While this is slanted toward cover crops for use in no-till
farming, it does provide a good general guide to different
kinds of cover crops:
http://newfarm.rodaleinstitute.org/f...ll/chart.shtml
This document from UCDavis provides an index to good descriptions
of individual cover crops:
http://www.sarep.ucdavis.edu/database/covercrops
Down south, lots of farmers like crimson clover (kind of expensive
seed, perhaps because of demand, and often out of stock in spring)
and cowpeas or Austrian winter peas. A mix of winter rye and hairy
vetch is popular with no-till tomato farmers, but it matures in late
April and can be inconvenient where you want to plant crops earlier
in spring. Bell beans (fava bean relative) may be a better choice where
you need to cut it down earlier and either amend the soil with the top
growth or leave it on top for mulch if using no-till methods.
Here is a document on cover crops from U of Kentucky:
http://www.ca.uky.edu/agc/pubs/id/id113/id113.htm