There are nearly as many bonsai soil recipes as there are bonsai trees in the world. It seems that just about everyone has their own mix. The important thing is to find a recipe that works for you and your trees. Your soil mix should provide enough water retention to sustain the tree between watering, yet be loose enough to allow for adequate drainage. You must also consider your fertilizing routine when planning a soil mix.
Soil mixes with organic components tend to retain more moisture, as well as retain more fertilizers. This means that if you are fertilizing often then you will need to be careful that you do not overload the soil with fertilizer. Too much fertilizer may burn the fine feeder roots.
Inorganic or "soil-less" mixes are another alternative. I've been using a soil mix that I adopted from Boon Manakitivipart:
- 1/3 Akadama
- 1/3 Lava
- 1/3 Pumice
- 5% Horticultural Charcoal
- 5% Decomposed Granite
For deciduous trees and trees that prefer more moisture, use a smaller/finer mix with a little more Akadama added to retain more moisture.
An inorganic mix such as the one detailed above, allows you to water and fertilize a little more often. Watering more often will help flush the soil of excess salts and other build-ups that may occur from municipal watering. Fertilizing more often will help produce ample growth during our growing season.
"You can grow plants in anything if you change your watering, fertilizing, and other cultural habits to match your soil" – Brent Walston
There are a number of trace elements that a tree will need to survive. By using a soil-less mix you control when the tree receives fertilizers, vitamins, and minerals. A good way to add the amendments to the soil is with organic fertilizer cakes. Organic fertilizer cakes will have many of the nutrients that a tree will need, and will deliver these on a continual basis with each watering.
With a well draining soil mix, liquid fertilizer may be lost too quickly. That isn't to say that liquid fertilizers shouldn't be used. Trees such as pines and junipers will do well with an extra dose of an acidic type fertilizer a few times during the growing season. This will increase the acidity level enough for the trees.
Since switching to the inorganic soil mix I've seen increased root development, and have stronger, healthier trees. In fact, watering has become easier, as it's nearly impossible to over-water with this rapidly draining soil.