Pouring It On

Nitrate Contamination of Drinking Water

February 1, 1996

Pouring It On: Solving the Nitrate Problem

Use nitrogen, but use it wisely. This is the resounding refrain from researchers around the world. Since 1980, hundreds of published, peer reviewed papers, based upon the experiences of thousands of farmers, have shown that farmers can substantially reduce nitrogen fertilizer use, and better manage manure -- without reducing crop yield, and often while increasing profits. So why don't farmers just do it? Because nitrogen fertilizer is relatively low cost crop insurance that, up to a point boosts yields, and because farmers pay none of the costs to remedy the environmental and public health damage that they cause by excess and poorly managed nitrogen fertilizer and manure applications.

The most authoritative review of the issue was completed in 1993 by the Board on Agriculture of the National Research Council of the National Academy of Sciences. The committee concluded that each year in American agriculture there are six to nine million metric tons more nitrogen in farm fields than can be used by the crops growing in those fields. In other words, one fifth to one third of the nitrogen in the environment is unneeded. If farmers converted freely available nitrogen into reductions in fertilizer use, they would reduce contamination of water supplies and save an estimated two to three billion dollars annually1.

In the carefully understated words of the NAS committee,

"The magnitude of estimated positive (nitrogen) balances does help to explain the prevalence of elevated nitrate concentrations in surface water and groundwater in intensive agricultural watersheds" (NRC 1993 pg. 262).

This committee's consensus recommendations for "reducing the mass of residual nitrogen in the soil-crop system..." were:

  • Accounting for all sources of nitrogen in the system,
  • Refining estimates of crop nitrogen requirements,
  • Refining yield goals,
  • Synchronizing the application of nitrogen with crop needs, and
  • Increasing the seasonal nitrogen uptake in the cropping system.

Accounting for All Sources of Nitrogen

On a regional and farm level, farmers are ignoring major sources of nitrogen and overapplying billions of pounds of fertilizers each year. According to the National Academy of Sciences, "the nitrogen applied to corn in synthetic fertilizer exceeded that removed in the grain (corn) by 50 percent or more every year since 1968" (NRC 1993 p. 60). These calculations did not include additional nitrogen available to crops in these field from legumes and manure.

Overall, in all crops, nitrogen application including fertilizer use, manure, and plant residue, exceeds crop needs by approximately 33 percent, or eight billion pounds of excess nitrogen per year (NRC 1993). In the words of the Academy committee, "The single most important way to improve nitrogen management is to reduce supplemental applications of nitrogen to account for nitrogen supplied by legumes and manure" (NRC 1993 p. 67).

Animal manure is the most often overlooked source of nitrogen for crops. Many growers apply manure to their fields, and nationally, economically recoverable nitrogen from animal manure accounts for approximately eight to nine percent of total nitrogen inputs, and even higher in some regions of the country. Surveys indicate, however, that many farmers do not adequately account for the nitrogen in the manure they apply to their fields (Duffy and Thompson 1991). As a result, they then overapply nitrogen fertilizer.

Another substantial source of nitrogen is that made available from nitrogen-fixing crops such as alfalfa or beans. When these crops are grown, they leave available nitrogen in the soil, and when the next crop is planted, farmers do not need to apply as much fertilizer. As with manure, however, most producers do not fully account for this crop-supplied nitrogen. A 1990 study concluded that 56 percent of fields where corn was grown following alfalfa had more than twice as much nitrogen as necessary for economically optimal yields; 86 percent had more than the optimal amount of nitrogen (El Hout and Blackmer 1990).

These, and numerous other analyses show that there is ample room for growers to improve nutrient management by accurately accounting for all nitrogen inputs. In the Corn Belt, alfalfa is grown on approximately 8 percent of cropland and contributes approximately 1.1 million tons of nitrogen annually (NRC 1993). If farm practices accounted for the nitrogen available from alfalfa and manure, fertilizer-N applications in the Corn Belt could be reduced by eight to fourteen percent (Peterson and Russelle 1991). This would reduce nitrogen fertilizer applications by nearly one billion pounds per year, reduce the risk to water supplies, and save the region's farmers approximately $150 million annually in production costs. In some states with greater alfalfa production, fertilizer could be reduced even more than the regional average. For example, by simply accounting for nitrogen from alfalfa, fertilizer applications could be reduced by 20-36 percent in Michigan, and by up to a whopping 66 percent in Wisconsin (Peterson and Russelle 1991). And these conservative estimates do not include inputs from soybeans and all sources of manure.

