Hi Natalie...I amend regular potting soil with about equal amounts of chicken grit #2 and find this works well in my rock garden. Grit is basically small crushed rock and it provides excellent drainage. I pay about 11 dollars per 50 pound bag at the feed store.
Whatever soil you use, you need to make sure it drains freely. I have agaves and small cushion plants thriving in my garden with this soil mixture.
