Personally, I prefer ceramsite. In North China, it is recommended to use bluestone and ceramsite. Charcoal is too light and alkaline, so it needs to be used with broken bricks. Personally, it is not recommended to use the latter two. Broken bricks are too heavy and angular, which is easy to cause root deformation. Sapphire is the best substrate, because it has moderate weight, can hold the basin down, and has good water retention and air permeability.