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.
Container grown cactus plants may require root pruning if you want to keep them small. Outdoor plants also require a root trim if you dig them to plant them in a new area. Dead roots appear dry and brittle or soft and rotten. These require removal back to the healthy root section. Healthy roots also require trimming.