Question: What kind of cover crop is best to use on clay soils?
Published in the Contra Costa Times on October 27, 2007
Question: What kind of cover crop is best to use on clay soils?
Answer: Cover crops are a wise choice when you want to improve soils. Cover crops are plants that are chosen to be deliberately turned into the soil in the spring, in order to increase the percentage of organic material in the soil. They improve soil tilth, water-holding capacity, provide competition with weedy plants and improve nutrient availability. Some cover crops can assist with pest and disease control.
When selecting a cover crop, consider what you are hoping to achieve. Legumes are often planted when you want to increase the amount of nitrogen in the soil. Legumes that have been inoculated with a nitrogen-fixing bacteria can add up to 200 pounds of nitrogen to an acre of soil, often as much as plants might need the following season. Annual legumes are a good choice for annual vegetable and flower gardens.
Vetches are good winter cover crops on clay soils. The viney plants grow rapidly, with tendrils that allow the slender branches to climb on nearby plants for height. Lana woolypod vetch is one of the best varieties, returning the most nitrogen to the soil. It does not tolerate frequent mowing, and the climbing ability can be a maintenance headache. Avoid hairy vetch, which does best in sandy soils.
Several other legumes can also be used, notably fava bean and field peas (Pisum sativum ssp. Arvense); however, the nitrogen return is not as great.
Clover is another possibility, with Berseem clover (Trifolium alexandrinum) most recommended. This plant can be cut back multiple times, with the clippings used as green manure and mulch. Clovers will fix nitrogen, but their advantage is the percentage of organic material they provide.
The time to plant cover crops is right now. The first winter rains have moistened the soil, but temperatures are not as cold as they will be in another month, encouraging more rapid germination. The cover crop plants will not appear to grow much in the winter months, but come spring, they will grow rapidly. The latest you should plant is early November.
The Master Gardener office has information on specific seeding rates for different plants, and calculations for determining the amount of nitrogen you will be adding.
Bethallyn Black is director of the Contra Costa Master Gardeners, a program associated with the University of California Cooperative Extension and supported by Contra Costa County. Reach her at bblack@ucdavis.edu, or 925-646-6586. General questions for the Master Gardener should be sent using our Question Form