Prairies are typically associated with continental, mid-latitude climates that are designated as subhumid. Precipitation in these climates ranges from 20 to 40 in (510 to 1,020 mm) per year, and is almost entirely offset by evapotranspiration (see Appendix 2, climate diagram for Fargo, North Dakota). In summer, air and soil temperatures are high; soil moisture in the uplands is inadequate for tree growth, and deeper sources of water are beyond the reach of tree roots. Prairie forms a broad belt extending from Texas northward to southern Alberta and Saskatchewan. Forest and prairie mix in a transitional belt on the eastern border of the division.
The prairie climate is not designated as a separate variety in the Koppen-Trewartha system. Geographers' recognition of the prairie climate (Thornthwaite 1931, Borchert 1950) has been incorporated into the system presented here. Prairies lie on the arid western side of the humid continental climate, extending into the subtropical climate at lower latitudes. Temperature characteristics correspond to those of adjacent humid climates, forming the basis for two types of prairies: temperate and subtropical.
Prairie vegetation is dominated by tall grasses associated with subdominant broad-leaved herbs. Trees and shrubs are almost totally absent, but a few may grow as woodland patches in valleys and other depressions. Grasses are deeply rooted and form a continuous cover. They flower in spring and early summer, with forbs appearing in late summer. In the tall-grass prairie of Iowa, for example, typical grasses are big bluestem and little bluestem; a typical forb is black-eyed Susan.
Because there is less rainfall in the grasslands than in forest, there is also less leaching of the soil. The pedogenic process associated with prairie vegetation is calcification, as carbonates accumulate in lower soil layers. Soils of the prairies are Mollisols, which have black, friable, organic surface horizons and a high content of bases. Grass roots deeply penetrate these soils. Bases brought to the surface by plant growth are released on the surface and restored to the soil, perpetuating fertility. These soils are the most productive of the great soil groups.