The soil in which you choose to grow your Bonsai is in effect its sole life support and as such you cannot pay too much attention to its preparation. It should be considered very carefully in terms of the function it has to perform for a particular species of tree at a particular age.
Before going into the specifics of soil preparation there are certain key points to take into account. The soil has to physically as well as nutritionally support the tree; it must be able to drain freely; it needs to contain oxygen in the form of air; it should have the capacity to remain comfortably damp without becoming waterlogged; it should have within its properties a 'buffer' capable of reserving nutrients in a dissolved state; it should be possible to control its pH value; it should not be unsightly in its appearance; it should be capable of retaining its physical state for as long as possible to reduce any tendency to compact.
Having said this, there is no single Bonsai 'magic mix,' and most Bonsai enthusiasts tend to arrive at their own conclusions concerning the right soil. This, to a great extent, is dependent on the local availability of the necessary ingredients. What is important is that the function of the soil is fully understood and some of the points mentioned will serve as a useful guide.
I use Grimson stone and crushed lava rock for my aggregate. I have heard of people using cat litter, Turface, or Oil-Dri which are all calcinated clay. My experience with calcinated clay here in Florida is with our summer rains it stays too wet and breaks down too quickly. Clay is good at holding nutrients in the soil but it also holds the salts and other unwanted ingredients from your fertilizer which could toxify your soil, so I found a little clay is good but to use it as your main aggregate I didn't care for. I found the natural rough aggregate works good.
I sift the aggregate through a 1/4" screen, what stays becomes my aggregate for my coarse mix, what goes through is sifted again through a 1/8" screen, what stays becomes my medium mix and what goes through is for my fine mix.
For my organics I use dried, aged, chipped pine bark. I sift it through a 1/4" screen, what stays goes into my coarse mix and what goes through is sifted again through a 1/8" screen, what stays is put into my medium mix, what goes through is my fine mix.
The proportions of each are as follows:
25% pine bark
The proportions of each of the basic ingredients, however, will vary according to the species of tree grown and as to which stage of development the tree is in. Younger trees in early stages would need more organics for faster growth, whereas your more mature trees will do better in less organics to control growth. Conifers generally prefer more aggregate for better drainage, maples and other deciduous trees prefer more organics. Experimentation, followed by observation, is the best way of finding out which soil mix is best suited to a particular species and location. Although one can have good results by using just a standard mix.
For the hobbyist with a few trees one would probably be better off finding a source of standard, ready made soil, suitable to your area and then amending it to your particular species and age of tree. If you have a large collection then doing all the work to make your own soil might be beneficial.