Getting More Out of Your Cover Crop
UVM Ext. Agronomy Outreach Professional
Vermont farmers are on target to plant over 20,000 acres of cover crops this year. The majority of these acres will be 
planted to winter rye, but there is still time (even now) to get a little more from your cover crop.
Legumes are unique because of their ability to fix nitrogen, utilize that nitrogen themselves reducing fertilizer requirements, and contribute it back to the soil for use by the following crop. Agricultural legumes are plants that are in the family Fabaceae. Most farmers are familiar with the list of legumes that comprise their forage legume species like alfalfa, clover, and trefoil, or those that are grown for grain like soybeans, peas, lentils and even peanuts. And don’t forget the vegetable legume crops like green beans and snap peas.
Legumes also have a much lower carbon to nitrogen ratio (C:N) than cereal grains, so they decompose quicker making that nitrogen more available to the subsequent crop. If you have ever plowed down (or killed) a nice stand of alfalfa and then planted corn, then you know just how beneficial a legume in your crop rotation can be. Legumes can provide over 100 pounds of nitrogen credit per acre, which is why they are often called ‘green manures’.
The legumes themselves are not responsible for nitrogen fixation, however. This happens as a result of a symbiotic relationship between the nitrogen-fixing bacteria that invade the plant root and store nitrogen in root nodules. The plant provides the bacteria with nutrients and energy, and the bacteria provide the plant with a usable form of nitrogen. These bacteria, called Rhizobia, are able to take nitrogen gas from the atmosphere (N2) and convert it to ammonia (NH3), which is then converted to ammonium (NH4+) and nitrate (NO3–) which are the forms of nitrogen usable by plants. In order for good root nodulation and maximum nitrogen production, it is important to inoculate your legumes with the appropriate species of Rhizobia bacteria at planting. Some seed is available pre-inoculated, but many times you will need to apply the inoculant yourself. Whoever you get your seed from should have inoculant available as well. Beware, however, inoculants have a short shelf-life and are also species specific. Using clover inoculant on peas or vetch will not be successful.
Plant Available Nitrogen (PAN)
The ability of your legume cover crop to supply nitrogen to your subsequent crop depends on how much biomass and when you terminate the cover crop. This plant available nitrogen (PAN) becomes available roughly 4-6 weeks after cover crop termination. Oftentimes, a cereal grain terminated at or beyond the boot stage can actually immobilize nitrogen and create a PAN deficit, making it necessary to increase fertilizer/manure nitrogen applications. This is because microbes are tying up nitrogen temporarily as they break down the carbon rich material. Conversely, a cover crop terminated too early will provide only minimal PAN. Below is a simple explanation of the differences between cereal grain and legume cover crops and the implications of when you terminate them.
Considerations When Planting Legume Cover Crops
Legume cover crops will need to be planted earlier than cereal grains to survive winter and maximize N production. For clovers, you’ll want them established by August 15th in Vermont, so this limits them to being interseeded or planted after a cereal grain harvest. The winter annual legumes can be planted as late as September 1st through 15th, which means you can still plant them after a timely corn harvest.
If you are planting legume cover crops only to replace nitrogen, the economics may or may not pencil out. Usually in organic systems, this is a preferred practice. However, when commercial nitrogen fertilizer is $45 for 100 pounds of nitrogen and a legume cover crop could cost you $70 per acre for that same 100 pounds the nitrogen benefit may not be financially rewarding. Certified organic fertilizer, however, could run you $150 per acre, making the cover crop a wise investment. However, a legume cover crop is more than just nitrogen, and these additional benefits are harder to quantify. According to USDA this includes “yield improvements beyond those attributable to nitrogen alone. These may be due to mulching effects, soil structure improvements leading to better moisture retention and crop root development, soil biological activity and/or enhanced insect populations below and just above the soil surface.” (Clark, SARE). They are also great soil conditioners, and can provide early weed suppression.
There are many legume cover crops, but the table below gives a list of the most common ones planted in the northeast.
References:
Clark, A. 2007. Managing Cover Crops Profitably. College Park, MD: SARE.
Carbon to Nitrogen Ratios in Cropping Systems. 2011. USDA Natural Resources Conservation Service.
Other Resources:
- Our Factsheet: “Manure & Cover Crops: A Winning Combination
- Penn. State Extension: “Inoculation of Legumes for Maximum Nitrogen Fixation”
Do you have questions about cover crops? Would you like to conduct a trial on your farm? Contact Kirsten [802-388-4969 ext. 347, kirsten.workman@uvm.edu]
