By Don Donovan and Amanda Kautz

Corn thrives when it has adequate available nitrogen in the soil, but where does that nitrogen come from?  It’s common to think nitrogen comes from whatever nitrogen product is used, but is that always the case? 

Four studies from 2019 to 2023 by the University of Illinois show that about 67% of nitrogen found in corn-grain and biomass comes from sources naturally occurring in the soil. Researchers labeled nitrogen fertilizers with N15 to identify naturally occurring nitrogen versus synthetic nitrogen. In all cases, no matter application method or timing, most nitrogen found was unlabeled, meaning it came from the soil.

This also means that biological processes are required to make synthetic fertilizers available to corn. If these processes are biologically broken, it means that more nitrogen must be added. Knowing this, the question of the day becomes: How can I get more natural nitrogen from my soil? 

Promote soil health

The key to nutrient cycling is a functioning soil food web. This food web is responsible for decomposing crop residue, cover crop residue and dead microbes, creating a source of plant-available nutrients. To build habitat for your biology, minimize disturbance, and maximize cover, diversity and continuous living roots.

These principles will ensure the soil structure has pore space for biology to inhabit and has access to air and water. They also guarantee a food source for biology over the winter to keep it from going dormant or dying.

Cover crops are key to building these systems. Legumes, such as clovers and peas, fix nitrogen, and any excess not needed by the plant is sequestered in the soil. Grass cover crops, such as cereal rye or annual ryegrass, are scavengers of nitrogen and can capture excess nitrogen applied to the last crop. 

The cover crop must decompose to release nitrogen. This depends heavily on both the functionality of your soil biology and the carbon-to-nitrogen ratio of the material. A high carbon-to-nitrogen ratio means that the material will take longer to break down and release nutrients. This breakdown process also may cause a temporary tie-up of nitrogen. A low carbon-to-nitrogen ratio material will decompose faster, releasing available nitrogen.

However, nitrogen could leave the field through leaching or other means if the crop is not ready to uptake it or too much is released at once. It’s a delicate balance that depends on many factors, and you may have to try several cover crop combinations before you find the one that works best for your field. Manure also could be a source of fertilizer that could boost soil nitrogen and provide support for the soil food web.

Use natural sources

This study doesn’t mean that you shouldn’t be applying synthetic nitrogen fertilizers to corn, but it makes the case for using natural sources to add nitrogen to the soil and for taking soil nitrogen into account when figuring your crop-nutrient budget.

Richard Mulvaney, co-author on all four studies, sums it up well. “If the soil is the main source of nitrogen for crop uptake, which it almost always will be, we need to take the soil into account,” Mulvaney says. “… Otherwise, with factors like timing, rate, placement and form, we’re tweaking, but [we] probably won’t find a miraculous increase in efficiency using those approaches.”

Donovan is a district conservationist with the Natural Resources Conservation Service. Kautz is the state soil health specialist with the NRCS. They write on behalf of the Indiana Conservation Partnership.