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HUMUS |
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The
ground The
ground is usually considered a simple support for human activity and the
place where the plants grow and the animals live; really it’s itself the
environment for the life of a lot of living organisms (fungus, bacteria,
algae, earth-worms, insects, ecc.), equivalent to a weight coming to 2.000
kilos easily for hectare; to make a comparison we consider that in a densely
populated country like Italy little more than 100 kilos of men and 200 kilos
of animals live in a hectare. The organic substance, which has to be added to
all this life, is in a very different percentage at an average of 1-3%,
30.000-120.000 kg/ha and results both from the living organisms in the ground
and from the vegetables. So the ground is an immense living environment but
also the holder of big quantities of carbon in organic form.
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Humus In the
ground the organic substance is in a percentage of 10-15% in form of
vegetable or of animal residual products which don’t have undergone deep
alterations. The remaining part of it is composed by humus which appears
under colloidal state and which exceeds all the other inorganic colloids of
the ground because of its considerable capacity of imbibition and of its soft
quality. Humus comes from all the organic residual products that reach the
soil and the absolutely predominant material of them results from the
superior vegetables. When the vegetables reach the ground, the numerous
micro-organisms slowly transform them by mechanical, chemical and
chemical-biologic action. Humus is constituted by a whole of substances
containing more carbon and less oxygen and hydrogen than the original
substance (cellulose, lignin, proteins,ecc.); it results from a process of
partial decomposition but especially from a process of synthesis, which flows
into the production of substances characterized by a major complexity than
the one concerning the original material: humus is a polycondensation product
of aromatic, heterocyclic, aliphatic molecules. Moreover humus combines
itself to the clayey minerals in units which become bigger and bigger and
structuralize the ground, making the mineral salts available; so, if on one
hand the molecule of humus tends to become more and more complex, thanks to
new reactions of condensation which involve organic and clayey
macromolecules, on the other one it is subdued to a slow but continuous
catabolic process, both oxidative and destructive. In this way it originates
carbon in form of CO2 and the nutritious elements connected to it. Humus has
particular characteristics among the organic substances; it’s the only case
in which complex organic molecules are directly and tightly joined together
with the mineral world, having at the same time continuous exchanges with it
and holding a relatively stable structure, but developing during the time. Generally
the base chemical components of humus are the following: 55% of carbon, 36%
of oxygen, 5% of azote and 4% of hydrogen. Therefore humus is characterized
by a ratio between carbon and azote equivalent to10 and the weight of the
first one represents the 50%.
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LOSSES
OF HUMUS The
first 30 cm.(centimetres) of the ground weigh normally 4.000 tons pro
hectare; considering the 2% of humus we obtain 80 tons which, with an annual
mineralization of 2%, product 1,6 tons equivalent to 0,8 ton of carbon. It
returns to the atmosphere in form of CO2, so without the contribution of
organic substances, the contents of carbon are reduced to the half every 35
years. In the same conditions, more stable humus derives from vegetables
which high contents of lignin. The index of mineralization could reach higher
levels if one practises wrong agricultural techniques such as excessively
deep and/or too much frequent cultivations, if one lets the soil stay without
vegetation, expecially in summer, if one burns residual products of the
cultivations ecc. This can bring to the disappearance of humus in a few
years, as it happens during the process of desertification or of laterization
which can result from the removal of the vegetable covering and of the
cultivation of tropical lands. The IPCC estimates that the CO2 discharged in
the atmosphere because of this last action amounts to hundreds millions of
tons annually. |
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UTILITY
OF HUMUS As we
have already seen, humus creates and holds the structure of the ground, makes
soluble the mineral salts and originates the azote through its own
mineralization; moreover, it allows the passage and the change of air in the
ground, essential both for the root and the aerobic organisms, thanks to its
much lower density (with 2% in weight , it occupies the 6-8% in volume). Its
colloidal characteristics allow it to hold big quantities of water,
obstructing the erosion and then gradually giving back the water to the
tillages. It works as a water fly-wheel tank in the ground, a function which
will become more and more necessary, considering the forecast rise of
precipitations and their accentuated variability in consequence of greenhouse
effect. |
