Soil composition is a pretty complicated matter. The best article I have ever seen on the subject was done by Brent Walston at Evergreen Garden Works. Everyone should read this article at least once. It is a real eye opener and scientifically sound.
Here is the URL.
http://www.evergreengardenworks.com/index.htm
At the home page click on Bonsai Articles then on Soils in the list of articles. As a matter of fact there are three articles on soils and all are worth reading.
Ed