Came across this a while back and found it to be a good introduction to soil biology and health.
http://www.nrcs.usda.gov/wps/portal/nrcs/detailfull/mt/home/?cid=nrcs144p2_057307
There are multiple resources here, in particular I enjoyed the "Soil 101" course.
http://vimeo.com/channels/soils101/videos
and the .pdf "managing cover crops profitably
http://www.sare.org/Learning-Center/Books/Managing-Cover-Crops-Profitably-3rd-Edition
These are resources more geared toward the general audience, and a more mechanized way of farming. However, the basic data is still relevant for those going a permaculture route.
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