With cold, wet spring weather, planting has been delayed in much of the Midwest. As farmers hustle to get seeds in the ground, the next big question is: 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 Jim Friedericks, outreach and education adviser 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. This means growers don’t need to apply excessive amounts of fertilizer to get the best yield at harvest. It’s all about the 4Rs — right place, right time, right source and right rate.
There are obvious environmental reasons for reducing nitrogen applications, including the protection of groundwater 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 predicted yield times 1.2 pounds of nitrogen for each bushel of corn, then deduct any nitrogen credits from prior soybean or alfalfa crops or residual nitrogen in the soil. Friedericks suggests also using an economic nitrogen rate calculator as a way to review those recommendations. There are a few tools to help with this, including the corn nitrogen rate calculator available online from Iowa State University at cnrc.agron.iastate.edu.
This calculator determines the economic return of nitrogen fertilizer using different nitrogen and corn prices to find the most profitable application rates. These calculations are based on recent nitrogen rate research data in the Upper Midwest Corn Belt states, and calculations are unique to each state.
“Profitability starts with the fertilizer application,” says Friedericks. “And the nitrogen rate calculator will help you evaluate your recommendation.”
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 to 12 inches tall. Soil cores should be taken at a depth of 1 foot. About 15 to 20 cores should be collected from similar field areas no more than 10 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 preplant and a sidedress 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 pounds preplant) 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.
Source: AgSource Laboratories