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Pouring It On

Nitrate Contamination of Drinking Water

Nitrogen Use and Sources of Nitrate Contamination

February 1, 1996

Pouring It On: Nitrogen Use and Sources of Nitrate Contamination

In 1995, America's agricultural producers added 36 billion pounds of nitrogen to the environment -- 23 billion pounds of nitrogen fertilizer, and 13 billion pounds of nitrogen in the form of animal manure. Twenty percent of this nitrogen -- or 7 billion pounds -- was not used by the crops for which it was intended (NAS 1993). Instead, this excess nitrogen remained in the environment, where much of it eventually entered the reservoirs, rivers, and groundwater that supply us with our drinking water.

Nitrogen is a naturally occurring compound. In natural ecosystems nitrogen gains and losses are in balance, and remain so unless additional nitrogen is added to the system, upsetting the equilibrium. Unfortunately, in vast regions of the country, this balance has been upset, meaning water supplies are contaminated or placed at risk. The primary cause of this problem is the nitrogen that is added to the environment from agricultural activities, mainly overapplication of fertilizer.

Nitrogen Inputs From Agriculture

Agricultural activities, primarily row crop and livestock production, account for over 80 percent of all nitrogen added to the environment (Figure 1). Fertilizer is the single largest source of nitrogen; in 1995 American farmers used 23 billion pounds of nitrogen fertilizer, primarily for production of corn and wheat (Terry, et al. 1996). This represents a 25-fold increase in total annual nitrogen fertilizer use in the fifty year period between 1945 and 1994. Not surprisingly, nitrogen from fertilizer is considered to be the most important preventable source of nitrate contamination of water supplies (Hallberg 1986a, Bouchard 1992, NAS 1993, Puckett 1994, Keeney 1986, Keeney 1989). And not surprisingly, agricultural areas have the highest rates of nitrate contaminated water.

Animal manure is the second largest source of nitrogen in the environment, accounting for 13 billion pounds per year. When animals are confined to high density feedlots, the nitrogen produced can be an important localized source of water contamination. And, in many of the regions where manure input to the environment is highest, fertilizer inputs are also high, making what is often already a high risk situation even riskier.

Non-Agricultural Sources of Nitrogen

Non-agricultural sources of nitrogen contribute less than 20 percent of the nitrogen released into the environment. Six percent is released from point sources (basically pipes) into water bodies, while fourteen percent is deposited from atmospheric sources.

Point sources, primarily in urban watersheds, can cause significant localized nitrate problems in surface waters or individual wells. They are not responsible, however, for nitrate contamination problems on the scale of those caused by agriculture. A variety of point sources contribute approximately 2.6 billion pounds of nitrogen to the environment, primarily into surface waters, each year (Figure 1). Municipal sewage plants account for 80 percent of point source nitrogen discharges; individual septic tanks and a number of industrial sources account for the rest.

Nitrogen is also deposited in soil and water from the atmosphere, where it enters from an array of sources, primarily as nitrogen oxide emissions from coal or oil burning electric utilities or other industries (53 percent of atmospheric nitrogen emissions) or from automobiles, trucks or buses (38 percent of atmospheric emissions). And nitrogen enters the atmosphere as it volatilizes from manure or fertilizer. Each year, 3.2 million tons of atmospheric nitrogen are redeposited into watersheds in the United States, with the largest inputs in the northern and midwestern regions.

Agriculture Is The Major Source of Widespread Nitrate Contamination

Agriculture is the chief cause of widespread groundwater and surface water contamination with nitrate in the United States (Hallberg 1986a, Bouchard 1992, NAS 1993, Puckett 1994, Keeney 1986, Keeney 1989).

