mollisol

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(Image source USDA NRCS. Used with permission)

Rich-dark organic horizon is common to mollisol soil 
like this one in central Iowa

Mollisols develop under Born under grassland vegetation, these soils are well-known for their dark brown to black organic rich surface layers. Mollisols are high in nutrients and rich in calcium soils among the most fertile soils on the earth. Mollisols are found in the drier portions of the humid continental climate through the steppe climate.