it is a symbiotic relationship
They have nodules on the roots which has nitrogen fixing bacteria
Legume plant. Root nodules are formed on the roots of legume plants through a symbiotic relationship with nitrogen-fixing bacteria. These nodules house the bacteria that convert atmospheric nitrogen into a form that the plant can use for growth.
Legume crops have the ability to form a symbiotic relationship with nitrogen-fixing bacteria in their root nodules. These bacteria can convert atmospheric nitrogen into a form that plants can use, leading to increased nitrogen levels in the soil. When legume plants decompose, they release nutrients back into the soil, further enhancing soil fertility.
Legume plants have a symbiotic relationship with nitrogen-fixing bacteria called rhizobia. These bacteria convert atmospheric nitrogen into a form that the plants can use for growth, which benefits both the plant and the bacteria. Examples of legume plants include peas, beans, and clover.
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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
Nitrogen-fixing bacteria in nodules on plant roots (legume plants; beans, peas, alfalfa) convert nitrogen in the air (ammonia) to nitrites then nitrates which is then absorbed by plants through their roots.
They are caused by a symbiotic bacteria which benefits the plant by fixing atmospheric nitrogen (which the plant needs to make proteins).
Generally leguminous plants i.e.,plants of Family Fabaceae have root nodules inhabited by nitrogen fixing bacteria such as Rhizobium leguminosarum,which retains and increases the nitrogen content in the soil.
Legumes do. Peas are a good example.
They absorb nitrogen from the air. Then nitrogen-fixing bacteria convert it to a useable form.