Nitrogen balances indicate that similar reductions can be achieved easily in other regions of the country. In the southeast, regional scale studies taking precipitation, fertilizer application, and input from legumes into account indicated that 47-75 percent of total nitrogen used was unaccounted for (Lowrance et al 1985, in NRC 1993).

Realistic Yield Goals

Only a minute percentage of farmers apply nitrogen to their crops based on a thorough assessment of availability, timing, needs, and profitability of nitrogen fertilizer in a given field. Reducing nitrogen pollution in the environment will require that farmers develop and implement thorough nitrogen management plans.

Realistic yield goals are an essential component of these plans. Currently a major percentage of crop yield goals in American agriculture are inflated beyond reason by a variety of factors including commodity program subsidies, the unrealistic expectation that the goal each year should be to achieve the absolute peak yield ever achieved in a given field, and the fact that farmers pay none of the cost of pollution caused by excessive fertilizer applications used to attain maximum yields. Yield goals should be rational, and based on the likelihood of maximum profitability. Currently the majority of them are not (NRC 1993).

Proper Timing of Nitrogen Application

Properly timing nitrogen application can substantially reduce nitrogen losses to ground and surface water. For the most efficient use, nitrogen fertilizer must be applied after the seedlings emerge from the soil -- at the time when crops are using the most nitrogen.

Pre-plant or pre-emergence (fall or spring) applications lead to substantial nitrogen losses because of the lag between application and use by the crop (Pereira and Dos Santos 1991; Peterson and Frye 1989). Nonetheless, large statistically stratified surveys conducted by the USDA's Economic Research Service show that many corn growers continue to use vast amounts of nitrogen fertilizer in the fall or spring, in spite of the fact that much of this nitrogen can be washed out of the root zone long before it reaches the crops for which it was intended (USDA 1992). In 1992, 55 percent of over 5,000 corn growers surveyed applied nitrogen fertilizer in the fall or spring. Forty percent apply fertilizer in the spring, eighteen percent in the fall (four percent applied fertilizer in both the fall and the spring).

Not surprisingly, growers who apply fertilizer in the fall or spring also use more fertilizer than those who do not. For growers not applying fertilizer in the fall or spring, the average nitrogen application rate was only 95 pounds per acre. For those applying in either the fall or spring, the average application rate was 134 pounds of nitrogen per acre. And for those applying fertilizer in both the fall and the spring, the average application rate was the highest of all, 149 pounds of nitrogen per acre.

Nitrogen Soil Tests

A vital tool to improve the timing and reduce the amount of nitrogen fertilizer applied are soil tests that can estimate how much nitrogen is available and needed in the soil at any given time. Among the most refined of these tests is the Pre-Sidedress Nitrogen Test (PSNT) for corn, which measures the amount of nitrogen available to plants in the top foot of soil in the late spring, when corn is 6 to 12 inches high. These tests are designed to help farmers sample early enough to apply additional nitrogen if it is needed but late enough to account for spring weather conditions. Armed with this knowledge, producers can fully account for all available nitrogen in their fields, and apply the amount of fertilizer necessary to obtain the optimal yields.

