Thats why i love buying potted plants.. I forgot i had some and went to pull out a plant that had branched off of a mother pot.. and pulled the whole thing out by mistake :Z lol. Im talking about the small plastic pots that are like 2"x2" or whatever.
Works GREAT especially for stems that keep floading out of the soil (ended up buying a few on ebay for that reason :O
To answer your question, your best bet is black sand. as previously stated anything bigger might let the dirt go through.
Problem will be that when you move stuff around you can still kick it up. Perhaps putting down black garvel and sand on top will work better, idk.