I've been reading about a product called Azomite. It's something you might want to look into:
AZOMITE® is a uniquely natural material, mined in Central Utah, USA. For over seventy years, crop producers have used AZOMITE® to support plant growth and vitality. Typical assays reveal that the material contains a broad spectrum of over 70 minerals and trace elements. AZOMITE® requires no mixing. It is odorless, will not burn plants and will not restrict aeration or water penetration. AZOMITE® is not manufactured or chemically prepared. It is 100% natural and completely free from additives, synthetics or fillers.
http://azomite.com/about.html