sooby said:Basically what happens is that water does not want to cross the junction between finer material and coarse material, so the flow of water comes to a stop when it gets to the coarse material. It won't cross that boundary until it is basically "forced" to do so when the top fine material becomes so saturated that it can't hold any more water.
This phenomenon is called a "perched water table". It's the principle used to build sand-based golf greens and was discovered over a hundred years ago but still we see coarse material recommended at the base of pots.
I'm a believer in perched water table theory, but lately I've realized that most, if not all of the old-school advice regarding 'drainage material' recommended using pot shards, large stones, or gravel, which most definitely impede the flow of water. The perched water table experiments I've read were done with similar material.
If the coarse matter used in the bottom of the container is permeable and can wick water away, I think the perched water table principles don't really apply. Chunks of bark will absorb and wick water. Also, the finer bark and potting soil above will wash down between the chunks to a certain degree and provide more of a drainage path. This could be plant-specific, in that daylily roots will happily invade and thrive in the lower regions of a container full of bark. Daylilies are somewhat of an anomaly, in that they love a lot of water, and frequent irrigation/fertigation as long as there is plenty of percolation and air space in the root zone. Putting a section of log in the bottom of a container isn't the same thing.
I'm thinking that if the container is tall enough, (18" or taller?) some kind of filler might be desirable, conserving water by keeping the water table high enough. It should also conserve growing mix, which simply degrades into anaerobic muck in the bottom of a deep container.
