Lava Rock is the best and natural.
I like to think it retains a bit of residual fertilizer through the years as it trickles down, and benefits the plants.
Any soil that does in time trickle down through it only adds to a more natural substrate that some plants roots may benefit from.
The upside is that as rock goes;...Lava is very porous, cheap, natural, and light! It will be here longer than us. Leave the plastic for the recycling centers is my thought.
