Wallaces Farmer

Tips on adjusting nitrogen rates for 2021

Corn Source: Unusual conditions in 2020 raise questions about how much nitrogen to apply for next year.

Virgil Schmitt

September 18, 2020

4 Min Read
Cornfield with reduced yields
CARRYOVER N: Fields with reduced yields will likely have more nitrogen carried over in the soil for the next crop. Rod Swoboda

As harvest 2020 gets underway, with downed corn and disappointing yields in many fields, farmers are looking forward to the 2021 crop year. With that come decisions about nitrogen rates for corn.

For those with a limited corn harvest due to drought, there are questions on nitrogen carryover where 2021 corn will follow 2020 corn. I’ll begin by talking about nitrogen rates where there is a relatively normal 2020 corn harvest or where 2021 corn will follow soybeans. This column also addresses potential nitrogen carryover.

An excellent tool to help farmers make nitrogen decisions is the Corn Nitrogen Rate Calculator. You tell the tool the field location, price of nitrogen fertilizer and price of sold corn, and it responds with the most profitable rate of nitrogen to apply year in and year out.

It will also provide a range of nitrogen rates that are within $1 per acre of the most profitable rate. Several universities, including Iowa State University, are partners in sharing data used in this tool. The calculation is based on hundreds of nitrogen rate response trials at various locations in each state over many years.

Location makes a difference

Iowans will note they can select being in southeast Iowa or anywhere in state. Work at the ISU research and demonstration farms showed that on the poorly and very poorly drained soils in southeast Iowa, slightly higher nitrogen rates are needed, compared to the rest of Iowa, and that fact is reflected in the recommendations provided by the tool.

Farmers in southeast Iowa with better drained soils may want to use the rates recommended for the remainder of Iowa. More detail is in Nitrogen Use in Corn Production.

Commonly, farmers will know the price of nitrogen but not the selling price of corn, so there is the ability to use multiple prices. Farmers can then see where the profitable rate ranges 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.

It’s tempting to apply extra nitrogen “as insurance” in case the early growing season is especially rainy and nitrogen is lost. In most years, that extra nitrogen is a cost without a benefit and serves only to add nitrates to water running down the Mississippi River. Work done by ISU’s John Sawyer suggests monitoring total rainfall through the months of April, May and June (in all but southeast Iowa) will provide excellent insight into the need, or lack thereof, for supplemental nitrogen.

Account for field conditions

Sawyer’s work suggests if total rainfall for that three-month period exceeds 15.5 inches, farmers should consider adding about 50 units of supplemental nitrogen fertilizer in very early July. And if rainfall amounts are less than 15.5 inches, sufficient nitrogen should remain in the soil. Following that guideline will be the correct course of action nearly 80% of the time.

The remaining 20% of the time is divided almost evenly between putting on nitrogen when it’s really not needed and not putting on nitrogen when it would have been beneficial. For southeast Iowa, the “trigger” is 17.8 inches of rainfall during the four months of March, April, May and June.

Farmers who are concerned about nitrogen losses due to rainfall after nitrogen (either commercial or manure) application may want to use the Late Spring Soil Nitrate Test to verify either the presence or absence of adequate nitrogen for the crop. The LSNT is taken when plants are 6 to 12 inches high at the whorl. If the test shows nitrogen is lacking, a sidedress application can be made. Details can be found in at Use of the Late-Spring Soil Nitrate Test in Iowa Corn Production.

If the corn plant vegetation or grain yield was drastically affected by drought conditions, then nitrogen uptake was reduced, and unused nitrate-N could be accounted for in determining the nitrogen fertilization rate for the 2021 corn crop following a 2020 corn crop. However, remember if fall or spring precipitation is well above normal, or if soils are coarse, carryover nitrate is unlikely.

Estimating carryover N

There are two methods to estimate carryover nitrogen in the fall. The direct method is to sample the soil profile (a minimum of 2 feet) after harvest and measure the nitrate-N concentration. The second method is to estimate carryover nitrate-N based on 2020 corn grain yield. However, ISU’s Sawyer and Antonio Mallarino recommend always applying a minimum of 50 units of nitrogen per acre.

Spring soil profile sampling, to a minimum of 2 feet, for nitrate-N is an option that eliminates some of the loss concerns after the fall and winter. Details on these approaches can be found in an article by Sawyer and Mallarino.

In the long run, the most profitable use of nitrogen fertilizer can be achieved by using the Corn Nitrogen Rate Calculator and then using the Late Spring Soil Nitrate Test and/or monitoring spring rainfall amounts to determine if supplemental nitrogen should be applied. Where 2021 corn will follow a drought-reduced 2020 corn crop, residual nitrate may reduce nitrogen application requirements if fall and spring rainfall is normal or below normal.

Schmitt is the ISU Extension field agronomist covering parts of southeast and east-central Iowa. Contact him at [email protected].



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About the Author(s)

Virgil Schmitt

Virgil Schmitt is the Iowa State University Extension field agronomist for east-central and southeast Iowa.
Serving the Iowa counties of Cedar, Clinton, Delaware, Des Moines, Dubuque, Henry, Jackson, Lee, Louisa, Muscatine, and Scott. His areas of expertise include agronomy, Integrated Pest Management and crop management. He's worked as an Extension 4-H leader and has taught high school and college-level ag education.

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