Ch11

Chapter 11 - EFFECTS OF AGRICULTURE ON THE ENVIRONMENT

The Big Picture

The landscapes of the arable portions of the Earth have been greatly altered by agriculture. Centuries ago, when the global human population was considerably smaller than today, the conversion of the natural landscape (e.g., forests and grasslands) to agricultural landscapes was localized. The eastern Mediterranean, portions of northern Europe, and population centers of eastern Asia were drastically altered by grazing or intensive crop production. Today, human populations are exponentially greater and extend to most of the inhabitable portions of the planet. The effects of agriculture are not only local but global, as well. Agricultural production is necessary to maintain modern human cultures but there are serious environmental trade-offs. Agricultural production entails the production of plants and animals for food and fiber. Agriculture seeks to promote the growth of species of plants and animals that provide these services. The most efficient way to maintain or increase agricultural production is to produce crops and livestock in monocultures or low-diversity systems. Thus, we are replacing ecosystems with relatively high biodiversity with agricultural ecosystems of relatively low biodiversity. Similarly, species that are not valued as agricultural commodities or interfere with the efficient production of agricultural species may be considered competing species or even pest species. In order to maximize agricultural production and minimize economic expenditure, modern agriculture in the United States and other more developed nations often relies heavily upon the use of pesticides, fertilizer supplements, and mechanized production. Using these techniques, the agricultural sector of our society is able to produce enough food for millions of people in non-agricultural sectors (i.e., manufacturing, industrial, governmental, and services). Agricultural production in the less developed nations still relies heavily upon human energy input but large tracts of land are often necessary to satisfy demands for food and fiber production. Most farmers have long recognized the importance of protecting soil, water, and biological resources and many are good stewards of the land. Still, as the demands for agricultural products increase the stresses on natural resources will be exacerbated. Only through a combination of governmental regulation, incentive programs, and personal initiative by farmers can natural resources be sustained.

Frequently Asked Questions

What are examples of on-site and off-site effects of agriculture?

On-site environmental effects are those which take place on or in the immediate vicinity of the agricultural activities. These effects include: the conversion of land to agricultural production thus a loss of biodiversity and habitat, erosion and soil loss, soil contamination, and possible degradation of water quality and reduction of water quantity. These on-site effects would be considered local effects.

Off-site environmental effects are those which occur downwind or downstream from the agricultural operation. Off-site effects may be expanded to include those effects that result from the combined effects of farming practices in a region, such as aquifer depletion or the loss of habitats. Globally, agricultural production has the potential to alter chemical cycles and climatic conditions.

How are soils affected by agricultural production?

There are over 17,000 soil types recognized worldwide. They vary widely in structure, erodibility, fertility, and ability to produce crops. A generalized soil profile for a humid, temperate climate is depicted in Figure 11.2
When the natural vegetation is cleared for agriculture, soils become exposed to erosion and loss of soil fertility.
The removal of the above-ground natural vegetative cover and subsequent plowing and preparation for row crop planting, also destroys plant roots which would otherwise help stabilize the soil. Soils disturbed by plowing and cultivation are prone to erosion by water runoff and wind. The erodibility of a particular soil depends upon soil type, topography (steepness), organic matter, and intensity of the erosive processes.
In the United States, soil erosion is estimated to be as severe or worse today as during the Dust Bowl years of the 1930's (Figure 11.4). One-third of the nation's topsoil has been lost to erosion.
Much of the eroded sediment eventually is deposited in streams, rivers, lakes, and the ocean. In the United States, approximately 4 billion tons of eroded sediment is deposited in waterways each year, 75% of this originates from agricultural lands.
When the sediments enter waterways, habitat quality for aquatic plants and animals may decline, as well as water quality for human use.

How can we reduce soil erosion?

