I started only changing the top part of the soil when I realized that the roots only used the top part, and that the bottom part was probably still as good as new. For using up space in the bottom of a pot, a plastic jug of water works just fine. The plastic lasts a long time if it's not exposed to the sun. Another thing that is wierd in potting soil mix is those water absorbing granules. They pop out on the surface and look like jelly ice cubes! (or ghost poop!)