Is it cold enough for fall anhydrous?
Sigurd Lindquist, CCA, Chetek, Wisconsin
Extension says to wait on fall anhydrous until soil at 4 inches stays below 50°F. Pick your county or type a ZIP. You get the measured soil temperature at the nearest station, its last 7 days and how far away it is. No station within 50 miles, no number.
Station readings through Oct 16, 2026. 133 stations in 12 states. A station counts as “under 50°F all week” when every one of its last 7 daily readings was under 50°F.
| State | Stations | Under 50°F all week | 50°F+ now | Now |
|---|---|---|---|---|
| Illinois | 2 | 0 | 2 | 60–67°F |
| Indiana | 1 | 0 | 1 | 68°F |
| Iowa | 3 | 0 | 3 | 57–63°F |
| Kansas | 86 | 6 | 80 | 18–71°F |
| Michigan | 2 | 0 | 2 | 52–53°F |
| Minnesota | 4 | 0 | 3 | 49–54°F |
| Missouri | 17 | 0 | 17 | 65–77°F |
| Nebraska | 5 | 0 | 5 | 51–59°F |
| North Dakota | 4 | 0 | 1 | 47–50°F |
| Ohio | 2 | 0 | 2 | 63–66°F |
| South Dakota | 4 | 0 | 4 | 51–56°F |
| Wisconsin | 3 | 0 | 3 | 51–59°F |
What extension says, state by state
- IowaIowa State: wait until soil at 4 in is below 50°F and trending cooler. North Iowa usually gets there in early November; central and south Iowa in the second and third weeks of November. Iowa State Extension
- IllinoisIllinois Agronomy Handbook: apply fall N once the daily high of bare soil at 4 in is below 50°F. No fall N for corn south of a line near Illinois Route 16, and none on sandy, organic or very poorly drained soils. This page shows the daily mean, which runs cooler than the daily high, so a mean under 50°F does not meet the Illinois rule by itself. Handbook excerpt
- MinnesotaMinnesota’s nitrogen best management practices: wait until soil at 6 in stays below 50°F. From September 1, fall nitrogen is restricted by rule on vulnerable groundwater land (coarse soils, shallow bedrock, karst) and in some drinking water supply areas. Check the MDA map before you apply. MDA Groundwater Protection Rule · restriction map · MDA 6 in soil stations
- KansasK-State: on silt loam and heavier soils, wait until soil at 4 in is below 50°F. No fall N on sandy soils prone to leaching. K-State suggests a nitrification inhibitor with fall ammonia and notes much of Kansas can stay above 50°F well into late fall. K-State Agronomy eUpdate
- NebraskaNebraska CropWatch: wait until soil at 4 in is below 50°F and trending cooler, and keep fall ammonia off coarse soils. Your Natural Resources District may restrict fall nitrogen in a groundwater management area; ask before you apply. UNL CropWatch
- Indiana, Michigan, Missouri, North Dakota, Ohio, South Dakota, WisconsinThis page has not checked your state’s own fall nitrogen guidance yet. The 50°F at 4 in line is the one Iowa State, Nebraska and K-State use. Ask your extension office or agronomist what applies to your ground.
How this page reads the soil
Every number is a measured daily mean from a soil sensor, never a model. The stations come from four networks: NOAA’s Climate Reference Network (10 cm, 3.9 in), USDA NRCS’s Soil Climate Analysis Network (4 in), the Iowa State Soil Moisture Network (4 in) and the Kansas Mesonet (10 cm). The depth of each station is printed next to its reading.
For a county, the page uses the station closest to the middle of the county, up to 50 miles away. For a ZIP, the station closest to the ZIP. A station drops off the page when its newest reading is more than 3 days old, when it has fewer than 5 of the last 7 days, or when a reading jumps more than 18°F in a day (a sensor problem, not weather).
Under 50°F all week means every one of the station’s last 7 daily readings was under 50°F. That is the condition extension describes. It is not a forecast. A warm week can push the soil back over 50°F, and your field is not the station.
Every station, newest reading
Questions
When is it cold enough to apply fall anhydrous?
Iowa State, Nebraska and K-State all say to wait until soil at 4 inches is below 50°F and trending cooler. Illinois uses the daily high of bare soil at 4 inches. Minnesota’s best management practices use 6 inches. One cold morning is not enough; the soil has to stay there.
Why 50 degrees?
Soil microbes turn ammonium into nitrate, and nitrate can leach or be lost to the air when soils are wet. That work slows a lot as soil cools below 50°F. It does not stop, so later and colder is still better.
Can I start at 55 degrees with a nitrification inhibitor?
Older Nebraska CropWatch advice put the line at 55°F when a nitrification inhibitor goes on with the ammonia. The current Iowa State, K-State and Nebraska pages cited here still give 50°F and suggest an inhibitor on top of waiting, not as a reason to start sooner.
Where should fall nitrogen not go on at all?
Sandy and other coarse soils, organic soils and very poorly drained soils, in the guidance from Illinois, Kansas and Nebraska. Illinois says no fall N for corn south of a line near Illinois Route 16. In Minnesota, fall nitrogen is restricted by rule from September 1 in vulnerable groundwater areas and some drinking water supply areas.
Why is my county blank?
No station on this page is within 50 miles of the middle of the county, or the nearest one has not reported in the last 3 days. The page does not fill the gap with a model or a guess.
Is the station reading the same as my field?
No. Residue, moisture, soil color and texture all change soil temperature. Bare ground warms and cools faster than ground under residue or sod. Use the station to see the trend, then check your own field with a soil thermometer at 4 inches in the afternoon.
Guidance: Iowa State Extension; Illinois Agronomy Handbook, fall N timing; Minnesota Department of Agriculture, Groundwater Protection Rule; K-State Agronomy; Nebraska CropWatch; Nebraska Water, the science behind 50 degrees.
Soil data: NOAA U.S. Climate Reference Network; USDA NRCS Soil Climate Analysis Network; Iowa State Soil Moisture Network (Iowa Environmental Mesonet); Kansas Mesonet. Data file: /data/fall-nitrogen.json.