How much nitrogen is left in your cornfield? That’s a popular question farmers are asking. “We’ve had a lot of rain in areas of Iowa this spring and even standing water in some fields,” notes Mark Johnson, Iowa State University Extension field agronomist in central Iowa.

Winter was fairly warm, and some nitrogen converted from ammonium to nitrate in the soil. The ammonium form is more stable, while the nitrate form leaches away easily. Nitrogen in the nitrate form can move down through the soil profile and leach away with water drainage. Or the nitrogen can sit in a pothole in a field and denitrify, volatilize and be lost to the air.

Should you apply some nitrogen fertilizer as a sidedress application in June, so the corn doesn’t run short of N later in the growing season when it really needs it? To answer such questions, ISU’s Johnson suggests you pull some soil samples, as outlined in publication CROP 3140; take the samples to a soil testing lab; and have them tested using the late-spring soil nitrate test (LSNT).

So how much N is left in the soil now?
Every calibrated soil test has a critical test value, above which there is little expectation of response to applied nutrients. For the LSNT that value is 25 parts per million of nitrate-N. This means that if the test result is more than the critical value, there is adequate plant-available N in the soil system and no fertilizer nitrogen application is needed. If the test result is below the critical value, then N application is needed.

For the LSNT, as there can be a variation in soil nitrate with recent high rainfall amounts, the critical value can be reduced by 3 to 5 ppm if rainfall is more than 20% above normal between April 1 and the time of soil sampling (critical concentration would be adjusted downward to 20 to 22 ppm). Reasons for this downward adjustment are to account for nitrate that may move below the soil sample depth shortly before sampling but is still in the crop rooting zone. Another reason to adjust the 25 ppm downward to 20 to 22 ppm is because of weather effects on organic N volatilization.

Making a nitrogen application rate decision
There are two steps in interpreting of LSNT results and making N rate decisions. First step is to evaluate the soil test results relative to critical level. If test results are above the critical level, no sidedress N application is suggested. If below the critical level, the second step is to estimate the fertilizer application rate by subtracting the test result from the critical level and multiplying by 8. This gives the fertilizer rate in pounds of N per acre.

For more information and instructions on using the test, you need to read the new ISU publication on this topic.

A new ISU Extension 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. ISU agronomists John Sawyer and Antonio Mallarino have updated the publication. Sawyer and Mallarino are experts in soil fertility and offer the following explanation and observations.

Provides update on how to use N test
The revised publication of CROP 3140 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 focus of CROP 3140 is now just on late-spring soil nitrate testing.

Considerations for use in spring 2017
The late-spring soil nitrate test sampling time is when corn is 6 to 12 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 Iowa this spring with greater-than-normal rainfall. Thus, you should consider using the adjusted soil test critical value of 20 to 22 ppm instead of 25 ppm if there has been more than 20% above normal precipitation since April 1 in the growing area. This adjustment is explained in the test result interpretation section of CROP 3140.

More online resources are available to help farmers decide the correct nitrogen application rate for their particular fields and situations:
ISU Extension and Outreach Soil Fertility
Concepts and Rationale for Regional Nitrogen Rate Guidelines for Corn