US Soil Taxonomy (ST)
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 general information see the Soil Taxonomy page on Wikipedia.
For a copy of the main Soil Taxonomy document, go to 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 (NRCS).
There also is a Keys to Soil Taxonomy document providing the taxonomic keys necessary for the classification of soils in a form that can be used easily in the field. It also acquaints users of soil taxonomy with recent changes in the classification system (The twelfth edition of the Keys to Soil Taxonomy incorporates all changes approved since the publication in 1999 of the second edition of Soil Taxonomy: A Basic System of Soil Classification for Making and Interpreting Soil Surveys).
Horizon Definitions are provided in Chapter 3, and the Identification Key starts in Chapter 4 and then branches to the appropriate chapter for further distinction within each soil order.