How does nitrogen fixation by legumes improve soil fertility?

Legumes (members of the Fabaceae family) improve soil fertility through a mutualistic symbiosis with Rhizobium bacteria.12

These bacteria invade the plant's root hairs, stimulating the formation of nodules where they convert atmospheric nitrogen into ammonia, a form of nitrogen that plants can metabolize. While the host plant uses these compounds for its own development, the nitrogen becomes available to the broader soil ecosystem when the plant's roots and other tissues decay and release these products into the ground.12

This biological process is essential for maintaining soil health, particularly in nitrogen-deficient environments. Agricultural practices such as crop rotation—alternating legumes with other crops—or using legumes as green manure allow farmers to naturally replenish soil nitrogen levels without relying solely on synthetic fertilizers. Additionally, legumes used as cover crops help prevent soil erosion, further contributing to the long-term sustainability of the land.2