Put N and P in the Crop, Not the Water

Robert Hoeft

Professor and Extension Agronomist in Soil Fertility

Office: N-305 Turner Hall (UIUC)
Phone: (217) 333-4424
E-mail: rhoeft@illinois.edu


Nutrient management is essential for U.S. grain producers to maintain both a competitive advantage in the world marketplace and, at the same time, a quality environment. Over the last 19 years, average annual Illinois corn prices have varied considerably, averaging $2.44 per bushel. Unfortunately, prices in the last three years have been tending down, and, based on the world grain supply, there is little hope that they will improve in the near future. The price paid for ammonia by U.S. farmers has also tended down over the last three years, but there are strong indications that it will make a dramatic upturn in the spring of 2001. The data given in Figure 1 for the crop year 2001 are not forecasts, but they could be reality. If so, nitrogen management will be even more crucial than it has been in the past.

At the same time that economics are becoming tighter, pressure to improve nutrient management because of environmental concerns is being stepped up by regulatory agencies. This past year, the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force reaffirmed the commitment to reduce N loss to the Mississippi River by 30 percent, with some suggesting that most of the gain will come from reduction in fertilizer use. While this is an admirable goal, the relationship between fertilizer sales and the size of the hypoxia zone is not strong.

The EPA, under provisions of the Clean Water Act (section 303d), has required states to submit lists of water bodies that do not meet water quality standards. These lists must be based on TMDLs (total maximum daily load), which is a calculation of the maximum amount of a pollutant that a water body can receive and still meet water quality standards. Illinois listed 738 water bodies with a total of 2,863 impairments. The two most frequently listed impairments were nutrients (634) and siltation (563). In most states, the implication of these activities has not been realized, but it is likely that this will lead to further restrictions on nutrient use.

Do these regulatory activities and tight economics mean that nutrient use will be dramatically curtailed in the future? Not likely, but they do mean that we must use the best management practices that have been identified through research and practical experience.

Nitrogen Best Management Practices

1. Use the proper rate.
Most agronomists recognize that rate of application is one of the most important factors (if not the most important factor) affecting N loss to the environment. Unfortunately, there is no system that will accurately predict the optimum rate for each field each year. Lack of an accurate prediction system is due in large part to climatic variability, which affects the optimum rate for any given year.

Years of low N need are often associated with poor growing conditions in that year or the year(s) before. In the example given in Figure 3, two years (1985 and 1990) had identical yields, but the amount of N needed to attain it was 50 pounds higher in 1985 than in 1990. The 1985 yield followed a good yield in 1984, but the 1990 yield followed two years of low yield. It is surmised that the residual N remaining in the field for the 1990 crop year was adequate to give the same high yield as in 1984, with much less N. In some fields, especially those that have a history of manure application, use of the nitrate test will help predict where lower N might be needed. Unfortunately, it does not always work.

A new test is being developed at the University of Illinois that appears to predict those fields that will not respond to applied N, and we are hoping that it will assist in identifying fields that need less than the recommended rate of application. Until such new tests are developed, economic analysis of the data indicates that use of the current recommendation system will provide a higher rate of return than using the N rate that is needed to maximize yield in the best of growing years. Fortunately, recently collected data indicates that use of the recommended rate will minimize the potential for N loss through tile lines as compared to those fields that have a history of excessive use over time.

2. Take credit for home-grown nitrogen.

Research has shown that corn after corn needs more N than corn after a legume. The suggested "credit" for soybean is 40 lb. N/acre. More recent data suggests that this is a conservative figure and may be closer to 50 lb. N/acre. If manure or sludge has been applied, obtain an accurate measure of the amount of N applied and reduce the rate accordingly.

3. Take credit for "incidental" nitrogen.

Nitrogen is often applied as a part of another fertilizer treatment or as a part of another farming operation. For example, phosphorus is often applied as an ammoniated phosphate (nitrogen-containing material), starter fertilizers almost always contain nitrogen, and many use UAN solutions to apply herbicides. The nitrogen in all of these materials needs to be credited toward the total N need for the crop.

4. Apply nitrogen at the proper time for your soil type.

The closer the time N is applied to the time N is needed by the crop, the lower the potential for N loss. This is not to say that earlier applications should never be used, as there is data to suggest that the difference due to time of application is minimal if done properly, at least as compared to rate of application (Figure 5). Delaying fall applications until soil temperatures are cool enough will reduce the rate of conversion of ammonium to nitrate.

5. Use nitrification inhibitors to reduce the rate of conversion of ammonium to nitrate.

Minnesota results have shown that use of a nitrification inhibitor with fall-applied nitrogen will reduce the amount of nitrogen lost in tile lines.

Phosphorus Best Management Practices

1. Do not maintain soil test P levels above that necessary for optimum crop production.
Soluble phosphorus concentration in runoff is related to soil test level, increasing rapidly with an increase in soil test level above 100 ppm P. Soluble P loss is greater with no-till than conventional till, but total P loss is greater with conventional than no-till.

2. Incorporate or inject manure and fertilizer whenever possible.

Surface-applied manure will result in soluble P losses five to 20 times greater than that observed with injected manure. When applied at equivalent rates, soluble P loss is greater from manure than from fertilizer.


The presence of nitrates and phosphorus in surface waters occurred prior to the influence of man. However, it appears that improper use of fertilizers has enhanced the concentrations of both of these elements in surface waters. Through proper management, farmers can minimize the potential for contamination.

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