Go here:
http://www.clemtex.com/ and get a back of their Black Beauty sand blasting sand. It's wonderful stuff, and cheap! It's got iron in it, but I don't know if it's available to the plants or not. I've been using it in small tanks with good results and just replaced the substrate in two of my larger tanks with it. Visit my blog vis the link in my signature for pictures of tanks I just redid with it.
Clemtex carries three sizes: #6 super fine, really too fine in my opinion, but the fish store owner I learned about it from uses this size. #4 is close to sand, and #3 is closer to gravel. It's $13 for 100lb bag.
Michael