I found that roots cling to sand and seem to be much easier to transplant when I put some sand at the bottom of my seed starter set ups.! I even enclose my boxes inside a white plastic garbage bag! I put a few holes in the bottom of the bags to drain! It seems to hold the moisture better than the wood or Styrofoam containers I use!
Sand is cheap and seems to work with all my mixes! Any suggestions! What do you think of 1/2 sand and 1/2 strained garden soil for a mix! I also like dry leaves at the bottom of my containers about a 1 inches since I am adding a few worms to all my boxes this year! I do like to mix a cup of cow manure compose or mushroom compost that I get in 40 pound bags from Agway! Its really nice stuff!