When it comes to bonsai supplies, bonsai soil is one of the most difficult to determine. There are many different formulas and no simple answers. Why special soil for bonsai?
Bonsai are essentially container plants we've made look like small trees. Because we are asking these plants to live as long as they can in a pot, soil is a major consideration.
You can either mix your own or start by purchasing a basic bonsai soil mix. Today, many hobbyists are using “soil-less mixes.” This just means they do not contain any earth or dirt. (Never use dirt from your yard for any potted plants.)
If you decide to make your own bonsai soil, remember all components should be free of dust and be similar in size. This can be resolved with sifting and rinsing. (Dust and very small particles can clog the drainage.)
Try using one of each - organic and inorganic.
Discover which works best for you and your bonsai. Never repot all your bonsai in a recommended mix, without testing it first.
Some people use peat moss to add acidity for certain plants. However, it retains a lot of water and should be used sparingly.
"Potting soil" from garden centers is a last resort for organics and should only be used with the addition of an inorganic aggregate. If you choose to purchase potting soil, do not buy the cheapest! It will most likely be the wettest and heaviest, which is what you do not want!
You will hear about many seemingly odd materials to add to your mix including ‘chick grit’ (a crushed granite available in farm feed stores) and 'kitty litter' a type of calcined clay, as well as imported akadama clay.
Used properly they all work. Lava rock and pumice are other additions used with good results. Overall, a good mix looks like small gravel!