Groundnut

Biological Nitrogen Fixation

• With ever – increasing cost of nitrogenous fertilizers greater emphasis has been laid on the use of alternative and renewable sources of nitrogen.
• Biological nitrogen fixation through micro organisms as biofertilizers comes to the rescue of the farmer as a cheap and supplementary source of nitrogen.
• Certain micro – organisms present in the soil have the ability to convert the abundant atmospheric nitrogen into ammonia (N fixation) which is readily absorbed by the plants.
• Rhizobium, the root nodule bacterium, in mutually beneficial association (Symbiosis) with legumes fixes nitrogen.
• Rhizobia penetrate the roots of legumes and give rise to highly specialized organs, root nodules.
• Groundnut (Arachis hypogaea Linn.) is nodulated by a large group of rhizobia classified as ‘cowpea miscellany’.
• Most of the cultivated soils in India have large population of this Rhizobium and ensure normal to abundant nodule formation in the groundnut crop.
• However, sufficient nodulation does not mean better nitrogen fixation.
• Observations have revealed that some of the nodules are found to be ineffective and do not fix adequate nitrogen required by the groundnut plants.
• The effective nodules are big and pink and concentrate on the tap root and the top lateral roots; while ineffective nodules are small, green or white and are distributed throughout the root system.
• To ensure effective nodules, the crop has to be provided with highly efficient Rhizobium in the vicinity of its root system.
• This can be achieved by artificially inoculating preselected effective and efficient Rhizobium.
• It is estimated that a well nodulated groundnut under normal growth conditions is capable of fixing about 180 kg N / ha.

• The objective of the inoculation technology is to introduce preselected Rhizobium culture into the vicinity of emerging root in such a way that they have competitive advantage over many indigenous rhizobia.
• Higher rate of Rhizobium inoculation is required to obtain sufficient nodulation by inoculant strain to overcome the competition by the native Rhizobium.
• Inoculum containing 105 cells / seed is the minimum requirement.
• This is achieved by a proper method of Rhizobium inoculation.
• In general, 200-500 g of carrier – based Rhizobium inoculum is recommended per hectare.
• The technology involves strain selection, preparation of inoculants and suitable method of application of Rhizobium.

• Rhizobium selected for inoculation must fulfill the following criteria for efficient biological nitrogen fixation in groundnut
• The strain should be an effective nitrogen fixer,
• It should be able to outcompete the native Rhizobium population in forming maximum number of nodules,
• It should be able to survive on seed and soil for a longer period and should be able to multiply faster in the vicinity of the roots of host plants, under a variety of environmental conditions,
• The host specific strain will have an added advantage due to its competitiveness over the native strains.

• As the nitrogen fixation is a sensititve process the environmental factors like soil type, temperature, light and agronomic practices determine the success of inoculation.

• Nodulation and nitrogen fixation show rapid decline under drought conditions.
• Prolonged dessication leads to nodule loss with partial inability to further form nodules.
• Proper soil moisture should therefore be maintained by supportive, irrigation, wherever possible.

• Groundnut is grown during the winter (rabi) / summer in certain parts of the coastal belts of the country.
• Survival of rhizobium under waterlogged conditions is reduced which leads to poor nitrogen fixation.
• Hence groundnut cultivated after the rice needs rhizobium inoculation.

• Soil physical constituents influence the availability of moisture, aeration and regulation of soil temperature.
• Proper land preparation and addition of organic matter should be ensured for good plant growth and efficient nitrogen fixation.
• Low or high pH directly or indirectly affects the success of the Rhizobium inoculation, either by inhibiting the growth of Rhizobium or by making the nutrients non-available to the plants.
• Soil ammendments as recommended should necessarily be used to correct the situation.

• Soil nutrients, both major and minor, not only contribute to the growth of the plants but also to nitrogen fixation.
• Nitrogen fixation begins when the plants are in quadrifoliate stage(25-30 days after sowing).
• To support the crop during this initial stage of growth and also further support N fixation, the groundnut crop requires 12.5 and 25 Kg N/ha during the rainly (Kharif) and summer seasons respectively.
• The excess of N fertilizer, however, tends to reduce the N fixation.
• Requirement for phosphorus in nodulating legumes is more as compared with other nodulating crops.

• Recommended doses of insecticides and fungicides do not unduly harm the nodulation.
• However, care should be taken that soil insecticides should not come in contact with rhizobium inoculum.
• Thiram, a seed-treating fungicide affects the Rhizobium population considerably.
• Therefore, one should preferably inoculate groundnut through other methods like liquid inoculation, seed pelleting or rhizobium mixed in compost.
• Higher plant population, as a result of competition of moisture, nutrients, light and space, affect the process of nodulation and nitrogen fixation.