When you put fine material over coarse material in a pot, the fine material will not drain into the coarse material until the fine material on top is so wet that it cannot hold any more water. The water does not easily cross the interface between the two different textured materials. Yes it is counter-intuitive so I tested it myself with potting mix over polystyrene packing peanuts and it was absolutely true, the mix in that pot held more water than the same sized pot with all potting mix. This has been known for a very long time yet so many articles still recommend using gravel or other coarse material at the bottom of a pot for drainage. There are some old pictures on the web that show what happens when water is poured into a container of fine medium over coarse, but I'm not sure if the link will work, let's try:
https://www.google.ca/search?q...
