This is our standard mix that we use ourselves.
It comprises the basic Japanese origin clay particle Akadama.
A hard fired clay particle produced in the UK called expanded clay. This is extremely porous and capable of holding excess water and then when dry, a lot of air. Remember roots need air about as much as they need water.
Thirdly it includes fine sifted pine bark to provide a more organic particle but one that breaks down only very slowly so as not to impede drainage.
The proportions are approx. 40% akadama, 40% expanded clay and 20% bark. This gives a really free draining mix that will not remain waterlogged even in very wet conditions. The bag contains very approx. 15litres