How close are farmer-applied nitrogen fertilizer rates to the economic optimum?

Determining how much nitrogen fertilizer (N) to apply is a constant challenge facing farmers. The amount of N needed to maintain yield and healthy soils is highly dependent on weather and agronomic management, and therefore differs across years and fields (Table 1). 

Here we used information from 755 on-farm Iowa Nitrogen Initiative trials conducted across three years (2023-2025) to compare economic optimum nitrogen rates (EONR) with farmer-applied nitrogen fertilizer rates. 

Across the 755 trials, we found that roughly 1/3 of fields were under-fertilized (Figure 1). However, this was highly dependent on year due to differences in weather: in 2023 and 2024 roughly 41% and 47% of fields didn’t receive enough N to satisfy crop demand, and the average difference between the EONR and farmer-applied rates was +20 and +2 lbs nitrogen per acre (Figure 1). By contrast, we found that 83% of trials received more than enough nitrogen to satisfy crop demand in 2025, with the average difference being +39 lbs nitrogen per acre. 

This pattern wasn’t due to a large increase in fertilizer applied – actually the opposite – the average farmer-applied rate in 2025 was the lowest of the three years, but so was the EONR (Table 1). In 2025, the EONR was 171 lbs of nitrogen per acre, which was 55 lbs nitrogen per acre lower than the EONR in 2024. The exceptionally low EONR in 2025 was attributable to a near-record amount of soil nitrogen mineralization starting in late June and continuing through July, which reduced fertilizer needs. If farmers could have forecasted this lower need, they certainly would have applied less.

Overall, our results do not show any consistent evidence of over-fertilization in Iowa corn fields. With three years of data from the Iowa Nitrogen Initiative, we don’t see a consistent pattern – we see variability, which is expected. 

The Iowa Nitrogen Initiative is working with farmers, ag retailers, and agronomists to reduce this variability and increase fertilizer use efficiency through decision support tools such as N-FACT.ag. As we continue to partner with Iowa farmers to monitor and forecast optimum nitrogen fertilizer rates, we’re identifying management practices that increase fertilizer use efficiency and reduce the uncertainty in optimum nitrogen fertilizer rates, helping to maximize the amount of nitrogen used by the crop while minimizing environmental losses and maintaining healthy soils. 

Table 1. Average ± 1 standard deviation of economic optimum N rate (EONR), farmer-applied N rate, and the difference between the two rates per year.

Figure 1. Comparison of estimated economic optimum N rates vs the farmer-applied N rate across 755 INI trials in 2023, 2024, and 2025. Values to the left of the 1:1 line indicate economic optimum N rate is higher than the farmer-applied rates while values to the right of the line indicate the opposite.