order [soil] . A group of soils in the broadest category of the U.S. soil taxonomy. There are 12 orders, differentiated by the presence of diagnostic horizons and special properties. They are: Alfisols, Andisols, Aridisols, Entisols, Gelisols, Histosols, Inceptisols, Mollisols, Oxisols, Spodosols, Ultisols, and Vertisols. Orders are divided into suborders. See also: soil classification.