I think at the beginning of this train ride I mentioned that the most important aspect of a bonsai soil is structure. Structure is the physical nature of the soil as to how well it remains the same over a prolonged period of time. Water and climate have their effects on the structure of a soil mix in that the soil structure will break down and change, causing the soil to hold more water and expell less than it used to over a determined length of time. I have also found that a good soil has to breath. This is why I have my success with Mugo Pines. Mugos are a paradox in that they don't like to be wet but they like to be watered a lot. This means that you have to have a soil mix that drains well. Mugos like to be watered a lot because they need to breath. Essentially they liked the soil ball flushed with clean water and air.
Breathing for a soil mix is the process of water driving air out --- and--- gravity pushing and pulling the water through and out, pulling fresh air into a soil mix in the process. This is what hinders the formation of root rot, the circulation of fresh air in a soil mix. The fungus that forms phytotoxins cannot flourish in this kind of condition. When structure breaks down this process changes and when the structure changes it is usually not for the best causing the arability to slow. When the airability changes there is a tendency to over water because we all default to bad habit at times. We water this many times a day because we always have, not realizing that our soil is holding onto a lot more water than it used to. This in turn can cause root rot to start. A technical term is field capacity and can be measured if you are so interested in the debate.
What is important is that you understand what your soil is doing and know what to do when it stops doing it. Consistency is important in maintaining the root health of a bonsai and if you cannot achieve a consistency you are never going to be able to figure out what the hell is going on. Sometimes we get lucky,--- sometimes we don't. It is for the sake of structure that my mix is as it is. I don't like akadama because it breaks down too quickly, I don't like garden dirt because it breaks down too quickly and unless I have some lab tell me what is in the garden dirt I have no clue what I am doing. If I am just digging dirt out of the ground I have no clue what's in it, it could contain Triox.
In the beginning when I started doing bonsai we used collected garden/field soil,-- essentially dirt. We did not have access to pumice, coarse sand, composted Pine bark, Akadama, or even Turface and a host of similar products. We used good black dirt gathered from a foot or two below ground dried and sifted out to particles about 1/8 inch. We collected clay, red and gray driving distances to find it. We sifted this stuff and we did use construction sand. We also used forest loam and leaf mold. It was a major project to get a soil mix together and it did not hold on to its structure more that two years. Sometimes these soil mixes worked fine and occasionally they failed forcing us to repot a lot of trees or lose them because there was something wrong with the latest batch of soil. Finding consistent elements that can be put together like a kitchen recipe for apple pie was a major sea change in the world of bonsai.
The short story: You can use what you want for a soil mix as long as you have the time and talent to recognize when something is going wrong and know what to do about it. It is my belief that it is better to know what you are putting a tree into and what it is going to do over time than it is to of a sudden discover that your tree is not looking right. The problem with garden dirt is you don't know up front what is in it unless it is you that made that dirt up. I am not sure what the garden dirt recipe really is, I suspect it is more man made than is being portrayed.