Lava rock in the bottom of pots seriously messes with drainage and should be avoided - unless we're talking about large chunks of lava that serves as ballast as opposed to a "drainage layer". Most commercial potting media based on fine particles like peat/ compost/ coir, et al, support 3-6" of 100% saturated soil at the bottom of the pot after a thorough watering. The 3 line drawings below are pots filled with soil. The shaded area of the saturated soil, aka a perched water table. Perched water is very limiting and the longer it persists in the pot, the more limiting it becomes. The drawing on the left is a conventional pot set-up with perched water at the bottom. The middle drawing shows how water perches in the soil ABOVE a "drainage layer". This takes a big bite out of the volume of healthy (well-aerated) medium available for root colonization. The drawing on the right illustrates how bricks or other large objects used as ballast will displace soil that would otherwise be filled with water. Ballast, used effectively can eliminate as much of 90% of the excess water a planting can hold, and transform the growing experience of those who for whatever reason find themselves using a medium that holds too must water to allow good root health.
Al
