Other
N.e. Soil Formation: Genesis
This comprehensive site explores a number of soil related topics. A glossary of soil terms, formation of soils, and classification of soils are just of a few of the topics found on this this.
Biology Pages
Kimball's Biology Pages: Soil Horizons
This site is a personal site from John W. Kimball, a Harvard Professor, that contains a brief definition of both topsoil and subsoil. A drawing supplements the text.