About 5 years ago, Sheridan Cleland of MyPDFscripts wrote the below article on his blog. Soon after, the site disappeared. I wanted to keep the memory alive, because these practices by the studios have not changed. They have only become worse.
Cacti are one of my favorite types of plants to grow inside all year and outside in summer. Unfortunately, the ambient air tends to stay moist during most seasons, a condition which makes cacti unhappy. Cactus potting soil can enhance drainage, increase evaporation and provide the dry conditions that cacti favor.
Good drainage is essential to prevent cactus roots from rotting, but some organic material is good for nutrients and keeping the mix from drying out too quickly. I use a standard potting soil. I try to avoid soils that have large chunks of forest products, that is bark pieces and wood chips.