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How Much Nitrogen Does Corn Get From Fertilizer? Less Than Farmers Think
Illinois Ag Connection - 06/01/2023

Corn growers seeking to increase the amount of nitrogen taken up by their crop can adjust many aspects of fertilizer application, but recent studies from the University of Illinois Urbana-Champaign show those tweaks don't do much to improve uptake efficiency from fertilizer. That's because, the studies show, corn takes up the majority of its nitrogen—about 67% on average—from sources occurring naturally in soil, not from fertilizer.

The evidence for soil as corn's major nitrogen source came repeatedly over the course of four studies, the first published in 2019 and the rest more recently.

In all four studies, researchers in the Department of Natural Resources and Environmental Sciences (NRES) in the College of Agricultural, Consumer and Environmental Sciences (ACES) at U. of I. labeled fertilizers with a naturally occurring isotope of nitrogen, known as 15N, and applied it in the field at different rates, forms, placements, and timings.

After each harvest, the researchers analyzed corn biomass and grain for its nitrogen content, attributing labeled 15N to fertilizer and unlabeled nitrogen to soil sources. In all four studies, which included both poor and fertile soils in Central Illinois, most of the nitrogen in corn at harvest was unlabeled.

"My hope would be that producers would just realize the magnitude of these numbers. They're purchasing this nitrogen and it's not all getting into the crop," said Kelsey Griesheim, who completed the studies as an NRES graduate student and is now an assistant professor at North Dakota State University. "It's important to make them aware of it, so that when they're looking at their bottom line and how much they're spending on nitrogen, they realize the situation."

Griesheim's 2019 study found only 21% of fertilizer nitrogen made it into the grain when applied in the fall as anhydrous ammonia. The result made some sense, as fall-applied fertilizer lingers in the soil for months before corn is planted, and then has to last throughout the season to nourish the growing crop.

Incidentally, the study also found nitrification inhibitors, often applied with anhydrous to slow transformation from ammonia to more-leachable nitrate, didn't help to enhance nitrogen uptake from fertilizers.


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