From Penn State University
Nitrogen provided by legume cover crops is an important source of fertility for many vegetable growers, but cover crops are also important for recycling nitrogen and building long-term soil nitrogen reserves in soil organic matter (SOM).
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
Oats as a cover crop have a high C:N ratio.
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.