a surface horizon of mineral soil that is dark and relatively thick, contains at least 5.8 g kg^–1 of organic carbon, is not massive and hard or very hard when dry, has a base saturation of > 50 percent, has < 110 mg P kg^–1 soluble in 0.05 M citric acid, and is dominantly saturated with divalent cations This definition last updated 10/27/2008