How do legumes and green manure help replenish nitrogen in soil?

Legumes and green manure replenish soil nitrogen through a biological process involving symbiotic nitrogen-fixing bacteria.

The Role of Legumes

Leguminous plants, such as peas, beans, and clover, form a mutualistic relationship with bacteria of the genus Rhizobium. These bacteria invade the plant's root hairs, stimulating the formation of root nodules. Within these nodules, the bacteria convert atmospheric nitrogen into ammonia, a form of nitrogen that the host plant can utilize for its development.12

The Role of Green Manure

Green manuring involves growing specific crops—typically annual legumes or grasses—and plowing them into the soil before they reach maturity. This practice replenishes soil nitrogen in two ways:34
* Fixation: During the growth phase, the nodule bacteria on the roots of leguminous green manure crops actively fix atmospheric nitrogen.35
* Incorporation: When the crop is turned under in the spring, the decomposition of the plant matter releases the accumulated nitrogen and other nutrients back into the soil, thereby increasing overall fertility for subsequent crops.342