Lava rock is a commonly used in bonsai soil mix to prevent compaction. Lava rock will not break down over time. It is non toxic and completely reusable which makes it an ideal component for soil mixes. The porous structure of the lava rock also allows for water retention.- Amount: 5 Dry Qt- Ingredients: Black Bonsai Grade Lava Rock. Fill approximately a 12-14" bonsai pot.