Fertile soil is inhabited by the
root system of many higher plants, by many animal forms (e.g., rodents, insects
and worms), and by tremendous numbers of micro-organisms. Few environments on
earth have a great variety of micro-organisms as fertile soil. Bacteria, fungi,
algae, protozoa and viruses may be composed on the soil which may reach a total
of billions of organisms per gram.
Bacteria:- The bacterial population of
the soil exceeds the population of all the other groups of micro-organisms in
both number and variety. Direct microscopic counts of bacteria as high as
several billions per gram, have been reported. Autotrophs and heterotrophs,
mesophiles, thermophiles and psychrophiles, aerobes and anaerobes, cellulose
digesters and sulphur oxidizers, nitrogen fixers and protein digesters and
other kinds of bacteria are present in soil microflora.
Large
number of Actinomycetes are present in warm dry soils. The most predominant of
this genera are Nocardia, Streptomyces and Micromonospora.
Fungi:- Hundreds of different species
of fungi inhabit the soil. They are most abundant near the surface, where an
aerobic condition is likely to prevail. They appear both in mycelial and spore
stage. Their count ranges from thousands to hundreds of thousands per gram of
soil.
Yeasts are likely to be more prevalent
in soils of vineyards, orchards and apiaries, where special conditions,
particularly the presence of sugars, favour their growth.
Algae:- The population of algae in soil
is generally smaller than that of either bacteria or fungi. The major types
present are the green algae and diatoms. Cyanobacteria,
the oxygenic photosynthetic bacteria are known to grow on the surfaces of
freshly exposed rocks where the accumulation of their cells results in
simultaneous deposition of organic matter. This establishes a nutrient base
that will support growth of other bacterial and fungal species.
Protozoa:- Most soil protozoa are
flagellates or amoebas. The number per gram of soil ranges from a few hundreds
to several hundred thousand in moist soils rich in organic matter. The protozoa
maintain the equilibrium of micro-organisms in the soil by digesting the
bacteria.
Viruses:- Bacterial viruses
(bacteriophases) as well as plant and animal viruses, periodically find their
way into soils through addition of plant and animal wastes. Also, soil
micro-organisms themselves may harbor viruses.
The Rhizosphere:- The region where the
soil and roots make contact is designated as rhizosphere. The rhizosphere represents a tremendously complex
biological system and there is a great deal yet to be learned about the
interactions which occur between the plant and the micro-organisms intimately
associated with the root system.
The
microbial population on and around root is considerably higher than that of
root free soil. The differences are both quantitative and qualitative. Bacteria
predominate, and their growth is enhanced by nutritional substances from the
plant tissue, e.g., amino acids, vitamins and other nutrients. In return the
growth of the plant is influenced by the products of microbial metabolism that
are released into the soil. It has been reported that amino acid requiring
bacteria exist in the rhizosphere in larger numbers than in the root free soil.
It has been demonstrated that the microbiota of the rhizosphere is more active
physiologically than non-rhizosphere soil.
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