Legume-Rhizobia Symbiosis

“Free” Nitrogen from the atmosphere


A root nodule of a legume containing N-fixing bacteria. The bacteria “give” the plant nitrogen in exchange for photosynthate.

Legumes are unique plants that form relationships with soil bacteria. The bacteria take atmospheric nitrogen, unavailable to plants, and convert it to plant-usable forms. In exchange, the plants give the bacteria some of their photosynthate. If there is a lot of nitrogen already in the soil system, plants will regulate their relationships with bacteria because the nitrogen from bacteria comes at an expense to the plant (photosynthate).

Legume cover crops can be used to bring “free” nitrogen to the farm, especially in nitrogen-limited situations. Different legumes form symbioses with different bacteria; not all of the bacteria are present in all soils, but seeds can be inoculated with bacteria before planting. To learn more about using legume cover crops effectively, see:

Legume Cover Crops from SARE’s Managing Cover Crops Profitably, 3rd Ed., the definitive handbook on cover cropping.

Managing Cover Crops for Maximum Carbon and Nitrogen Contributions: by Julie Grossman: A presentation filled with information on managing legume cover crops by one of the leading Rhizobia cover crop researchers.

Continue to Explore the Cover Crop Web of Functionality