The
organic matter contents of a good mineral soil are 2-5% which consists of
organic matter in the form of living and dead organic matter. Living organic
matter consists of soil microorganisms (fungi, bacteria, nematodes and earthworms),
plant roots and insects.
Dead organic matter is added after the death of these
living organisms and addition of leaves, bark of trees and dead animals. Farmyard manure, composts and mushroom compost are also source of dead organic
matter. All of this organic matter is ultimately broken down into carbon
dioxide, water and minerals. Humus is the collection of organic compounds that
persists in soil for longer duration.
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| Soil section from a grass field |
LIVING ORGANISMS IN
THE SOIL
Living
organisms in the soil ecosystem for food webs and food chains.
1. The source of converting
sunlight and simpler compounds into organic compounds are the Producers, which
are involved in photosynthesis (green plants mostly in agricultural soils).
Next are the primary, secondary and tertiary consumers. The most important
after producers are ‘decomposers’ which decompose and convert dead organic
organisms and organic matter to simpler compounds for reuse; and successfully
complete the nutrient cycle of many nutrients which also become available to
the plants.
Decomposers
can be further distinguished into primary decomposers which attack the freshly
dead organic matter such as earthworm and some species of arthropods and fungi.
Secondary decomposers live on the wastes decomposed by the primary decomposers
and include many species of fungi and bacteria in particular.
1.
Plant Roots
Plant
roots are the main source of organic matter and aeration in the soil. Roots
evade the soil and when the plant is uprooted or dead, the roots decompose and
create space for aeration of soil plus they add to the soil organic matter. The
penetration of soil by roots is decreased when the soil becomes too compact. Otherwise
roots play a vital role in the development and improvement of soil structure.
2.
Earthworms
Earthworms
are the ‘farmers' friends’ which are very important for the primary decomposing
of soil organic matter as well as improving the soil structure. The casting
activity of soil is only done by few selected species of earthworms. The casting
species of earthworms eat both soil and organic matters and the excreta from
their intestines consist of intimately mixed, partially digested, finely
divided organic matter and soil.
Earthworm
burrows created by their activity in the soil are important source of improving
soil structure in uncultivated areas and grasslands but sometime the burrows
are troubling the lawns. Some species of the earthworms are present in the
topsoil layers while others go on to make vertical burrows up to 2 meters
depth.
Earthworm
activity is largely affected by soil organic matter, soil moisture contents,
soil pH, temperature and soil type. Large populations of earthworms and greater
activity is seen in the soils with high organic matter contents; slightly higher
calcium reserves, basic pH and light and medium loams soils. Small populations
of earthworm exist in acidic soils; soils with low organic matter contents,
clayey soil, peat and gravelly soil.
Snails,
slugs, nematodes and arthropods also have role in the decomposition of the
organic matter of soil but some are also important horticultural pests.
3.
Bacteria
Bacteria
are the most abundant living organisms in soil. About 1000 million bacteria are
present in one gram of fertile soil. Bacteria have strong roles in the
decomposition of organic matter, weathering of the rocks, detoxification of pesticides
and herbicides. Some bacteria are also fixing nitrogen in the soil and others
are denitrifying bacteria; root nodulating bacteria form association with the roots
of some plants to help uptake of nitrogen. Bacteria are also important for the completing
the nutrient cycles in the soil environment.
Bacterial
population in the soil increase with the presence of high organic matter
contents, high food supply, pH of 5.5-7.5 and temperature close to 350C.
4.
Fungi
Fungi
along with bacteria are most important decomposers in the soil. Fungi are more tolerant to harsh environment than other micro ogranisms in the soil. They can exist in soils with
low calcium; soils with acidic/neutral pH and low organic matter. There are
different species of fungi, some can decompose only simpler organic matter,
others decompose cellulose and some also have ability to hydrolyze lignin. Hence,
fungi are most important primary decomposers in the soil.
Fungi
also form mutualistic association with algae in ‘lichens’ and roots of higher
plants in ‘mycorrhizae’.
DEAD ORGANIC MATTER
IN THE SOIL
The
freshly dead organic matter in the soil has effect on the physical properties
of the soil but nutrients are released only after the decomposition of the
organic matter. The decomposition of the green leafy organic matter is usually
very fast and completes quickly as compared to the decomposition of the fibrous
or woody organic matter which takes longer for decomposition and forms brown humus.
Humus
Humus
forms in the soil as the result of decomposition of fibrous or woody organic
matters which does not completely decomposes due to lignin contents and other
decomposition-resistant organic matter. These ‘brown’ contents form humic acids
which form a collection of black colloidal-gels that surrounds the soil
particles and imparts characteristic brown color to the soil.
This
gel has cation exchange capacity (CEC) and add to cation holding capacity of the
soils. It also holds the soil aggregates together. In sandy soils, with low
clay contents, humus holds the soil particles together. The humus is decomposed
completely by action of bacteria in the soil so new organic matter must be
added to soil to replace the decayed to retain the soil properties.
NUTRIENT CYCLES IN
SOIL
All
the nutrients in the soil are in constant circulation between soil, plants,
animals and air.
Mineralization is the process of formation of simpler
inorganic compounds such as ammonia, nitrites, nitrates and sulphates.
Humification, on the other hand, is the process for the formation of humus in
the soil through decomposition of fibrous/woody dead organic matter.
Nutrient
cycles on-going in the soil include;
1.
Carbon cycle
2.
Nitrogen cycle
3.
Sulphur cycle
Carbon
to Nitrogen Ratio (C:N)
Carbon
and nitrogen are two most abundant elements in soil. The nutrient cycles of
both are continued at the same time in the soil. The change in the quantity of
one of them will have an effect on the amount of other. C:N ratio is much wider
in plant organic matter as compared to other materials; this organic matter is
utilized in decomposition by the microbe. Carbon serves as the energy source to
the microbes but nitrogen is utilized by microbes in the formation of proteins.
Thus there is an imbalance in the C:N ratio.
