My soil mixes is based on Sphagnum Peat Moss, and gravel.
This has shown to be very good for especially Shohin and Mame-bonsai, but I also use it for the few larger bonsai I have.
The Sphagnum Peat Moss available in Northern Europe is of a very high quality (Probably it is also available in many other countries). It has a very beneficial influence on the root development.
Most important it also precipitates nutrients well, releasing these slowly. This is an important feature of a soil.
The structure is broken down over time, but when dealing with bonsai, it works well for three or four years. In mixes with much gravel it will last even four too five years before broken down too much. The best quality to look for is a light colour and reasonable roughness, making it last longer before broken down, securing oxygen in the soil.
The PH value (acidity) of the soil differs according to the type of Sphagnum but this can be regulated by adding lime or acid to the soil.
Akadama, which I do not use, also has the ability of precipitating nutrients. I do not know of how well this works for that type of soil, but it probably works good based on the structure of the gravel.
Gravel has two main purposes. To secure drainage in the bottom of the pot, and to add drainage to the soil mix.
Depending on the physiological structure of the gravel it may or may not be able to precipitate nutrients, and releasing these in a shorter or longer time span.
Most gravel has a life time unbreakable structure, and others wither with time.
Using pure gravel or anything with the structure as gravel will not at all be able to bind any nutrients. This means that any nutrients will be washed out immediately, and nothing will be stored for later use, when watering without adding nutrients to the water.
For threes that doesn’t need much feeding in a period that might work well on a shorter time span. But the health of the tree and the vigour and growth will be set back over time, because lacks of the main nutrients and micro and macro-nutrients will influence on the ability to develop healthy roots, back budding, flowering i.e.
Repotting a tree into pure gravel has only one purpose. Saving a tree with rotted or other vice harmed roots, by adding as much oxygen to the roots as possible using pure gravel, but on a short time span.
Afterwards, when the tree has re-established the root system, it is necessary to use a soil mixture with ability to precipitate and release nutrients.
Fertilizers and their influence on the soil
Using soil with the ability of storing and releasing nutrients can have a side effect when using chemical fertilizers. Over feeding will have a negative effect, so be careful not to do this when using chemical fertilizers. Chemical fertilizers also break down the structure of the soil faster than when using organic feeding, making it necessary to repot more often.
Storing to high concentrations of fertilizing salts will cause roots burn. Organics fertilizers are secure to use, because these will not store salts in the soil. The nutrients form organic feeding pellets are also released slowly and thereby high concentrations are avoided.