There are three key characteristics of good bonsai soil.
First, the soil must have good water retention. The potted soil must be able to hold and retain sufficient water to continuously supply moisture to the growing bonsai between each watering session. This, however, does not mean that you need a super-absorbent type of soil mix for your bonsai. No bonsai species must be soaked in a lot of water for long periods of time. Take note that although it is in a miniaturized form, it is a tree that you are dealing with and not a water-borne plant.
Second, good aeration is a must for any bonsai soil mix. The soil particles must be of sufficient size to allow air pockets or gaps between each soil particle for air to circulate. The bonsai roots must always have access to air so that your miniature tree will grow healthily. One indicator of a poorly aerated soil is this: upon lifting the bonsai root ball out of its pot during repotting time, you notice that the roots grow around the base of the pot and within the soil itself. This only means that the soil inside the pot is airless and that it is very water retentive.
Third, you need soil with good drainage. If the soil has excellent drainage, then any excess water drains right away from the pot and will not end up flooding the tree roots. If the soil has poor drainage, then it is too water retentive for the bonsai tree. The tree will not be properly aerated and it will lead to damaging salt build up.
The composition of the soil is an ongoing issue among bonsai enthusiasts. Soil mixes are marketed commercially, but they are quite expensive. Many experienced bonsai growers learned the art of mixing bonsai soils on their own. So, as long as the three bonsai soil essentials €" good aeration, good water retention, and good drainage -- are present in the soil, then your bonsai will grow and train properly.
For beginners, simply use inorganic soil so that you can start cultivating easily. Inorganic soil is already well-structured, particle-based, and has all of the three characteristics discussed above.
For a much more detailed and updated information about maple bonsai cultivation, styling, and maintenance techniques, visit http://www.maple-bonsai.com/€">Maple Bonsai.