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Over 440 million acres (19.5 percent of land) is dedicated
to growing crops in the U.S., and another 587 million
acres (26 percent) is in pasture and range, largely used
for domestic livestock production. Agricultural activities
on these lands produce a plentiful, diverse, and relatively
inexpensive supply of food and fiber for people here
at home and abroad. However, agricultural production
practices can degrade the environment. Transformation
of undisturbed land to crop production can diminish habitat
for wildlife. Soil erosion, nutrient and pesticide runoff,
and irrigation can pollute the air and water, degrade
soil quality, and diminish water supplies. The extent
and degree of the environmental problems associated with
agriculture vary widely across the country. Concern over
these problems has given rise to local, State, and Federal
conservation and environmental
policies and programs to address them.
Soil Quality
Soil, as a plant-growing medium, is the key resource
in crop production. Soil supports the fundamental physical,
chemical, and biological processes that must take place
in order for plants to grow; it regulates water flow
between infiltration, root-zone storage, deep percolation,
and runoff; and it acts as a buffer between production
inputs and the environment. Soil can also function as
a "degrader" or "immobilizer" of
agricultural chemicals, wastes, or other potential pollutants,
and can mitigate climate change by sequestering
carbon from the atmosphere (when the rate of organic
matter production exceeds the rate of oxidation). How
well soil performs these functions depends on soil
quality. How soil
is managed has a major impact on soil quality, and
on the potential for various pollutants to leave the
field and affect other resources.
Soil quality can be defined
as the capacity of a specific kind of soil to function,
within natural or managed ecosystem boundaries, to
sustain plant and animal productivity, maintain or
enhance water and air quality, and support human health
and habitation. Soil quality depends on attributes
such as the soil's texture, depth, permeability, biological
activity, capacity to store water and nutrients, and
organic matter content. Soil quality can be maintained
or enhanced through the use of appropriate crop production
technologies and related resource management systems.
Poorly managed fields can lead to soil degradation through
three processes: physical degradation, such as via wind
and water erosion and soil compaction; chemical degradation,
such as toxification, acidification, and salinization;
and biological degradation, such as loss of organic matter
and decline in the activity of soil fauna. Poor management
can also increase runoff of nutrients and pesticides
to surface and groundwater systems. Thus, soil degradation
can have both direct and indirect negative effects on
agricultural productivity and the environment. Even on
high-quality soils, overuse of chemical inputs can result
in soil toxicity and water pollution.
Water Quality
Agriculture is widely believed to have significant impacts
on water quality. While no comprehensive national study
of agriculture and water quality has been conducted, the
magnitude of the impacts can be inferred from several water
quality assessments. A general assessment of water quality
is provided by EPA's 2002 Water
Quality Inventory.
Based on State assessments of 19 percent of river and stream miles, 37 percent
of lake acres, and 35 percent of estuarine square miles, EPA concluded that
agriculture is the leading source of pollution in 37 percent of river miles,
30 percent of lake acres (excluding the Great Lakes), and 8 percent of estuarine
waters found to be water-quality impaired, in that they do not support designated
uses. This makes agriculture the leading source of impairment in the Nation's
rivers and lakes, and a minor source of impairment in estuaries. Agriculture's
contribution has remained relatively unchanged over the past decade.
(For additional information on agriculture's contribution
to water quality problems, see AREI
2.2).
Major Agricultural Pollutants
Sediment is the largest contaminant of surface water by weight and volume, and is identified
by States as the leading pollution problem in rivers and streams and the fourth
leading problem in lakes. Sediment in surface water is largely a result of soil
erosion, which is influenced by soil properties and the production practices
farmers choose. Sediment buildup reduces the useful life of reservoirs. Sediment
can clog roadside ditches and irrigation canals, block navigation channels,
and increase dredging costs. By raising streambeds and burying streamside wetlands,
sediment increases the probability and severity of floods. Suspended sediment
can increase the cost of water treatment for municipal and industrial water
uses. Sediment can also destroy or degrade aquatic wildlife habitat, reducing
diversity and damaging commercial and recreational fisheries. Regions with the
greatest potential to discharge sediment from cropland to surface waters include
parts of the Heartland, Mississippi Portal, and Prairie Gateway regions
(see ERS Resource Regions for a description of the regions used here).
