10.2: Benefits of Cover Crops

Cover crops provide multiple potential benefits to soil health and to the following crops, while also helping to maintain cleaner surface water and groundwater (Figure 10.1). They prevent erosion, improve soil physical and biological properties, supply nutrients to the following crop, suppress weeds, improve soil water availability, and break pest cycles. Some cover crops are able to break into compacted soil layers, making it easier for the following crop’s roots to more fully develop. The actual benefits from a cover crop depend on the species and productivity of the crop you grow and how long it’s left to grow before the soil is prepared for the next crop. In this chapter we focus on the principles of cover cropping, which are more comprehensively discussed in a companion book by the same publisher, SARE, titled Managing Cover Crops Profitably.

Purposes Of Cover Crops

The term “cover crop” refers generally to plants that are grown but not harvested. While this term is used generally, different types of plants are grown as cover crops to achieve a number of primary purposes:

Catching and cycling nutrients: typically grasses such as cereal rye and oats. Especially useful in high-nutrient environments.

Fixing nitrogen via symbiotic relationship with Rhizobium bacteria (green manures): typically legumes (e.g., hairy vetch and red clover). Especially useful on organic farms or by others who want to “grow” their own nitrogen.

Smothering weeds: typically competitive, fast-growing species (e.g., buckwheat, sorghum-sudangrass, cereals). Especially useful when weed control is a challenge.

Biofumigating pests with glucosinolates and isothiocyanates: typically brassicas (e.g., mustards and radishes). Especially useful when growing disease-susceptible crops with limited chemical control.

Loosening compacted soil: typically strong-rooted crops (e.g., cereal rye, radishes, hairy vetch, alfalfa). Especially useful to improve a degraded soil.

Growing biomass and organic matter: typically fast-growing crops (e.g., sorghum-sudangrass, cereal rye, sunn hemp). Especially useful when soils are low in organic matter or when you aim to capture carbon.

Providing cover for the soil surface: typically crops that establish quickly during the off season to protect the soil, like rye or oats in cool climates.
Plant ecologists separate these into canopy functions (where benefits are primarily derived from the aboveground biomass) and root functions (where benefits are from the belowground biomass), and the selection of a cover crop may be based on the specific desired traits. If there are particular problems that need to be addressed, it certainly influences the choice of cover crops. However, most farmers grow cover crops specifically because of their multiple benefits (Figure 10.1).

Organic matter. Grass cover crops are more likely than legumes to increase soil organic matter. The more surface residue and roots provided to the soil, the better the effect on soil organic matter. In that regard, we generally don’t fully appreciate their rooting systems unless we dig them up because some cover crops grow as much or more biomass underground than above, thereby directly benefiting the soil.

Good production of hairy vetch or crimson clover cover crops may yield from 1 1/2 to more than 4 tons of dry weight per acre if allowed to grow long enough. Likewise, if a vigorous grass cover crop like cereal rye is grown to maturity, it can produce 3–5 tons of residue. However, the amount of residue produced by an early terminated cover crop may be very modest, as little as half a ton of dry matter per acre. While small cover crop plants add some active organic matter, they may add little to long-term build-up of soil organic matter if not enough root growth and residue are allowed to develop.

A five-year experiment with clover in California showed that cover crops increased organic matter in the top 2 inches from 1.3%–2.6% and in the 2- to 6-inch layer from 1%–1.2%. Researchers found, when the results of many experiments were examined together, that including cover crops led to an organic matter increase of 8.5% over original levels and an increase of soil nitrogen by 12.8%. The longer the cover crop grows and the less tillage that is used, the greater the increase in soil organic matter. In other words, the beneficial effects of reduced tillage and cover cropping can be additive, and the combination of practices has greater benefits than using them individually. Low-growing cover crops that don’t produce much organic matter, for example, cereal rye that’s killed before it has much chance to grow in the spring, may not be able to counter the depleting effects of intensive tillage. But even if they don’t significantly increase organic matter levels, cover crops help prevent erosion and add at least some residues that are readily used by soil organisms.

Beneficial organisms. Cover crops help maintain high populations of mycorrhizal fungi during the period between main crops and thereby provide a biological bridge between cropping seasons. The fungus also associates with almost all cover crops (except brassicas), which helps maintain or improve inoculation of the next crop. (As discussed in Chapter 4, mycorrhizal fungi help promote the health of many crop plants in a variety of ways and also improve soil aggregation.)

