Wallaces Farmer

How to know if your corn needs supplemental nitrogen

The Late-Spring Soil Nitrate Test can hold some answers about whether farmers should apply supplemental nitrogen.

Gil Gullickson, editor of Wallaces Farmer

June 10, 2024

5 Min Read
young corn plants
CORN COLOR: Color can be an indicator of whether corn needs more nitrogen, although it’s not foolproof. Pale corn can also indicate water stress. Gil Gullickson

Do you need to add additional nitrogen to your corn crop?

That’s the million-dollar question these days among Iowa farmers, says Richard Roth, Iowa State University Extension nitrogen science specialist. Prolific May rainfall has caused concerns about N deficiencies in Iowa corn.

Roth has spent some time assessing the situation and has written a three-part blog that digs into the situation. The sections include:

Where we are

Potential N losses through leaching and denitrification are where we are in early June.

Under leaching, nitrates migrate below the root zone with soil drainage. Since nitrate is a negatively charged ion, it is highly mobile and follows the flow of water, particularly after the heavy rainfall events much of Iowa endured in May. In tile-drained landscapes, the risk of nitrate leaching is particularly pronounced, Roth says.

Conversely, denitrification is a biochemical reaction, catalyzed by soil bacteria in oxygen-depleted, saturated conditions, that transforms nitrate into nitrogen gas. This primarily occurs on fine-textured, poorly drained soils or areas prone to ponding, where the absence of oxygen prompts soil bacteria to metabolize nitrate to nitrogen gas that is released into the atmosphere. This process typically begins after two to three days of soil saturation, with the rate of denitrification accelerating as soil temperatures exceed 55 degrees F.

What to do?

Corn color can indicate N needs. Still, it’s not foolproof — particularly early in the season. Roth points to a late-May drive he took in Story County in central Iowa.

“I drove past a lot of fields where the corn was yellow, with saturated soils and areas of standing water. At that point, it was tough to tell what was going on in the field. I suspect most of the light-colored corn was from soils being too wet rather than lacking nitrogen.”

At this point, it’s a waiting game. If yellow areas green up following 10 days of dry weather with sunshine and warm temperatures, it indicates that the problems were caused by water stress vs. lack of nitrogen.

“If it stays wet, though, it’s more difficult to use color as an indicator,” Roth says.

Another factor compounding the situation is delayed planting. In late May, a lot of corn in central Iowa had not passed the V2 or V3 stage.

“By mid-June, we may not have much corn past mid-calf height,” Roth points out.

Nitrogen deficiency typically doesn’t show up until we get later into the season, at the V6 to V8 stage or maybe even a little later. Up until that point, soil can supply the N the corn needs, Roth says.

Test it out

One tool that can take the guesswork out of supplemental N applications is the Late-Spring Soil Nitrate Test, Roth says.

According to ISU Extension and Outreach (CROP 3140), the LSNT aims at when corn is 6 to 12 inches high. However, Roth points out that farmers can take samples within the first three weeks of June, even if corn has not reached this height. While the test primarily aims at 6- to 12-inch-high corn, it is calibrated more to the time of year than the corn stage. The LSNT is not calibrated for, and should not be used for, interpretation of samples taken beyond this time frame.

The LSNT measures nitrate-N in the top foot of soil to guide sidedressed N applications. It accounts for residual nitrate, mineralized organic matter and any nitrate converted from fall- or early spring-applied N. Because nitrate can leach below the top foot with excess rainfall, test reliability varies — particularly with this year’s wet spring, Roth says.

To use the LSNT results, first compare them to the state’s calibrated critical level of 25 ppm. If results are above the critical level, no additional N application is suggested. If below — while not a perfect calibration — it can estimate supplemental nitrogen rates by subtracting the test result from the critical level and multiplying by eight. For example, if the test result is 15 ppm, subtract 15 from 25 to get 10 ppm, then multiply by eight to get a suggested application rate of 80 pounds of N per acre.

Given this spring's wet conditions and the effect they may have on soil nitrate levels, consider adjusting the critical value to 20 to 22 ppm, especially if rainfall at your farm has been more than 20% above normal from April 1 to the sampling time, Roth says.

Where to place it

Injecting N — whether in the form of anhydrous ammonia, UAN or urea — is preferable to surface-applied nitrogen.

“Corn can use it [injected N] at a faster rate than N placed on the surface, since it [surface N] has to work its way through the soil down to the corn roots,” Roth says. “The other reason why injecting is preferred over other methods is that unless we get about a half-inch of rain within two to three days of application, there’s a good chance a portion of the nitrogen will be lost through volatilization,” Roth adds.

Late-season N and modern hybrids

“Not all hope is lost, even if we do need more nitrogen,” Roth says.

Research shows split N applications that include June sidedressed applications can result in yields just as good, if not better than, fall N applications and spring preplant applications. Increasing the odds of a late-season N payoff is that today’s hybrids use N deeper into the growing season than they used to.

“In general, if you look at pre-1990s hybrids vs. post-1990s hybrids, the newer-era hybrids take up about 7% more N during the late growth stages and post-tasseling vs. older hybrids,” Roth says.

About the Author(s)

Gil Gullickson

editor of Wallaces Farmer, Farm Progress

Gil Gullickson grew up on a farm that he now owns near Langford, S.D., and graduated with an agronomy degree from South Dakota State University. Earlier in his career, he spent 13 years as a Farm Progress editor, covering Minnesota and the Dakotas.

Gullickson is a widely respected and decorated ag journalist, earning the Agricultural Communicators Network writing award for Writer of the Year three times, and winning Story of the Year four times. He is a past winner of the International Federation of Agricultural Journalists’ Food and Agriculture Organization Award for Food Security. He has served as president of both ACN and the North American Agricultural Journalists.

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