Leguminous plants do not require nitrogenous fertilizers. Why is it so?

Adding a high-nitrogen fertilizer at planting time gives your plants a boost to produce abundant and healthy leaves. However, because of special bacteria at their roots, some plants don't require additional nitrogen. These plants also make nitrogen more available to flowers and vegetables planted in the same soil. In addition to a few other groups of plants, legumes are known as "nitrogen fixers."

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 This is because Legumes themselves add nitrogen to the soil.