A 1990-91 national water quality summary analyzed nitrate transport in surface waters and land use and found that the highest transport rates occurred in corn and soybean production areas. This study found that the average annual yield of nitrate contamination1 on agricultural land was 0.93 tons per square mile. In contrast, the average yield in urban areas is significantly lower, 0.547 tons per square mile. Forest and rangeland had even lower annual nitrogen contamination yields, 0.26 and 0.03 tons per square mile per year (Smith and Alexander 1993). The impact of this discrepancy between urban and agricultural land as a source of nitrogen is even more dramatic when one considers how much more land is used in crop production than for urban space. Corn alone accounts for 12 times more land area than all urban land in the United States (Department of Commerce 1993). Nationally, agricultural regions contribute approximately 20 times more nitrate contamination to surface waters than does urban land. Similarly, corn and soybean acreage is responsible for 11 times more nitrate contamination than acreage used as rangeland.

In most regions, agriculture is also the major source of groundwater pollution with nitrate. In many areas where groundwater is heavily contaminated, there are few other significant sources of nitrate besides agriculture (Hallberg 1986). Regional scale studies have shown a three to sixty-fold increase in groundwater nitrate concentrations as land uses change from forest, to pasture or grass land, to agriculture (Hallberg 1989) Intensive studies over 40 years in the Corn Belt have shown that increases in groundwater contamination by nitrate correspond closely with increases in nitrogen fertilizer use (Hallberg 1989, Hallberg 1984, Figure 2). The most recent studies by the United States Geological Survey have found that groundwater wells in agricultural regions are much more heavily contaminated than wells in urban, forest, or rangeland regions (Mueller, et al. 1994).

Although a variety of factors, from urban sewage to atmospheric fallout, may be responsible for localized instances of nitrate contamination, on a regional and national scale nitrogen inputs from agricultural activities are the single most important source of ground and surface water problems.

A recent authoritative study of the problem concurred:

"although these [nonagricultural] sources can contribute to nitrate contamination of groundwater in any given area, there is a general consensus that agriculture activities constitute the most important anthropogenic source of nitrate..." (Bouchard, et al 1992)

Solving the nitrate contamination problem will require reducing and refining agricultural use of nitrogen fertilizer, as well as vastly improved management of manure, both as a point source of pollution and when used as a fertilizer in the field. Unfortunately, the most recent data indicate exactly the opposite trend. Instead of reducing their use of nitrogen fertilizers, America's farmers continue to increase nitrogen fertilizer use, thereby increasing production costs, environmental risks and the costs to taxpayers to solve contamination problems.

Fertilizer Use In The United States

Fertilizer Use Has Increased Dramatically Since 1945, Reaching An All Time High In 1994

Widespread, massive application of nitrogen based fertilizers is a product of the post-war era in American farming. Before 1945, little fertilizer was applied to U.S. cropland; over the next 15 years, farmers began to rely upon ever-larger amounts of fertilizer (Commoner 1977). By 1960, farmers were applying approximately 2.5 million tons of nitrogen fertilizer to their crops, and between 1960 and 1981, fertilizer use raced to a then-record high of 11.9 million tons (Figure 3; Berry 1994).

Nitrogen fertilizer, along with high yielding seeds and chemical pesticides, was part of a package of inputs that produced dramatic productivity increases in the 1950s, 60s, and 70s. It is now generally agreed, however, that the continuing increases in fertilizer use provide diminishing yield gains, and present serious risks to the environment and human health (NRC 1989; NRC 1993). Throughout the 1980s and 1990s the rate of increased use slowed, but total fertilizer use continued to rise. In 1994, years after virtually all experts had recognized overuse of fertilizer as a threat to ground and surface waters, nitrogen fertilizer use in the United States reached an all-time high of 12.6 million tons (Berry 1994). In 1995, use dropped slightly again, to 11.7 million tons. This slight drop did little to dent the trend of continual increases in nitrogen fertilizer since 1945 (Terry 1996).

Nitrogen Use Is Heaviest In The Corn Belt

Nine of the top eleven nitrogen using states -- Iowa, Illinois, Nebraska, Minnesota, Kansas, Indiana, North Dakota, Ohio, and Missouri -- are in the Corn Belt, and 50% of the nation's fertilizer use occurs in these nine states (Table 9, Figure 3). Texas, which ranks first in the country in fertilizer use, and California, which ranks ninth, are the only non-Corn belt states among the top ten in fertilizer use Table 9.

