US Soil Taxonomy (UST)
Developed by United States Department of Agriculture and the National Cooperative Soil Survey.
Soil taxonomy provided by United States Department of Agriculture/Natural Resources Conservation Centre provides an elaborate classification of soil types according to several parameters and in several levels: Order, Suborder, Great Group, Subgroup, Family, and Series.
For more information see: Soil Taxonomy on Wikipedia.
For a copy of Soil Taxonomy: Soil Taxonomy, A Basic System of Soil Classification for Making and Interpreting Soil Surveys at the United States Department of Agriculture - Natural Resources Conservation Centre