Region 1: The Central Lowland
The Central Lowland is a broad and mostly flat expanse of the North American interior, stretching approximately 2400 kilometers (1500 miles) across its east-west diameter. Recent glaciation has repeatedly ground down any preexisting topographical relief, burying the region’s pre-glacial geology in a layer of unsorted sediment and windblown loess that was carried and processed by the advance and retreat of continental glaciers. The combination of low levels of topographical relief, recent glacial deposits of till, and the dominance of a tall grassland ecosystem has produced remarkably rich and fertile soils with high agricultural value.
Mollisols are the dominant soil type in the Central Lowland region, formed where organic matter accumulates beneath prairie grasses and in poorly drained forests. In many cases, these soils are underlain by thick deposits of glacial loess, which has contributed to their rich nutrient content (see Figure 8.13). Mollisols are highly productive dark soils (Figure 8.14), 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 for agriculture. The eastern Dakotas and eastern Nebraska contain some of the most productive land in the world. Thanks to fertile Mollisols, these states are national leaders in the production of corn, soybeans, wheat, flaxseed, rye, sorghum, oats, hay, alfalfa, and barley. All three states are generally ranked at or near the top ten in annual yield of these crops, and also support a robust livestock and dairy industry along with the bulk of the nation’s honey production.
Figure 8.14: A farmer ploughs a field of rich, dark Mollisols in North Dakota’s Red River Basin.
The Mollisols of the Central Lowland reflect a climatic gradation from wetter to drier conditions. The dominant Mollisols found in the region, especially in the Red River Valley and near the Missouri and Platte rivers, are wetter and occur close to the water table. Southeastern South Dakota and northeastern Nebraska contain drier Mollisols that form under semi-arid climates.
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 prevalent along the Platte and Missouri rivers in Nebraska.
Wet Vertisols, which remain saturated for large parts of the year but occasionally dry out enough to form cracks, can be found all throughout North Dakota’s Red River Valley.
