Higher corn yields don't always mean more nitrogen

Growing corn field, green Nitrogen is one of the primary nutrients required for plant growth.

BONDUEL - With the cold, wet spring weather, planting has been delayed in much of the Midwest. As farmers hustle to get seeds in the ground, we’re looking ahead to the next big question – is there enough nitrogen? 

“Although corn yields have risen steadily in the past 50 years, these higher yields don’t necessarily require more nitrogen,” says Dr. Jim Friedericks, Outreach and Education Advisor for AgSource Laboratories.  

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. Along with the improvements in corn yields, the efficiency of nitrogen use by the crop, NUE, has improved over the last 50 years too, which means you don’t need to apply excessive amounts of fertilizer to get the best harvest. It’s all about the 4Rs – right place, right time, right source and right rate.

There are the obvious environmental reasons for reducing nitrogen applications, including the protection of ground water and waterways, but there’s also the economics of it: nitrogen is expensive and overuse cuts into farm profits.

“As markets continue to be less then optimal, every dollar counts, which is why NUE is important to your crop and to the bottom line,” notes Friedericks.

Nitrogen application rates peaked around 1980 and have been steady since then, according to the USDA. Although corn yields have improved since then, the corn that is being grown contains more starch and less protein per bushel, so more nitrogen is not necessary.

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 of alfalfa crops or residual nitrogen in the soil. This straight forward approach works well to estimate the nitrogen requirement. Dr. Friedericks suggests also using an economic nitrogen rate calculator as a way to review those recommendations. There are a few tools to do this, including the corn nitrogen rate calculator that is available online from Iowa State University at

This calculator determines the economic return of nitrogen fertilizer using different nitrogen and corn prices in order to find the most profitable application rates. The calculations are based on recent nitrogen rate research data in the Upper Midwest Corn Belt States and is unique to each state.

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

Take spring nitrate soil samples when corn is 6 inches tall (measuring from the ground surface to the center of the whorl).

Once the crop is growing, a late spring soil test measures the amount of nitrogen available and helps predict the amount needed for the rest of the growing season by checking the nitrate concentrations present in the soil. Samples must be taken when corn is 6-inches to 12-inches tall. Soil cores should be taken at a depth of one foot with one sample containing 15 to 20 cores, from similar field areas that are no more than 10-acres to 20-acres in size.

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

The following conditions make late spring nitrogen testing a good investment:

  • Nitrogen applied last fall was less than the expected crop requirement.
  • The long, cool spring has delayed nitrogen transformations in the soil impacting availability of applied nitrogen.
  • Split applications (<125 lbs pre-plant) means more is needed to finish crop growth.
  • Manure applied since harvest has been subject to leaching and the same cold soil conditions that slow down nitrogen transformations.

AgSource Laboratories is a leader in agricultural and environmental laboratory analysis and information management services, with facilities in Iowa, Nebraska, Oregon and Wisconsin.