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.
| Year and number of locations | Economic Optimum N Rate (lbs N/acre) | Yield at the Economic Optimum N Rate (bushel/acre) | Efficiency at the Economic Optimum N Rate (lbs N/bushel) |
|---|---|---|---|
| 2023 (n=180) | 196 ± 35 | 228 ± 30 | 0.87 ± 0.15 |
| 2024 (n=330) | 224 ± 31 | 242 ± 32 | 0.94 ± 0.16 |
| 2025 (n=420) | 171 ± 40 | 228 ± 27 | 0.76 ± 0.17 |
Across the 34 fields were soil was sampled (a subset of the 420 fields in Table 1):
- 60% had less than 20 lb N/acre
- 35% had 20–70 lb N/acre
- 5% had more than 70 lb N/acre
For reference, 20 lb N/acre is the lowest residual soil nitrogen level used in the N-FACT decision support system. In conventional soil test terms, such as the Late Spring Nitrate Test (LSNT), 20 lb N/acre of nitrate-N in the top 12 inches corresponds to roughly 6 ppm NO₃-N.
Both Iowa Nitrogen Initiative field data and N-FACT simulations show that residual soil nitrogen strongly influences optimum fertilizer N rates. Fields with higher residual soil N typically require lower fertilizer N rates to achieve maximum yield.
What this means for 2026
Given the recent increase in nitrogen prices with already tight margins, we encourage farmers and agronomists to consider measuring residual soil inorganic N to help refine nitrogen rate decisions going into the 2026 crop year. A general protocol for nitrate sampling is available in the publication “Use of the Late-Spring Nitrate Test in Iowa Corn Production.” These measurements can be incorporated into the N-FACT tool to estimate how existing soil nitrogen may affect fertilizer requirements for the upcoming season.
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- Research Scientist II
- Professor
- Integrated Cropping Systems
- Associate Director Research Administration