Observed soil moisture data are collected at 19 sites in Illinois by the Illinois State Water Survey’s Water and Atmospheric Monitoring program. The soil moisture is measured at 5 cm (2 inches), 10 cm (4 inches), 20 cm (8 inches), and 50 cm (20 inches). All amounts are reported as the fraction of water by volume of the soil. For example, a value of 0.35 means that 35% of the soil is occupied by water.
Silty clay loam soils, common in Illinois, are considered saturated at values of 0.45 to 0.50. Field capacity is in the neighborhood of 0.35. Values below about 0.20 would be at the permanent wilting point. As the name implies, that is the level where plants wilt and do not recover. Plant roots are unable to recover any water below this level because it is tightly bound to the soil particles. Of course, soil moisture is very dynamic with layers drying or wetting at different rates so it would take low levels of soil moisture at all layers in the root zone to kill a plant.
In general, soil moisture is highest in spring and slowly declines in the summer months because of the heavy demands from evaporation and from transpiration of growing vegetation. In the fall, cooler temperatures and the end of the growing season allow soils to recover quickly.
The map below shows the May 14, 2012, soil moisture status at 10 cm (4 inches). Values in the 0.2 and 0.3 range are widespread. The noticeable exception is in Mason County where the site is located on a very sandy soil. As a result, the moisture drains right out of the soil after a rainstorm.