Recently many people have become concerned over the levels of nitrate in surface water supplies. Concern has been expressed that modern farming practices are contributing to the pollution of our state's and nation's drinking water, and some have gone so far as to suggest radical changes in farming techniques in hopes of solving the problem. An understanding of the relationship between agriculture and nitrate is necessary before reasonable decisions can be made.
Nearly all surface waters show a low level of nitrate throughout the year and this has not been deemed a public health hazard. When nitrate nitrogen concentrations rise above 10 milligrams per liter, however, water suppliers in Ohio are required to issue an alert, and steps are normally taken to minimize the exposure of susceptible individuals to these higher concentrations. Elevated nitrate levels often occur in May and/or June. Concentrations normally peak and decline within a few days. The realistic problem facing the public with regard to surface water is not how to eliminate nitrate, but rather how to minimize the frequency and severity of those peak concentrations which exceed 10 milligrams per liter.
The connection between agricultural activity and stream nitrate loadings cannot be denied. Elevated levels of nitrate have often been noted in streams draining watersheds with high levels of corn production, nitrogen fertilizer application, and tile drainage. Runoff from uncontained livestock operations can also be a contributing factor. The relative importance of cropland and livestock contributions will vary among Ohio streams. In the case of livestock operations, improved waste handling to minimize runoff can help significantly, but the situation regarding cropland is much more complex.
Studies have shown that most of the nitrate lost from crop fields moves through subsurface tile flow rather than in surface water runoff. This indicates that direct loss of applied nitrogen fertilizer in surface runoff is not the major contributor to nitrate levels in streams. While a small portion of applied nitrogen may move directly into tile flow on some soils in some years, most accumulates in the soil over a relatively long period of time and is then flushed into tiles and into streams by spring rainfall.
Nitrate in water is derived from three primary sources: rainfall, decomposition of soil organic matter, and nitrogen amendments (fertilizers, manures, etc.). Ideally, most of the nitrate in soil is removed by the harvested crop. Problems occur when the crop does not remove sufficient nitrate, and the excess is flushed into streams after harvest, or when no growing roots are present to intercept it as it moves toward the tile.
The solution to the excess nitrate problem involves developing cropping systems that remove the nitrate added to, or produced within a given field in a given year. Farmers usually choose a yield goal and then add adequate fertilizer nitrogen to supplement the amount normally released from the organic matter, to achieve the desired yield goal. Actual nitrate release from organic matter cannot be predicted at planting time. Farmers add extra nitrogen based on EXPECTATIONS of release. If more nitrate is released than expected, an excess condition is created, and leaching into tile is likely to occur. If weather or management conditions result in lower than expected crop yields, nitrate removal is reduced and leaching is again possible. The relatively dry summers which Ohio has experienced in recent years have reduced removal by crops, contributing significantly to the nitrate problems seen in the past several springs in many areas.
It is unlikely that nitrate peaks can be eliminated completely, because farmers must make their fertilizer decisions with no knowledge of what type of growing season will develop. There are, however, management strategies which may assist the farmer in reducing this problem. Fertilizing for a realistic yield goal is one method. Yield is determined by the total crop production management system, not nitrogen alone. If the remainder of the production system limits production capacity to 130 bushels per acre, adding additional nitrogen to produce 170 bu is not only detrimental to the environment, but is also expensive. Management programs should be improved, where necessary, to utilize better the fertilizer applied, or fertilizer rates should be reduced to coincide with anticipated yields. In addition, farmers can adopt sidedressing programs in which most of the nitrogen fertilizer is applied after the crop emerges. It is easier to determine potential yield once planting date and plant stand have been established. This approach can aid in developing nitrogen rates for potential yields and should reduce accumulations of excess nitrate.
Fortunately, such management options may reduce the cost of production for farmers, as well as reduce the quantities of nitrate potentially available to enter streams. While the adverse effects caused by nature may not be completely overcome, and excessive nitrate peaks may not be eliminated (completely), the steps which may reduce the severity of the problem should be beneficial to the crop producer as well as the total public.
Prepared by:
Donald J. Eckert
Professor, Natural Resources
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