Cover crop

Cover crops protect the soil during periods when a cash crop would otherwise leave it exposed. Their living roots and surface residues reduce the force of rainfall and wind, improve soil aggregation, and can provide habitat and food for soil organisms.¹ Unlike a cash crop, the cover crop is usually terminated before planting the next commercial crop, although some are grazed, harvested for forage, or maintained as living ground cover in specialized systems.

Farmers choose species according to the service required. Grasses such as cereal rye and oats produce abundant fibrous roots and residue; legumes such as crimson clover, hairy vetch, and field peas can fix atmospheric nitrogen through symbiosis with Rhizobium bacteria; and broadleaf species such as forage radish may create large channels as their roots decay. A mixture can combine these functions, but it also complicates termination, seed handling, and prediction of nutrient release.

Successful cover cropping depends on fitting establishment, growth, and termination into the surrounding crop rotation. Sowing commonly occurs after harvest, before a fallow period, or into a standing crop by interseeding; termination may involve winterkill, mowing, rolling, grazing, herbicides, or incorporation into the soil. The appropriate method depends on species, growth stage, weather, equipment, and the planting date of the following crop.

Timing creates important trade-offs. A cover crop allowed to grow longer generally produces more biomass and takes up more nitrate, but it may consume soil water, delay field preparation, or become difficult to kill. Legume residues often decompose relatively quickly and release nitrogen, whereas mature grass residues can temporarily immobilize mineral nitrogen as microbes break down their carbon-rich tissue. Termination is therefore a nutrient-management decision, not merely a mechanical operation.

• Winter-hardy species are useful where protection through spring is needed.
• Non-winter-hardy species can provide summer or early-autumn cover and may be killed naturally by frost.
• Seed mixes should account for compatible planting depths, maturity, herbicide restrictions, and the next crop’s disease risks.

Cover crops can improve several soil and water outcomes, but their effects vary with climate, soil type, species, and management. Ground cover commonly reduces runoff and erosion, while living plants can scavenge residual nitrate and lower the amount vulnerable to leaching. Added roots and residues may improve aggregate stability and soil organic matter; a global synthesis found that cover crops can increase soil carbon, although the size and persistence of the gain differ among sites.²

The practice is not automatically beneficial in every field. Cover crops may compete with the following crop for water, harbor pests or diseases, interfere with planting, or require additional seed, labor, and termination costs. A vigorous cereal rye stand, for example, can suppress weeds but may also delay emergence of a subsequent crop if residue or nitrogen management is poor. Local trials and measurements of soil moisture, biomass, and nitrate help determine whether the intended service outweighs these costs.

Cover crops can serve as more than an erosion-control measure: they are also tools for nutrient cycling, weed suppression, grazing, pollinator support, and experimentation with reduced tillage. “Scavenger” crops are selected to capture nutrients left after fertilization, while “green manure” crops are deliberately incorporated to return organic material and, in the case of legumes, nitrogen to the soil.³ These terms overlap, but they emphasize different purposes rather than entirely different kinds of plants.

Some effects appear below ground and are easy to miss. Root exudates can alter microbial communities, and decomposing roots can create biopores that improve infiltration; outcomes depend on root architecture and soil conditions rather than on biomass alone. Cover crops can also produce unintended allelopathic effects, in which chemical compounds from residues inhibit germination of other plants. In conservation agriculture, cover crops are often combined with no-till and diversified rotations, but each practice has separate costs and should not be treated as interchangeable.

Cash crop

A crop grown primarily for sale or commercial processing rather than chiefly for soil protection.

Green manure

A crop grown to be returned to the soil, usually by incorporation, to add organic material and nutrients.

Nitrogen fixation

The biological conversion of atmospheric nitrogen into forms plants can use, commonly performed by bacteria associated with legume roots.

Biomass

The total living or recently living plant material produced by a crop, often measured above ground in cover-crop studies.