the houdek soil series is deep, well drained, loamy soil that represents many soils formed in south dakota under grass vegetation. the dark color of the surface layer is a result of decomposition of biomass from vegetation and other materials that have been deposited over thousands of years. prairie conditions form a thick, dark colored, humified surface horizon or layer that is humus rich (1 to 4% organic c). this is a key characteristic that makes these soils fertile.