High Quality Content by WIKIPEDIA articles! An Umbrisol in the FAO World Reference Base for Soil Resources is a soil with a dark topsoil and in which organic matter has accumulated within the mineral surface soil (in most cases with low base saturation) to the extent that it significantly affects the behaviour and utilization of the soil. Umbrisols are the counterpart of comparable soils with a high base saturation (Chernozems, Kastanozems and Phaeozems). Many of these soils are classified in the USDA soil taxonomy as Great Groups of Entisols and Inceptisols, as Very dark-humus soils in the Russian soil classification.