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Serving: IA
corn on cornstalks
CALCULATE RATES: An updated online tool helps determine the most profitable nitrogen application rate for corn in Iowa fields.

Analyzing this year’s nitrogen usage

Corn Source: Are you applying too much nitrogen fertilizer, or not enough?

By Josh Michel

With 2018 harvest soon in full swing, planning for 2019 crops and decisions about nitrogen applications for corn won’t be far behind. This growing season certainly had its weather challenges, with northern Iowa being too wet and southern Iowa too dry. The weather and Mother Nature can have a big impact on what is the “right” nitrogen rate.

While fall is a great time for soil sampling for nutrients like phosphorus and potassium; unfortunately, there isn’t a good way to predict how much crop-available nitrogen will be in fields next spring. However, tools and resources are available to assist producers in evaluating their nitrogen management this year and in planning for their growing season next year.

End-of-season cornstalk test
As the 2018 season comes to a close, evaluate your nitrogen management plan. An end-of-season cornstalk nitrate test can help you figure out if you may have overfertilized your corn crop. This test is performed between one and three weeks after black layer formation in corn kernels.

The test measures the nitrate concentration in the lower part of the stalk (an 8-inch segment of stalk taken from 6 to 14 inches above the ground). This year, black layer formation may be earlier than normal.

Corn plants that have more nitrogen than needed to attain maximum yields accumulate nitrate in the lower part of the stalk at the end of the season. Stalk nitrate concentrations consistently greater than 2,000 ppm-N indicate excess nitrogen was available during the growing season. Stalk nitrate concentrations of less than 250 ppm-N may indicate inadequate nitrogen throughout the season. Stalk nitrate concentrations ranging from 250 to 2,000 ppm-N are considered sufficient.

It should be noted that stalk nitrate concentrations can be greatly influenced by both external and internal plant factors. For this reason, it is recommended to monitor fields for multiple years and use the data collected over the years to adjust your N management. For instance, if high levels are found for several years, and drought was not an issue during those years, then you can conclude that nitrogen application rates are too high, and that rate should be lowered.

More information on this test can be found in ISU publication CROP 3154, Use the End-of-Season Corn Stalk Nitrate Test in Iowa Corn Production.

Corn N rate calculator
An excellent tool to help farmers make future decisions on nitrogen application is the revised Corn Nitrogen Rate Calculator. The University of Illinois, University of Minnesota, University of Wisconsin and Iowa State University, as well as several other universities, are partners in sharing data used in this tool.

You need to tell the tool where you are located, the price of nitrogen fertilizer and the price of corn marketed. The tool will respond with the most profitable rate of nitrogen to apply year-in and year-out. It will also provide a range of nitrogen application rates that will be within $1 per acre of the most profitable rate. This calculation is done based on hundreds of nitrogen rate response trials conducted at various locations in each state over many years.

Consider farm’s location in Iowa
A farm’s location can be narrowed to southeast Iowa or the rest of the state. Work at the ISU research and demonstration farms shows that on the poorly and very poorly drained soils in southeast Iowa, slightly higher nitrogen application rates are needed compared to the rest of Iowa. However, the farmers in southeast Iowa with better drained soils may want to use the rates recommended for the remainder of Iowa. More details are in ISU publication Crop 3073, Nitrogen Use in Corn Production.

Commonly, farmers will know the price of nitrogen but not the selling price of corn, so the tool allows you to use multiple prices. Farmers can then see where the profitable application rate ranges mentioned above will overlap. An application at a rate where the ranges overlap will provide profits within $1 per acre of maximum profit from nitrogen fertilizer, regardless of corn price at the end of the season.

Other important factors to consider are weather, field history and experiences in determining a nitrogen rate. Farmers should also remember that if they plan on making fall N applications, soil temperatures should be 50 degrees F or cooler to minimize the amount of leaching and denitrification that occurs in the conversion to nitrate.

Reviewing this year’s nitrogen usage and looking ahead to next year using the Corn Nitrogen Rate Calculator will help enable Iowa producers to make the most profitable use of nitrogen fertilizer.

Michel is the ISU Extension field agronomist for southern and southeast Iowa. Contact him at

TAGS: Fertilizer
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