Root cells need, like other plant cells, oxygen.
For that your bonsai soil should be well-drained and should have sufficiently large drainage holes in the bottom of the bonsai pot.
Especially with large drainage holes the substrate trickles after repotting from the drainage holes. Therefore, these openings are covered with plastic grids.
How to do that ? Quite simply. Just follow this step by step guide:
It should be slightly larger than the drainage hole of the bonsai pot.