Look for a new publication at Iowa State University Extension county offices by May 15 that shows updated recommendations for nitrogen applied to corn.
Although the new recommendations are not intended to influence rates of nitrogen applied before crops emerge this spring, they should be used with the late-spring test for soil nitrate this year. The new recommendations in the brochure, Nitrogen Fertilizer Recommendations for Corn in Iowa, are being released at this time to give users maximal time to plan for next year's crops.
The new recommendations replace all earlier guidelines for using the late-spring test, and all previous nitrogen (N) fertilizer recommendations based on corn yield goals and credits for N supplied by legumes or animal manures. Yield goals (or potentials) are no longer used because research has shown no relationship between optimal rates of N fertilization and yields at these rates. The use of legume and/or manure credits has been eliminated, but the effects of these sources of N are addressed by giving separate recommendations for different field categories that reflect previous crops and history of manure applications.
Recommended rates of N application are provided as appropriate ranges for each field category. The user selects a rate within the appropriate range after considering prices of fertilizer, expected prices of grain, soil moisture conditions, and, if available, site-specific information provided by the late-spring soil test and the end-of-season cornstalk test in previous years. Of course, concentrations of soil nitrate are considered when the late-spring test is used to guide fertilization after plants are six inches tall.
The new recommendations encourage all producers to use the late-spring test and the end-of-season cornstalk test to evaluate and improve their N management. These tests should be used to evaluate N management practices even in fields where all fertilizer N is applied before crop emergence. Information provided by the tests in such fields can be used to identify failures in N management. One common failure is consistently applying too much N fertilizer for some field categories. Another common failure is using times or methods of application that often result in large losses of N and yields. The need for using these tests has been intensified by the advent of applicators that vary rates of fertilization as they move across fields. Mounting evidence suggests that the old recommendations are not good enough for profitable use of the new application technologies.
The new recommendations should not cause a substantial change in average rates of N fertilization during the next year. However, they represent a major change in the process used to select rates of fertilization in a given field in a given year. It is expected that the new recommendations will result in gradual improvements in N management that help corn producers increase their profits while reducing environmental problems associated with the use of N fertilizers.
Single copies of the new publication, Nitrogen Fertilizer Recommendations for Corn in Iowa, Pm-1714, are available free of charge.
This article originally appeared on pages 52-53 of the IC-478 (7) -- May 5, 1997 issue.