AMES, Iowa — Questions about effective management of nitrogen fertilizer continue to plague agriculture, even after decades of practical use. For example, when is the best time to fertilize? And how much fertilizer is needed to optimize production and minimize losses to the environment?
There are no simple, one-size-fits-all answers, but the questions are important. Their answers can be the difference between higher yields and profits for farmers and ag retailers, or higher costs and more pollution for little benefit.
Traditional approaches, like applying a fixed amount of fertilizer early in the season, do not account for how crop needs change over time, according to Richard Roth, an assistant professor of agronomy at Iowa State University. Too much fertilizer or poorly timed applications often lead to wasted input costs, along with water pollution and increased greenhouse gas emissions.
As the state’s new Extension nitrogen science specialist, Roth provides leadership to help answer farmers’ questions about growing crops profitably and responsibly.
Can monitoring through the growing season improve N efficiency?
Roth is developing a new guidance tool, a Crop Nitrogen Vigor Index (CNVI), in collaboration with others in the Department of Agronomy, including Associate Professor Mark Licht, Postdoctoral Research Associate Larousse Dorissant, and Research Scientists Aaron Sassman and Fernando Marcos.
Their tool will combine drone-based aerial imagery with a range of data to fine-tune knowledge of when and where fertilizer applications are needed throughout the growing season to improve corn yields.
“Remote sensing tools exist, but they are more predictive. This tool is meant to be more responsive to actual needs during the growing season to help farmers synchronize the supply of nitrogen to the times when the crop has the highest demand,” Roth said.
“The tools we have generally only use limited data and don’t fully capture important relationships between the crop, soil and environment,” he continued. “This CNVI project seeks to improve on that to include more factors, such as additional data about soils.”
The goal is to create maps that separate fields into subfield sections for tailored nitrogen rate applications, guided by Real Time Kinematics systems, he said. “Those using precision equipment will be able to make the best use of this technology. Even so, better knowledge of how crop nitrogen needs change throughout the season will be useful to most of Iowa’s farmers.”
Researchers believe that this tool can have broader benefits as well, which is why the Iowa Nutrient Research Center is funding the two-year project. The INRC grant will provide preliminary data to test the CNVI concept and leverage future funding to field test and refine the tool.
“This type of tool could be really useful for reducing nitrogen loss,” said INRC Director Matt Helmers. “If it were easier to tailor fertilizer application to the needs of growing crops, in some years, farmers could reduce nitrogen rates, allowing them to maintain optimal yields while trimming their costs and cutting N losses to water.”
Do Iowa corn crops need sulfur?
Roth is also working to better understand corn crops’ need for sulfur and the potential relationship between nitrogen and sulfur.
“I get almost as many questions about sulfur as I do about nitrogen,” Roth said.
For a corn crop to thrive, it needs small amounts of the trace element sulfur, which occurs naturally in many soils. For many years, Iowa crops also have received sulfur from coal-fired power plant emissions. However, sulfur can leach from soils over time, and recently, air pollution controls have greatly reduced atmospheric deposition. Higher levels of nitrogen fertilization are also thought to deplete sulfur in soils.
“We may be getting to a place where many fields need supplementation with sulfur,” Roth said. “Fortunately, if sulfur is overapplied and lost, it doesn’t have the same kind of negative impacts on water quality that excess nitrogen can have.”
A new project on sulfur dynamics that Roth is collaborating on with partners in nine states came out of discussions at a Nitrogen Use Efficiency workshop in 2024. His part is managing six field trials around Iowa.
What other practices can improve nitrogen use efficiency?
In 2025, Roth coordinated the annual Nitrogen Use Efficiency workshop, which took place in Ames. It drew 189 attendees – academics, students, retailers and farmers — from 19 different states, where a big topic of conversation was enhanced efficiency fertilizer products that aim to reduce nitrogen loss.
The topic of enhanced fertilizer products poses a whole new set of questions, Roth said. Are these products effective at boosting profits? Will they reduce nitrogen loss to waterways, and if so, by how much and under what conditions?
“There are some studies on enhanced fertilizer products, but there are still a lot of questions that deserve independent research, and there are a number of products out there to investigate,” he said. “I’ve already started work in the lab on this and expect to move into the field this spring. Stay tuned.”
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Contacts
Richard Roth, Department of Agronomy, 515-294-0506, rtroth@iastate.edu
Ann Y. Robinson, Iowa Nutrient Research Center, 515-294-3066, ayr@iastate.edu