I am looking for a good book or online course that explains soil chemistry and how it interacts with plants, I have taken some undergraduate chemistry courses so I some basic understanding of chemistry.
I've not seen one book that explains soil chemistry very well, yet. The Soil Biology Primer does a pretty good job of explaining the life in soil, however.
Here is a link that might be useful: Soil Biology Primer
Environmental Soil Chemistry by Sparks is a good source of information on the various interactions in soil. The UC Davis folks have some courses dealing with soil chemistry as well. See http://lawr.ucdavis.edu/classes/ssc102/ for an introductory course with lots of good notes online. Several other universities also have university courses with online notes.
Here is a link that might be useful: Soil Chemistry
Thanks for the help.
By the way, I found the Sparks book available electronically in a library a while ago. This is really one of the best books on soil chemistry, but is fairly expensive. Many of the books out there are quite elementary, and fail to address some of the fascinating chemical pathways in the soil. Sparks will take you through an information rich trip full of interesting tidbits. You might check at your local city or university library. As I looked at some of the current university offerings on soil chemistry I noted that many still use this book as a text.