More on Soil Temperatures in Illinois

Here are the average dates in fall when the 4-inch soil temperature falls below 50 degrees (left panel) and below 60 degrees (right panel). Consider these as a “rule of thumb” for planning purposes. Consult the current soil temperatures, as discussed in the earlier post, for decisions on applying nitrogen in the fall.


Average date when 4-inch soil temperature drops below 50 and 60 degrees.
Average date in fall when the 4-inch soil temperature drops a) below 50 degrees, and b) below 60 degrees.


Daily Soil Temperatures Across Illinois

The Illinois State Water Survey operates a network of 19 sites that report daily soil temperatures at 4 and 8 inches.

Soil temperatures in the fall are critical for the application of nitrogen (N) fertilizer. According to the Agronomy Handbook (University of Illinois), “fall N applications should be done when daily maximum bare soil temperature at 4 inches is below 50 degrees.”

The daily maximum and minimum soil temperatures are measured over grass. In addition, the values are adjusted using regression to estimate the bare soil temperatures, to better represent the soil temperatures in a cultivated field. Grass tends to insulate the soil so the daily temperature swing is a little smaller than over bare ground. In addition,  soil temperatures within a particular field may vary due to soil color, soil moisture, and crop residue on the surface.

You can see the daily data and maps at

Frost in Illinois

Frost is defined as ice crystals that form on a freezing surface as moist air comes in contact with it. Farmers, landscapers, and gardeners are interested in frost in both spring and fall.  However, frost is usually not measured directly at weather stations. Instead, we choose dates when the air temperature crosses the threshold of 32°F.

The Midwestern Regional Climate Center keeps tabs on what places have hit 32 degrees so far this fall. Click to enlarge.

Locations that have experienced a freeze so far.  Image courtesy of the Midwestern Regional Climate Center.

Frost Climatology

The average date ranges from October 7 in far northern Illinois to October 21 or later in far southern Illinois (see map below). The actual date varies from year to year. For tender plants in the fall, subtract two weeks from the average date to protect against an early frost.

Although 32°F is the temperature traditionally used to identify frost, visible frost can be seen on the ground and objects when temperatures are slightly above 32°F. This occurs on calm, clear nights that allow cold, dense air to collect near the ground. Under these conditions, the temperature near the ground actually can be a few degrees cooler than at the 5-foot height of the official National Weather Service thermometer.

Open, grassy areas are usually the first to experience frost, while areas under trees are more protected because the trees help prevent the heat from escaping. Homeowners can protect tender plants by providing this same type of protection if they cover their plants when a frost is expected. Plants near heated buildings sometimes are spared too. Those living in the country tend to see frost earlier in the fall than those who live in town, because of the many warm buildings and trees in town may ward off frost in some cases.

Fall Frost
Average date when temperature first drops below 32 degrees in Fall.

September Climate Summary

Illinois experienced temperatures close to normal for September. The statewide average temperature was 66.8 degrees, just 0.6 degrees above normal.

During the month, many stations in southern Illinois had reported high temperatures in the mid to upper 90s. Fairfield and Grayville both reported the highest temperatures in the state with 99 degrees on September 21 and 22, respectively.

The statewide average precipitation was 4.0 inches, 0.8 inches above normal. However, the rainfall  was unevenly distributed throughout the state. The largest rainfall totals for the month occurred across west-central Illinois. Quincy Lock and Dam 21 reported 8.10 inches, the most rainfall in the state. This was followed closely by Springfield with 7.94 inches.

Meanwhile, far southern Illinois continued to struggle with drought conditions. The U.S. Drought Monitor lists several counties in southeast Illinois in the category of moderate drought.

September rainfall
Rainfall departures from normal for September 2010 in Illinois.