Those soils we think of as rich, meaning those most able to sustain plant life because of their inherent nutrient content, are soils best described as middle-aged. They've not yet been weathered and had the nutrients leached from them, but those nutrients are close to the surface. A few examples of rich soils include udepts, udolls and ustolls.
Udepts are suborders of inceptisols that appear in areas of recent geomorphic activity. In the United States, that is limited to the Appalachian Mountains and some forest areas of the Pacific Northwest. More specifically, udepts are well-draining, loamy soil types that have high quantities of nutrient material that remain relatively untapped given the uncultivated nature of these areas. Part of their development is due to the udic climate regime, in which the environment stays humid with cool temperatures and frequent, heavy rainfall. This may have aided erosion and movement of soil to create more homogenous mixtures better suited for growing plants.
Udolls are suborders of mollisols that appear in the humid portions of America's breadbasket, specifically Iowa, Illinois and South Dakota where they are taken advantage of by the cultivation of corn and similar vegetables. They are the result of deposits left behind by the glaciers of the last ice age. This combined with an initial subsurface horizon, horizontal layer running parallel to the surface, of decayed grasses and other organic matter makes udolls excellent for cultivating plants.
Ustolls appear in subhumid areas, areas with somewhat lower annual rainfall such as Oklahoma and Nebraska. Because of this, they are used to cultivate drier grain products like wheat, oats and sorghum. They possess all the same physical characteristics as udolls, but can be exhausted much more quickly. In order for ustolls to replenish themselves, they are reliant on heavy rainfall or man-made irrigation to grow native seeds and grasses. These grasses, once plowed under, add to the nutrient-rich layer of organic matter beneath the soils' surface.