A new ISU Extension and Outreach publication, Use of the Late-Spring Soil Nitrate Test in Iowa Corn Production (CROP 3140), has replaced the previous publication (PM 1714). The publication is available from the ISU Extension Store.
The publication provides an overview of the test, research on correlation and calibration, specific procedures for using the test, and interpretation of test results. 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.
Figures have been included to display research on correlation and calibration of the test over time. Also, additional information was 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.
Considerations for use in the spring of 2017
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). That timing is 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.
Resources for nitrogen rate decisions
Corn Nitrogen Rate Calculator
ISU Extension and Outreach Soil Fertility Web Site
Nitrogen use in Iowa Corn Production (CROP 3073)
Concepts and Rationale for Regional Nitrogen Rate Guidelines for Corn (PM 2015)
Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on May 25, 2017. The information contained within may not be the most current and accurate depending on when it is accessed.