Total nitrogen fertilizer use is lower in other regions of the country because less acreage is devoted to field crops. However, even in states outside the Corn Belt, fertilizer use can be extremely high on a local or regional basis. In the fruit and vegetable growing regions of central Florida, California, and the Atlantic Coastal Plain, as well as the corn growing regions of the Chesapeake Bay, nitrogen fertilizers are heavily used and contribute to significant nitrate contamination problems (Figure 4).

Fertilizer Use On Corn

Corn growers use more nitrogen fertilizer than producers of any other crop. Since the mid-1970s, at least 95 percent of the nation's corn acres have received nitrogen fertilizer each year, and in 1994 corn producers applied a total of 7.9 billion pounds of nitrogen to 62.5 million acres of corn, at an average rate of 129 pounds per acre (USDA 1995). This accounted for more than thirty percent of all national nitrogen fertilizer use. Fertilizer application rates were highest in Illinois, Ohio, and Indiana.

Nationwide, nitrogen application rates on corn have remained virtually unchanged since 1989 -- although between 1993 and 1994, application rates increased by six pounds per acre. But in four major corn producing states, Ohio, Indiana, Missouri, and Michigan, nitrogen fertilizer application rates increased between 1989 and 1994.

In three of the top five corn producing states, Illinois, Indiana, and Nebraska, application rates exceed the national average by at least ten pounds per acre (Table 10). The experience of Iowa farmers, meanwhile, provides strong empirical evidence that corn producers in these states can significantly reduce their use of nitrogen fertilizer (Iowa State University 1993; Hallberg, et al. 1991; updated, 1995).

In 1985, nitrogen fertilizer application rates in Iowa were 145 pounds per acre -- on par with those of farmers throughout the Corn Belt. However, between 1985 and 1994, average application rates in Iowa dropped by 16 percent, to 122 pounds of nitrogen fertilizer per acre. Meanwhile, fertilizer application rates for farmers in the remaining Corn Belt2 states remained at the same high rates.

In spite of significant statewide reduction in nitrogen fertilizer use, Iowa's corn yields remained higher than those of farmers throughout the Corn Belt (Figure 5). In an average year between 1989 and 1994, Iowa farmers used sixteen percent less fertilizer than farmers in other Corn Belt states -- and still achieved higher yields. In fact, Iowa farmers obtained record yields in 1992 and 1994, while dramatically decreasing fertilizer use (Hallberg, et al. 1991; updated 1995). As a result, Iowa farmers reduced their costs by 31-39 million dollars per year, and reduced the threat to water supplies considerably (Hallberg et al. 1991; Iowa State University 1993). Unfortunately, farmers in most other states have not followed Iowa's lead. For most farmers throughout the Corn Belt, fertilizer use can be dramatically and easily reduced.

Fertilizer Use On Wheat

Wheat is an increasingly important source of nitrogen contamination in the environment. In 1994, more than three billion pounds of nitrogen fertilizer, thirteen percent of all applications, were applied to wheat. And in 1994, nitrogen fertilizer was applied to 87 percent of the nation's 53 million wheat acres, at an all-time high application rate of 68 pounds per acre (USDA 1995). These represent an increase of fourteen percent in the wheat acreage to which fertilizer was applied, and application rate increases of ten percent in the ten year period between 1984 and 1994.

Much like corn, wheat application rates vary dramatically from state to state. Illinois , Idaho, and Minnesota had the highest fertilizer application rates on wheat, and in six other states -- Texas, Missouri, Ohio, North Dakota, Washington, and Oklahoma -- wheat application rates were higher than the national average (Table 11). In Minnesota, application rates on wheat were nearly identical to application rates on corn.

