What is bonsai muck and how would I use it? There are times when you’d like to mound the soil in your pot. Usually the soil will just erode to a level plane. Muck pressed out to about a quarter inch thick can be used to fabricate walls to contain soil. Think of it as Play Doh for adults.
There are all sorts of recipes on the internet, but, the simplest and best is just clay from your backyard and long fiber sphagnum moss. If you don’t have clay you can buy more than you can lift from potter’s supply stores for next to nothing. It comes in shoe box sized bricks for a couple of dollars each.
Just grab a baseball sized lump of clay and mix in as much sphagnum as you can while still keeping the clay sticky. It is the same principle as reinforced concrete. Press it out into manageable size strips. Pile soil in your pot and wrap it up like pie crust. Leave an open space on top so you can water your tree. If you wish, you can cover the clay with moss. It makes a great base for that. A couple of things are going to happen in time. Tree roots will penetrate the clay from inside and form a stable root ball in the shape of the mound. If you didn’t coat the clay with moss then the outer surface will erode leaving the exposed sphagnum holding the mound together. Either way you win.
One caveat, if you’ve covered a lot of the soil surface the soil will retain more moisture, so don’t over water.
I've been doing bonsai since the late 70's. I'm passing on what I've learned in those 35+ years. These are real life techniques and tips that worked for me, but, may not for you. Remember, this is free advice and worth every cent of it.