The U.S. Dept. of Agriculture, Natural Resource Conservation Service (which used to be the Soil Survey) has links to soil surveys and maps of soils all over the country as well as information about farming programs and congressional actions.
NSCSS has soil science links to sources of information in soil biology, farming, and physical soil sciences.
A wonderful source of soil and forest information is available at http://www.metla.fi/info/vlib/soils/old.htm including a glossary.
The Advanced Biotech site sounds like they make mutant plants but they have a good source of links to world wide soil science societies and agencies and an electronic library.
Cornell Univ. Dept. of Agriculture has a good site with a list of soil science software and links to sites where desert soils are treated.
Biological & Agricultural Index
Elsevier ScienceDirect (group login, search, subject=agricultural & biological sciences, env. sciences)
Wiley InterScience (journals in subject category=earth science)
JSTOR (Select Journals to Search=Ecology)
(look them up in Snoopy and you'll find a link to the electronic text)
Soil genesis and classification [computer file] / S.W. Buole BOOKS; S591 .B887 1997eb
Soil science simplified [computer file] / Milo I. Harpstead,eBOOKS; S591 .H28 1997eb
The Encyclopedia of soil science, Ref S592 .E52
Glossary of soil science terms. Ref S592 .S62 1997