4-Inch Soil Temperature in Illinois

The current average 4-inch soil temperature across Illinois is 75°F, measured at 3 USDA and NOAA stations. Readings range from 68°F to 79°F depending on location and elevation. Soil across Illinois is right at the seasonal normal of 73°F for mid-August, based on 3 of 3 stations with historical records.

The 4 inch soil temperature shows conditions deeper in the root zone than the 2 inch germination reading. It is commonly used for established lawn, garden, and agricultural timing because it changes more slowly than the surface layer, so it is a steadier signal for fertilizer, aeration, and weed-control decisions.

Use 2 inches for germination decisions, since that is where seeds sit. Use the 4 inch soil temperature for established lawn and garden timing: it reads the root zone, changes more slowly than the surface, and is the depth most fertilizer, aeration, and disease guidance references.