For an even cheaper solution, you can check your local superstore for cat litter. Most cat litter that are unscented and non-clumping are made of clay and diatomite. This type of litter product is also indistinguishable from clay based oil spill absorbent product used in many car mechanic garages. Some litter products are made of corn and sand, these litter products are not suitable bonsai. You must find a litter that is unscented and non-clumping and composed of clay and diatomite. Many bonsai adept have successfully reported using 100% cat litter as bonsai soil with great success. )
...and since I have a few willow trees to spare I thought I'd try an experiment.
Here are 3 trees..
one in 100% diatomite( napa oil dry),One in half diatomite half potting soil
and one in 100% potting soil. I will solve once and for all, and purely out of curiosity, which is best.
This is what happens when I have too much time on my hands
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You need to make sure the cat litter is diatomaceous earth. If it isn't it will turn in to clay when you water it. That will be pretty bad for anything planted in it.
Oooopps! Sorry about that, Lance. That's what happens when I stay up too late! I'm still chuckling at myself. Here is the link from the Empire site. www.bonsaiempire.com/care/repotting You may have read it already, or not however, at the end of the article there is a link to watch a video on Bonsai soil and what he has to say about using standard potting soil with Bonsai. Hope you find this helpful.
Mushroom Compost has high water and nutrient holding capacity. I'm not convinced that would be good for bonsai since they need free draining potting medium. If you want to try it, do it on something you wouldn't mind losing.