Nitrogen production is the key benefit of growing legume cover crops. Legumes have a symbiotic relationship with rhizobia that take gaseous nitrogen from the air and feed this back to the plants; the reason why legume cover crops are said to “fix” a certain amount of nitrogen in the soil. This natural system is less expensive than the energy-intensive manufacturing and application of inorganic nitrogen fertilizers. Legumes are well-known for their superior quality as forage. Palatable and easily digestible, legumes provide higher levels of crude protein and are less fibrous than grasses resulting in faster digestion and increased intake of nutrients by both livestock and wildlife.
Brassicas represent a smaller group of broadleaf plants belonging to the mustard family. This group includes cabbage, collards, mustard, kale, radish, turnip, and rapeseed. While brassicas provide excellent disease suppression, they are not a mychorrizal fungi host and should be used in mixes with a grass specie. Brassicas are generally small-seeded and require a very low seeding rate.
Grass cover crops including annual cereals, forage grasses, and warm-season grasses are versatile in their benefits and form the foundation of most cover crop programs within the soil health community. The extensive root systems of grasses are especially adept at scavenging nutrients (including nitrogen) and preventing erosion. Large amounts of residue remaining after termination will add organic matter to the soil. Crops with large amounts of fall residue, such as cereal rye, may have high C: N ratios that can cause nitrogen deficiency for subsequent crops. Proper management is crucial to avoid setbacks to the cash crop. Early termination, supplemental nitrogen, proper seeding of cash crops, and mixing grasses with legumes and/or brassicas can be beneficial.