The organic soils that have to be PH neutral for most trees (except acid loving specimens like Azaleas) is available in good quality in Northern Europe. It has a very beneficial influence on the root development because of its ability to store and release nutrients and water.
Disadvantage is that it decompose quickly, and will be broken down in three to four years depending on quality. Larger trees with slow root development needs to stay in the pot for six to eight years to develop roots all over, so a greater amount of Akadama and structural improving lava or gravel is recommended.
Most important it also precipitates nutrients well, releasing these slowly. This is an important feature of a soil.
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.
This type of soil do not have the ability to help roots divide into a fine pattern, extremely important for bonsai growing. Adding a higher amount of fine structured lava pellets will help this to some extend.
Akadama, which I have not used in big quantities so far, also has the ability of precipitating nutrients. Releasing these and water optimal.
Akadama seems very good for large trees, and warmer climates. In colder areas it breaks down too fast, before the roots do their job, and therefore a limited amount should be used.
The main advantage of Akadama, which is a mineral, is its stability and active breakdown by roots that develop a dense and fine ramification of the root system. When roots grow through the Akadama particle the spaces gets smaller between the particles and this increases the development of finer roots. Akadama clay pellets (the good quality) has microscopic pores that adds a high amount of oxygen crucial for the health of the tree.
When Akadama breaks down in smaller particles it also decrease the amount of oxygen, but the fine pores helps expanding the volume of oxygen for a time. For some reason Junipers seems to dislike Akadama, and the same goes for Scots Pines.
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.