Nitrogen and phosphorus
are important crop nutrients, and farmers apply large
amounts to cropland each year. They can enter
water resources through runoff and leaching. The
major concern for surface-water quality is the promotion
of algae growth (known as eutrophication),
which can result in decreased oxygen levels, fish
kills, clogged pipelines, and reduced recreational
opportunities. The
U.S. Geological Survey (USGS) has found that
high concentrations of nitrogen in agricultural streams
are correlated with nitrogen
inputs from fertilizers and manure used on crops
and from livestock
waste. EPA reported in its Water
Quality Inventory that nutrient pollution is
the leading cause of water quality impairment in
lakes, and a major cause of oxygen depletion in estuaries.
USGS
modeling of total nitrogen and phosphorus export
from watersheds in major
water resource regions of the U.S. found that, nationally,
the median contribution of nitrogen from agricultural
sources was 36 percent, compared to 0.8 percent for
point sources (sewage treatment plants, factories).
For phosphorus, the median contribution from agricultural
sources was 43 percent, compared to 3 percent for
point sources. Watersheds with a high potential to
deliver nitrogen to surface water are primarily in
the Heartland and Southern Seaboard regions. Watersheds
with a high potential to discharge nitrogen to ground
water are primarily in the Southern Seaboard, Fruitful
Rim, Heartland, and Prairie Gateway regions. Watersheds
with a high potential to discharge phosphorus to
surface water are located primarily in the Heartland,
Southern Seaboard, and Northern Crescent regions.
Eutrophication and hypoxia (low
oxygen levels) in the northern Gulf of Mexico
have been linked to nitrogen loadings from the Mississippi
River. Agricultural sources (fertilizer, soil inorganic
nitrogen, and manure) are estimated to contribute about
71 percent of the nitrogen
loads entering the Gulf from the Mississippi Basin,
and 80 percent of phosphorus loads. The Gulf of Mexico
is not the only coastal area affected by nutrients.
Recent
research by the National Oceanographic and Atmospheric
Administrations has found that 29 estuaries (20
percent of major U.S. estuaries) exhibit highly eutrophic
conditions, caused primarily by nitrogen enrichment.
Farmers apply a wide variety of pesticides to
control insects (insecticides), weeds (herbicides), fungus
(fungicides), and other problems. Well over 500 million
pounds (active ingredient) of pesticides have been applied
annually on farmland since the 1980s, and certain chemicals
can travel far from where they are applied. Pesticide
residues reaching surface-water systems may harm freshwater
and marine organisms, damaging recreational and commercial
fisheries. Pesticides in drinking water supplies may
also pose risks to human health. Pesticide concentrations
exceeded one or more human-health benchmarks in about
10 percent of agricultural streams examined by USGS as
part of the National
Water Quality Assessment Program, and in about 1
percent of sampled wells used for drinking water in agricultural
areas.
Watersheds with a high propensity to discharge pesticides
to surface water are located primarily in the Heartland
and Mississippi Portal regions. Watersheds with a high
propensity to discharge pesticides to ground water are
primarily in the Heartland, Prairie Gateway, and Southern
Seaboard regions.
Some irrigation
water applied to cropland may run off the field
into ditches and receiving waters. These irrigation
return flows often carry dissolved salts as well as
nutrients and pesticides into surface or ground water.