If farmland is allowed to recover by allowing it to remain fallow and letting natural succession take place or by actively restoring the original vegetation type, soil forming processes (pedagenesis) will proceed. However, soil forming processes can be very slow.
Removing highly erodible land from production is one strategy to reduce soil erosion. State and federal incentive programs are in place to promote this strategy .
Obviously, much farmland must remain in cultivation to satisfy the demand for agricultural products.
A variety of plowing and cultivation techniques can be used to reduce soil erosion. These include contour plowing (following topographic contours) and no-till agriculture (minimal disruption of soil surface). Other strategies include timing plowing and cultivation to time when the potential for erosion is reduced, crop rotation, planting multicultures, strip-cropping, terracing, and "grassed" waterways (these are drainage ditches with grass cover) (Figure 11.3).
The success of the techniques depends on local soil conditions and the type of crops being planted.

How does agriculture affect soil fertility and when do plant nutrients pose an environmental problem?

Nutrients cycle between soil, water, air, and the biota (see Chapter 4). Agricultural production interferes with the rate of nutrient cycling and ultimately redistributes nutrients, depleting soils of some nutrients and concentrating nutrients in eroded sediments and waterways.
Many crops have high nutrient demands thus deplete the soil of nutrients at a faster rate than native plants.
Soil cultivation disturbs the soil and increases the rate of nutrient loss via erosion.
Harvesting crops removes nutrients from the system thus they are not recycled.
Nitrogen and phosphorus are nutrients that crops require in large amounts but that are readily depleted from the soil. If soils are of low fertility, farmers have to artificially supplement soils with inorganic and/or organic fertilizers. (Commercial fertilizers used in the United States typically contain nitrogen (N), phosphorus (P), and potassium (K), as well as inert materials and possibly trace nutrients. The numerals appearing on a bag of fertilizer correspond to the percentages of N-P-K.)
Intensive livestock production in feedlots and concentrated animal production facilities tend to concentrate nutrients in animal wastes.
An excess of nutrients applied to cropland or concentrated in animal wastes can result in off-site environmental problems. Through erosion and water movement, nutrients may be transported by surface waters and groundwater. Eutrophication and water quality degradation often results from nutrient mismanagement (see Chapter 20).

What are some of the environmental effects of grazing?

In arid (desert) and semi-arid (grassland) regions where rainfall is insufficient to support row crop production, land is often used for grazing (Figure 11.6). In the United States, 40% of the land area is classified as rangeland.
The grasslands of the Great Plains were well-adapted to grazing by migratory bison and other ruminants. Today, bison have been replaced with cattle, sheep, and other non-migratory animals that forage differently than bison, thus affect plant community structure and composition.
Overgrazing by livestock alters the plant community by removing species that are intolerant of intensive grazing and promoting the proliferation of plant species that are unpalatable. Additionally, livestock are often directly or indirectly responsible for the introduction of exotic plant species that may become pest species.
Landscapes that were once forested have been converted to rangeland. In eastern North America, Europe, Southern Africa, and, increasingly South America and Central America, forests have been cleared and converted to pasture (Figure 11.5). This conversion results in a loss of habitat and biodiversity, and a change in ecosystem structure and function.
Game ranching is an alternative to conventional grazing practices. Native or other "wild" herbivore species are allowed to free-range and are harvested at a sustainable rate. This non-conventional practice is controversial and would have many cultural and economic obstacles to overcome.

What is desertification and how can it be prevented?

Deserts account for approximately one-third of the total land area on earth. The five major deserts lie between 15 and 30 latitude, north and south.
As a result of historic overgrazing and inappropriate land use practices, some desert areas are expanding into surrounding landscapes and other semi-arid regions have been and continue to be converted to desert. This process is called desertification.
Additional desertification may be prevented by monitoring vegetation, water table, surface and soil moisture, soil organic content, and soil salinization.
The restoration of lands than have experienced desertification will entail reducing grazing pressure and re-establishing vegetation and habitat.

How are pests and weeds controlled in modern agricultural practices?

"Pest" is a general term referring to any undesirable species of plant, animal, or microbe. Agricultural pests include any organisms that compete with or otherwise damage crops and livestock.
The goal of agriculture is the production of economically valuable crops and livestock in an efficient manner. Pests reduce efficiency thus there is considerable effort and monetary expenditure toward combating pests.
In the U.S., economic losses of $16 billion/year can be attributed to plant pests (weeds). Weed control costs about $3.6 billion/year.
Following World War II, sophisticated pesticides with complex chemical structures were developed. Pesticide usage in the United States and worldwide increased dramatically. In the United States today more than 300kg of pesticides are used per year (661 million lb/yr).
Pesticides include bactericides, insecticides, herbicides, fungicides, and rodenticides.

