I understand that most people have a basic idea of what constitutes a bonsai (specific) soil. I'm slowly learning what will work best for my tiny trees in my environment. My question is more on the theoretical side of bonsai care.

Why do bonsai need a special soil? What is wrong with regular potting soil to use for trees? What is the purpose of using a substrate that is almost devoid of nutrients and barely holds water? Regular trees grow in soil, often with compost from dead foliage.

Please remember that I'm not asking about what should be in a substrate. I'm asking why use bonsai specific substrate.

Pots are not the ground; pots will hold a column of water inside, and if nursery soil is to be used, great care should be taken so the roots don't get overwet, leading to root rot eventually.
It can be done though.
If the mix is draining, then overwatering is almost impossible, but care must be taken as underwatering can be an issue.
Please read the link below:
Characteristics of good bonsai substrate

Boy, an I going to get creamed for this.

Dirt is dirt. The woods are full of it. If trees could not grow in the dirt on the forest floor there would be no forest.

You can grow trees in sand if you check moisture level frequently and if you fertilize often with the nutrients the tree isn't getting from dirt the trees are used to. Or, use dirt and let the tree grow the way it evolved. Well, it isn't that easy. You do need to get big chunks out and you do need to mix sand or something up for drainage. You do need to fertilize because the tree will suck nutrients out of the small amount of soil. But, trees have been growing in plain old dirt for millions of years. Trees grew in plain old dirt in Japan for a long time before people started selling that stuff.

If you examine dirt in the forest, it isn't like potting soil, however. But none of that matters. Growing in free-draining soils makes watering easier. The less organic material the less water the soil will retain, depending on which materials you use. I use a mix of akadama, lava rock, other types of rock, and some organic materials, because I am not at home all day. So, usually I only had to water twice in the summer, when it got really hot and dry here. I'm moving from an apartment to a house next month, so it will be interesting to see if this mix will work for me in the future. I may go to a larger organic part, when I repot in the spring... I'm going to have to give some thought to that... but first I will need to get a real greenhouse.

Ivan wrote:

Boy, an I going to get creamed for this.

Dirt is dirt. The woods are full of it. If trees could not grow in the dirt on the forest floor there would be no forest.

You can grow trees in sand if you check moisture level frequently and if you fertilize often with the nutrients the tree isn't getting from dirt the trees are used to. Or, use dirt and let the tree grow the way it evolved. Well, it isn't that easy. You do need to get big chunks out and you do need to mix sand or something up for drainage. You do need to fertilize because the tree will suck nutrients out of the small amount of soil. But, trees have been growing in plain old dirt for millions of years. Trees grew in plain old dirt in Japan for a long time before people started selling that stuff.

Yet ... A bonsai is not a regular tree. With bonsai you try to grow small trees in even smaller pots. Yet you expect them to get a dense canopy, good growth and develop. To do this, the plant needs to cram way more root growing tips in a small space then normally. Putting added stress on oxygen provision to the roots. This is why you need an open substrate. Besides that, it is a lot easier to not overwater and regulate fertilizer than when using dirt.

So in other words.. CAN bonsai grow in normal soil? Sure. Will you run into many problems? Probably, yeah.