field with young corn plants
NEED NITROGEN? Using the late-spring soil nitrate test field by field tells you if you need to apply more N as a sidedress application for corn.

Will your corn have enough N?

When plant is 6 to 12 inches, use the late-spring soil nitrate test to check nitrogen.

With the cold, wet weather earlier this spring, planting was delayed in much of Iowa and the Midwest. Farmers have had to hustle to get seed in the ground, and they’re now looking ahead to the next big question: Is there enough nitrogen?

You don’t want your crop to run short of N and limit yield potential. On the other hand, achieving high corn yields doesn’t always mean more nitrogen is needed.

“Although corn yields have risen steadily in the past 50 years, these higher yields don’t necessarily require more nitrogen,” says Jim Friedericks, agronomist and education adviser for AgSource Laboratories, a soil testing and consulting service at Ellsworth in central Iowa.

How do you know how much nitrogen fertilizer is enough? Nitrogen-use efficiency (NUE) is a term used to indicate the ratio between the amount of nitrogen removed from the field by the crop and the amount of nitrogen applied, he explains. Along with improvements in corn yields, the efficiency of nitrogen use by the crop has improved over the last 50 years, which means you don’t need to apply excessive amounts of fertilizer to get the best harvest. It’s all about the 4R’s: right place, right time, right source and right rate.

Don’t overapply N
There are obvious environmental reasons for reducing nitrogen applications, including protection of groundwater and waterways. But there is also economics to consider: Nitrogen is expensive, and overuse cuts into farm profits.

“As market prices for crops continue to be less then optimal, every dollar counts, which is why NUE is important to your crop and to the bottom line,” Friedericks says.

Nitrogen application rates peaked around 1980 and have been steady since then, according to USDA data. Although corn yields have improved during that time, the corn hybrids that are being grown today produce grain that contains more starch and less protein per bushel, so applying more nitrogen isn’t necessary.

Fine-tune application rates
The traditional way to calculate nitrogen fertilizer requirements is to multiply your predicted yield times 1.2 pounds of nitrogen for each bushel of corn. Then deduct from that amount any nitrogen credits from prior soybean or alfalfa crops, or residual nitrogen in the soil. This straightforward approach is still being used by some farmers to estimate the nitrogen requirement. However, Friedericks suggests also using an economic nitrogen rate calculator to review, fine-tune and double-check those recommendations for your fields.

There are a few tools to do this, including the corn nitrogen rate calculator from Iowa State University at This calculator determines the economic return of nitrogen fertilizer using different nitrogen and corn prices to find the most profitable application rates. The calculations are based on recent nitrogen rate research data in the Upper Midwest Corn Belt states, which makes this tool unique to each state.

“Profitability starts with the fertilizer application,” Friedericks says. “And the nitrogen rate calculator will help you evaluate your recommendation.”

Using soil nitrate test
Once corn is up and growing, you can use a late-spring soil test to measure the amount of nitrogen available. By checking the amount of nitrate concentration present in the soil, the test will help predict the amount of N needed for the rest of the growing season.

TIMING: Take soil samples when corn is 6 to 12 inches tall (measuring from ground surface to center of whorl) for the late-spring soil nitrate test.

Soil samples must be taken when corn is 6 to 12 inches tall. Soil cores should be taken at a depth of 1 foot, with one sample containing 15 to 20 cores, from similar field areas that are no more than 10 to 20 acres in size.

“To make the most of a nitrogen application, it must be the right amount, applied at the right time and at the right rate per acre,” Friedericks says. “Splitting the application between preplant and a sidedress application may be one of the ways to get the right rate at the right time. A late-spring soil nitrate test can help you ensure this year’s crops have adequate nitrogen levels for top production and maximum yields.”

The following conditions, he says, make late-spring nitrogen testing a good investment:

 Nitrogen applied last fall was less than the expected crop requirement.

 The long, cool spring delayed nitrogen transformation in the soil, impacting availability of applied nitrogen.

 Split applications (less than 125 pounds applied per acre preplant) mean more is needed to finish crop growth.

 Manure applied since harvest was subject to leaching, and cold soil conditions slowed down nitrogen transformations.

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TAGS: Fertilizer
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