A big chunk of Southern Illinois is called the Clay Pan District. The soils here are old; formed in glacial till from the Illinoisan Glacier, dating back about 180,000 years. The prairie soils north of this area date from less than 20,000 years ago, and are formed in Wisconsinin Glacial till. All those extra years have moved minerals downward in the soil profile, forming a tough layer called a fragipan, and it is very restrictive to movement of water, and root development. These soils will have a perched water table in the winter and spring, and can be very hot and dry in the late summer. Tree species location is closely governed by these soils. The amount of slope, and the depth to the fragipan are two very important factors on clay pan soils that have to be considered when you are planting trees.
This tree was tipped over by a severe storm, and it illustrates the limitations imposed on root growth by clay pan soils. The roots of this tree were also being limited by changes in the site due to construction and lawn mowing. Large mature trees need to be evaluated regularly when they are within striking distance of homes; especially on sites with restricted root zones. Trees that are planted on clay pan soils must be able to tolerate the wet conditions in the spring and hot, dry conditions during summer.
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