by Mitchell Baum and Melissa Miller
Of the four R’s of nitrogen management (right place, right product, right rate and right time), the most difficult to pin down each year is the right nitrogen rate.
Every farmer knows that applying nitrogen fertilizer improves corn yield, but applying just enough to maximize profit — the so-called “optimum nitrogen rate” — is highly context dependent. Recent experiments from Iowa State University have found the optimum N rate can change by over 100% across fields within a single year due to differences in crop management, soil types and weather conditions.
Iowa Nitrogen Initiative
The Iowa Nitrogen Initiative is a partnership between Iowa’s farmers and Iowa State University that deploys hundreds of scientifically robust yield responses to nitrogen trials each year on farmer-operated production fields (Figure 1). INI goals include:
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developing a better understanding of the key drivers of variability of the optimum N rate
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providing Iowa’s farmers with the information they need to apply the optimum rate for productivity, profitability and environmental performance
TRIAL LOCATION: This map shows trials placed in Iowa from 2022 to 2025. Shaded areas represent major land resource areas, each associated with differing predominate soil series.
Since 2022, we have partnered with more than 200 Iowa farmers to deploy over 1,400 trials. In 2025, 141 farmers conducted 620 trials across Iowa. Collaborating with Iowa’s farmers has allowed us to build a database over six times larger than its predecessor in a much shorter timeframe.
NITROGEN RATES: This graph shows the distribution of economic optimum N rates in crop year 2025 for Iowa. Values in the top right corner of the figure represent the mean ± standard deviation.
With 63% of the 2025 trials analyzed, the mean economic optimum N rate — the rate that maximizes return on investment — for last year’s corn crop was 172 pounds of N per acre, with individual trial results ranging from 36 up to 390 pounds of N per acre (Figure 2).
Compared with previous years, the mean optimum N rate is down 21% from 2024 and 12% from 2023. Additional trial data may change those numbers, but we expect the trend to remain the same.
Lower nitrogen rates were likely the result of high mineralization rates during July, when corn demand for nitrogen is high. Summer mineralization rates are commonly limited to moisture in the topsoil, but in 2025, these rates got a boost from the excess July precipitation.
This mineralization supplied the plant with more N from the soil than usual. As a result, less N fertilizer was needed to maximize returns despite also having unusually high environmental losses.
West-to-east pattern
In all years, we’ve seen a pattern of optimum N rates increasing from west to east across the state. In 2025, the average economic optimum N rate was 173 pounds per acre in western Iowa and 206 pounds per acre in eastern Iowa. This gradient mirrors the precipitation pattern across the state; the wetter it is, the higher the optimum N rate.
As we continue to add hundreds of trials to the INI database each year, we will improve our ability to benchmark and forecast the optimum N rate for different farming systems under changing conditions. This information is available on the Nitrogen Fertilizer Application Consultation Tool, a free decision support website with two resources: Explore On-Farm Nitrogen Rate Trials and Discover Your Optimum N Rate.
The Explore On-Farm Nitrogen Rate Trials tool allows users to view anonymized data from the hundreds of trials in the INI database. Data can be sorted by location and crop rotation, with other filters (product, timing, precipitation) to be added soon.
The Discover Your Optimum N Rate tool is an interactive simulation tool that visualizes the impacts of different environmental and management factors to the optimum N rate. Users select location, crop rotation, precipitation, residual soil nitrate, planting date, and nitrogen and corn prices for modeled estimates of optimum N rates. From the results page, users can play around with combinations to see the optimum rate and potential yield change or compare different scenarios side by side.
Visit n-fact.ag/start to use both tools.
Baum is an ISU technical project specialist. Miller is project director for the Iowa Nitrogen Initiative.