Increased salinity levels in irrigation water can reduce
crop yields or damage soils such that some crops can
no longer be grown. Increased concentrations of naturally
occurring toxic minerals—such as selenium, molybdenum,
and boron—can harm aquatic wildlife and impair
water-based recreation. Increased levels of dissolved
solids in public drinking water supplies can increase
water treatment costs, force the development of alternative
water supplies, and reduce the lifespans of water-using
household appliances. The possibility of pathogens
contaminating water supplies and recreation waters
is a continuing concern. Bacteria are the largest source
of impairment in rivers and streams, according to EPA's
water quality inventory. Potential sources include
inadequately treated human waste, wildlife, unconfined
livestock, and animal
operations. Diseases from micro-organisms in livestock
waste can be contracted through direct contact with
contaminated water, consumption of contaminated drinking
water, consumption of crops irrigated with contaminated
water, or consumption of contaminated shellfish. Bacterial,
rickettsial, viral, fungal, and parasitic diseases
are potentially transmissible from livestock to humans.
Fortunately, proper animal waste management practices
and water treatment minimize this risk. However, protozoan
parasites, especially Cryptosporidium and Giardia,
are important sources of waterborne disease outbreaks. Cryptosporidium and Giardia may
cause gastrointestinal illness, and Cryptosporidium may
lead to death in persons with compromised immune systems.
These parasites have been commonly found in beef
herds and Cryptosporidium is widespread
on dairy
operations.
Air Quality
Ever since farmers began raising animals and cultivating crops, agricultural production practices have generated a variety of substances that enter the atmosphere with the potential of creating health and environmental problems. The relationship between agriculture and air quality became a national issue in the 1930s with the severe dust storms of the Dust Bowl. Although dust storms of this magnitude no longer occur in the U.S., soil particulates, farm chemicals, and odor from livestock are still carried in the air we breathe. These emissions can harm human health and pollute the environment. Air quality in most rural areas is not a cause for concern, but there are some farming communities where ozone and particulates have impaired air quality to the same extent as in urban areas.
Ammonia is a gas and one of the most abundant nitrogen-containing compounds emitted to the atmosphere. Animal farming systems contribute about 50 percent of the total anthropogenic emissions of ammonia into the atmosphere in the U.S. Ammonia is a health hazard to humans and animals in high concentrations. Once in the atmosphere, ammonia is rapidly converted to ammonium particles by reactions with acidic compounds such as nitric acid and sulfuric acid found in ambient aerosols. These ammonium particles can be carried long distances in the atmosphere and contribute to fine particulate pollution and haze. Ammonium is redeposited to the earth's surface by both wet and dry deposition, contributing to eutrophication of water resources.
Nitrous oxide is another nitrogen
compound of concern. It is a greenhouse gas and contributes
to ozone depletion. Nitrous oxide forms primarily in the soil during the microbial
processes of nitrification and denitrification. Agricultural sources include
manure from livestock farming and commercial fertilizer. Agriculture contributes
about 72 percent of total anthropogenic emissions of nitrous oxide in the U.S.,
mostly from the fertilization of cropland.
Methane is an important greenhouse gas.
It is produced by microbial degradation of organic
matter under anaerobic conditions. The agricultural
sector is the largest anthropogenic source, with
livestock production being the major component.
Enteric fermentation (within the stomachs of cattle,
sheep, goats and other ruminants) and manure management
contribute 27 percent of methane emissions in the
United States.
Carbon Dioxide is the primary greenhouse
gas emitted in the U.S., mostly from the combustion
of fossil fuels. Carbon dioxide is also a primary
input in plant growth. Agriculture can sequester
(store) carbon in soils and biomass, thus offsetting
greenhouse gas emissions. Carbon entering the soil
is stored primarily as soil organic matter. Agricultural
soils sequestered an estimated 12.4 million metric
tons carbon equivalent in 2004, less than 1 percent
of U.S. emissions. Studies indicate that it may
be technically possible to sequester an
additional 89-318 million metric tons of carbon
annually on U.S. croplands and grazing lands through
various management practices, such as conservation
tillage, crop rotations, and fertilizer management.
Shifting cropland to grasslands or forest could
increase sequestration even more.
