Goemar
http://www.goemar.com/cgi-bin/client/entree.pl?shop=goemar&langue=2

The Fertilizer Institute
http://www.tfi.org/

Soil Fertility Information
http://www.back-to-basics.net/index.htm

IPGRI - International Plant Genetic Resources Institute
http://www.ipgri.cgiar.org/index.htm