If you feel behind this fall, you are not alone. Just under half of Iowa's corn is harvested, 4-inch soils are still in the upper 50s, and a very strong El Niño is likely to keep them warm well into November.

For fall anhydrous ammonia, the message is simple: it's not time yet. The recommendations haven't changed. What has changed is when the window is likely to open, and how much patience it will take to wait for it.

Where we stand: harvest is behind, soils are warm

Harvest picked up last week, with 6.4 days suitable for fieldwork, but it is still well behind (Figure 1). As of Oct. 25, Iowa corn harvest trailed the five-year average by about 20 points (USDA NASS, via Iowa Capital Dispatch).

Current and historical crop harvest progress as of October 25, 2026

Figure 1. Current and historical crop harvest progress.

While crop progress is behind, especially compared to the last 5 years, the current date of 47% of corn acres harvested is not unprecedented (Figure 1). Years of late harvest have happened before. One difference, however, is that soil temperatures this year are exceptionally warm. Since 2009, only 2023 had soil temperatures somewhere in the state in the 60s (degrees Fahrenheit) on October 26th. The winter of 2023-24 was the last time we had an El Niño. Keep in mind, soil temperature variability is expected at this time of year. 

With that much crop still in the field, there will be pressure to put anhydrous on right behind the combine. Soils are not ready for it. As of October 27, 2026, 4-inch soil temperatures across Iowa were mostly 51 to 63°F (Figure 2).

Daily average 4-inch soil temperature by county, October 27, 2026. Source: Iowa Environmental Mesonet.

Figure 2. Daily average 4-inch soil temperature by county, October 27, 2026. Source: Iowa Environmental Mesonet.

You can check current conditions on the Iowa Environmental Mesonet's county 4-inch soil temperature maps. The page shows the past three days of observations and a 15-day soil temperature forecast.

What a typical fall looks like

In a typical year, 4-inch soils are above 50°F for the last time around Nov. 6 in northern Iowa, Nov. 8 to 10 in central Iowa, and Nov. 12 to 14 in southern Iowa (Figure 3A).

To get those dates, we analyzed 13 years (2013–2025) of daily 4-inch soil temperatures from 24 ISU Soil Moisture Network stations. For each station and year, we found the last fall day with an average soil temperature above 50°F. After that day, soils stayed at or below 50°F through the end of the year.

Map depicting the last fall day with daily-average 4-inch soil temperature above 50°F

Figure 3. Last fall day with daily-average 4-inch soil temperature above 50°F. (A) Typical date by location, adjusted for year. Dashed lines divide the north, central and south regions used in B and C; thin lines are county boundaries. (B) Share of years in which that day had passed, by date and region. (C) Range across stations each fall, with regional medians.

Three patterns stand out:

  • South is later than north. The date moves about 3 days later for every degree of latitude, roughly 70 miles, you go south. East–west location made little difference (Figure 3A).
  • Late warm spells happen. In central Iowa, half of years had their last day above 50°F by Nov. 10, and 90% by Nov. 19. A few stations saw a day above 50°F as late as December (Figure 3B).
  • The year matters more than location. Year-to-year weather explained about 65% of the variation in the date; location explained about 5%. Statewide, the median date ranged from Oct. 21 (2019) to Nov. 20 (2020 and 2024) (Figure 3C).

Why this fall will likely run late

Expect this year's dates to land a week or more later than the typical dates in Figure 3.

The current El Niño is one of the strongest on record. NOAA's Climate Prediction Center reported Pacific sea surface temperatures (Niño-3.4) 2.1°C above normal in September. It gives an 83% chance that October–December reaches a strength not seen since records began in 1950 (CPC, Oct. 8).

Strong El Niño events tend to bring warm falls and late winters to Iowa. During the 2023 event, Iowa's September–November temperatures ran 2.7°F above normal, and December ran 9.7°F above normal (ISU ICM).

Our soil records show the same thing. The two El Niño falls in the dataset were among the latest:

  • 2015 (the last very strong El Niño): statewide median last day above 50°F was Nov. 18.
  • 2023 (a strong El Niño): Nov. 16.
  • All 13 years: about Nov. 10.

Wet soils add to the delay. State climatologist Justin Glisan has noted that wet soil holds heat, so soil temperatures will "hang on longer" this fall (KCRG). Colleagues have estimated the 50°F date could arrive 10 to 15 days later than usual. For central Iowa, that is closer to Thanksgiving week than to Nov. 10.

One cold day is not the signal

The first day soils dip below 50°F is almost never the last day they are above it. Soil temperatures swing a lot in late October and November as cold fronts come and go.

In our 13-year record:

  • After the first day below 50°F, soils warmed back above 50°F in 94% of station-years. The median rebound came 19 days later.
  • Even after five straight days below 50°F, soils warmed back above 50°F in two of every three station-years. Four in ten rebounded two weeks or more later.

Fall 2023 at the ISU research farm near Ames shows how this plays out. The 4-inch soil temperature fell from 64°F on Oct. 26 to 44°F on Oct. 28 and stayed below 50°F for nine days. It then climbed back above 50°F on four days between Nov. 6 and Nov. 16, reaching 55°F on Nov. 16.

That is why the guideline is "50 and falling," not just "50." Wait until soils are at or below 50°F and the forecast keeps them there. Use the 15-day soil temperature forecast on the Mesonet page alongside the 6- to 10-day air temperature outlook.

Warm days matter because nitrification doesn't stop at 50°F. At 50°F, ammonium still converts to nitrate at about a third of its maximum rate (KCRG). Every warm spell after application adds nitrate that can be lost before next year's crop needs it.

The recommendations haven't changed

A late window doesn't call for new practices. It makes the usual ones matter more.

  • Wait for 50 and falling. Apply fall anhydrous only after 4-inch soils are at or below 50°F with a cooling forecast. Watch the soil, not the calendar.
  • Use anhydrous ammonia only. Don't fall-apply urea, UAN or other nitrate-containing sources for next year's corn.
  • Add a nitrification inhibitor. An inhibitor slows the conversion of ammonium to nitrate. It buys time in a warm fall but doesn't make an early application safe. More than two-thirds of Iowa farmers already use one with fall anhydrous (ISU ICM).
  • Keep rates in check. Use the Iowa Nitrogen Initiative's N-FACT tool to set rates. Consider applying part of the N in the fall and the rest in spring or sidedress.
  • Plant cover crops. A cover crop takes up nitrate that would otherwise leave the field over winter and early spring.
  • Consider waiting until spring. On fields with the highest loss risk, or if soils never cool enough this fall, spring or in-season N is a sound choice.

The same patience applies to fall manure. Early fall manure applications can cost more than 40 bushels of corn per acre compared with late-fall or spring applications (ISU ICM).

Bottom line

It's not time yet. Harvest is behind, soils are warm, and this El Niño will likely push the 50°F date at least a week past normal. Finish harvest, watch the 4-inch soil temperature maps, and apply fall anhydrous only once soils are 50 and falling and staying there.