Introduction: The worldwide spread of inflation, initiated by several fold rise in Petroleum price thereby depicting its striking influence on the prices of chemical nitrogenous fertilizers, the prices of nitrogenous fertilizers have nearly doubled during the last 3-4 years. This has necessiated to search for cheaper source of nitrogen to meet the needs of crops. This has rejuvenation of soil microbiology to tap out the biological fixation of nitrogen.
Azotobacter spp: These are free living bacteria which grow well on a nitrogen free medium. These bacteria utilize atmospheric nitrogen gas for their cell protein synthesis. This cell protein is then mineralised in soil after the death of Azotobacter cells thereby contributing towards the nitrogen availability of the crop plants.
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.
Azotobacter in soil: In Indian soils, the population of Azotobacter is not more than 10 thousand to 1 lakh/g of soil. The Population of Azotobacter 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.
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.
Besides, nitrogen fixation, Azotobacter also produces, 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. In order to obtain most efficient strains of Azotobacter one has to conduct different tests or experiments in glass house, earthern pots and field under the guidance of micro-biologists, some of them are explained below;
Strain found efficient in acetylene reduction test are selected and multiplied in a pure form. The broth is mixed with liquite and inoculant so prepared is used to inoculate the seeds. Seeds are then dried in shed and sown in pots. Suitable inoculated control plants are kept for comparision. Plants are watered as and when required and allowed to grow for about 45-60 days. Differences between inoculated and uninoculated plants in respect of height, nitrogen content of plant and soil, dry weight of plants are noted. Efficient strains are used for field tests.
After maturity yield figures are noted and comparision is made. Such experiments are repeated for 3 to 4 years at different places. From this data an efficient strain is selected and used for the production of Azotobacter on a large scale. Such strains are stored under cold storage conditions or in refrigerator. For production of bio-fertilizer, it is always advisible to use more than one strain on safer side.
Experience of biofertilizer in Russia: Field experiments conducted in Russia with Azotobacter as a biofertilizer for wheat, barley, oat maize, sugarcane, etc. revealed that the performance of Azotobacter was medium to poor. Experiments on the use of Azotobacter for seed inoculation of various crops have also been conducted in other countries including India. The results of these experiments indicate that benefits obtained from inoculation of seeds with biofertilizer were marginal in soil with poor organic matter content. While in rich soils results were quite encouraging.
The field soils are inhabited 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.