[IBC] Bonsai Today article on drainage
 

For a large or deep pot, there is nothing wrong with having a layer of coarser
material on the bottom. Actually, it has been found that a narrow deep pot,
like a cascade pot, may drain faster than a wide shallow pot.
The reaction to the drainage layer came after some research revealed what is
called the perched water table. If you have a layer of dense potting soil with
a layer of very loose soil or gravel below it, the water stays in the potting
soil and doesn't drain into the bottom, due to the surface tension of the
water. However, this does not really apply to bonsai, for two reasons. Even our
tropical soil is very gravelly and drains well, no matter what. Also, when you
are potting, you jab a chopstick and your fingers into the soil, so there is no
hard and fast distinction between layers of soil. So if a coarse drainage layer
on the bottom works for you, use it.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)

[IBC] Bonsai Today article on drainage
 

I haven't read that article but on the subject of perched water tables, try
this experiment:
Put a sponge under water and bring it out into the free air, holding it
horizontally. Now, turn it vertically and note the amount of water coming
out. This is in the air. A drainage layer is a bad idea if the difference
between a sponge and air holds water,how does that bode for your already
shallow bonsai pot? Essentially, you have just made it shallower.
Jim Gillespie
Defrocked Master Gardener and dirty person in Napa.

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