Spring 2010
How It Works
Cover Crops
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Soil is arguably the most prized component of agricultural ecosystems. Healthy soil drains well, is rich in organic matter, and teems with life, including many beneficial microbes and insects. One of the most effective methods for developing healthy, productive agricultural soil—which has important benefits for the environment as well—is the use of cover crops.
A mainstay of organic farming systems, cover crops (often grasses, legumes, or cereal grains such as rye and winter wheat) are not meant to be harvested but rather to stabilize and improve soil that would otherwise remain bare when crops for harvest and sale—"cash" crops—are not growing. Because they are often planted in fall, cover crops have been described as a winter blanket for soils, helping to reduce erosion and water runoff by buffering the soil from rain and wind. They also suppress weeds and increase the soil's water-holding properties, thereby improving the ability of cash crops to withstand drought. And when cover crops are plowed under in the spring, their organic matter improves soil structure to enhance drainage and root growth, and provides nutrients for both the subsequent cash crop and a wide variety of beneficial organisms.
Among the most useful cover crops are legumes (e.g., beans, peas, vetches, clovers), which provide significant amounts of nitrogen to subsequent crops naturally, reducing the need for synthetic fertilizers. They do this by "fixing" nitrogen—converting it from the abundant but unusable form in the atmosphere into forms that plants can use—through a symbiotic partnership with common soil bacteria (see the sidebar). When the cover crop is plowed under, microbes break down its molecules, releasing nitrogen into the soil for use by the subsequent cash crop. In this way, leguminous cover crops can supply most or all of the nitrogen needed for subsequent crops to produce high yields, allowing farmers to substantially reduce—or even eliminate—their use of synthetic nitrogen fertilizers, which cause serious pollution problems.
Reducing Pollution
Plants are unable to absorb more than half of the nitrogen fertilizer currently applied on U.S. farms, and much of the excess leaves the soil. This excess nitrogen threatens both the environment and public health in a number of ways.
For example, nitrogen overuse in agriculture is the largest domestic, human-caused source of nitrous oxide, a global warming gas nearly 300 times more potent than carbon dioxide. Additionally, nitrogen runoff or leaching from farms in the Mississippi River watershed is the largest contributor to the Gulf of Mexico's "dead zone"—an area the size of Connecticut and Delaware combined, where algae that flourish in conditions of excess nitrogen start a cycle that robs the water of oxygen, making it uninhabitable for fish and other marine life.
Nitrogen in the form of nitrate can also become a threat to human health when it seeps into drinking water. And finally, airborne ammonia (formed from the nitrogen in fertilizer) contributes to smog, respiratory diseases, and acid soil.
In response to mounting nitrogen pollution, the biotechnology industry has suggested that it could genetically engineer crops to use nitrogen fertilizer more efficiently. But a recent UCS report, No Sure Fix, found that this technology has yet to produce any nitrogen-efficient crops, and the prospects of it happening in the foreseeable future are uncertain.
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A Bacterium's-Eye View of Cover Crops Underground, nature's tiny nitrogen factories are at work. Producing fertilizer with high nitrogen content requires an extremely energy-intensive industrial process—at least it does if you're human. Certain soil-dwelling bacteria, on the other hand, can "fix" nitrogen (converting nitrogen gas in the atmosphere into ammonia, a form usable by plants) all by themselves. And soil bacteria known as rhizobia have evolved to fix large quantities of nitrogen by working in partnership with leguminous plants during the growing season.
The process begins when legumes release chemicals that stimulate the growth of rhizobia living in the soil around the plant roots. The bacteria then attach to the plant's root hairs, which secrete compounds called flavonoids that activate "nod" genes in the bacteria. This, in turn, causes the rhizobia to excrete chemicals that curl the plant's root hairs, allowing the bacteria to invade the plant and penetrate its root cells. The rapidly reproducing rhizobia induce the root cells to multiply, forming nodules that become visible to the naked eye within days. Each root nodule contains thousands of rhizobia that produce nitrogenase, an enzyme that catalyzes the transformation of nitrogen gas into ammonia that the plant uses to grow. Once the legume is plowed under, its organic matter and stored nitrogen become rich fodder for the cash crop planted in its wake. |
By contrast, advanced forms of traditional breeding have produced crops that are more nitrogen efficient, and we also found significant evidence indicating that cover crops can greatly reduce nitrogen pollution. For example, in experiments in which cash crops fertilized with synthetic nitrogen were rotated with non-leguminous cover crops, the cover crop reduced nitrogen leaching by an average of 70 percent, without reducing cash crop yields. When cash crops were rotated with leguminous cover crops and no synthetic nitrogen fertilizer was added, the cover crop reduced leaching by 40 percent. Though cash crop yields fell an average of 7 to 10 percent in these cases, the benefits of building soil quality could outweigh the loss in yield.
Maximizing the Benefits
In addition to reducing fertilizer use and nitrogen pollution, cover crops can play an important role in reducing global warming pollution. As the crops grow, they remove heat-trapping carbon dioxide from the atmosphere; when plowed under, the carbon in the plant is transferred into the soil. Non-leguminous cover crops such as rye and winter wheat are particularly good at storing carbon because they typically produce more biomass, or plant matter, than legumes. (More biomass also means more organic matter will be returned to the soil before cash crops are planted.)
An ideal cover crop system would provide the benefits of both legumes and non-legumes. One multi-year study found that hairy vetch and rye (a legume and non-legume, respectively) grown together yielded greater biomass and greater carbon and nitrogen content compared with either one grown alone. Yields and nitrogen uptake of some subsequent cash crops were also greater with this cover crop combination. Researchers are also looking at other legume/non-legume pairings. For example, cover crops in the Brassica family (e.g., mustard, radish) are deep-rooted and therefore useful for breaking up hard soils; combined with legumes, they might prove beneficial in soils that are both nitrogen-deficient and heavily compacted.
Cover crops do have limitations, and additional research is needed to maximize their potential for farmers. For example, cover crop growth depends on the weather; low rainfall or cold autumn temperatures can reduce their growth—and thus their benefits. And farmers will need incentives and support to take on the seed and labor costs involved with growing them. Still, cover crops represent a major underutilized opportunity to reduce nitrogen pollution, combat global warming, and build healthy, productive soils that will be good for American farmers and consumers alike.
Karen Perry Stillerman is a senior analyst in the Food and Environment Program.
Learn more about solutions for reducing nitrogen pollution.
Photos: USDA/Stephen Ausmus (rye); USDA/Scott Bauer (hairy vetch); Pennsylvania State University/Jennifer Dean (rhizobia)
