange, decolmatation and excretion of the fabrics racinaire and the mushrooms symbions there alive play a fundamental role in the production process and of maintenance of the grounds and the Humus, which can be strongly affected by the human activities.

To know the grounds, it is to learn how to identify, locate and chart the elements essential to the life on Earth and to give the possibility of identifer geographical sectors to quality and stakes varied to produce the foodstuffs, the protection of water and biodiversity.

The pedologist can locate grounds favorable or unfavourable at certain organizations and produce charts of Pédopaysage S . The botanist and the phytosociologist also can, by means of plants bioindicatrices, to identify the characteristics of certain grounds: for example plants of dry limestones mediums, group in which one will be able to locate some emblematic orchises.

The production of Agrocarburant S is one of the new vocations, which the interest and the ecological assessment remain discussed, that certain industry groups and states want to give on the ground.

Definitions

There exist several definitions of the ground:

the Agronome S name sometimes ground the arable part (surface film) homogenized by the Labor S and explored by the root S of the Plante S. one considers that a good agricultural ground consists of water 25%, 25% of air, 45% of mineral matter and organic matter 5%. The compressing and the sole of ploughing can induce a loss of output from 10 to 30%.
the pedologists estimate that the arable part constitutes only the surface part of the ground. The pedologist Albert Demolon defined the ground as being “the natural formation of surface, with movable structure and variable thickness, resulting from the transformation of the subjacent bed rock under the influence from various processes, physics, chemical and biological, in contact with the atmosphere and of the living beings”.
the persons in charge of the Town and country planning distinguish the agricultural grounds, the timbered grounds, the built grounds and the other grounds.

The science which studies the grounds, their formation, their constitution and their evolution, is the Pédologie.

Components of the grounds

Mineral fraction

The mineral fraction represents the whole of the products breakdown physical then chemical of the bed rock.

One can classify them by decreasing diameters:

sands
the silt S
the granulometric Clay

See the detailed article Granulometry .

All these elements constitute the “skeleton” of the ground in which one finds them in various phases: gas, liquid, solid, and with the state of plasma.

Organic fraction

The organic matter can be defined like a carbonaceous matter coming from the decomposition of vegetable and animal living beings. It constitutes the Humus.

It is made up of principal elements (the Carbone - C, the Hydrogène - H, the Oxygène - O and the Azote - NR) and of secondary elements (the Soufre - S, the Phosphore - P, the Potassium - K, the Calcium - Ca and the Magnésium - Mg).

It is divided into 4 groups:

the Organic matter alive, animal and vegetable, which includes the totality of the biomass in activity,
remains of vegetable origin (vegetable residues, exsudats) and animal (dejections, corpses) called “fresh organic matter”,
of the intermediate organic compounds, called organic matter transistoire (evolution of the fresh organic matter),
of the stabilized organic compounds, organic materials, coming from the evolution of the preceding matters.

The vegetation provides vegetable remains which constitute the litter of the Ao horizon. Its decomposition is done under the action of the Microflore and of the Microfaune of the ground, and produces the Humus and the mineral compounds of horizon A. the two processes are on the one hand the Minéralisation (producing the mineral compounds such as, it, the Nitrate S and the Carbonate S) and the Humification (polymerization in amorphous organic compounds which bind to clays). The process of humification leads to the formation of the humus. (anas)

In active medium not very, the decomposition of the litters is slow, the organic horizon Ao is brown black, fibrous and acid. One speaks about Mor or ground of heather.
In medium biologically more active, the Ao horizon is less thick and constitutes a Moder.
In very active medium biologically, the decomposition is very fast, the Ao horizon disappears and appears a A1 horizon, made up of argilo-humic aggregates with Fer and Aluminum. One speaks about mull.

The texture of the ground

One of the characteristics of the grounds is the size of the biogenic salts which compose it.

the stones or blocks are the elements of size higher than 2mm.
the elements of size lower than 2 mm are defined by class of texture (Sable S, silt S and Argile S).

These rocks belong to the group of the Silicate S.

Of the ions (Ca²⁺, Mg²⁺, K⁺, NH₄⁺, NO₃^-,…) arrive in the ground in solution in infiltrated water, or fixed at the colloidal particles quoted above.

The colloidal particles negatively charged can be presented at the dispersed state or flocculate.

In a dispersed state, the particles are pushed back because of their polarity, and occupy all the interstices of the ground. This last becomes poison gas, and water does not infiltrate there more. The ground is difficult to work.
In a flocculated state, the colloidal particles are neutralized by the positively charged ions, and are bound with those. The formed flakes leave a lacunar, permeable ground with water and the air. It is a ground with a good structure.

profile of the ground

To describe a ground, it is necessary to observe it in sections parallel at surface, called horizons. Two types of horizons are usually superimposed: a succession of humus-bearing horizons, above the mineral horizons. In short, the structure rescpecte this fitting (see profile of the ground for more precise details). The humus-bearing horizons are the horizons richest in being alive.

L - litter. The litter includes/understands the whole of the rough remains (remainders of wood, grass and faded flowers).
O, including/understanding the organic matters in the course of transformation:

O_f - horizon of fermentation. The temperature and moisture are optimal there, because of the solar protection provided by Ol.

O_h - humified horizon. This horizon is made up exclusively of organic matter directly usable.

A_h - horizon mixed. Composed of biogenic salts and humus. Its structure depends on more or less fast incorporation of the humus.

The mineral horizons are the least rich in living organisms.

