When it comes to the relationship between cover crops and nitrogen, there are two types of cover crops. Those that can fix nitrogen from the atmosphere, and those that can scavenge nitrogen from the soil. Your decision on which group to plant should be based on what you are trying to accomplish.
Nitrogen-fixing cover crops, such as vetch and clovers, form a mutualism with rhizobia bacteria, which capture nitrogen from the air and transform it into available soil nitrogen. This group of cover crops can add nitrogen to the system that would otherwise not be there.
If you are trying to reduce the amount of nitrogen fertilizer applied every year without jeopardizing your yields, planting nitrogen-fixing cover crops, or legumes, can help you to achieve this goal. Some legumes, such as white clover, if very well managed, can fix between 80-200 pounds of nitrogen per acre, but only about 50% of this amount becomes available to the next crop.
Alternatively, the scavenger cover crops, such as cereal rye and radish, can trap nitrogen that is free in the soil and release it later for the next crop. Nitrogen moves in the soil and can be easily lost through leaching and runoff. As a result, nitrogen that otherwise could be used by next year’s cash crop (lowering your nitrogen input costs) could now potentially contaminate water with nitrate and cause algae bloom.
Scavenger cover crops can help to keep nitrogen in the field by gathering and releasing it to the next cover cash crop. Scavenger cover crops planted in fields where manure is applied could scavenge up to 80 pounds of nitrogen per acre.
Seeding rates will vary depending on which cover crop you choose. Field peas, a nitrogen-fixing cover crop, might require planting rates up to 90 pounds per acre PLS (pure live seed), while you need to plant only 3-5 pounds per acre PLS of rapeseed (scavenger cover crop).
One last thing to remember is knowing when to terminate your cover crop is very important. The older your cover crops get, the more difficulty it will be to break down, and more nitrogen will be tied up. More plant-available nitrogen will be delivered within four to six weeks if you terminate your cover crop during the vegetative stage.
When and where practical, consider cover crop mixes (multiple species of nitrogen-fixing and scavenger cover crops) since mixtures can help to maintain and increase soil health.