Some microorganisms live in the roots of leguminous plants. Are they harmful or useful to plants and why?

Microorganisms living in the roots of leguminous plants are extremely useful and establish a mutually beneficial symbiotic relationship with their host plants. The bacteria Rhizobium and related species colonize root nodules of legumes like peas, beans, lentils, clover, and alfalfa, where they perform biological nitrogen fixation—converting atmospheric nitrogen gas into ammonia and other nitrogen compounds that plants can absorb and use for growth. This process is invaluable because although nitrogen gas makes up 78% of the atmosphere, plants cannot use it directly and typically depend on nitrogen compounds from soil.

In return for providing usable nitrogen, the leguminous plants supply the bacteria with carbohydrates (sugars) produced through photosynthesis, creating a win-win partnership. This symbiotic relationship reduces or eliminates the need for synthetic nitrogen fertilizers in legume cultivation, making these crops environmentally sustainable and economically beneficial. Farmers have long practiced crop rotation, alternating legumes with other crops to naturally replenish soil nitrogen depleted by previous harvests. The nitrogen-fixing ability of these root bacteria is so significant that legume plants actually enrich the soil, improving its fertility for subsequent crops and supporting sustainable agriculture practices worldwide.