Particulates from agriculture result from
a variety of activities. Wind erosion can carry
soil particles directly into the atmosphere. Many
areas west of the Mississippi River experience
low average rainfall, frequent drought, and relatively
high wind velocities. These conditions, when combined
with fine soils, sparse vegetative cover, and agricultural
activity, make some western regions susceptible
to wind erosion.
Wind erosion can produce short-term levels of particulate
pollution in rural areas that exceed urban levels. Particulates
from wind erosion can impose costs on those living in
affected areas, including cleaning and maintenance of
businesses and households, damage to nonfarm machinery,
and adverse effects on health. Another source of particulates
is open-field burning. Open-field burning is used as
a means of removing crop residue after harvest and controlling
disease, weeds, and pests. Diesel engines from farm equipment
and irrigation pumps are also a source of particulates.
A source of fine particulates (particles smaller than
2.5 microns, also known as PM2.5) is gaseous emissions
of ammonia and nitrogen oxides (NOx, or nitric oxide,
and nitrogen dioxide). Ammonia and NOx in the atmosphere
react with other compounds to form fine particulates,
such as ammonium. Fine particulates pose a health risk
because they can be inhaled deep into lungs. Fine particulates
are also a source of haze, which detracts from views
in many popular
national parks.
The atmosphere is now recognized as a major pathway
by which pesticides can be transported and deposited
far from their point of use. Pesticides can enter the
atmosphere directly from the spray cloud during application,
from evaporation after application, and attached to windborne
soil particles. As much as 80 percent of some pesticide
applications evaporate. And many of these pesticides
across different chemical groups have been detected in
the atmosphere. The U.S.
Geological Survey found that the most frequently
detected pesticides in the atmosphere are DDT, methidathion,
diazinon, heptachlor, malathion, and dieldrin. Even though
some of these have been banned for years, they continue
to be detected.
Deposition of airborne pesticides will also affect water
quality, but an understanding of the extent to which
airborne deposition of pesticides contributes to water
quality impairments requires additional monitoring. The
highest concentrations of pesticides detected in air
and rain occur in those areas where they are applied
most frequently and in the highest amounts. Atmospheric
concentrations of pesticides also vary seasonally, and
not surprisingly, the highest concentrations in air and
rain usually occur in the spring and summer months, when
most pesticides are applied and temperatures are warmest.
Odor is a major nuisance associated with animal
production facilities. The expansion of urban and suburban
development into rural areas has brought more people
into contact with animal operations. People who are agricultural
novices living near farms often object to the traditional "smells" of
farming. However, farmers too are objecting to odors
from large animal feeding operations, particularly hog
operations with large lagoons. Odorous gases consist
of a host of compounds (over 300) that originate from
animal housing, manure storage units, and land application
of manure. USDA's Agricultural Research Service is currently
conducting research on interactions between agricultural
production practices and air quality, and new approaches
for addressing air
quality problems.
Wildlife Habitat
Habitat is a combination of environmental factors that
provides the food, water, cover, and space that a living
organism needs to survive and reproduce. Agricultural
land use can benefit some species, harm others, and sometimes
do both. Potentially harmful effects of farming include
plowing up habitat, farming riparian buffers, fragmenting
habitat, diverting water for irrigation, and diffusing
agricultural chemicals into the environment. In addition,
specialization in agriculture reduces landscape diversity
by creating more of a monoculture. This reduces the presence
of ecological niches, which can limit wildlife populations
and biodiversity on farms. Historically, the conversion
of native forests, prairies, and wetlands to cropland
has diminished wildlife. Habitat loss associated with
agricultural practices on over 400 million acres of cropland
has been identified as a primary factor depressing wildlife
populations in North America. Agriculture is thought
to affect the survival of 380 of the 663 species listed
by the Federal Government as threatened or endangered
in the conterminous 48 States.