Cover crop pollen and nectar can be important food sources for predatory mites and parasitic wasps, both of which are important for biological control of insect pests. A cover crop also provides good habitat for spiders, and these insect feeders help decrease pest populations. Use of cover crops in the Southeast has reduced the incidence of thrips, bollworm, budworm, aphids, fall armyworm, beet armyworm and white flies.

Earthworm populations may increase markedly with cover crops, especially if combined with no-till. Aggressive tillage harms earthworm populations and destroys their burrowing channels—as well as those from old roots—that reach the surface, reducing infiltration during intense rainfall.

Farmers Say Cover Crops Help The Bottom Line

A 2019–2020 national cover crop survey, which included perspectives from 1,172 farmers representing every U.S. state, found new insights into farmer experiences with cover crops. Most producers, working with their seed dealers, are finding ways to economize on cover crop seed costs, with 16% paying only $6–$10 per acre for cover crop seed, 27% paying $11–$15 per acre, 20% paying $16–$20 per acre, and 14% paying $21–$25 per acre. Only about one-fourth were paying $26 or more per acre.

This survey was conducted annually beginning in 2012 (except for 2018–2019). On average, reported yields were higher as a result of planting cover crops in all years, and most notably in the drought year of 2012 when soybean yields were improved by 12% and corn yields were 10% better. Yield gains were more modest in the wet year of 2019, when the average increase was 5% for soybeans and 2% for both corn and wheat. Farmers also reported significant savings on fertilizer and/or herbicide production costs in the 2019–2020 survey for the following crops:

• soybeans: 41% saved on herbicide costs and 41% on fertilizer costs
• corn: 39% saved on herbicide costs and 49% on fertilizer costs
• spring wheat: 32% saved on herbicide costs and 43% on fertilizer costs
• cotton: 71% saved on herbicide costs and 53% on fertilizer costs

In this survey, 52% of farmers “planted green” into cover crops on at least some of their fields. (“Planting green” is the term for seeding a cash crop into a standing cover crop and terminating the cover crop soon after.) Of those, 71% reported better weed control and 68% reported better soil moisture management, with 54% indicating that cover crops allowed them to plant earlier.

Of the horticulture producers surveyed, 58% reported an increase in net profit. Only 4% observed a minor reduction in net profit, and none reported a moderate loss in net profit.

Survey participants indicated an increase of 38% in land devoted to cover crops over the previous four years and the use of a range of cover crop seed and mixes to address their individual needs. This survey showed many positive aspects of cover crop integration and that farmers continue to find benefits to their use.

Source: CTIC-SARE-ASTA National Cover Crop Survey 2019–2020 (www.sare.org/covercropsurvey)

Selecting Cover Crops

Before growing cover crops, you need to ask yourself some questions:

• What are my goals in planting cover crops?
• What cover crops should I plant?
• When and how should I plant the cover crops?
• When should the cover crops be killed or incorporated into the soil?
• What is my next cash crop and when should it be planted?

When you select a cover crop, you should consider the soil conditions, climate and what you want to accomplish by answering these questions:

• Is the main purpose to add available nitrogen to the soil, or to scavenge nutrients and prevent loss from the system? (Legumes add N; other cover crops take up available soil N.)
• Do you want your cover crop to provide large amounts of organic residue?
• Do you plan to use the cover crop as a surface mulch or to incorporate it into the soil?
• Is erosion control in the late fall and early spring your primary objective?
• Is the soil very acidic and infertile, with low availability of nutrients?
• Does the soil have a compaction problem? (Some species, such as sudangrass, sweetclover and oilseed (forage) radish, are especially good for alleviating compaction.)
• Is weed suppression your main goal? (Some species establish rapidly and vigorously, while some also chemically inhibit weed seed germination.)
• Which species are best for your climate? (Some species are more winter hardy than others.)
• Will the climate and waterholding properties of your soil cause a cover crop to use so much water that it harms the following crop?
• Are root diseases or plant-parasitic nematodes problems that you need to address? (Cereal rye, for example, has been found to suppress a number of nematodes in various cropping systems. Brassica cover crops may also reduce populations of certain nematodes.)

In most cases, there are multiple objectives and multiple choices for individual cover crops and for cover crop mixes.