The Great Plains, some 280 million acres in extent, are bounded by the Rocky Mountains on the west and approximately the 99th meridian on the east. They occupy an area from 150 to 500 miles wide, extending from southern Canada nearly to the Rio Grande in Texas, including between 70 to 80 million hectares of range, 75% of which is in private ownership.
Topography. Comparatively level plain, the Black Hills being the only important mountainous uplift. Some of the main streams, particularly the Missouri, the Platte, and the Yellowstone Rivers in the northern Great Plains, and the Cimarron and Canadian in the southern Great Plains, have deeply eroded drainage channels, forming comparatively level valley floors 2 to 8 kilometers wide. Elevation of the Great Plains ranges from about 600 to 1800 meters, with a gradual slope to the east averaging about 1.8 meters per kilometer.
Climate. Rainfall of the region is of the summer type, ranging from about 25 cm in its northwest to as much as 65 cm along its eastern edge. Seventy to 80% falls in the months between April and September. The major advantage of summer rainfall is that it comes when needed, during the growing season. Disadvantages lie in the high evaporation during hot weather and low storage efficiency. Showers of less than 1-2 cm are mostly lost by evaporation before they can do much good, especially where a significant portion of the rain may be intercepted by foliage. Torrential rains, which often have much runoff, are common. Evaporation is about twice as high in the southern as in the northern Great Plains, and water requirement of the growing vegetation is proportionally higher. The frost-free season ranges from about 100 days at the Canada border to 200 days in Texas. Although the growing season is short in the north, summer days are long. This makes for rapid growth of vegetation, resulting in total seasonal yields that, contrary to expectations, are not much lower than those further south.
Soils. Soils in this region vary from mollisols in the east and north to alfisols and aridisols in the south. Entisols are scattered throughout. On finer-textured soils, moisture does not ordinarily penetrate more than about 60 cm below the soil surface under native sod, and it is there that the so-called "lime layer" is accumulated. There usually is no storage of moisture from year to year in the western Great Plains grasslands, being used each year by the vegetation, so the subsoil is permanently dry in the shortgrass plains grasslands. There is no loss of soil nutrients by leaching, hence many Great Plains soils are extremely fertile when plowed. In some of the lighter, more sandy soils along streams and draws there is greater than average penetration of moisture, and such areas were originally populated by the taller grasses commonly found in the true prairie (little bluestem, sand bluestem, (Andropon hallii, switchgrass and indiangrass.
Vegetation. The climax vegetation is mixed prairie but most range is now characterized by the short grasses, chief of which are blue grama (Bouteloua gracilis) and buffalograss (Buchloe dactyloides). In central and northern areas, western wheatgrass and needlegrasses are relatively abundant. Mixed prairie in western Kansas and Nebraska and portions of southwestern South Dakota, eastern Wyoming and Colorado, and the panhandle of Oklahoma has lost most of its overstory of mid grasses. Much of the Great Plains is now a shortgrass disclimax due to long-continued close grazing. Its original cover varied from nearly pure stands of shortgrass in the driest sites and finest-textured soils to mid and some tall grass in the more favored sites. The general appearance was one of a rather complete shortgrass cover with a varying overstory of midgrasses. The tall grasses were present in favored lowlands.
Management. Most of the central and southern mixed prairie supports cow-calf operation year-long. Northern areas are primarily used for summer grazing. Winter grazing in the region requires protein and energy supplementation. Heavy use in the southern portion of the region promotes buffalograss at the expense of blue grama. Carrying capacities vary widely from 4-5 ha per animal unit year-long in the wetter areas of the eastern and northern parts to 10-12 ha per animal unit in the drier southern and western parts.