Background info on Todd's Question: Traditional Earth Boxes/Global Buckets do have a layer of black plastic sheeting over the top of the soil to restrict evaporative loss of water from the soil. Holes are cut in this plastic for planting seedlings of whatever you will be growing.

I currently do not have plastic over the soil as I'm growing stuff straight from seeds which doesn't work well with a black plastic sheet over the top. I may put clear plastic covers over the tops to get a greenhouse effect if the weather stays cool. I'm growing wild edible plants (aka weeds) in these buckets and and they don't need quite the level of babying as regular plants.

-Blast