Fertilizer Use On Cotton

Approximately 1 billion pounds of nitrogen fertilizer are used on cotton in the United States each year, accounting for 4 percent of all annual use. Virtually all of this use occurs in six states: Arizona, California, Arkansas, Louisiana, Mississippi, and Texas. As it has on most other crops, nitrogen use on cotton has increased since 1985. Nationwide, 86 percent of the cotton crop receives nitrogen fertilizer, an increase of ten percent since 1985. The average application rate in 1994 was 110 pounds per acre, up 28 pounds per acre since 1987 -- a thirty-four percent increase in seven years. Again, in many states, cotton application rates are higher than the national average. In Arizona, application rates were 223 pounds per acre; in California, 188; and in Louisiana, 157. Because of its importance as a crop and the high nitrogen application rates, cotton often accounts for a major part of the nitrogen fertilizer used in agriculture areas like the South. For example, in Mississippi, 156 million pounds of nitrogen fertilizer were applied to 1.2 million acres of cotton -- accounting for 40 percent of the state's annual fertilizer use.

Fertilizer Use on Fruits, Vegetables, and other Crops

While the majority of the nation's fertilizer is used on high acreage crops such as corn, wheat, and cotton, there are numerous fruits and vegetables that may account for significant amounts of fertilizer use, particularly on a local basis. For most fruits and vegetables, nitrogen fertilizer application rates are higher -- often significantly higher -- than application rates on corn and wheat. In 1993, 8.8 billion pounds of nitrogen -- 39 percent of total national agricultural use -- were applied to fruits and vegetables and other "non-commodity" field crops (Vroomen and Taylor 1995). In Florida, Arizona, California, and Washington, these crops account for a major part of statewide fertilizer use, and can account for more nitrate contamination than fertilizer use on corn or wheat.

These crops are also major contributors to the recent increase in fertilizer use. Unlike corn, where application rates have either stabilized or increased only slightly over the past ten years, the use of nitrogen on the so-called "minor use" crops has increased dramatically. Between 1983 and 1993, the last year for which data are available, nitrogen applications on crops other than corn, wheat, or soybeans increased by over a third, over one million tons, from 3.2 million tons to 4.4 million tons.

Among field crops, fall potatoes account for a national use of 210 million pounds, with an average application rate of 195 pounds per acre -- a far higher rate than any other major crop. Rice -- primarily grown in Arizona and Louisiana -- accounts for 200 million pounds per year, with an average application rate of 115 pounds per acre. And although only 16 percent of all soybeans receive nitrogen fertilizer, a significant -- and growing -- amount of fertilizer is applied this crop. Since 1985, application rates have increased by 67 percent, from 15 to 25 pounds per acre, for a total of 144 million pounds per year (USDA 1995).

Many fruits and vegetables are also intensely fertilized (Table 12). Tomatoes, at 264 pounds per acre, and lettuce, at 262 pounds of nitrogen fertilizer per acre, are the two most intensively fertilized crops. Five other fruits or vegetables -- celery, bell peppers, cauliflower, lemons, and broccoli -- have nitrogen fertilizer application rates that exceed 200 pounds per acre, and 15 additional fruits and vegetables receive between 100 and 200 pounds of nitrogen fertilizer per acre.

In some states, such as Arizona, Florida, and California, nitrogen fertilizer use on fruits and vegetables is even higher (Table 13). For example, Arizona watermelon growers have the nation's highest fertilizer application rate for any crop -- 414 pounds per acre. Lettuce growers in the state use 357 pounds per acre. In California, nitrogen fertilizer application rates on bell peppers are 320 pounds per acre, and application rates on celery are 317 pounds per acre. In Florida, tomatoes, the state's largest vegetable crop, receive an average nitrogen fertilizer application rate of 311 pounds per acre. In total, there are 45 different crop and state combinations where fertilizer application rates exceed 150 pounds per acre.


1 This refers to the amount of nitrogen running off into surface waters draining the growing area (Smith and Alexander 1993).
2 Illinois, Indiana, Missouri, Ohio, and Nebraska.