The development of modern agricultural practices is one of the great success stories of applied sciences. Improved plowing techniques, new pesticides and fertilizers, and better strains of crops are among the factors that have resulted in significant increases in agricultural productivity.
Yet these improvements have not come without cost to the environment and sometimes to human health. Modern agricultural practices have contributed to the pollution of air, water, and land. Air pollution may be the most memorable, if not the most significant, of these consequences. During the 1920s and 1930s, huge amounts of fertile topsoil were blown away across vast stretches of the Great Plains, an area that eventually became known as the Dust Bowl . The problem occurred because farmers either did not know about or chose not to use techniques for protecting and conserving their soil . The soil then blew away during droughts, resulting not only in the loss of valuable farmland, but also in the pollution of the surrounding atmosphere .
Soil conservation techniques developed rapidly in the 1930s, including contour plowing , strip cropping, crop rotation, windbreaks, and minimum- or no-tillage farming, and thereby greatly reduced the possibility of erosion on such a scale. However, such events, though less dramatic, have continued to occur, and in recent decades they have presented new problems. When top soils are blown away by winds today, they can carry with them the pesticides, herbicides, and other crop chemicals now so widely used. In the worst cases, these chemicals have contributed to the collection of air pollutants that endanger the health of plants and animals, including humans. Ammonia, released from the decay of fertilizers, is one example of a compound that may cause minor irritation to the human respiratory system and more serious damage to the health of other animals and plants.
A more serious type of agricultural pollution are the solid waste problems resulting from farming and livestock practices. Authorities estimate that slightly over half of all the solid wastes produced in the United States each year—a total of about 2 billion tons (2 billion metric tons)—come from a variety of agricultural activities. Some of these wastes pose little or no threat to the environment. Crop residue left on cultivated fields and animal manure produced on rangelands , for example, eventually decay, returning valuable nutrients to the soil.
Some modern methods of livestock management, however, tend to increase the risks posed by animal wastes. Farmers are raising a larger variety of animals, as well as larger numbers of them, in smaller and smaller areas such as feedlots or huge barns. In such cases, large volumes of wastes are generated in these areas. Many livestock managers attempt to sell these waste products or dispose of them in a way that poses no threat to the environment. Yet in many cases the wastes are allowed to accumulate in massive dumps where soluble materials are leached out by rain. Some of these materials then find their way into groundwater or surface water, such as lakes and rivers. Some are harmless to the health of animals, though they may contribute to the eutrophication of lakes and ponds. Other materials, however, may have toxic, carcinogenic, or genetic effects on humans and other animals.
The leaching of hazardous materials from animal waste dumps contributes to perhaps the most serious form of agricultural pollution: the contamination of water supplies. Many of the chemicals used in agriculture today can be harmful to plants and animals. Pesticides and herbicides are the most obvious of these; used by farmers to disable or kill plant and animal pests, they may also cause problems for beneficial plants and animals as well as humans.
Runoff from agricultural land is another serious environmental problem posed by modern agricultural practices. Runoff constitutes a nonpoint source of pollution. Rainfall leaches out and washes away pesticides, fertilizers, and other agricultural chemicals from a widespread area, not a single source such as a sewer pipe. Maintaining control over non-point sources of pollution is an especially difficult challenge. In addition, agricultural land is more easily leached out than is non-agricultural land. When lands are plowed, the earth is broken up into smaller pieces, and the finer the soil particles, the more easily they are carried away by rain. Studies have shown that the nitrogen and phosphorus in chemical fertilizers are leached out of croplands at a rate about five times higher than from forest woodlands or idle lands.
The accumulation of nitrogen and phosphorus in waterways from chemical fertilizers has contributed to the acceleration of eutrophication of lakes and ponds. Scientists believe that the addition of human-made chemicals such as those in chemical fertilizers can increase the rate of eutrophication by a factor of at least 10. A more deadly effect is the poisoning of plants and animals by toxic chemicals leached off of farmlands. The biological effects of such chemicals are commonly magnified many times as they move up a food chain/web . The best known example of this phenomenon involved a host of biological problems—from reduced rates of reproduction to malformed animals to increased rates of death—attributed to the use of DDT in the 1950s and 1960s.
Sedimentation also results from the high rate of erosion on cultivated land, and increased sedimentation of waterways poses its own set of environmental problems. Some of these are little more than cosmetic annoyances. For example, lakes and rivers may become murky and less attractive, losing potential as recreation sites. However, sedimentation can block navigation channels, and other problems may have fatal results for organisms. Aquatic plants may become covered with sediments and die; marine animals may take in sediments and be killed; and cloudiness from sediments may reduce the amount of sunlight received by aquatic plants so extensively that they can no longer survive.
Environmental scientists are especially concerned about the effects of agricultural pollution on groundwater. Groundwater is polluted by much the same mechanisms as is surface water, and evidence for that pollution has accumulated rapidly in the past decade. Groundwater pollution tends to persist for long periods of time. Water flows through an aquifer much more slowly than it does through a river, and agricultural chemicals are not flushed out quickly.
Many solutions are available for the problems posed by agricultural pollution, but many of them are not easily implemented. Chemicals that are found to have serious toxic effects on plants and animals can be banned from use, such as DDT in the 1970s, but this kind of decision is seldom easy. Regulators must always assess the relative benefit of using a chemical, such as increased crop yields, against its environmental risks. Such as a risk-benefit analysis means that some chemicals known to have certain deleterious environmental effects remain in use because of the harm that would be done to agriculture if they were banned.
Another way of reducing agricultural pollution is to implement better farming techniques. In the practices of minimum- or no-tillage farming, for example, plowing is reduced or eliminated entirely. Ground is left essentially intact, reducing the rate at which soil and the chemicals it contains are eroded away.
[David E. Newton ]
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