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The ‘Cover Cropping’ Renaissance

Using Crops as Ground Cover Over the Winter Months Improves Agricultural Yield, Soil Health, and Helps the Environment

The following article is an edited summary of Professor Ray Weil’s Poster Session remarks for the Twenty-Fifth International Conference on the Unity of the Sciences (ICUS XXV) in Seoul, Korea, February 12–14, 2019. His remarks were titled: “Cover Crops for Soil Health, Environmental Quality, and Farm Profitability.”

No till cover crop emerging from plant residue, South Dakota (US).  ©USDA/Wikimedia. (CC BY-SA 2.0)
No till cover crop emerging from plant residue, South Dakota (US). ©USDA/Wikimedia. (CC BY-SA 2.0)

Using cover crops—plants grown for the protection and enrichment of soil—in agriculture is nothing new, but the steady march of science is showing farmers new ways to use these crops to improve yields, profits, and the environment.

The Soil Quality Lab at the University of Maryland (UMD) started conducting cover crop research in the 1980s. It has spent the last two decades studying how cover crops can improve soil health compared to the common practice of leaving soil bare (and vulnerable to runoff and loss of nutrients) during part of the year.

One of the guiding principles of the lab’s research has been that cover crops can offer multiple benefits to the farmer, soil, and environment. In addition, the combination of several benefits allows cover crops to provide a positive return on investment.

Different cover crop species provide different ecosystem services, so the choice and mixtures of species are critical. The following is a summary of highlights from two decades of research.

Brassicas, Grasses, and Legumes

The Soil Quality Lab at UMD pioneered Brassica cover crops (such as cabbage, radish, and mustard), documenting their ability to alleviate soil compaction as well as send roots rapidly down to the subsoil to capture nutrients before they leach away during the winter.

These Brassica species, especially daikon radish, proved valuable in reducing fertilizer requirements by pumping the nutrients nitrogen (N), sulfur, phosphorus, potassium, calcium, and boron up from deep soil layers and making them available in the surface soil.

Daikon radish penetrates deeply into subsoil and brings nutrients to the surface.  ©okugawa/iStock.
Daikon radish penetrates deeply into subsoil and brings nutrients to the surface. ©okugawa/iStock.

Cover crop species in the grass family (e.g., ryegrass, rye, wheat, oats, and barley) also proved to be valuable for capturing nutrients, protecting the soil from erosion, improving soil structure, and sequestering carbon.

Species in the legume family are generally slower growing and less able to provide the above-mentioned benefits, but they have the distinct advantage of biological nitrogen fixation, which can add additional nitrogen to the cropping system. In temperate humid regions, this nitrogen addition occurs principally in spring before cash crops are planted.

Multiple Species Create Synergies

In addition to the performance of individual cover crop species, the soil lab examined interactions among species and found that there were many synergies that could be harnessed to improve cropping systems. For instance, it was discovered that mixing a small amount of a cereal with a legume improved biomass production and nitrogen fixation.

Another example of synergy is that early planting of a mixture of radish, clover, and cereals produced excellent fall Brassica growth, weed suppression, and nutrient capture, while promoting the growth of the legume in the spring, thus maximizing the amount of nitrogen fixation.

Intercropping of oat and rye in Sweden.  ©W.Carter/Wikimedia. Public Domain
Intercropping of oat and rye in Sweden. ©W.Carter/Wikimedia. Public Domain

Timing is Key

Much of the scientific literature on the effects of cover crops is based on single species, often with very low levels of cover crop productivity. Low cover crop performance is often due to poor timing of both planting and crop termination. This prevents the species from achieving its full potential. That is why, over the past several years, the lab’s research has focused on the timing part of cover crop management, both for planting in late summer/early fall and for termination in the spring.

Low cover crop performance is often due to poor timing of both planting and crop termination.

Maryland [where the lab is located] has a unique subsidy program to encourage farmers to grow cover crops. Its goal is to reduce the nitrogen that makes its way from farmland to the Chesapeake Bay estuary where dense plant growth kills marine animal life from lack of oxygen. The dense plant growth is caused by too much nitrogen in the water.

Although adoption of cover cropping has been widespread in Maryland, cover crops could be much more effective if the rules required earlier planting and later termination. UMD’s soil lab research shows that nitrate leaching over the winter is not significantly reduced if the cover crop is planted too late for growth before winter dormancy sets in or death occurs. This discovery challenges farmers to adapt and plant cover crops much earlier than in the past. This may require using earlier-maturing cash crops or changing crop rotations to include winter small grains that allow cover crop planting in late July or early August.

Inter-seeding and ‘Planting Green’

Other soil lab research involves inter-seeding (planting seeds where a crop already exists) with airplanes [and helicopters and drones] or machines that can drive over a standing cash crop of, say, corn or soybean, and seed the cover crop several weeks before the cash crop is harvested. The soil lab researchers have looked at what cover crop species and mixtures perform best under these inter-seeded conditions.

Many cover crops are underperforming because they are killed too early in the spring. One problem with allowing cover crops to grow late into the spring is that cash crops must be planted early to achieve high yields. Typical agricultural practice is to kill the cover crop with herbicide about three weeks ahead of cash crop planting.

