Nitrogen is an essential plant nutrient which is one of the most expensive inputs in agriculture. Chemical nitrogen fertilizers applied to the soil is taken up by crops and about 65% of the applied nitrogen is lost through drainage or converted back to gas in the atmosphere. Despite 78% of the earth's atmosphere is made up of N2, its inert nature makes it metabolically unavailable to higher plants. However, some species of microorganisms can make it available through biological nitrogen fixation (BNF), in which atmospheric nitrogen is converted to ammonia. These microorganisms are capable of colonizing the root surface and providing benefits to the crops while utilizing the carbon supply made available to them in the form of root exudates. Contribution of BNF is estimated in the range of 175 million tons of nitrogen annually. Among the different N fixing microorganism, Rhizobia, the special bacteria which form symbiotic association with leguminous crops contribute 120 Mt N /yr to the global nitrogen cycle.
Legume - Rhizobia symbiosis
Pulses (family Leguminosae) are the important source of high quality protein, high in bone building minerals and vitamins essential to good health. Unlike cereals, pulses are grown under marginal land with minimum fertilizer inputs, therefore the legume fixed nitrogen play important role in rain fed agriculture. The economic success of pulses such as pigeon pea, chickpea, mung bean, urd bean, and lentil depends upon symbiotic association with Rhizobia in nodules on their roots. Rhizobia are soil bacteria that infect the root hairs of the leguminous plants, develop nodules and become small nitrogen factories on the legume roots. Inside the nodules, Rhizobia transform atmospheric nitrogen to the plant available form. The plant provides a home for the bacteria and energy to fix nitrogen (N2). In return, the plant receives fixed nitrogen from the nodule and produce food and protein.
How to assess the effective inoculation?
Nodules can be seen in 21-28 days after sowing. Effective nodules are generally large and are clustered on the primary and upper roots. The effectiveness of the Rhizobium nodulation can be determined by slicing the nodules of the legume plant during the early flowering period and noting the nodule colour. Effective nodules contain red pigment leghaemoglobin inside, which is associated with active nitrogen fixation. In contrast, ineffective nodules are small, numerous, and usually distributed throughout the root system. Under such conditions, a very large inoculum of competitive and highly effective strains of Rhizobia is needed to counteract the aggressive native Rhizobia.
Method of seed inoculation:
Commercial preparation of Rhizobium cultures is available as charcoal or lignite based preparations in powdered forms. These cultures can be procured from state department of agriculture and state agricultural universities. One packet (200 g) of Rhizobium is sufficient for 10 kg seed. The most common method of legume inoculation is by mixing the Rhizobium uniformly with the seeds with minimum amount of sticker solution (10% sucrose). Inoculated seeds should be dried under shade and sown immediately.
Benefits of Rhizobium
• Low cost inputs
• Increase N availability and N uptake
• Improve the soil health and benefit the succeeding crop
• Most environmental friendly
Do's and Don'ts:
Different legumes require specific Rhizobia. Appropriate Rhizobia should be selected for each leguminous plant (Table 1)
Pigeonpea, Mungbean, Urdbean - Bradyrhizobium sp
Lathyrus, Lentil, Pea - Rhizobium leguminosarum bv. Viceae
Chickpea -Mesorhizobium ciceri
Rajma -Rhizobium leguminosarum bv. Phaseoli
Inoculant should be fresh.
Inoculant should be kept in a cool place until ready to use. It should be kept away from hot sun shine to protect from excessive drying.
When the seeds are treated with fungicides, Rhizobium coating should be the last.
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