E - washed horizon. It is drained by the water which infiltrates, which makes it low in ions, in composed of clays, humic, and in aluminum and iron hydroxides.
B - horizon of accumulation. Intermediate horizon appearing in the advanced grounds. It is also called structural horizon or of deterioration and is rich in fine or amorphous elements: clays, oxides iron and aluminum, humus, stopping their descent on his level.
C - not very faded rock-mother.
R - nonfaded rock-mother. Geological layer in which the hydrocarbons were formed.

Each profile of ground has a history, that the pedologists try to recall thanks to the characteristics of the various horizons.

Various types of grounds

There exists a great number of the types of grounds, among which the brown ground, the podzol, the hydromorphic ground, the red ground, ground isohumic, ground ferralitic, ferruginous ground. See the List of the grounds for more details.

Functions of the grounds

The ground is a Ressource natural, little or slowly renewable, overall in the process of degradation (especially in the poor countries, where this one is not compensated by the rises of productivity permitted by mechanization, the Engrais and the Pesticide S). This inheritance is also in quantitative regression according to UNO (FAO), primarily devoted to agriculture with the Sylviculture or the ecosystem S, but also and more and more with the " humains" establishments; (cities, dwellings, zones of activity, carpark, etc).
Des new functions is recognized to him of which an important role in the cycle of water, for the Public health (cf polluted grounds) and of the ecosystem S, as well as an importance in the biogeochemical cycles of CO2, of Nitrogen, the Potassium, the Calcium, the Phosphore, the metals. The grounds of quality limit the risks of erosion and Salinisation.
Sa function “ well of carbon ”, is still badly included/understood or badly evaluated, and seems strongly varied according to the biogeographic conditions , but the Protocole of Kyoto stresses the importance of the ground like important carbon well, especially in moderate zone. The work group of the European Program on climate change (PECC) devoted to the carbon wells related to the agricultural grounds for example estimated that this potential represented the equivalent from 1,5 to 1,7% of the CO2 emissions of the EU. A European conference recalled in 2001 the importance between carbon Puits and Biodiversité.
In France the Grenelle of the Environment proposed in 2007 the concepts of green Trame of environmental Remembrement which could both contribute to restore the grounds. A “ environmental Bail ” had been created (decree of March 2007). This lease is worth however only in certain geographical areas specified by the decree and for private financial backers (if their pieces are located in particular natural spaces and the clauses in conformity with the official document of management into force in these zones). This bali makes it possible to impose a restrictive list of cultivation methods likely to protect the environment. Their nonrespect by the transferee of the lease can involve his resilisation.

Threats

When the grounds are not simply " consommés" by construction (Urbanization, Périurbanisation, Road S, Carpark S.), they are especially threatened by some practical agricultural which induce various forms of Régression and impoverishment of the soil;

- a reduction in the organic matter rates (Ploughing, intensive cultures);

- compaction and asphyxiation, appearance of a Sole of ploughing;

- the Acidification, the Salinisation and possibly the Turning into a desert;

- the erosion (hydrous or wind)…

In the low-grounds, they can also be threatened of marine immersion (cf assembled of the oceans).
Les atmospheric repercussions, the Clarification sludge, some Manure (Phosphate S rich person in Cadmium in particular), the Pesticide S and sometimes water of irrigation brings there significant quantities of Heavy metals (not dégradables, bioaccumulable) and various Polluant S or contaminant microbial, of which Pathogène S sometimes.

Pollution

The agricultural grounds often contain micropolluants which originate in the geochemical bottom, the Séquelles of war, or more often the atmospheric repercussions (45 000 t/an of zinc and 85.000 T of plomb/an estimated in the years 1990 per Juste, 1995, for Europe of the 12) and sometimes water of irrigation.
En France, the INRA studied seven metals (Cd, Cr, Co, Cu, Ni, Pb and Zn) in 460 horizons of agricultural grounds, by finding some a rate median of 0,22 mg/kg, against 0,10 in equivalent forest grounds. In an industrial area (Northern of France), P. Six (1992, 1993) confirms these results with on 1000 plowed horizons analyzed in the department of north, not having undergone mud contributions, for which the median value was of 0,37 mg/kg, 60% of the samples ranging between 0,12 and 0,58 mg/kg. Except in the case of species Métallophyte S, natural export by the plants is weak (less than 1% of the waste mud contributions in the harvests studied on 15 to 10 years)
Certains forest grounds, in condition of being exposed to repercussions of pollutants or if they contain some naturally airs to however better preserve some of these pollutants than the agricultural grounds (it is the case of the radionuclides). The pollutants are more or less biodisponibles according to the grounds. They are it generally more (up to 100 times more) in France showed that the coefficient of transfer of the cadmium of a ground towards cereal grains varied from 0,01 to 1 according to the sites, whereas in the 1st case ground by containing 4 to 10 times more) ) in the acid grounds.

Quality of a ground

It relates to the aptitudes of a ground to fill its functions of agricultural production, forestry or ecological and its impact strength. It is measured by its biological components (Bioindicateur S, such as Vers of ground), the Fertilité, its medical condition (in the broad sense), to compare with a stage climacic or ideal, which varies according to the biogeographic Zone and altitude and the context considered. One now seeks to measure the environmental externalities on water, the air and the risks related to the Inondation S Sécheresse S, nitrates, pesticides, Aérosol S, etc One differentiates the impacts of biodegradable pollutants (nitrates) of pollutants not dégradables (metal elements traces or ETM), and one is interested in their ways of dissemination or the Synergie S which they can develop with other pollutants or elements of the system ground.

See too

List of the grounds
Pedology
Pedogenesis
Regression and Parent directive impoverishment of the soil
for the protection of the grounds
Classification of the grounds
Humus
Érosion
Dépollution of the grounds
Hydroponie
Aéroponie
Ultraponie
Terra lent

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