Cover Crops: Regenerating and Sustaining Healthy Soils
by Steve Peters

Human beings have been cultivating and harvesting plants for food, medicines, fiber, and other purposes for over 10,000 years. This has resulted in extensive depletion of carbon and essential plant nutrients such as nitrogen, phosphorus, and potassium from the soil. Many early farmers were nomadic and practiced slash and burn agriculture with little regard to long-term sustainability; however, a few keen observers, particularly those who farmed the same land every year, recognized that if we take from the land, we must also give back to sustain our soil, our plants, and ultimately ourselves. Continual crop removal without sufficient replenishment eventually leads to an impoverished environment. Plants grown specifically to improve depleted soils are referred to as "cover crops" or "green manures." Weeds, growing randomly in a field and then turned back into the soil before next year's planting, must have been the first cover crops. As farmers increased their knowledge over time, they began to understand that particular species of plants were better suited for improving the soil and effective cover-cropping became a finely honed skill. With the advent of the chemical fertilizer industry in the late 19th century, many farmers began to abandon the time-honored practice of cover cropping because fertilizers were easier to apply and they were seductively effective--at first. Over time, however, soil carbon levels dropped and serious nutrient and biological imbalances began occurring to the detriment of plant and soil health. Recently, cover crop usage has seen a great resurgence, giving us hope that we can achieve a balanced, sustainable, and healthy agricultural environment, whether it's on a commercial farm or a home garden.

Feeding the Soil with Organic Matter

Cover crops create numerous synergistic and beneficial effects that enhance the soil and its surrounding landscape. Most importantly, because they produce carbohydrates from sunlight, water and CO2 (photosynthesis), and their residues are largely returned to the field, they feed and energize a living, breathing soil containing everything from one-celled bacteria, to foot-long nightcrawlers. Initially, it is primarily the soil micro-organisms (bacteria, fungi and actinomycetes) that devour easily digestible simple sugars and proteins from freshly incorporated plant residues. These materials, which contain high levels of nitrogen, as well as some phosphorus, potassium, and other essential nutrients, are then immediately available for subsequent crop plants. After consuming the easily digestible portions of cover crop residues, certain specialized soil microbes begin munching on tougher, structural plant materials such as cellulose and lignin. The resulting byproducts contribute to a more stable, humus portion of soil organic matter, and remain a part of the soil for a much longer time. These fibrous, hard-to-break-down substances release nutrients to plants much more slowly than the succulent materials rich in nitrogen and protein, but are more important for creating long lasting organic matter.

As soil microbes digest organic matter, they produce complex sugars known as polysaccharides, which are glue-like substances that cement soil particles into aggregates or "crumbs." This crumb structure, along with the build-up of stable organic matter, leads to improved water and nutrient-holding capacity and to water infiltration. The soil begins to take on a spongy quality and is less prone to compaction.

One of the great wonders of the soil world is the fungi known as mycorrhizae. These fungi produce long, rootlike strands called hyphae, which take up sugars from the roots of green plants; in return, they provide plants with phosphorus, micro-nutrients, and stabilized soil aggregates. Leguminous cover crops such as red clover and hairy vetch are particularly adapted for supporting large populations of mycorrhizae; hence, they are effective for improving soil structure.

Mining Nutrients from Air and Subsoil

Cover crops are sometimes referred to as "catch crops" because they absorb nutrients that may otherwise be lost by leaching into the groundwater. This is particularly true for nitrogen (N), which is needed in copious amounts by most plants. Nitrogen is very soluble and easily lost unless a cover crop is present to take up the excess for its own needs. Upon cover crop decomposition, this nitrogen is available for subsequent crops. Among the best nitrogen catch crops are winter rye and sorghum. Rye, which is planted in the fall, grows rapidly in cool soils, and its densely fibrous root system holds up to 100 lbs. of N/acre until the following spring. Sorghum has similar positive attributes, but is grown in the warm months only. Both rye and sorghum also produce plentiful above-ground biomass, greatly improving soil structure and organic-matter levels.

