This may seem off topic, but it is at the heart of the entire soil issue and though it is a short and incomplete over view many reading this should be encouraged to research the issue further. Most bonsai failure can be traced back to problems with watering, and most of those issues are traced back to a lack of understanding of how a soil interacts with the growth of a bonsai. So if you think This is off topic don't read it and go to the next post.
I have said it before, and gotten into a debate for doing so, but I will say it again; a bonsai pot is not a natural environment. In order for a tree to do well in this setting there are several things that need to be done. Most important is the careful cultivation of a fibrous root system devoted to the biological needs of the tree more than the support functions. Second, is a soil mix that will retain enough moisture the tree has a chance to utilize it but will not retain a large quantity of water to the point the tree becomes waterlogged. Third, this soil mix must retain its structure over a long period of time.
Structure is the basic grittiness of the soil where by the particle size and relationship remain more or less intact over a long period of time. This is why the Clay components such as Haydite, Akadama and the different Turface type products are important, these are the most likely to break down but the most critical in maintaining a stable cation exchange capacity(look it up).
There is another function of soil, called field capacity. Field capacity is a measurement of the amount of water a given soil mix will retain over a given time. As a soil starts to break down the field capacity will change for the worse. A good soil will drain off the excess water once field capacity is reached. An old soil, or a bad soil will continue, from the point of break down, to retain more and more water or it's field capacity will increase. This change makes it difficult to judge a watering regimen and also makes it possible to dangerously over-water a tree that you may not be watching careful. It is bad enough that the increase of root activity and growth will tend to slow down drainage, it is even a worse problem if this is coupled with a changing soil mix that is breaking down. This is why it is imperative to pick good materials for whatever soil mix you decide to utilize.
Gravity also has an affect on drainage. In a small shallow bonsai pot gravity does not pull the excess water out of the pot as effectively as it drains water away from a tree in the ground. This is why some experts will tell you to tilt your pots to assist in draining away excess water from rain storms and over watering, it is the effect of gravity along a longer water column.
These are some of the things you have to keep in mind when selecting a soil mix, and of course the area where you grow your trees and the subsequent climates. A soil mix used in Oregon will probably not work well in Arizona.
It is true you can grow a tree in almost anything if you are willing to take the time to make sure that all of the trees needs are met, but it is better to have a soil mix that does not take rocket science on a daily basis to deal with. Hope this helps.