Even though I said to use potting soil without fertilizer, I am not so sure that Miracle-Gro at 1/3 vol. would do any harm. The fertilizer is so doggone weak in their potting soil that I cannot imagine it over-feeding these plants. But I could be wrong. I have never experimented with planting one with no-fertilizer and planting another with weak, 6-month feed fertilizer, on plants needing little fertilizer. Commercial growers might have an answer to this question. When in doubt, I would not use the slow-release fertilizer potting mix.
I would not add any organic matter if you are using 1/3 potting mix and 2/3 "cactus" mix. The cactus mix will have the sand and/or gravel, perlite, and some organic material. There is enough organic material in the potting mix plus what's in the cactus mix. Ken
