Rainfall this spring and into midsummer wasn't unusual in total amount, but it was unusual in how it arrived in central Iowa. Cumulative rainfall data from Boone, Iowa (April 1–July 20) was in the upper-middle range of the 30-year record at 19.5 inches (range: 6.5” in 2011 to 26.5” in 2008; Figure 1).
What stands out is how the rain arrived: there were long stretches of dry periods punctuated by large precipitation events. Central Iowa received about 10 rain events of more than 0.5 inch per day in 2026 (Figure 1), a middling number compared with the historical range of 3 to 17 events. However, of those 10 events, 2 delivered more than 2 inches in a single day, a disproportionately large share of “heavy hitters” relative to the total event count. That distinction matters a great deal for how nitrogen behaves in the soil.
Where Is the Nitrogen Now?
Using the APSIM cropping systems model, we simulated a continuous corn field near Boone, Iowa (planted May 1, with 225 lbs N ac-1 of UAN applied April 10) to track where nitrogen stands today. As expected, the fertilizer application caused a sharp increase in topsoil inorganic N, reflected in the May 13 soil N profile (Figure 2). Successive rainfall events then pushed nitrogen into deeper layers, and some was lost to denitrification and leaching while crop uptake removed another share from the soil.
By July 20, the model indicates that the 2- to 4-foot soil layer contains more inorganic N than the top 2 feet (Figure 2). This should not be read as bad news. Corn roots have already reached 4 to 5 feet deep and can readily access this subsoil nitrogen. At the same time, the crop has already taken up slightly more than 50% of its total seasonal N requirement, so there is still plenty of N uptake to occur, though it will gradually slow as the crop moves through grain fill.
What If It Had Rained More or Less?
Because precipitation is highly variable even within central Iowa, we ran additional simulations with 5, 10, and 15 inches more or less precipitation. The results from this scenario analysis are summarized in Figure 3. The 2026 season is already running about 5 inches above the 30-year normal for this period. That extra simulated rain led to more N loss, lower topsoil N levels, essentially no effect on crop N uptake, and only a small decrease in maximum rooting depth, which stays deeper than 4 feet. This kind of quantitative framework helps illustrate just how complex the fate of soil N really is, and how many processes act on it at once.
The Take-Home Message
- Soils have sufficient nitrogen for high corn yields.
- Roots are deep and capable of extracting nitrogen from across the soil profile.
- Large amounts of nitrogen currently reside in the subsoil.
Additional heavy rainfall events could move some of this subsoil nitrogen below the rooting zone and increase leaching losses. On the other hand, light to moderate rainfall is generally beneficial because it is absorbed within the soil profile, replenishes soil moisture, and helps transport nitrogen to the roots without causing substantial leaching.
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- Professor
- Integrated Cropping Systems
- Postdoc Research Associate