Recommendations for spring
When spring arrives, and if you didn’t apply fertilizer in the fall, “follow the strategy you planned if you have soil tests,” Hergert says. “If you don’t have soil tests, get them. If you have a problem getting a good sample in dry soils, use a screw auger. Get samples down to 2 or 3 ft.”
For corn following corn, “N loss really happens in the spring. Until you get close to planting, you really don’t know what you have. If you suspect high levels of residual N, test your soils in the spring,” says Fernandez.
“If you don’t sample, consider using the low end of the Corn Nitrogen Rate Calculator,” Sawyer says. “Use the bottom end of the range if it is dry and you expect average rainfall. You can make an upward adjustment if you receive well above-normal rainfall in the spring.”
Hergert adds, “For irrigated fields, you have a projected yield. For rain-fed corn, shoot for average. Split the N between preplant and a sidedress application. Then, you can manage the nutrients effectively.”
Another tool for evaluating residual N is the Adapt-N online computer model. “Even if you didn’t use the program during 2012, you can sign up and evaluate the season after the fact and determine how much N remained in the soil profile at the end,” says Harold Van Es, professor of soil science at Cornell University. “This tool simulates water and N dynamics in a corn field.”
Van Es led the development team for the model that was available for the Northeast, Iowa, Minnesota, Wisconsin, Illinois, Indiana and Nebraska in 2012. “In the late winter, we’ll activate the 2013 season information. We hope to add all states by the coming growing season.”
In general, for corn following low-yielding soybeans, “we recommend that producers use the same rates as their normal application program,” Sawyer concludes. “Iowa research shows that soybean yield in the prior year does not relate to optimal corn N fertilization rate or the differences between optimal rate for continuous corn and corn following soybean.”