Cover crops provide a wide range of ecological and environmental benefits. Depending on cover crop type and grower needs, each cover crop can be utilized to provide a specific ecological benefit.
Both the living foliage and the residue from dead cover crops protect the soil from rain drop impact and slow water and air flow across the soil surface, which reduces dislodging and movement of soil particles. The cover crop root system helps to hold soil in place by enmeshing and anchoring soil aggregates.
Another advantage of growing cover crops is the addition of organic matter to the soil. Cover crops also promote populations of soil macrofauna such as earthworms, millipedes, beetles and spiders, which help create air pore spaces in the soil. Examples of cover crops that can add substantial organic matter to soil include cereal rye, oats, sorghum-sudangrass and triticale.
Leguminous cover crops such as clovers and vetches have the added advantage of fixing nitrogen for their growth and the following crops. Upon death of the legume, around 40 – 60% of the nitrogen in the cover crop is made available for the next crop. The amount of nitrogen contributed by legumes varies by species. It is advisable to inoculate legume seeds with the proper nitrogen-fixing bacterium strain for efficient nitrogen fixation. Research has shown significant increase in cover crop biomass and nitrogen-fixing potential in inoculated legume cover crop systems.
Cover crops planted in autumn can scavenge and use unused soil nitrogen left at the end of the growing season which may have otherwise leached during autumn or spring. Certain cover crops tend to be very efficient at recycling or scavenging excess nutrients such as oilseed radish, cereal rye, yellow mustard, etc. These species are well adapted to cool autumn and spring conditions, and continue growing after nutrient absorption by the crop has slowed or stopped.
Cover crop roots can help alleviate the effects of soil compaction by penetrating a compacted layer and creating macropores or root channels that allow air, water and crop roots to penetrate deeper. In general, cover crops such as oilseed radish have large diameter taproots and are more effective at penetrating compacted soil layers than species with smaller diameter roots. Once these taproots penetrate the restricting soil layer they bring up nutrients from deeper soil layers to upper layers of the soil.
Cover crops can be used to suppress problematic nematodes, bacteria and fungi in the soil. Certain cover crops in the Brassicaceae family produce biologically active compounds, called glucosinolates, that have shown activity on soil-borne pests. Glucosinolates are present in plant roots, shoots, stems and leaves. When incorporated into the soil they break down into compounds called isothiocyanates (ITCs) and other chemicals. ITCs are known to suppress soil-borne diseases, nematodes and weed seeds. Some cover crops that belong to the Brassicaceae family include oilseed radish, canola and Indian, brown and yellow mustards.
Additionally, there is variability in biofumigation capabilities among varieties of cover crops. Oilseed radish cultivars such as Adagio and Ultimo are reported to have better nematode suppression (especially cyst nematodes) than other cultivars.
Cover crops can be used to manage weeds in vegetable production systems. They can reduce weed establishment by competing and/or producing allelochemicals, which suppress weed seed germination. Cover crops such as cereal grains and grasses establish quickly in autumn, cover the soil and grow throughout the winter, thereby suppressing weeds.
The benefits of crop rotation in terms of weed, pest and disease management are well documented. Cover crops can be used in crop rotation plans to break pest cycles, add organic matter and improve soil quality and health. Vegetables have many potential seasons of production, and given the choices available with long- and short-term cover crop life cycles, cover crops can easily fit into any crop rotation plan.
Carbon to Nitrogen Ratios
Knowing the carbon to nitrogen ratio (C:N) in a substance is very important before deciding what kind of cover crop to grow and how it will decompose in the soil. For example, a C:N of 10:1 means there are 10 units of carbon for each unit of nitrogen in the substance. Since the C:N ratio of everything in and on the soil can have a significant effect on crop residue decomposition, particularly residue cover on the soil and crop nutrient cycling (predominantly nitrogen), it is important to understand these ratios when planning crop rotations and the use of cover crops.
C:N ratios are important in determining nitrogen availability or tie-up by affecting mineralization when cover crop residues decompose. Mineralization is the process where organic nitrogen, which is largely not available to plants, is converted by soil microorganisms into inorganic (or “mineral”) nitrogen that is readily plant available.
When C:N ratios of plant material are below about 20:1 these microorganisms release excess nitrogen into the soil which plants can use. When ratios are above 20:1, microorganisms tie-up nitrogen from the soil which can result in plants being nitrogen deficient. Clovers, peas and radish have low C:N ratios while oats and sorghum-sudangrass have high ratios.
Ratios of canola, cereal rye, triticale and annual ryegrass are highly variable. This has to do with when the cover crop is terminated. If it is terminated when it is still young and lush, before it has produced flowers and seed, C:N ratios are lower than if terminating when the cover crop is mature. Cover crop mixtures from growers’ farms and those tested at the Penn State research farm also had variable C:N ratios. When the mixture had more rye in it the C:N ratio was on the higher end of the scale (meaning it would tie-up nitrogen upon decomposition).
When using rye as a cover crop, growing it for maximum above-ground growth is a good strategy when your goal is to harvest it or leave it on the soil surface as a mulch. When rye is tilled into the soil it can inhibit the growth of vegetables by tying up nitrogen and causing allelopathy (producing harmful substances to vegetables). To add nitrogen to the soil, till in rye while it is still lush and green. Also consider planting it with hairy vetch to add more nitrogen to the mix. Research at Penn State and elsewhere suggests that a seeding rate for non-legumes in a mixture that is 20 – 30% of the typical monoculture seeding rate provides a good balance between soil nitrogen scavenging by the non-legume and atmospheric nitrogen fixation by the legume, with C:N ratios generally staying below the critical 20:1 threshold.
A seeding rate of the non-legume species greater than 30% is likely to smother the legume companion and increase the C:N ratio. For rye, which has a typical monoculture seeding rate of 120 pounds/acre, use a seeding rate of between 24 and 36 pounds/acre with a 70 – 80% monoculture seeding rate for the legume companion.
Information in this article has been adapted from “Cover crops in vegetable production systems” by Ajay Nair, Iowa State University, and “Growing cover crops for nitrogen on vegetable farms” by Elsa Sanchez and Charles White, Penn State.