The Great Plains, a broad plateau that is home to intermediate and short grasslands, stretches for 3200 kilometers (2000 miles) from the Canadian interior south to the Mexican border. In the Southwest, this relatively flat region runs along the eastern edge of Colorado and New Mexico. Here, the Great Plains tends to be at a higher elevation than it is farther east; the area of lowest elevation runs through southeastern New Mexico, in the broad valley occupied by the Pecos River. Most of the region’s soils are derived from Mesozoic- and Cenozoic-aged sedimentary rock that mantles the area.

Mollisols are the most common type of soil in the Great Plains. The region is dominated by dry Mollisols belonging to the suborder Ustolls, which form in semi-arid conditions. These soils 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. Decreased precipitation and lower soil fertility provides for a localized agricultural economy based heavily in rangeland livestock (Figure 7.20), and crops here often require irrigation from local aquifers or various surface water impoundments. Despite these limitations, fruits, nuts, vegetables, and wheat are all grown in the Great Plains’ Mollisols.

Figure 7.20: Cows graze in New Mexico rangeland underlain by dusty Mollisols.

Entisols are also common throughout the Great Plains, especially at lower elevations and along river courses where they are generated by slope erosion. Sandy deposits of wet Entisols can be found all along the tributaries and banks of Colorado’s Platte River, and the Pecos River in New Mexico frequently deposits alluvial materials on its floodplains. Layers of sandy soil in the northeastern part of Colorado provide a productive foundation for rangeland and pasture.

Alfisols occur mostly in the high-elevation areas of the Great Plains, especially in southeastern Colorado, where open forests of juniper and other conifers grow.