The science of potting

A woman named Sue Bergeron in Ontario did a little experiment on drainage:

Therefore, in these two experiments, coarse 'drainage' material in the bottom of the pot resulted in the potting mix above it staying wetter per cup of medium than in a container of potting mix alone.

This is so frequently misunderstood that you need to read the whole thing. Also see Paul Cumbleton's reply [Feb. 23 entry in Jim Shields's unpermalinked blog]:

As we learned earlier, small pores hang on to water more strongly than large ones. Because of this, when you have a medium with smaller pores above one with larger pores, the water has difficulty crossing the boundary. There is insufficient "strength" in the larger pores to pull the water out of the smaller ones above where they are held more strongly by capillary action. So instead of the water draining evenly from the pot, it drains to the interface between the two layers then slows down or may even be stopped altogether until a sufficiently large hydraulic head has built up again to force it across the boundary. This of course means when the compost above is completely saturated! Since the stated goal for using a layer of coarse material is to improve drainage", it is ironic that this practise actually causes the very state it is intended to prevent!

Help translating the latter (and, indeed, all UK garden writers) is to be found in this entry from Ian Young, who explains, among other things, the various meanings of compost, and what exactly grit is.