And don't forget the plants. What do they get out of this mess of soil life? Nutrients, of course. At each
stage in the decomposition process, some nutrients leach out into the water and get hungrily sucked up
by the plants whose roots run through the whole ecosystem.
breakdown products of plants
and those generated by microorganisms.
The easily decomposable sugars,
starches and proteins are quick and
easy for fungi and bacteria to decompose, hence the C and energy
they provide is readily available.
Most of the microbes living in the
soil can secrete the enzymes needed to break up these simple chemical compounds. The larger mites
and small soil animals often help in
this first stage of degradation by
breaking up the organic matter into
smaller pieces, thereby exposing
more of the material to colonization by bacteria and fungi.
Some of the energy or nutrients
released by the breakdown of molecules by enzymes can be used by
the bacteria and fungi for their own
growth. For example, when an enzyme stimulates the breakdown of
a protein, a microbe may be able to
use the C, N and S for its own physiological processes and cell structure. If there are nutrients that the
microbes do not use, they will be
available for other soil organisms or
plants to take up and use. When
microbes die, their cells are degraded and the nutrients contained
within them become available to
plants and other soil organisms.
The second stage of decomposition
involves the breakdown of more
complicated compounds by many
fungal and bacteria species. These
compounds take longer to decompose because they are larger and
are made up of more complicated
units. Specific enzymes, not commonly produced by most microorganisms, are required to break
down these compounds.
Decomposition only takes place
where conditions are suitable. Abiotic conditions have a considerable
effect on the rate of breakdown.
There must be some moisture available, soil temperatures must be
suitable (usually between 10 and 35
°C) and the soil must not be too
acidic or alkaline. Decomposition
also occurs at higher temperatures,
as in composts, or under waterlogged conditions through anaerobic processes. Thus, the types of
organisms involved in breaking
down the organic matter also depend on the conditions.
The type of organic matter, the way
it is applied or incorporated into
soil and the way it is decomposed
influence the physical, chemical and
biological balances in the soil and
determine the various impacts. It
can change:
• the amount of N available to
plants;
• the amount of other nutrients
available;
• how the soil binds together (soil
aggregation);
• the number and type of organisms present in the soil.
Micro-organisms can access N in
the soil more easily than plants.
This means that where there is not
enough N for all the soil organisms,
the plants will probably be N deficient. When soils are low in organic
matter content, application of organic matter will increase the
amount of N (and other nutrients)
available to plants through enhanced microbial activity. The number of microbes in the soil will also
multiply, as they can use the organic matter as a source of energy.
Where the number of fungi and
bacteria associated with the breakdown of organic matter increases,
there may be some improvements
to the soil structure. Adding organic matter can also increase the activity of earthworms, which in turn
can also improve soil aggregation.
Text abstract from: Alexandra Bot and José Benites: The importance of soil organic matter: Key to droughtresistant soil and sustained food production, Rome 2005 http://www.fao.org/docrep/009/a0100e/
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