A hierarchical system that is used in United States to classify soils. This system has six levels: order, suborder, great group, subgroup, family, and series. At the order level, 12 types of soils are recognized: gelisols, oxisols, aridsols, mollisols, alfisols, ultisols, spodsols, entisols, inceptisols, vertisols, histosols, and andisols.