First - I believe the science behind perched water tables.

Second - I use a drainage layer in my pots.

Method - My drainage layer is composed 1 - 3 layers of 1/4 - 3/8" (6 - 9 mm)
crushed lava with the main function to keep the finer soil mix above from
sifting out through the 1/4" (6 mm) hardware cloth I use to cover the
drainage holes or the bottoms of my grow boxes. My finer soil (1/16" - 1/4"
= 1 - 6 mm) goes on top of this and settles down into the top of the
drainage layer.

Observation - The drainage layer is generally very heavily colonized with
roots, sometimes far more heavily than the rest of the soil. This agrees
with some of the pictures that I see of repotting in Bonsai Today of the
articles translated from Japan.

Hypothesis - Roots grow well in the drainage layer because it is moist, but
not wet. The slightly perched water table supplies the drainage layer with
moisture, but the coarser soil in the drainage layer prevents pooling of
water at the bottom of the pot (I water almost every day).

Conclusion - With my soil mix (lava and bark), climate, types of trees, and
watering schedule the thin drainage layer works. Your results may vary and
will depend upon your soil mix, climate, trees, and watering schedule.

Marty
Spokane, WA

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