An innovation that solves this problem is a practice called “planting green,” by which a no-till planter seeds the cash crop into a living green cover crop. In this way, biomass and carbon sequestration can be quadrupled by allowing cover crops to grow up to, or even a week or two beyond, the planting of the cash crop. [See video here: …]

Although this idea seemed quite challenging because of the tall and heavy cover crop growth that would be present at planting time, the experience of farmers and the UMD Soil Quality Lab research group shows that the practice of “planting green” works very well in terms of both the mechanics of the no-till planter and the growth of the cash crop.

A seeding helicopter.  ©BLM/Wikimedia. Public Domain
A seeding helicopter. ©BLM/Wikimedia. Public Domain
High biomass, multi-species cover crops also provide habitat for beneficial insects and arachnids, as well as for a wide range of wildlife species.

An added benefit of this late termination management is that many of the cover crop species will go to flower and provide pollinating insects with early spring food resources. This kind of high biomass, multispecies cover crop also provides habitat for beneficial insects and arachnids, as well as for a wide range of wildlife species.

Why There’s a Cover Cropping Renaissance

Although not a new concept, cover cropping is experiencing a renaissance, especially among conservation tillage farmers in North and South America. Annual cash crops keep the soil permeated with living roots and covered by foliage for only three or four months of the year. Cover crops give annual crop rotations many of the advantages of perennial vegetation, including carbon sequestration, weed suppression, and nutrient cycling. Adding cover crops to a crop rotation also changes everything about the soil-plant-water-atmosphere system. The heavy residue protects the soil from erosion, overheating, and evaporative water loss all summer.

Cover crops can also be used as soil improvement tools. One major mechanism is called bio-drilling, by which cover crops with strong roots growing during the wet, cool time of the year penetrate compacted soil layers, leaving behind permanent root channels that later cash crop roots can follow when the soil is dry and hard. Thus, bio-drilling gives crops with weaker roots, such as soybean, access to the great water and nutrient stores in the deeper layers.

Cover crops such as forage radish (FR), rapeseed, and rye roots have differing abilities to penetrate compacted layers into the subsoil. Corn crop roots permeate the compacted subsoil in proportion to the number of cover crop roots that had grown there during the previous winter. The effect can be extended to next year’s soybean crop as well.

Cash crops stop taking up N from the soil a month or more before harvest maturity, and residual nitrate commonly leaches out of the soil between fall and spring in the humid, mid-Atlantic region.

Cover crop of tillage radish.  ©Ethanstuckey/Wikimedia (CC BY-SA 4.0)
Cover crop of tillage radish. ©Ethanstuckey/Wikimedia (CC BY-SA 4.0)

Over half of the remaining N was found to be 90–210 cm (35–82 in) deep, which is likely to be out of reach for next year’s summer crop. The residual mineral N comes from both fertilizer and soil organic matter and may be even greater following soybean than following corn, due to more mineralization.

Research by the UMD soil lab and others shows that properly managed cover crops can clean up nearly all the soluble N from the soil profile in the fall. Ideally, the cover crop will cycle that N back to the surface soil so it becomes available for the following cash crop, thus reducing fertilizer requirements. However, grass cover crops tend to keep the N in unavailable forms because of microbial immobilization.

In order to evaluate the ability of cover crops to capture N remaining deep in the soil profile before it leaches away over winter, UMD soil lab researchers buried stable isotopes (non-radioactive forms of atoms) in the soil to act as a nitrate tracer. They buried the tracer at various depths at the end of the cash crop N uptake season.

Next, they planted various cover crops over the buried tracer on various dates. When planted in early September, both radish and rye cover crop roots could reach the nitrate tracer buried at 180 cm (70 in) deep. Cover crops planted a month later were able to reach only the 60 cm (23 in) deep tracer. If cover crops are planted early enough in the fall, they can greatly reduce the concentration of nitrate in the leaching water all winter.

Research at many sites revealed that planting cover crops after the cash crop harvest is usually too late for effective reduction in wintertime N leaching. Cover crops planted by early October, common in Maryland, were not much more effective than no cover crop.

The planting date also influences the type of cover crop vegetation produced. The species that dominates a mix of cover crops changes dramatically with just a few weeks’ difference in planting date in the fall. It is frustrating that many research papers simply state that a cover crop was used, but not the planting dates and biomass achieved. Extending cover crop growth in spring is just as important as planting early in the fall. Both the fall planting date and spring termination date affect the nature of the cover crop.

In summary, research is advancing cover crops technology using multiple species, early planting, late termination, and no-till management to enhance ecosystem services while using fewer inputs, maintaining high profitability, and increasing yields of cash crops. This innovation is part of what is sometimes referred to as the “no-till cover crop revolution.”

Maryland USA “no till” method—Soybeans planted in wheat residue.  ©USDA/Wikimedia. Public Domain
Maryland USA “no till” method—Soybeans planted in wheat residue. ©USDA/Wikimedia. Public Domain

*Ray Weil, PhD, is Professor of Soils, Department of Environmental Science & Technology, University of Maryland, USA.


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