Cover Crops and Green Manure - September 9, 1998
Jeff Schalau, County Director, Agent, Agriculture & Natural Resources
Arizona Cooperative Extension, Yavapai County
Erosion and deposition are natural phenomena and very common throughout Arizona. Most of us have visited the Grand Canyon and have seen the effects of erosion firsthand. Materials that were once rocks and minerals on the Colorado Plateau, now exist as deposits of fertile soil in the lower Colorado River Valley. In most cases, erosion was responsible for creating the world's fertile agricultural regions. However, now that productive, arable soil is where we want it, we often employ conservation practices to help keep it on site.
Cover crops are usually non-food crops planted primarily to reduce erosion (it can't ever be totally prevented). They do this by adding stability to the soil with their roots, reducing soil raindrop impact which detach soil particles, and sheltering the soil from wind erosion with their foliage. Cover crops also slow overland flow of water and allow suspended sediments to be deposited before they are transported away from the field.
Most commonly, cover crops fall into one of two categories: legumes or grasses. Legumes include peas, beans, clover, alfalfa, vetch, trefoil, fenugreek, and medic. Legumes can be annuals, biennials, or perennials and have the added advantage of being able to fix nitrogen (converting nitrogen in the atmosphere to plant available nitrogen in the soil). Some legumes can fix as much as 200 lbs of nitrogen per acre per year. To ensure an adequate level of nitrogen fixation, legumes must be inoculated with the appropriate strain of Rhizobacteria. These inoculants are inexpensive and the process is simple. Annuals are ideal for short-term plantings in fall or early spring. Biennials grow much more vigorously in the second year and are best planted on areas that will not be cultivated for one year. Perennial legumes, such as alfalfa, grow deep tap roots that are very good at improving compacted soil conditions but can be difficult to eliminate from the garden once they have served their purpose.
Grasses also make excellent cover crops. They do not fix nitrogen, but have a fine textured, fibrous root system that is efficient at stabilizing soils and is easily decomposed to add organic matter to the soil. Some common annual grass cover crops are brome, barley, millet, oats, rye, sudangrass, and wheat. Perennial grasses are usually used where tillage us unlikely to occur. These include fescues, orchardgrass, bluegrass, ryegrass, and even bermuda (I don't recommend using it unless you have very specific objectives).
There are some cover crops that are neither legumes nor grasses. These include buckwheat, mustard, oil seed radish, Phacelia, rape, and others. These are usually planted with a specific objective in mind. For instance, buckwheat attracts beneficial insects, mustard has a taproot that breaks up compacted soils, radish and rape are said to lower populations of crop damaging nematodes.
A cover crop containing a mixture of grasses and legumes will have many complimentary effects. Annual grasses germinate quickly and act as a nurse crop for the legumes. As time goes on, the grasses provide a scaffold for other plants, such as legumes, to grow upward and spread. A diverse mixture of plants also encourages a diversity of insects: both pests and beneficials. The pest species will prefer the dense vegetation of the cover crop to adjacent crop plants. The beneficial insects will be present if and when the pests decide to attack your crop plants.
The Verde Valley's growing season is ideally suited to fall cover crop plantings. Some warm season cover crops (alfalfa and sudangrass) are also well suited but these may require frequent watering until the monsoon season arrives. Given the intensity of our monsoon rains, cover crops also make good sense for reducing soil losses on areas of bare ground.
When the cover crop has served its purpose, it can be tilled into the soil as green manure. Green manure adds organic matter, nitrogen and other nutrients that were contained in the leaves, stems and roots of the cover crops. The green manure should be allowed to decompose for at least three weeks before planting. Here again the combination of grasses and legumes will have the greatest benefit. Legumes add nitrogen and decompose easily while grass roots add easily decomposed organic matter. The grass leaves are less easily decomposed and will contribute organic matter over a longer period.
The practice of cover cropping and green manuring are not just for organic gardeners. These are sound agricultural practices that decrease soil losses, increase soil fertility and organic matter, and lower inputs of nitrogen from commercial fertilizers.
The University of Arizona Cooperative Extension has publications and information on composting and cover crops. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at email@example.com and be sure to include your address and phone number.
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