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Azotobacter Vinelandii
is a free-living bacterium that can fix atmosferic nitrogen into the soil, being a great source to obtain a natural biofertilizer that can be used in the cultivation of most crops.
It is a great source of nitrogen to meet the needs of crops because also
has the capabilities to cause a rejuvenation of soil microbiology to tap out the biological fixation of nitrogen.
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Benefits:
It improves seed germination and plant growth
Azotobacter are tolerant to high salts.
It can benefit crops by Nitrogen fixation, growth promoting substances, fungi static substances.
Azotobacter is heaviest breathing organism and requires a large amount of organic carbon for its growth.
It is poor competitor for nutrients in soil and hence its growth promoting substances, fungistatic substances.
It thrives even in alkaline soils.
Azotobacter is less effective in soils with poor organic matter content.
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Characteristics of Azotobacter
Azotobacter is Gram negative bacteria, polymorphic i.e. they
are of different sizes and shapes. Their size ranges from 2-10x1-2.5 m ., young cell
possess peritrichous flegella and are used as locomotive organs. Old population of
bacteria includes encapsulated forms and have enhanced resistant to heat, desication and
adverse conditions. The cyst germinates under favourable conditions to give vegetative
cells. They also produce polysachharides. Azotobacter spp.,are sensitive to acidic pH,
high salts, and temperature above 350C.
There are four
important species of Azotobacter viz. A.Chroococcum, A.agilis, A.paspali and A.vinelandii
of which A.chroococcum is most commonly found in our soils.
Nitrogen fixation
by Azotobacter: The species of Azotobacter are known to fix on an average 10
mg.of N/g of sugar in pure culture on a nitrogen free medium. A maximum of 30 mg. N fixed
per gram of sugar was reported by lopatina. However, Azotobacter is a poor competitor for
nutrients in soil. Most efficient strains of Azotobacter would need to oxidise
about 1000 kg of organic matter for fixing 30 kg of N/ha. This does not sound realistic
for our soils which have very low active carbon status. Besides, soil is inhabitated by a
large variety of otherr microbes, all of which compete for the active carbon.
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Azotobacter in soil
Soils are habited by a very large number of microbial species. The co-existence of the relative
populations of each one of the species is determined by ecological factors prevailing in
the soil. These various species survive in soil while maintaining a balance of population
is between various microbial species within certain limits.
A normal population of Azotobacter could be about than 10 thousand to 1
lakh/g of soil and it is mostly influenced by other
micro-organisms present in soil. There are some micro-organism which stimulate the
Azotobacter population in soil thereby increasing the nitrogen fixation by
Azotobacter. On the other hand there are some micro-organisms which adversely affect the
Azotobacter population and hence nitrogen fixation process is hampered. For example
cephallosporium is most commonly found organisms in soil which restricts the growth of
Azotobacter.
| | Additional Benefits:
Azotobacter also
produces some substances which check the plant pathogens such as Alternaria, Fusarium and
Helminthosporium. Hence Azotobacter also acts as a biological control agent.
Functions of
Azotobacter: Azotobacter naturally fixes atmospheric nitrogen in the rhizosphere.
There are different strains of Azotobacter each has varied chemical, biological and other
characters. However, some strains have higher nitrogen fixing ability than others..
Azotobacter uses carbon for its metabolism from simple or compound substances of
carbonaceous in nature. Besides carbon, Azotobacter also requires calcium for nitrogen
fixation. Similarly, a medium used for growth of Azotobacter is required to have presence
of organic nitrogen, micro-nutrients and salt in order to enhance the nitrogen fixing
ability of Azotobacter.
Azotobacter also produces fixation of Thiomin, Riboflavin, Nicotin, indol acitic acid and
giberalin. When Azotobacter is applied to seeds, seed germination is improved to a
considerable extent, so also it controls plant diseases due to above substances produced
by Azotobacter.
Selection of
Azotobacter strains: After isolation of Azotobacter from soil its purity is tested in
the laboratory in a pure form. In fertile soil spp.A.Chroococcum is found, commonly. The
organism is aerobic in nature, requires oxygen for its growth. In old culture Malinin
chemical is formed which gives the blackish colour to the culture. The organism is
prominently found in alkaline or neutral soils. Strains of Azotobacters vary in their
nitrogen fixing ability which depends upon pH of soil, crop and atmosphere of soil.
Therefore nitrogen fixing capacity of strains is tested frequently.
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