Oats as a cover crop have a high C:N ratio. Photo: Emelie Swackhamer
Planting mixtures of cover crops that combine legumes (such as hairy vetch, clovers, or pea) and non-legumes (such as cereal rye, triticale, oats, and canola) can provide three important benefits to soil fertility:
- Supplying nitrogen to the following cash crop
- Taking up excess nitrogen from the soil
- Building soil organic matter
When deciding on the species and seeding rates to use in a mixture, it is important to plan for a favorable carbon to nitrogen ratio (C:N ratio) in the cover crop. The high C:N ratio (>25) of most non-legume cover crops can reduce the amount of nitrogen that is available to the following crop. In most locations, mixtures that are mostly legumes with some non-legumes will supply nitrogen to the next crop and do a better job at taking up excess soil nitrogen than a legume alone. If a mixture does contain more non-legumes than legumes, killing the cover crop early can help keep the C:N ratio low.
Another point to consider when planning a cover crop mixture is SOM content. Over time, using cover crops in a rotation increases SOM. High SOM levels lead to greater nitrogen availability for crops, but can also generate excess soil nitrogen. Under these conditions (around 4% SOM or higher), more non-legumes are needed in a cover crop mixture to take up excess nitrogen. While more non-legumes will raise the C:N ratio of the mixture, a slightly higher C:N ratio will not reduce nitrogen availability when SOM levels are high. One way to establish this type of "non-legume dominated" mixture is to seed non-legumes at 50-60% of their monoculture seeding rates.
To assure some legumes survive in the mixture use more competitive legume species such peas and crimson clover and seed them at 50-60% of their monoculture seeding rate. When SOM levels are low (around 2% OM), some non-legumes are still needed, but it is more important to have a lower C:N ratio in a cover crop mixture. Using low seeding rates of non-legumes in the mixture (around 30% of the typical monoculture rate) will help promote a "legume dominated" mixture. Also consider using non-legumes that are less competitive and slower maturing such as triticale and annual ryegrass when SOM is low.