Within its borders, Oklahoma has a number of different soils of varying fertility. The ultisols (red and yellow podzols) of the forested Ouachitas and Ozarks have been leached of much of their nutrients. Alfisols and mollisols (chernozems and chestnut soils) of the grassy prairies are known for their natural fertility although agricultural overuse and limited precipitation restrict their natural richness. Alluvial soils are found along the river valleys while loess, a wind-deposited soil, can be found on the uplands between the rivers.
Vegetation responds to variation in the water, temperature, elevation, slope, soil, drainage, and competition among native and introduced species. Three broad categories exist in the state—forest, woodland and savannas, and grasslands. The largest forested area can be found in eastern Oklahoma. Deciduous forest of oak, hickory and other species, mixed forests of pines and hardwoods, or pure stands of southern pine, are located here.
Woodlands and savanna cover the mid-section of the state with trees becoming less frequent moving westward. Tall grasses dominate the drier areas in this region. The largest woodland area was the “cross timbers” in the east central region, so called because the branches of the blackjack and post oak grew so close that their branches became intertwined, creating a barrier to passage.
Still farther west, the ground cover is dominated by short grasses, sagebrush, and eastern redcedar. The northwestern Panhandle has a piñon-juniper woodland like that of the Rocky Mountains. Here, too, though less dramatically than elsewhere in the state, the natural vegetation has been altered by cultivation and grazing. Flowering trees found in Oklahoma include dogwood and redbud. Among the flowers found throughout the state are the sunflower, goldenrod, wild indigo, verbena, violet, primrose, anemone, and phlox. "Oklahoma" © Emmanuel BUCHOT, Encarta, Wikipedia
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