Agriculture's negative effects on wildlife need not
be permanent. U.S. agriculture is in a unique position
with respect to the Nation's wildlife resources. The
management of land now controlled by U.S. farms and ranches
can play a major role in protecting and enhancing the
Nation's wildlife. In 2002, private farms accounted for
41 percent of all U.S. land, including 434 million acres
of cropland and 395 million acres of pasture
and range. Farms also account for 76 million acres
of forest and woodland, and 17 million acres of nonfederal
wetlands. Different types of habitat can be restored
or improved through conservation on agricultural lands.
Grassland Habitat
Grasslands constitute the largest land cover on America
's private lands. Privately
owned grasslands and shrub lands (including tribal)
cover more than 395 million acres in the United States. These lands contribute significantly to the economies
of many regions, provide biodiversity of plant and animal
populations, and play a key role in environmental quality.
Grasslands directly support the livestock industry. They
also provide habitat for many wildlife species, reduce
the potential for flooding, control sediment loadings
in streams and other water bodies, and provide ecological
benefits such as nutrient cycling, storage of atmospheric
carbon, and water conservation. Grasslands also improve
the aesthetic character of the landscape, provide scenic
vistas and open space, provide recreational opportunities,
and protect the soil from water and wind erosion.
Large expanses of grassland acreage are annually threatened
by conversion to other land uses such as cropland and
urban development. About half
of all grasslands in the U.S. have been lost since
settlement, much due to conversion to agricultural uses.
Wetland Habitat
Wetlands are complex ecosystems that provide many ecological
functions that are valued by society. They take many
forms, including prairie potholes, bottomland hardwood
swamps, coastal salt marshes, and playa wetlands. Wetlands
are known to be the most biologically
productive landscapes in temperate regions. More
than one-third of the United States' threatened and
endangered species live only in wetlands,
and nearly half use wetlands at some point in their lives.
Most freshwater fish depend on wetlands at some stage
of their lives. Many bird species are dependent on wetlands
for either resting places during migration, nesting or
feeding grounds, or cover from predators. Wetlands are
also critical habitat for many amphibians and fur bearing
mammals. Besides supporting wildlife, wetlands
also control water pollution and flooding, protect
the water supply, and provide recreation.
When the country was first settled there were 221-224
million acres of wetlands in the continental U.S. Since
then, about half have been drained
and converted to other uses, nearly 85 percent for
agricultural uses. Currently, there are about 111 million
acres of wetlands on nonfederal lands. About 15 percent
are on agricultural lands (cropland, pastureland, and
rangeland). For more information on wetlands and their
value to society, see EPA's Wetlands
Homepage.
Riparian Habitat
Riparian areas are the zones along water bodies that
serve as interfaces between terrestrial and aquatic ecosystems.
Riparian ecosystems generally compose a minor proportion
of the landscape, but they are typically more structurally
diverse and more productive from a wildlife perspective
than adjacent upland areas. This is especially true in
the arid West. Studies in the Southwest show that riparian
areas support a higher
breeding diversity of birds than all other western
habitats combined. In Arizona and New Mexico, at least
80 percent of all animals use riparian areas at some
stage of their lives. Western riparian habitats contain
the highest non-colonial avian breeding densities in
North America.
Riparian zones also support productive aquatic habitat.
They stabilize streambanks, thus reducing streambank
erosion and sedimentation. Detritus from streamside vegetation
provides energy to the stream ecosystem. Vegetation also
provides shade, preventing extreme temperature swings
that are detrimental to healthy stream ecosystems. Riparian
areas also filter out sediment, nutrients, and pesticides
in runoff, thereby protecting water quality.
No comprehensive national inventory has been completed
on the status and trends of riparian areas. However,
NRCS estimates that the conterminous U.S. originally
contained 75-100 million acres of riparian
habitats, and that between 25 and 35 million acres
remain.
Implications for Policy
Agriculture has wide ranging impacts on environmental
resources. Because of this, it also has the capacity
to provide a wide range of environmental services. Understanding
the links between agriculture and environmental quality
enhances our ability to design programs that best meet
the needs of producers and those who value the services
the environment can provide.
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