Dig IT! at the Smithsonian

Dig It! The Secrets of Soil is an exhibit at the Smithsonian's National Museum of Natural History on view from July 19 2008 – the corresponding website has copious interactive tools and links. 

Above is one of the many charts from the website- showing 8 of the 12 basic types of soil orders. 

(Soil orders reflect the environment in which they form, their age, and the ecosystems they support. Some soil scientists also propose to name a thirteenth order. Anthrosols would include soils formed or influenced by human activities. Scientists outside the United States use many other soil-naming systems.)