I am trying to develop a good Juniper soil that is fast draining, but without any rocky inorganic material like surface, akadama, or decomposed. Roots can’t grow into Lava rock. Organic potting compost includes peat moss, perlite, and sand. It has several disadvantages (it retains much water and doesn’t aerate/drain very well), but as part of a mixture, it can be used perfectly well. Fine gravel/grit helps to create a well-draining and aerated Bonsai soil.
Juniper trees tolerate a variety of growing conditions, making them excellent specimens for … A basic bonsai soil mixture may be made from 1 part loam, 2 parts. To go into further detail: Conifer bark as a soil conditioner can add bulk to the mix and can retain moisture; while allowing excess water to drain. Peat moss (humus) is what retains the most water and therefore should be used sparingly. Potting soil will add bulk to your mix as well as used as a bonding agent.
Conifer Bonsai Soil Mix information: