As was said earlier, the store bought cactus mix (just about all the premade mixes) seem to have a lot of wood chips in them (cheap filler I guess). If I have to use them, I strain the wood chunks out with a home-made straining screen, which helps some. For my trichos, I do 50% perlite, 20% sand, and 30% good organic soil. I fertilize with slow release 10/10/10 or so. Works for me.