Answer: Generally you would use a fast draining soil mix formulated for cacti, the same as you would plant the cactus in eventually. Alternatively you could use a soilless mix formulated for container plants with some clean coarse sand added. Based on my experience, I would spread a very thin layer of vermiculite on the soil surface before planting. This would help prevent the spread of disease during the germination process.
If your local store does not carry a cactus mix and you want to try this, you could mix your own based on the Cornell epiphytic mix recipe:
Good luck with your seeds!
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