By Andi Anderson
According to, Amy Cooper, Mitchell Baum, Sotirios Archontoulis, Mike Castellano, and Melissa Miller, Nitrogen fertilizer needs for corn change every year and vary widely across fields.
One major reason for this variation is the amount of nitrogen that remains in the soil after harvest. Understanding residual soil nitrogen can help farmers make better fertilizer decisions for the next growing season.
As part of the Iowa Nitrogen Initiative, researchers measured soil inorganic nitrogen levels after the 2025 crop season.
They analyzed 34 on-farm nitrogen trials across Iowa. Results showed an average of 22 pounds of nitrogen per acre in the top 12 inches of soil. This level was much higher than in 2024 and similar to values recorded in 2023. However, nitrogen levels varied greatly between fields, ranging from 5 to 80 pounds per acre.
Two main reasons explain these higher levels. First, Iowa experienced warm and wet conditions during the summer of 2025, which increased nitrogen mineralization. This process converts organic nitrogen in soil into forms plants can use.
Second, farmers applied lower fertilizer rates in 2025 compared to 2024, meaning more nitrogen remained unused after harvest.
Data from the Iowa Nitrogen Initiative also showed that fields with higher leftover nitrogen generally needed less fertilizer to reach maximum yield. This relationship highlights the value of measuring soil nitrogen before deciding fertilizer rates.
Looking ahead to 2026, rising nitrogen prices and tight farm margins make efficient nutrient use especially important. Researchers encourage farmers and agronomists to test residual soil nitrogen and use decision-support tools such as N-FACT. Soil testing methods, including late spring nitrate testing, can help adjust fertilizer plans to match existing soil nutrient levels.
Careful planning based on soil data can help farmers reduce costs while maintaining strong corn yields in the coming season.
Photo Credit: gettyimages-zoran-zeremski
Categories: Iowa, Sustainable Agriculture