Friday, October 12, 2012

A Case for Cover Crops and Green Manuring by Sally Berriman

Crimson Clover and Annual Rye Photo by Peg Tillery
A cover crop is simply a high number of plants, not grown for food but as plant material which is used to improve the soil.   When the cover crop is tilled into the soil, it is referred to as a green manure crop or green manuring.  The terms are often used interchangeably.  While cover crops have been used extensively in commercial agriculture, recently more home and community gardeners are starting to use cover crops on a smaller scale.

Cover crops are beneficial because they build soil structure, add organic materials, replenish soil nutrients, fix atmospheric nitrogen, protect the soil from wind and water erosion, suppress weeds and reduce insect pests.  Additionally, cover crops can provide a green and much more attractive alternative to an expanse of dry dirt during the off-season.  

Cover crops help with soil structure creation: the microorganisms that decompose plant material and the plant material itself produce substances that glue soil particles together.  These substances include slime, mucus and fungal mycelia, which contain gums, waxes and resins.  They aggregate the soil particles, enhancing the tilth, porosity and water holding capacities of soil.  Grasses are exceptional in their ability to do this.

Green manuring is beneficial because it enhances soil fertility by increasing the organic matter and feeding beneficial soil microorganisms and earthworms.    When fresh plant material decomposes in the soil, its carbon-to-nitrogen ratio becomes low, allowing the nitrogen to be easily released into the soil chemistry by bacteria. 

Cover crops can help with nitrogen fixation.  Legumes crops, inoculated with their specific Rhizobium bacteria, will take nitrogen out of the air that is present in the soil and store it in their plant tissues via nodules on their roots.  Some of this nitrogen is available as the roots die, but the majority becomes available when the legume is tilled under. They also reduce the need for nitrogen fertilizer or manure.  Inexpensive manure frequently has not been aged properly and contains thousands of viable weed seeds as well as salts which can accumulate in soil.
Photo by Ann Murray

  The primary erosive force in Colorado is wind and our winter winds are especially destructive.  A thick stand of a cover crop protects the soil from having small particles of topsoil blown away while the roots can hold soil in place during heavy downpours.  

Cover crops left in place for part of the growing season can suppress annual and some perennial weeds.  Cover crops encourage beneficial insect populations often minimizing or eliminating the need for other insect control measures.

Most home gardeners do not have sufficient space to forfeit to a cover crop for an entire growing season, so most will opt for a fall cover crop to be tilled under as a spring green manure.   A crop that is well established prior to winter temperature extremes should rebound from wildlife grazing in late winter /early spring.
Crown Vetch (Coronilla varia) photo from ScienceU

    Mid-October is a good time to plant winter cover crops for improving soils where vegetable crops are grown. A long term plan for building and maintaining good levels of both organic matter and nitrogen is desirable.  Non-legume cover crops such as rye help with organic matter while legumes can add both organic matter and nitrogen.  Nitrogen accumulation is greater with legumes than with grasses.  Winter rye is one of the hardiest cereal grasses and can be seeded later in the fall.  Rye growth is rapid in cool fall weather and its quick- growing, fibrous roots hold soil and leftover fertilizer well.  It can also be sown with legumes in fall.  The winter grain will protect the legume roots and maximize winter survival.  Good legumes for Colorado include hairy vetch, Austrian winter pea, crimson clover, field pea, and sweet clover. 

Before seeding, prepare the seed bed by clearing all garden debris, amending the soil (if necessary) and tilling as deeply as possible.  Broadcast the seed and water in.

In the spring, leave at least one month after tilling under your cover crop to decompose before planting your vegetables.  Decomposing plant material consumes soil oxygen and can create plant health problems if not tilled in ahead of time.  You can spend that down time reading garden catalogs and planning your garden.  For more information: