Legumes are plants in the Fabaceae family. Legume cover crops are the primary nitrogen fixers of the cover crop world. While other cover crop seed types return nitrogen through decaying matter, legume crops convert nitrogen from the air and return it to the soil with the aid of rhizobia bacteria. Legumes popularly require inoculation before planting, an easy process to ensure the plant performs nitrogen-fixing to its fullest capability after germination. Find out about all subcategories.