Many people feel that is important to place a “drainage layer” at the bottom of their flower pots – crockery, gravel, foam peanuts, etc. – so that their potting media will drain faster and more thoroughly.
The reality however, is that, depending upon the container and medium used, adding a flower pot drainage layer may actually result in a wetter, less airy medium above it. Let me explain why:
When irrigating a plant, water is retained within the substrate through two distinct mechanisms. Some moisture is absorbed directly by the potting medium particles, while the remainder is maintained between these particles due to surface tension. Water absorbed by the media particles persists until the surrounding environment starts to dry, leading to evaporation. Conversely, the interstitial water held by surface tension is subject not only to this balance of surface forces but also by gravity.
The taller the column of potting medium, the heavier the retained, interstitial water will be on the lower portions of the pot. If it is heavy enough, that gravitational force overcomes the surface tension in the lower portions of the pot, forcing the water out – as drainage. The shallower the pot, the less gravitational force is applied, so the medium will hold more water, not less.
To prove this to yourself, take a sponge and totally immerse it in water, squeezing and releasing it several times so that it absorbs the most it can. Now, with it flat on the palm of your hand, lift it out of the water, and wait until all of the excess has drained. Now, without squeezing the sponge, tilt it on its end and notice how much additional water drains out.
When you put a flowerpot drainage layer in, you are, in essence, shortening the water-holding column above it. That shortened column has less pressure from above, so will actually hold more water, just like the horizontal sponge.
If you want better drainage from your pot, use a coarser particle size in your mix, so that less water is retained by surface tension.