Some growers apply 50% to 70% of estimated N need early in the season, and then use a chlorophyll meter to compare the crop with overfertilized reference strips. The process can indicate the appropriate rate for a sidedress application to bring the crop over the finish line without wasting N.

Writing about the process in an Extension bulletin, Sylvia Brouder of Purdue University and Dave Mengel of Kansas State University noted, “It is important to remember that this particular tool is best used to manage the last 40 pounds of N, not the first.”

In fields where the primary source of N was manure or incorporated sod, a pre-sidedress nitrate test can provide insight on how much nitrate is available in soil reserves. The test is not accurate if N fertilizer had been broadcast, however. Nitrate from the application will throw off the readings.

For commercially fertilized fields, Adapt-N would be a better choice. An online computer model developed by Harold van Es and fellow Cornell University agronomists, Adapt-N analyzes local temperatures and precipitation, soil type, organic matter, slope, cropping history, and yield potential to recommend highly accurate sidedress rates in real time. The idea is to reduce preplant applications with the confidence that you can go back and provide what the crop needs later.

“You can also replay the growing season and see whether you did the right thing,” adds Jerry Hatfield, director of USDA’s National Laboratory for Agriculture and the Environment in Ames, Iowa.

Another way to review the season and learn for the future is with an end-of-season cornstalk nitrate test. The lower portion of the corn plant is like a reserve fuel tank in a truck — the plant will draw on it if the usual sources are deficient and leave it full if the roots had enough “fuel” to work with. Testing the nitrate levels of stalks when the corn is mature provides an idea of how N levels matched crop needs.

The test doesn’t allow corrective action, but it provides insight into the crop’s N needs — information that is increasingly vital as economic and environmental pressures on N use increase.