A new Iowa State University publication provides an overview of the late-spring soil nitrate test, including new research on correlation and calibration, specific procedures for using the test, and interpretation of test results. The new publication, Use of the Late-Spring Soil Nitrate Test in Iowa Corn Production (CROP 3140), replaces publication PM 1714.
The basics and interpretation of the test are generally the same as the past. Guidelines are now specific for interpretation in manure-applied fields and corn following alfalfa, and an additional soil test category was added for those interpretations.
In the new publication, figures have been included to display research on correlation and calibration of the test over time. Additional information was also added providing more detail explaining various aspects of the test and soil test reliability and precautions on use. A major change was removal of nitrogen rate guidelines for corn (non-test based) as those are now in the Nitrogen use in Iowa Corn Production (CROP 3073) and the Corn Nitrogen Rate Calculator . The publication focus is now just on the late-spring soil nitrate testing.
The late-spring soil nitrate test sampling time is when corn is six to twelve inches tall (measured from the ground to the center of the whorl), usually late May to early June. With some of the corn planted late this year, and the cool spring periods slowing corn growth, consider collecting soil samples in early June even if the corn is not to the suggested height. This sample timing caveat is described in the time and depth of sampling section of the publication.
There have been some areas of the state with greater than normal rainfall this spring. Therefore, consider using the adjusted soil test critical value of 20-22 ppm instead of 25 ppm if there has been more than 20 percent above normal precipitation since April 1 the growing area. This adjustment is explained in the test result interpretation section.
Other resources for nitrogen rate decisions include:
Nitrogen use in Iowa Corn Production (CROP 3073)