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.
Regular potting soil presents challenges. The plant needs a loose, well-aerated mix to thrive in a container. Compacted soil prevents roots from getting the air they need. Stress and wilting can result. Porous, sandy soil enriched with peat moss or leaf mold provides an ideal medium for Christmas cactus roots.