Gday Andy and welcome!
I'm no expert on
anything Bonsai related, including Chinese Elms, but I can tell you that they will require some portion of organic material in the potting soil. I use about 60% sifted potting mix to 40% sifted sharp gravel (basalt/'bluemetal'), plus a generous serve of pelletized organic fertilizer. Proportions of organic/inorganic ingredients will vary from tree to tree, and will differ according to your specific situation (location, climate, whether the tree is in a training pot).
By double clicking on the word 'soil' in your original post, you will open up the
Wikipedia where you should get some ideas.
Good luck.
FlyBri.
PS: Is that a
heater your tree is sitting on? Cute looking tree, BTW!