Placing one or two inches of small gravel in the bottom of a pot has been a common practice to improve soil drainage. This, however, is not the thing to do. The gravel actually causes poorer drainage because it shortens the soil column. This can cause excessive moisture to be held in the soil and cause root problems.
If the soil drainage is poor enough to need improvement, it is best to use addi tional additives such as coarse sand, perlite, peat moss, or vermiculite. Addition of any of these materials will improve soil porosity and improve drainage.