Nitrogen can also be replenished in the soil by growing legumes such as vetch, peas, or clover; all of which support specialized bacteria known as rhizobia on their roots. The legumes provide the rhizobia with carbohydrates, and in return the rhizobia provide the legumes and the surrounding soil with nitrogen from the air. Many other nutrients may be deficient because they are unavailable to most crops in their current form in the soil. Buckwheat, in particular, is effective at scavenging soil phosphorus, which is then taken up readily by subsequent plants after the buckwheat residues decompose. Deep-rooted cover crops such as sweetclover and alfalfa can mine calcium, potassium, phosphorus, and other nutrients from deeper depths and transport them to the surface for future crops.

Creating Habitat, Smothering Weeds, and Preventing Erosion

While the below ground action is compelling, there are many wonders to behold above the soil surface in a cover crop field: not the least of which is the swarm of insect activity—big-eyed bugs, lady beetles, syrphid flies, lacewings, wasps, and many others that feed on nectar, pollen, and insect pests that can damage vegetable, herb, and flower crops. Maintaining this dynamic, beneficial insect community requires that these cover crops are not fully incorporated into the soil. It is better to leave surface residues intact or allow standing strips of cover crops to remain for ensuring a year-round food source.

Protecting fertile topsoil from the damaging effects of water erosion is a huge benefit of cover crops, considering that a raindrop falling at high speed can dislodge a soil particle on bare ground as far as six feet. Cover crops cushion the impact of rain; and the leaves, stems, and roots greatly slow down water movement. The roots, in particular, increase the soil's ability to absorb water to prevent surface runoff. This thick layer of plant material also protects against the damaging effects of the wind and reduces surface crusting and soil compaction.

Cover crops are great for smothering weeds too. Overwintered cover crops such as rye and hairy vetch shade out early spring weeds such as mustard and quackgrass. In addition, rye releases toxins, or allelopathic compounds, which effectively suppress weed growth for several weeks. During the summer, buckwheat can grow two feet tall in just a few weeks, leaving the ground underneath virtually weed free and in excellent condition for planting the next crop.

Managing Cover Crops Through the Cycle of the Seasons

Now that we know about the wonderful benefits of cover crops, how do we go about integrating these plants into our farms and gardens? The overriding consideration is the primary functions that we want the cover crop to perform. Is it to add nitrogen? Prevent soil erosion? Attract beneficial insects? Suppress weeds? Or, is it a combination of the above? Once we have identified the purpose of the cover crop, there are four basic questions that must be addressed: 1) When do we plant? 2) What do we plant? 3) How do we plant? 4) How do we manage the cover crop, especially as it reaches maturity?

In Mediterranean climates such as coastal California, planting is possible almost year round, although summer plantings may not be an option if water is limited. In humid, temperate climatic zones there are three distinct time periods in which to sow our cover crop. The three time periods considered are: 1) early/mid spring; 2) late spring/early summer; and 3) late summer/early fall. Ideally, we plant cover crops during periods when our vegetables, flowers and herbs are not growing, or at least when the cover crop is not interfering with the primary crop. Consequently, we rely heavily on annual cover crops that grow and mature rapidly. Biennial and perennial plants are actually more effective at contributing toward long-term soil health, but they occupy a piece of ground much longer, so there may not be enough open land left for our desired crops.

The early/mid spring planting of cool weather cover crops such as fava beans, field peas, oats, and triticale will reach their maximum growth by early/midsummer. You can sow these crops in rows, although better soil coverage is achieved by broadcasting the seed. While a super fine seedbed is not necessary for successful germination, there must be good seed-to-soil contact; which can be achieved by pressing the seed into the soil with a lawn seed roller or by gently raking and tamping before watering. The ideal time to cut and incorporate cover crops is at early- to mid-flowering when they have accumulated nearly all their nutrients, but have not yet gotten woody or fibrous; hence, they will break down quickly. This is true for virtually all the annual cover crops. You can then rototill or spade in the residue in preparation for a seeding of a fall crop (e.g. broccoli, lettuce, and beets). You may also leave the residue on the surface; however, the following crop should be transplanted (avoid direct-seeding). If you do not intend to seed another crop until fall, you may cut the cover crop before it flowers, and it will regrow. This will extend its life, provide continuous cover throughout the summer, and accumulate more biomass over its life cycle. You may also cut the plants and use them as green matter for your compost pile, although this will reduce the amount of nutrients that are immediately returned to the soil.

