Bonsai soil is one of those subjects where everyone seems to have a different opinion- so I guess I get to share mine. I hope you find this helpful.
Why care about bonsai soil? Like really, isn't it basically the same as any houseplant soil?
Bonsai soil is different than regular houseplant soil and it needs to be.
One big reason is the bonsai pot. You see, in spite of how they appear, bonsai pots- because they tend to be disproportionately shallow - don't drain very well. In fact, they're kind of notorious for hanging onto water. They're often relatively wide and straight-sided compared to your typical houseplant pot that is deep and narrows at the bottom, which naturally promotes better drainage. Additionally gravity does a better job pulling water out the bottom of a deep pot, just because its further to travel. Huh? Yup. Gravity.
Lot's of things I learned about the movement of water in a pot turns out to be junk. You know how you were taught to pile some stones in the bottom of the pot to better promote drainage? That's a load of junk as well, because when suddenly faced with particles of a different size, water does not naturally head for the big holes, it gets held back. (I think that's hydrostatic something or other- I'll try to remember to look up the correct term before I finish this post).
Now, having said that I must point out that a layer of larger particles on the bottom of the pot actually is a good idea. That's because they allow for air to flow in the drainage holes on the bottom and provide oxygen into the root zone.
And that is what your bonsai needs - water and oxygen in a good balance that provides the necessary moisture but also lets air reach the roots so they don't rot.
Watering is a challenge and you can kill by overwatering -too much water and not enough oxygen slowly rotting the roots and by underwatering- not enough water, allowing the roots to quickly dehydrate.
Underwatering kills faster, which is why I don't tend to recommend "REAL" bonsai soil to novice bonsai growers. Real bonsai soil has no soil at all, its a blend of pumice, lava rock and a type of japanese clay called akadama. The ratios between these ingredients can change depending on the type of tree you're growing and the recipe can be altered with the addition of an organic component like pine bark, but the main feature of real bonsai soil is that it drains exceptionally well. Oh, and it costs a fortune
That's fine for someone who is used to watching over their trees with the care of a mother hen, but for someone who is just as likely to forget to water for a week, this stuff is the kiss of death. I feel fairly confident in that statement because I used to pot my trees for sale in "real" bonsai soil and my CKR (customer kill rate (forgive me- that wasn't really nice)) became quite noticable. The symptoms? "Leaves turned brown and crispy and all fell off " "Leaves turned pale green and crispy and all fell off" Its the crispy and falling off part that, to me, is the give away.
So while real bonsai soil drains a bit too well for the novice, you still must have something that will release water and allow for oxygen to reach the roots. As far a commercial options are concerned I'm a fan of cactus/orchid mix. Look for something that either has both or just cactus soil. Pure orchid mix is pretty much bark chips.
Another option is a 3-4:1 ratio of commercial seed starting mix ( which is easy to find and very affordable) with perlite ( not quite so easy and considerably more expensive). Don't substitute vermiculite for the perlite. They look the same but that's where the similarity ends. Vermiculite holds moisture, perlite does not.
If you really want to go with the "real" stuff, you'll probably need to buy the components separately and put them through a soil sieve so that you match the particle sizes and remove what we call "the fines" (superfine particles that don't allow for the movement of air).