One of the most common soil orders in Illinois is mollisols. Mollisols are the type of soil generally found underneath grassland areas. Approximately 45 percent of Illinois' land mass is composed of mollisols soil. Aquolls and udolls are the two suborders of mollisols usually found in Illinois. The wet mollisols soil is aquolls, which support the growth of vegetation, grass and sedges. Northern and central Illinois are where most of the state's mollisols are found.
The predominant soil order in hardwood forests is alfisols. In Illinois, alfisols make up 45 percent of the state's soil. The two primary suborders of alfisols in Illinois are aqualfs and udalfs. Aqualfs are wet soils within woodlands areas, while udalfs are found in humid climates. Central and northern Illinois are where aqualfs soil areas are found, while udalfs soil occupies the southern portion of the state.
Entisols are eroded soils not solid enough for rural or urban development. Most entisols are sandy or shallow. These soils are found underneath wetlands areas such as river banks and tidal mudflats, which is soil deposited on land by rivers. The majority of entisols in Illinois are in the southern part of the state and belong to the aquents suborder of entisols. Entisols make up approximately 7 percent of Illinois' land mass.
Illinois' organic soil is known as histosols. This soil order makes up nearly 1 percent of the soil found in Illinois' land area. When organic matter forms faster than it is destroyed, the creation of histosols is the result. Histosols are in Illinois' bog and marshy wetlands areas. Most of Illinois' histosols are in the western region of the state. Histosols are ecologically important because they contain large amounts of carbon.
The list of Illinois' wetlands includes all marshes, bogs, riparian areas (river banks) and watersheds. Since the 19th century, Illinois has lost approximately 90 percent of its original wetlands. Many of the remaining wetlands are protected by the federal or state government from urban or agricultural development. Some of Illinois protected areas are Lincoln Marsh Park near Wheaton and Pecatonica Wetlands Forest Preserve in northern Illinois.