By Amy Cooper and Mitchell Baum et.al
Optimum nitrogen (N) fertilizer rates for corn vary widely from field to field and from year to year. One reason is that the amount of nitrogen left in the soil after harvest is never the same. Measuring residual soil inorganic nitrogen—ammonium (NH₄⁺) and nitrate (NO₃⁻)—can help indicate whether fertilizer needs for the upcoming crop may be higher or lower than usual.
As part of the Iowa Nitrogen Initiative (INI)—a partnership between Iowa State University and Iowa farmers—we measured fall soil inorganic N levels in 34 on-farm N fertilizer trials in 2025. Across these fields, the average residual nitrogen in the top 12 inches (30 cm) of soil was 22 lb N/acre. This is three times higher than levels measured in 2024, but similar to those observed in 2023 (Figure 1).
In addition to higher average levels, we also observed substantial variability among fields. Residual soil N ranged from 5 to 80 lb N/acre in the top foot of soil. This range is roughly twice as large as what we observed in 2024, when residual N ranged from 1 to 37 lb N/acre.
Why were residual N levels higher?
Two factors likely contributed to both the higher residual N levels and the greater variability in 2025. First, much of Iowa experienced record-high nitrogen mineralization during the growing season (see: FACTS). In a typical year, soil moisture limits microbial activity and soil nitrogen mineralization during June and July, but in 2025 warm and wet conditions throughout this period allowed more organic nitrogen to be converted into plant-available forms.
Second, optimum fertilizer N rates in 2025 were approximately 24% lower than in 2024. Lower fertilizer needs combined with high mineralization likely also contributed to the higher levels of residual soil N observed after harvest.
Source : iastate.edu