Repotting season is here. I don't plan to repot as many trees as last year but there is still 2 maples, a crab apple, an euronymus, a carpinus and fir to take care of. This is what I use for planting medium in bonsai pots and also for houseplants:
Damolin A/S in Denmark. Cheap, works well and is easily obtained.
I have tried to mix in organics sometimes, bark & peat and it has worked well but I don't see any reason for it unless the tree need mychorizza. Usually I just pour it in the pot straight from the bag. It drains well and looks ok. See more info on cat litter for bonsai at bonsai4me.com.