Narrow spectrum pesticides target specific pest species that are considered less harmful to the environment. Broad spectrum pesticides affect a wider range of organisms including pests species as well as non-pest and beneficial species.
The rapid reproduction rates of many pest species may allow these species to evolve resistance to pesticides; this is called pesticide resistance. Consequently, new and more toxic pesticides must continually be developed.

What are some of the environmental effects of pesticide usage?

Unfortunately, many pesticides (even narrow spectrum pesticides) have environmental effects beyond their targeted use.
Non-target species may be eliminated by pesticide use. These species may include such beneficial species as pest predators and plant pollinators.
Some pesticides are not easily biodegraded in the environment. Long after their target use, these pesticides may persist in the tissue of plants and animals or in sediments, then re-enter the food web.
One of the more infamous examples of the environmental side effects of pesticide use is DDT, a chlorinated hydrocarbon that was widely used in the United States as an insecticide until its use was discontinued in the 1970's.
DDT was a fat-soluble compound that did not readily decompose in the environment. If consumed by animals, DDT concentrated and persisted in the oils and fats of the consumer organism. Then DDT could biomagnify and become transferred through food chains. Organisms at the top consumer levels of food chains and long-lived organisms tended to have the highest concentrations of DDT in their tissues.
Aquatic food webs were especially vulnerable to the harmful effects of DDT. Long aquatic food chains resulted in high concentrations of DDT in the tissue of top consumers, such as pelicans, ospreys, and bald eagles. These birds experienced damage to their reproductive systems and consequently they produced thin, easily damaged egg shells. Populations of these species declined significantly.
DDT is no longer used in the United States, however, chemical manufacturers are legally allowed to produce DDT and sell it overseas. DDT is often used in the tropics to control mosquitoes and other disease vectors.

What is Integrated Pest Management (IPM)?

IPM is a common sense strategy to manage pests, minimize environmental damage, and maximize farm profits.
IPM utilizes a suite of possible solutions rather than relying solely on a single method of pest control, such as relying heavily on pesticides (Figures 11.8 and 11.9).
IPM strategies include using pesticides judiciously and only after determining 1) the specific nature of the pest problem, 2) the proper application rates, 3) and the proper timing of application.

Ecology In Your Backyard

What are the principal agricultural products of your region?
Are there any specific environmental problems associated with agricultural production in your region?
What alternative agricultural practices are being used or may be used to ameliorate any harmful effects?
Have any environmental or human health problems associated with agriculture received media attention?

For example: Hog production in eastern North Carolina is an example of how a very intensive agricultural practice can have major off-site effects on the local and regional environment, and on human health and well-being. A Pulitzer Prize winning series of newspaper articles in the Raleigh, NC News and Observer describes the effects of the hog industry on air and water quality. Since the publication of the newspaper series, the hog farming issue has become much more acute. In the summer of 1995 and fall 1996, above average rainfalls resulted in saturated soil conditions and failure of the waste lagoon and spray field disposal system. In June 1995, millions of gallons of hog waste were inadvertently discharged into coastal rivers. In one instance, 25 million gallons of hog waste (more than twice the volume of contamination from the Exxon Valdez) flowed into a coastal river. Read about this dilemma by accessing the News and Observer hog farming website listed in the Ecolinks On The Web section that follows.

Soil Surveys (Natural Resources Conservation Service) have been published for most counties in the United States. These may be obtained from county extension agent offices or NRCS offices for a nominal fee or cost-free, or see your local or university library. Soil Surveys contain a wealth of information pertaining to local soil types and suggested land uses based on soil type.
Please respond to these questions or send your thoughtful examples and comments to:

BackYard@wiley.com

The best responses will be posted on the Wiley Environet Website, so check the page regularly for updates to see if your email is posted!

