According to the United States Department Of Agriculture Natural Resources Conservation Service http://www.nrcs.usda.gov/feature/backyard/benmulch.html
"Mulching can be one of the most beneficial things you can do for your soil and your plants: .
Mulches help to:
Organic mulches can consist of tree bark, mulched grass clippings, wood chips coffee bean shells, decayed leaves and even newspaper. We prefer colored dark, wood chips or coffee bean shells simply because they look nicer.
Mulch should be applied to a depth of of 3 to 4 inches. This would mean that a 10' by 10' garden would need about a cubic yard of mulch. ( This amounts to roughly 3/4 m³. Yes, we are metric, but old habits die hard.)