In Iowa, statewide surveys between 1987 and 1991 using the PSNT indicated that in 50% of corn fields, soil nitrate concentrations before the addition of fertilizer were equal to or greater than the critical concentration needed for optimal yields (Frable, et al. 1994). Nationally, farmers who use the soil test have reduced their nitrogen fertilizer application rates on corn by an average of 26 percent (Woodward, et al. 1995)

In Iowa, the nation's top corn producing state, three different studies under actual field conditions support the applicability of the PSNT (Frable, et al. 1994). All three found virtually no difference in yields between tested and non-tested fields, with yield changes that were non-existent or insignificant -- one to two percent at most. Growers using the test, meanwhile, reduced application rates by 36 percent, from 131 to 84 pounds per acre. And because they maintained yields while reducing their nitrogen use, farmers using the PSNT increased profits by $2.00-$16.00 per acre (Frable, et al. 1994). In the largest test, conducted by 70 farmers participating in an Iowa Natural Heritage Resourceful Farming demonstration project, fertilizer use was reduced by 46 percent, with less than a one percent decrease in yields. As a result, the producers who use the test obtained an average increase in profits of $5.75 per acre, with profits increasing for some farmers by as much as $46.00 per acre.

And these nitrogen soil tests are inexpensive. A 1994 USDA survey found that in virtually all cases soil nitrogen tests, which are often performed by state agricultural agencies, cost less than $10.00. In four states -- Maryland, Virginia, Connecticut, and Delaware -- the tests were free. In major midwestern farming states such as Ohio, Iowa, Wisconsin, Minnesota, and Michigan, the test cost less than $10.00. Indiana, where nitrogen soil testing costs $20.00, was the most expensive state for soil testing. Particularly when the savings due to using less nitrogen are taken into account, there are no financial burdens to growers using nitrogen tests before applying fertilizers.

Most Producers Are Not Using Nitrogen Tests

Although the soil tests have been very effective in helping farmers reduce nitrogen application rates and increase profits, use of the test is not widespread. Even in the Chesapeake Bay states of Pennsylvania and Maryland, where the nitrogen test was offered for free as part of Nutrient Management Programs, they are not widely used. In states where adequate data is available, no more than 16% of all corn acreage was tested for nitrogen content. In no state were more than 100,000 corn acres tested using the PSNT (Woodward, et. 1995). In large corn producing states like Iowa and Michigan, less than 25,000 acres were tested using the pre-sidedress nitrogen test. Clearly, increased use of the PSNT would go a long way to help farmers reduce nitrogen applications.

Farmers Can and Have Reduced Nitrogen Fertilizer Application Rates - The Iowa Example

No state is more evocative of the Corn Belt than Iowa, and in no state have farmers done more to try to reduce their overreliance on nitrogen fertilizer. In the mid-1980s, the state's scientists and policymakers began to understand the true scope of their nitrate contamination problem, and the need for solutions. The end result was an aggressive education effort by the State Extension Service to inform farmers how they could reduce their fertilizer use. The prescriptions were not new, but they have rarely been so vigorously pursued. They included the establishment of realistic yield goals, improving the rate and timing of applications, full crediting for manure and nitrogen fixing crops, and the use of soil samples to test for nitrogen content (Hallberg, et al. 1991, Iowa State University 1993).

The plan worked. In 1985, nitrogen fertilizer application rates in Iowa were 145 pounds per acre -- on par with those of farmers throughout the Corn Belt. 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 other Corn Belt states2 increased or remained at the same high rates. Iowa's corn yields remained consistent with those of farmers throughout the Corn Belt (Figure 6). In fact, Iowa farmers obtained record corn yields in 1992 and 1994.

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 (Hallberg 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). Most states have not initiated such an aggressive nitrogen reduction program, although Nebraska and Wisconsin have worked to reduce nitrate contamination of groundwater in certain vulnerable areas.

On most farms throughout the Corn Belt, and on many farms throughout the country, nitrogen is overapplied based on estimates that do not account for all sources of available nitrogen, and failure to time and measure applications for optimal profitability. Until nitrogen fertilizer is used more prudently, through the widespread adoption of well known and accepted nitrogen management practices, nitrate contamination of groundwater and surface water will continue, and the cost to fix the resulting drinking water and other water quality problems will rise.


1. Based on an estimated cost of $0.15 per pound of nitrogen applied as fertilizer (University of Iowa 1993)
2. Illinois, Indiana, Missouri, Ohio, and Nebraska.