I hadn't heard about the practice you mentioned. I will often use styrofoam peanuts at the bottom of the pot where the goal is to NOT absorb water.

Some people use a mix of bark, perlite and charcoal. I use different sized rocks or small bark, depending on the plant. Potting mixes vary greatly.

A lot of people add charcoal because it is an adsorbent and removes impurities from water. How the charcoal is smart enough to NOT remove the impurity we add, namely fertilizer, has always escaped me. I don't use it.

Nick

I'm with Nick. Most people who use charcoal claim that it removes impurities, but I've never heard anyone state what specific impurities need to be removed from potting soils. And even if they did know what they wanted to remove, it's very doubtful that a few chunks of charcoal added to a potting soil will have any significant effect. Charcoal used for air and water purification is typically of a much smaller particle size (to increase total surface area), and it is always used actively (air or water is forced through the charcoal with fans, pumps, etc). And even if it did have an effect in potting soils, there are different types of charcoal with different pore sizes, each of which is capable of adsorbing a different range of molecule sizes. Who knows if horticultural charcoal has the right pore size for the targeted impurities? And who knows what desirable substances it might also adsorb? Needless to say, I do not believe charcoal has any use as a soil "cleanser." I've never used it and don't see any reason to.