Once you have healthy garden soil, rich with healthy compost* you can make potting soil. But first you need the right compost. Compost is a substance called humus that "mediates ion exchange" to plant roots. Yet compost may contain either beneficial micro-organisms (aerobic decomposition) or harmful micro-organisms (from anaerobic decomposition). You can't buy organic compost from most garden supply stores. You have to buy it from an organic gardener (hard to locate) or make it yourself. If you like to mail order you can buy it from Peaceful Valley Farm Supply.
If you cannot buy it or make it you can steal it from a forest. That black, clean smelling stuff at the bottom of leaf piles is humus. In fact, early organic gardeners used to collect this stuff from forests to spread on their fields. But please do not think that cow manure is compost! Nor is "Redwood compost" sold at garden centers. Stay away from "mushroom compost" as it is a waste product. Sure, you can get compost from any number of people or stores that swear it is compost and I guarantee you it is either 1) not fully composted, or 2) contains wood chips that will rob nitrogen, or 3) contains harmful bacteria. Ok, enough about compost.
Want the real stuff? Order it from Peaceful Valley Farm Supply:
Fish meal (N), rock phosphate (P), green sand (K)
Your cacti need humus (compost) on the roots, plus fertilizer and minerals. Organic gardeners provide minerals with rock powders. Rock powders are a slow release source of minerals. Some last for several years. You can readily buy packages of them from most gardening stores.
General Info: http://www.groworganic.com/a/a.html?sCategory=342
Advice about soil "mixes"
Learn about organic soil from an Eliot Coleman book before you get too involved in complicated mixes of cow manure, sand, lime, etc. I do not recommend that you follow the path of commercial nursery plant production. Such operations grow in small containers—this is very different than what may be best for a San Pedro. Nursery production techniques remind me of raising chickens in cages. I'd rather let my chickens "free range" to be healthy.
The best cactus soil is rich, organic garden soil--the good earth one raises vegetables in. Add to this 50% perlite for drainage and you have ideal potting soil.
Allow your cactus lots of root space! Think "tree". This is not a small, slow growing barrel cactus. If San Pedro could walk they would leave the sandy, dry pots people put them in and plant themselves in rich soil with lots of water. But lots of water means you have to provide lots of drainage or it will rot.
After rooting (when you see plump new growth) you feed the plant like a fruit tree or garden vegetable with fish emulsion (high nitrogen) in the spring and Miracle Grow. Organic fertilizers are great if you can get them--slow release rock powers.