Keys to Soil Taxonomy
Soil Survey Staff. 2014. Keys to Soil Taxonomy, 12th ed. USDA-Natural Resources Conservation Service, Washington, DC.
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.
The document contains internal hyperlinks that can be followed from one key-step to the next.