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it is a symbiotic relationship
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They have nodules on the roots which has nitrogen fixing bacteria
Rhizobia bacteria are primarily responsible for pulling nitrogen out of the air and making it available to plants, which then use the nitrogen to create proteins. The legume family (the pea & bean family) of plants are able to form a mutually beneficial relationship with rhizobia bacteria.
Ground nuts are legumes. Groundnut being a legume crop utilizes the nitrogen in the atmosphere by nitrogen fixation in association with a group of bacteria belonging to Rhizobium family. The Rhizobium bacteria penetrate the roots of legumes and form root nodules .
S. F. Edwards has written: 'Legume bacteria' -- subject(s): Bacteria, Nitrifying, Field experiments, Inoculation, Legumes, Nitrifying Bacteria
Nodulation in the roots of plant is a symbiotic bacterial activity and is very common in Legumenous plants, other than legume plants do not allow bacteria to establish this relationship hence nodule formation is prohibited.
The bacteria live in the plant cells, located on the root, and form nodules. These nodules convert nitrogen gas in the soil to usable energy for the plant. In return, the bacteria have a safe place to live and reproduce.
Any legume will convert nitrogen to nitrates, as well as many bacteria found in the soil
Legume plants are notable for their ability to fix atmospheric nitrogen, thanks to a symbiotic relationship with certain bacteria known as rhizobia found in root nodules of these plants. The ability to form this symbiosis reduces fertilizer costs for farmers and gardeners who grow legumes, and allows legumes to be used in a crop rotation to replenish soil that has been depleted of nitrogen. The nitrogen fixation ability of legumes is enhanced by the availability of calcium in the soil and reduced by the presence of ample nitrogen. Root nodules. via Wikipedia
Bacteria =========================== Specifically a particular type of bacteria that live in nodules on the roots of certain plants (eg the Legume family) called "Nitrogen Fixing Bacteria" . Not other bacteria decompose organic material and release nitrogen back into the atmosphere.