Soil consists of multiple layers called horizons.

The USDA classifies soils into types based on texture, moisture, and other properties.

Most grassland soils are mollisols with a thick, dark A horizon.

Illinois also has significant alfisols in wooded areas that yield a thinner A horizon and clay-rich B horizon.

Warmer climate and faster decomposition in the southern plains result in less fertile aridisols with thinner A and B horizons.