As harvest draws near, farmers are also looking forward to the 2022 crop year. With that comes decisions about nitrogen rates for corn. Where drought-reduced corn yields, there are questions on nitrogen carryover where 2022 corn will follow 2021 corn. I will begin by talking about nitrogen rates where there is a relatively normal 2021 corn harvest, or where 2022 corn will follow soybean and then address potential nitrogen carryover.

An excellent tool to help farmers make nitrogen decisions is the Iowa State University Corn Nitrogen Rate Calculator. The user tells the tool the field location, the price of nitrogen fertilizer and the price of sold corn, and the tool 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 will be within $1 per acre of the most profitable rate. The Iowa calculation is based on hundreds of nitrogen rate response trials done at various locations over many years.

Iowans will note that they can select being in southeast Iowa or being in the rest of the state. Research showed that on the poorly and very poorly drained soils in southeast Iowa, slightly higher nitrogen rates are needed when compared to the rest of Iowa. Those in southeast Iowa with better-drained soils may want to use the rates recommended for the remainder of Iowa. More detail is in the ISU article 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. An application at a rate where the profitable 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.

Cost without a benefit

There is a temptation 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 nitrate to water running down the Mississippi River. Work by John Sawyer at Iowa State University suggests that monitoring total rainfall through the months of April, May, and June (in all but southeast Iowa) will provide guidance for supplemental nitrogen decisions. If the 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 is 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 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 that nitrogen is lacking, a sidedress application can be made. Details can be found in Use of the Late-Spring Soil Nitrate Test in Iowa Corn Production.

If the corn 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 2022 corn crop following a 2021 corn crop. However, remember that if fall and/or spring precipitation is well above normal, or if soils are coarse, carryover nitrate is unlikely.

Estimating carryover nitrogen

There are two methods to estimate carryover nitrogen in the fall. The direct method is to sample the soil profile after harvest and measure the nitrate-N concentration. The second method is to estimate carryover nitrate-N based on 2021 corn grain yield. However, Sawyer and Antonio Mallarino recommend always applying a minimum of 50 units of nitrogen per acre.

Spring soil profile sampling 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 this ISU 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  monitoring spring rainfall amounts to determine if supplemental nitrogen should be applied. Where 2022 corn will follow a drought-reduced 2021 corn crop, residual nitrate may reduce nitrogen application requirements if fall and spring rainfall is normal or below-normal.

Schmitt is an Iowa State University Extension field agronomist.