Cover crops are a vital tool in soil ecological management, providing
habitat, food and protecting the soil from erosion. Residues from cover
crops and crops, including stems, leaves and roots, provide the energy
and nutrition that feeds the entire soil food web (see soil
ecology link for more information on the micro and macro organisms
Cover crops have wide ranging effects on the farm or garden. It is important
to consider the primary goals for including a cover crop at a specific
location and time. Objectives to consider include -
- To insure soil cover to prevent erosion and build soil quality over
the winter. For many Michigan situations, a cover crop that establishes
easily, grows rapidly and can survive cold temperatures such as a winter
annual cereal (rye, winter wheat or oats) or a cool season grass (festulium,
annual rye grass) are good choices.
- To insure soil cover over the summer, a grass that is a warm season
grass adapted to hot temperatures will provide even more biomass and
organic matter residues than a cool season grass. An outstanding performer
as a summer cover crop is the warm season grass sorghum sudangrass.
- Enhancing cropping system diversity through cover crops is another
important goal. This can be achieved by using cover crop mixtures and
planting different cover crops over time. Diversity helps promote the
presence of beneficial soil microorganisms, insects and nematodes. For
example, growing leguminous (pea family) cover crops and buckwheat will
provide a source of flowers with nectaries that support pollinators
and other beneficial insects.
- Cover crops can be effective means to enhance nitrogen and phosphorus
availability to crops. These are the essential nutrients that often
determine crop yield. Legumes are able to biologically fix nitrogen,
working together with symbiotic Rhizobium bacteria to form specialized
root structures called nodules. Within the nodule, the plant and microorganisms
join forces to transform inert nitrogen from the air into available
forms of nitrogen, e.g., inorganic nitrate and ammonia which can be
taken up and incorporated into organic structures to form proteins.
Recent research provides exciting evidence that legumes may have a critical
role to play in improving phosphorus availability, as well as nitrogen
Legume seed tends to be relatively expensive, and a high seeding rate
of 50 to 80 lbs per acre may be required for larger-seeded legume cover
crops. If a legume cover crop is incorporated, as a green manure, and
the nutrients reduced requirement for fertilizer or manure application,
then the net benefits in soil quality improvement and nutrient supply
may outweigh the added expense associated with establishing a legume
cover crop. One of the most inexpensive legume cover crops is soybean,
either a grain soybean that is incorporated while still vegetative or
a forage soybean that remains vegetative longer and produces much larger
amount of residues for use as a green manure.
There are two outstanding species of short-duration legumes that have
shown consistent cold tolerance in Michigan, these are hairy vetch and
red clover. Long-duration legumes such as alfalfa are also cold tolerant,
but require early summer establishment. The annual hairy vetch and the
biannual red clover can be established in early fall (red clover growth
and winter survival is better if it is established earlier, often as
a frost-seeded underplanting into a winter wheat crop), and generally
survive the winter. A mixture of rye-hairy vetch or wheat-red clover
is often more productive and soil-improving than either cover crop grown
on its own.
- Disease suppression is a recent use of cover crops that has generated
considerable interest. Many members of the brassica family, such as
oriental mustard and oil seed radish have shown potential as cover crops
that suppress soil-borne diseases and enhance the root health of a crop
grown after the brassica cover crop. Recent findings at Michigan suggest
a range of species and management strategies that can be used to experiment
with bio-suppressive cover crops (links to oilseed radish new cover
crop bulletin of Mutch; and to biosuppressive article of Snapp).
For more information on individual cover crops, including management
information and seed sources use the cover crop website at http://www.covercrops.msu.edu/.
Information is also provided in Michigan cover crop extension materials
(link to MSU Vegetable cover crop bulletin and Michigan cover crop bulletin).