The Natural Resources Conservation Service (NRCS) defines a state soil as:
“a soil that has special significance to a particular state. Each state in the United States has selected a state soil, twenty of which have been legislatively established. These ‘Official State Soils’ share the same level of distinction as official state flowers and birds. Also, representative soils have been selected for the District of Columbia, Puerto Rico, and the Virgin Islands.
Areas with similar soils are grouped and labeled as soil series because their similar origins, chemical, and physical properties cause the soils to perform similarly for land use purposes.”
Source: U.S. Department of Agriculture (https://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/edu/?cid=stelprdb1236841)