Region 1: The Central Lowland

Mollisols are the dominant soil type in the Central Lowland region, formed where organic matter accumulates beneath prairie grasses and in poorly drained forests (Figure 8.16). In many cases, these soils are underlain by thick deposits of glacial loess, which has contributed to their rich nutrient content (see Figure 8.15). Mollisols are highly productive dark soils, and most of the native grassland that produces them has been converted to agricultural land. Tallgrass prairie once covered more than 69 million hectares (170 million acres) of North America, but today nearly 96% of it has been converted to agricultural land (Figure 8.17). The Mollisols of the Central Lowland have a distinct zonation in type, reflecting the region’s climatic gradation from wetter in the east to drier in the west. The dominant Mollisols in Missouri, eastern Kansas, and east-central Oklahoma are wetter and occur close to the water table, while those in west-central Oklahoma and north-central Texas form under semi-arid climates. These drier Mollisols, belonging to the suborder Ustolls, can become even more dusty and dry during drought conditions, limiting crop yields and leading to damaging dust storms such as those that occurred during the Dust Bowl of the 1930s.

Figure 8.14: Mississippi River floodplain and delta.

Alfisols are also very common in the Central Lowland, particularly in northeastern Missouri. They generally form in forested areas as a result of weathering processes that leach minerals from the surface layer into the subsoil, where nutrients are retained. Alfisols in the Central Lowland exhibit the same type of wet-dry zonation as the Mollisols.

Figure 8.15: Distribution of loess in the South Central and surrounding regions.

Entisols, young soils lacking in horizons, are found where erosion and deposition occur faster than the rate of soil formation. In the Central Lowland, they typically appear in floodplains where alluvial sediments are deposited. They are present along the Missouri River and are scattered along river courses throughout Oklahoma and northern Texas.

Inceptisols and Vertisols can be found scattered throughout the southern part of the Central Lowland region (Oklahoma and Texas).

Figure 8.16: An example of a Mollisol soil. These soils have a rich, dark surface horizon and are high in organic matter content.

Figure 8.17: Tallgrass Prairie National Preserve near Strong City, Kansas. This type of grassland is a typical environment for the formation of Mollisols. Today, most of it has been converted for use in agriculture.