Test Soil Nitrates to Determine Corn’s Nitrogen Needs

If you question whether sufficient nitrogen is available to your customers’ corn crops, perhaps as a result of too much rainfall this spring, now’s the time to figure that out, says Missy Bauer, Farm Journal Associate Field Agronomist. The good news, she adds, is that if crops do need N, you still have time to make rescue treatments.

“The first step you need to take is to review the type and amount of N that’s been applied to the crop so far and the application timing,” Bauer says. “Then, think back to what kind of weather was underway after the application was made, particularly any rainfall, and also consider what the temperature was at the time.” 

Soil temperature and the number of days the soil is water-saturated are two key factors in determining the amount of denitrification that’s occurred. The warmer the soil and the longer it’s saturated, the more denitrification losses typically occur. The opposite tends to be true as well.

According to data from the University of Nebraska-Lincoln, if soils are saturated for five days at 55° to 60° F, N losses will be about 10 percent, but at 75° to 80° F for only three days, losses could be 60 percent. If soils remain wet for two or more days, expect some amount of denitrification to have occurred. See Table 1.

“A plus for us this year is that lot of the big rains we had were in May when the temperatures were cooler, versus two years ago when we got a lot of rain in June and it was warm,” Bauer says.

Second steps. Once you’ve considered what occurred to any early N applications, you probably have a solid idea of whether the soil has lost significant nitrogen. If you opt to pull some nitrate samples, considering how earlier N applications were made can guide your sampling process. For instance:

• In areas where the grower applied N in broadcast applications, pull eight to 10 random cores per sample;
• In fields where N was applied in banded applications, pull four cores across the row evenly spaced, with one hitting the side-dress band.

To gather the samples, pull both 12” and 24” cores, if soil conditions permit. Bauer says the 12” sample will show you the level of nitrogen available in the root zone, while the 24” samples will tell you whether the nitrogen has moved deeper into the soil profile.

“As you pull those 12” samples, just go back into that same hole and pull another 12” sample, and keep them in separate containers for testing purposes,” she says.

As for how many samples you pull, Bauer says to let the field size and its soil variability guide you. “At the very least, do a minimum of two samples, one each on high ground and low ground, or sample where you have varying soil types and management zones,” she says.

Once you turn the samples over to your local lab, you can anticipate getting a quick turnaround on the results. Bauer says most soil labs will run nitrate tests within 24 hours.

Application rates. Once you get the nitrate data back from the lab, take into consideration how much soil supplied N may be available for the rest of the season prior to making any rescue treatments. That depends somewhat on soil mineralization rates, Bauer says.

“Where I’m based in southern Michigan we have sandy, low organic matter soil, and it doesn’t supply as much N to the crop as in central Illinois where they have high organic matter soils,” she says.

If you do decide to make a rescue treatment, consider using a stabilizer to protect that N from volatility, and if Mother Nature is cooperating, make the application ahead of a rain shower.

The other factor to consider, before you apply a rescue application of N, is the field’s stand and ear count. “If you only have two-thirds of a stand in the field, you’re not going to need to apply as much nitrogen,” Bauer explains. “Ultimately, you’re looking for the highest return on investment per acre for the farmer and what’s best for the environment.”