By Richard Roth
Nitrogen is a critical input for corn production that directly influences yield potential and profitability.
However, efficient nitrogen management is challenging due to variable weather conditions, soil characteristics and crop-uptake patterns. Fall- and spring-applied nitrogen can potentially be lost through leaching and denitrification, which leaves crops deficient at key growth stages.
To address these challenges, in-season nitrogen applications provide flexibility and enable farmers to adjust rates based on actual crop needs and environmental conditions. By using soil tests, decision-support tools and real-time observations, farmers can improve nitrogen-use efficiency and reduce excess application. Ultimately, this protects their bottom line and the environment.
Understanding nitrogen dynamics
Corn requires nitrogen throughout the growing season, with peak uptake occurring between the V6 and R1 growth stages. However, the amount of nitrogen available to the crop depends on soil type, weather and previous management practices.
Excess rainfall can lead to nitrate leaching, where nitrate-N moves below the root zone with soil drainage. This particularly occurs in tile-drained fields common in Iowa.
Conversely, saturated soils can create anaerobic conditions that promote denitrification, a process where soil bacteria convert nitrate into nitrogen gases lost to the atmosphere. Dry conditions, on the other hand, may slow microbial activity and nitrogen mineralization, which reduces availability.
Iowa’s unpredictable weather patterns make nitrogen management complex. Excess rainfall in spring can lead to nitrate leaching, while drought conditions can reduce nitrogen uptake. Utilizing real-time weather data and adaptive strategies — such as reducing early-season nitrogen rates and applying supplemental nitrogen as needed — helps mitigate these risks.
If fields experience prolonged saturation and warm temperatures, the potential for denitrification increases. This necessitates closer monitoring and possible adjustments to nitrogen management plans. Using rainfall thresholds, such as total rainfall from April through June and the frequency of heavy rain events, can help guide nitrogen decisions and reduce losses.
Relying solely on pre-plant nitrogen applications can lead to inefficiencies. Too much nitrogen early in the season increases the risk of loss, while too little can limit yield potential. In-season applications allow farmers to supplement nitrogen as needed, and ensure that the crop has an adequate supply at critical stages.
In-season N management tools
Some N management tools include:
- Late-spring soil nitrate test. The LSNT measures soil nitrate levels in late spring (around V6) and helps determine if additional nitrogen is needed. This test is particularly useful in fields with manure application or following legume cover crops, where nitrogen availability can be highly variable.
- Modeling and decision support tools. Digital platforms include the Nitrogen Fertilizer Application Consultation Tool developed at Iowa State University. Others, such as Adapt-N and Climate FieldView, integrate soil conditions, crop models and weather forecasts to provide nitrogen recommendations. These tools can help refine application rates, improving efficiency and yield potential.
- Rainfall triggers for nitrogen loss. Spring rainfall patterns can serve as important indicators of potential nitrogen loss. Current guidance recommends monitoring rainfall totals from April 1 to June 30 for most of Iowa and from March 1 to June 30 for southeast Iowa. If total rainfall exceeds 15.5 inches in most of Iowa or 17.8 inches in southeast Iowa during these periods, there may be significant nitrogen losses that require supplemental applications.
These thresholds provide a 76% chance of accurately identifying the need for additional nitrogen. This offers a valuable, though not perfect, decision-making tool. Farmers do not need to wait until the end of June to assess rainfall; ongoing monitoring allows for proactive planning and timely nitrogen adjustments.
Timing and application methods
Some timing and application methods to consider include:
- Sidedress applications (V3-V8). Applying nitrogen between V3 and V8 ensures that the crop has sufficient nitrogen as it enters its rapid growth phase. This method can reduce losses compared with fall or spring pre-plant applications.
- Late-season applications (V12-R1). High-clearance applicators allow nitrogen application closer to silking. While this approach can help correct deficiencies and potentially improve nitrogen-use efficiency, yield response is inconsistent and more variable than earlier applications.
- Split applications. Using a combination of pre-plant (or at planting also known as starter) and in-season applications could reduce the risk of nitrogen loss and allow for adjustments based on weather and crop performance.
Economic and environmental considerations
Applying the right amount of nitrogen at the right time improves profitability by maximizing yield while minimizing input costs. Excess nitrogen application not only wastes money, but also contributes to environmental issues such as nitrate contamination of ground and surface waters. Conservation practices such as cover cropping, conservation tillage and the use of nitrification inhibitors may enhance nitrogen-use efficiency and/or reduce losses.
Final thoughts
In-season nitrogen management could provide Iowa corn growers with an opportunity to improve efficiency and sustainability. By incorporating information from soil tests or decision-support models, farmers can make data-driven decisions that optimize nitrogen applications.
As research and technology continue to advance, these strategies will become even more critical in balancing productivity and environmental stewardship.
Roth is an ISU Extension soil fertility specialist.