Fertile soils cover much of Iowa. Deep fertile soils formed in glacial till, which are thick deposits of material remaining from the abrasive forces of the continental ice sheets. Several thousand years ago, the glacial till in many areas was covered by loess blown in from nearby flood plains along the major rivers. The soils were later enriched by the humus content derived from the luxuriant growth of densely rooted prairie grasses. These so-called tall grass prairie soils, with thick, black or very dark gray surface layers, cover most of central, western, and northern Iowa.
However, they no longer grow native prairie grass, as they have long since been put to agricultural uses. Among the most productive soils are those of the Prairie Pothole region in north central Iowa. In western Iowa, a strip of fertile alluvial soil extends the length of the Missouri River Valley.
The soils of southern and southeastern Iowa are classified dominantly as argiudolls and aquolls. They formed in older, more weathered glacial till with surface loess deposits generally thinner than the prairie soils to the north. The forces of erosion on this landscape have had more time to create relatively steeper hillsides. Consequently, the soils are generally less fertile, but are capable of producing high crop yields when effective farming practices are used. The soils in the Driftless region are least suited to cultivation. Soils classified as hapludalfs occur on steep hillsides and are generally covered by woodland or pastureland. Hapludalfs also occur farther south in eastern Iowa. "Iowa" © Emmanuel BUCHOT, Encarta, Wikipedia
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