Azospirillum is a gram negative, symbiotic, vibrioid soil bacterium. It occurs in large numbers in association with roots of cereals, grasses and tuber crops. Azospirillum fixes about 20-40 kg of the atmospheric nitrogen under microaerobic conditions. And it increases the vegetative growth and crop yield in many plants.
Azospirillum includes 4 species- Azospirillum lipoferum, A. brasilense, A. amazonese and A. halopraferns. Among them , A. lipoferum and A. brasilense are in common use as biofertilizers.
The production of Azospirillum inoculants involves the following important steps:
Azospirillum is present both inside and outside the plant roots. A proper host plant is uprooted from the soil and its root system is excised. The root system is washed with running water to remove the soil particles. It is cut into small pieces of 1cm in length. The root pieces are surface sterilized by dipping them in 0.1% mercuric chloride for one minute and then washed repeatedly with a phosphate buffer.
Semi- solid malic acid medium is formulated, sterilizes and distributed into culture tubes. One or two root pieces are aseptically inoculated and incubated at room temperature for 3-5 days. A white pellicle like layer of Azospirillum appears just below 1-2 mm from the surface of the medium. The medium becomes blue in colour.
Development of blue colour in nitrogen free malic acid medium is the confirmative test for Azospirillum.
A loopful of Azospirillum colony in the malic acid medium is transferred to okan's medium in culture flasks. The culture flasks are maintained at room temperature for 3-5 days. The bacterial cells grow into a starter culture.
Okan's medium containing a sufficient amount of ammonium chloride is filled in a fermenter. The starter culture is inoculated into the fermenter at the rate of 1% to the total volume of the medium. The temperature is maintained at 35ºC for a few days. Sterile air is pumped into the broth to supply oxygen. The broth is tested at specific intervals for its purity and cell number. When the culture attained 109 cells/ml, the inoculums is harvested to make carrier- based inoculant.
Powered farmyard manure (FYM) and soil or FYM alone or FYM plus charcoal is used as a carrier to make carrier- based Azospirillum inoculants. The harvested inoculums is mixed with the carrier till it attains 40% moisture.
The inoculants is then packed in polytene bags and stored at 4ºC for a week. The Azospirillum bags can be stored for about 31 weeks without any loss in the viability.
Azospirillum inoculants is applied to various crops by three ways-
Azospirillum inoculants is mixed with water in a container to form slurry. The seeds are soaked in the slurry overnight before sowing. About 2 kg inoculants is required for a hectare.
One kilogram of Azospirillum inoculants is mixed with 40 litres of water to make a slurry. The roots of seedlings to be transplanted are kept dipped in the slurry for 15 minutes and then transplanted in the main field. The remaining slurry is diluted with water and sprayed in the field.
In this method, 3 kg of Azospirillum inoculants is mixed well with 25 kg of FYM and 25 kg of soil. This mixture is broadcasted throughout the field by top dressing. This method is best suited for young seedling of rice in the main field.
Azospirillum fixes the atmospheric nitrogen and releases plant growth promoting substances. So the crops grow well and give higher yield.
Azospirillum increases the straw yield as well as grain yield in rice, wheat, barley, sorghum, bajra and fodder oats.
Azospirillum treatment saves about 25% of the recommended dose of nitrogen fertilizer for sugarcane. Meantime, it increases the sugar level in the canes.
Azospirillum treatment increases phenolic contents in sorghum. Hence becomes resistant to sorghum shootfly, and Antherigona soccata.
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