The classification of types of soil, in a manner similar to that used for biological classification. In one of the most widely used systems, devised by workers at the US Soil Survey, within the USDA, soils are divided into 11 orders: alfisols, andosols, aridisols, entisols, histosols, inceptisols, mollisols, oxisols, spodosols, ultisols, and vertisols. These orders are further subdivided into suborders, great groups, families, and soil series, defined by diagnostic horizons. The World Reference Base for Soil Resources is the principal international classification.