A layer of Hydroton on the bottom of containers would be about the same as a layer of marbles of the same size. Initially, when they were dry, they'd absorb some water, but after they were wet, they'd remain saturated. If the layer was shallow, it would have no impact on reducing the ht of the perched water table & probably nearly no impact on o/a water retention. If the layer was deep, soil would probably rest on top of the layer & cause the PWT to be even taller than w/o the Hydroton. They wouldn't have any notable impact on drainage (flow through rates), and if you were just looking for ballast to reduce o/a water retention, marbles or stones (something not internally porous) would actually be a better choice.

Al

Thank you Al

Mike