Warm-season, frost-tender cover crop varieties seeded in late spring/early summer include buckwheat, sorghum, and cowpeas. These crops fill in the summer fallow period between, for example, spring radishes and fall lettuce. Buckwheat is particularly useful in this regard because it reaches maturity in six weeks, allowing for a greater choice of both spring and fall crops. Buckwheat may actually be planted as late as six weeks before the first fall frost, to allow for longer maturing spring crops, and still provide enough time to plant certain fall crops. Creative gardeners will discover ways to overlap life cycles of different plants to maximize efficiency of time and space. For example, buckwheat may be planted around squash plants, and cut 4–6 weeks later for a weed-suppressing mulch. Sorghum requires 3–4 months to mature, although you can cut it and incorporate the residues at any time depending on when you need to sow your next crop. Cowpeas can be planted between rows of corn, without interfering with the corn's growth and still adding significant amounts of nitrogen to the soil.

The late summer/early fall planting time is appropriate for plants that can establish quickly in cool soils, go dormant during the winter, and resume vigorous growth in the spring. These include hairy vetch, crimson clover, Austrian winter peas, winter rye, and winter triticale. The leguminous species need to be planted no later than one month before the first fall frost, whereas the cereals (rye and triticale) can be planted after several fall frosts, as they are more cold hardy. An oat crop may also be planted at this time; however, it will not survive extremely cold winters. Oats, however, grow faster than almost any other crop in the fall and provide a thick, dead mulch for subsequent spring crops. Better performance is usually achieved if the legumes and grasses are planted as a mix, such as hairy vetch and rye, or Austrian Winter peas and oats. Plant coverage during the winter is better and more nitrogen and biomass is produced.

Perennial Cover Crops for Long-Term Soil Improvement

Although the focus of this discussion has been on annuals, the biennial and perennial cover crops can still play a role in your backyard garden, and are much more significant in perennial cropping systems such as orchards, vineyards, and pastures. The best all-purpose, adaptable legume is red clover, which is considered a short term (two to three years) perennial. It can be seeded from mid-spring through midsummer. It is best sown with oats, which grow more rapidly, thereby providing protection for the slow-growing clover. Once established, the clover can be grazed by animals or mowed to stimulate further growth. It is best to leave the clover in the ground for at least twelve to eighteen months for the most beneficial results.

After turning the clover into the soil, allow a few weeks of decomposition before planting a subsequent crop. Yellow Blossom Sweetclover is a spring-planted biennial: it completes its life cycle in about fifteen months and is treated much like red clover, although it should not be fed to animals. For longer-term perennials, the best choice is alfalfa, which should be planted with perennial grasses such as timothy or orchardgrass.

Whether you are growing a hundred square feet of salad greens in your backyard or hundred acres of mixed vegetables for market, integrating cover crops into your system can provide habitat for beneficial insects, help you manage your weeds, and most importantly, protect and enrich your soil so that it can provide a bountiful, sustainable harvest far into the future.

Steve Peters
Commercial Seed Manager

Cover Crop References:
Managing Cover Crops Profitably, 2nd edition.
USDA Sustainable Agriculture Research & Education Program (1998).

Overview of Cover Crops & Green Manures, Preston Sullivan.
ATTRA - National Sustainable Agriculture Information Service (2003).

Selecting the Right Cover Crop Gives Multiple Benefits,
C.A.Ingels, M.Van Horn, R.L.Bugg, & P.R.Miller.
University of California SAREP (1996).

Photo captions: (top)Irrigating mixed fall cover crops at the research farm. (middle) Intercropping nitrogen fixing legumes with a corn crop. (bottom) A buckwheat cover crop protects young squash plants before being cut and left for mulch.