Try adding mycorrhizae, pretty cool stuff
http://r.duckduckgo.com/l/?kh=-1&uddg=http%3A%2F%2Fwww.growersgold.net%2Fwhat-is-mycorrhizae
Be careful of adding to much nitrogen in the form of fertilizer,could burn the root system.
See what you have first and go from there.
Local extension is the county ag office. up here it is UCD cooperative extension, an awesome resource for all thing grown.