Hardcopy Links In The Library

Briggs, S.A. 1992. Basic Guide to Pesticides: Their Characteristics and Hazards. Taylor and Francis. Washington, D.C. 283 pp. This book, produced by the Rachel Carson Council, provides an overview of pesticides and their toxicity. Common, trade, and chemical names of pesticides are cross-indexed. An extensive chart, indexed by common name, lists the class, status, persistence, and adverse effects of each pesticide. Appendices include discussions of pesticide usage, carcinogenity, environmental and economic effects, federal legislation, and alternatives to pesticides (IPM).
Dunlap, T.R. 1981. DDT- Scientists, Citizens, and Public Policy. Princeton University Press. Princeton, NJ.
National Research Council. 1989. Alternative Agriculture. National Academy Press, Washington.
USDA-NRCS Soil Surveys may be obtained for your county at the local NRCS office or your library.

Farm Journal - A popular farming journal.

Progressive Farmer. Farming strategies, conservation, pesticide use, farm economics, and governmental regulations pertaining to farming are discussed.

Ecolinks On The Web

http://www.fao.org/ - The Food and Agriculture Organization is an agency of the United Nations. Explore this web site for international issues pertaining to agriculture and the environment.

http://www.usda.gov/ - This is the U.S. Department of Agriculture home page. Read about this federal agency or search for key words within the USDA web.

http://www.ctic.purdue.edu/ - This is the Conservation Technology Information Center. See the Farm Resources Management subsection.

http://www.nrcs.usda.gov/ - This is the National Resources Conservation Service home page. Go to the discussion of the 1996 Farm Bill and its discussion of programs designed to assist farmers, ranchers, and landowners in their conservation efforts.

http://www.blm.gov:80/ - The federal Bureau of Land Management (BLM) leases federal lands in the western United States to ranchers for the purpose of livestock grazing. Read about this agency and its programs at their home page.

http://www.inmind.com/schools/CVGS/sturesearch/Eyster/ - Results of study examining soil erosion in grassed and bare soil plots.

http://ipcm.wisc.edu/PAT/Categories/companion/Manual/RESIST.htm - This site provides a discussion of the problem of pesticide resistance.

http://atsdr1.atsdr.cdc.gov:8080/ - The Agency for Toxic Substances and Disease Registry of the Centers for Disease Control (CDC) provides a listing of the health and environmental effects of certain pesticides and other toxic substances, including DDT. Also see the "Science Corner" for interesting information and links.

http://www.sarep.ucdavis.edu/sarep/concept.html - This site provides a discussion of the basic concepts of sustainable agriculture.

http://www.nando.net/sproject/hogs/virtual.html - The growth of the hog industry in North Carolina is detailed in this Pulitzer Prize winning series in the Raleigh, NC, News and Observer entitled "Boss Hog":

http://www.nppc.org/ - This is the National Pork Producers home page. Go to "Especially for Producers" link, "Most Commonly Asked Questions About Pork Production and the Environment"

http://www.env.duke.edu/faculty/sigmon/agwaste/hogidea.html - Read policy recommendations, regarding the North Carolina hog industry, developed by students at Duke University. Go to: Policy Recommendations for Management...

Note: If any of these links are not working, please see if alternative links are available at the Ecolinks Update Site.

Ecotest Online

1. The soil horizon composed of a mix of organic and inorganic materials lying directly below the organic horizon is the _____ horizon. It is commonly referred to as topsoil.

a. A

b. B

c. C

d. D

2. Approximately _____ of the topsoil in the United States has been lost due to erosion.

a. three-fourths

b. one-half

c. one-third

d. one-quarter

3. Plowing and cultivation techniques that help reduce soil erosion include all of the following except ______.

a. contour plowing

b. pedegenesis

c. strip-cropping

d. no-till agriculture

4. Standard commercial fertilizers used by farmers and gardeners contain three of the following nutrients. Which of the following is not a plant nutrient supplied by fertilizer?

5. What percentage of the United States may be classified as rangeland?

a. 20%

b. 30%

c. 40%

d. 50%

6. ________ is the conversion of non-desert land to desert, usually as a result of inappropriate land uses such as overgrazing.

a. soil salinization

b. sclerophication