While both can contain ingredients such as moss, coconut coir, or perlite, there are several key differences. Unlike potting oil, potting mix contains no sand or topsoil. As a result, potting mix is usually much lighter and completely sterile of bacteria or fungi. Potting soil is heavier and less likely to be sterile. Potting mix is more likely to come with an added fertilizer whereas potting soil relies on natural composted items to be the fertilizing component. There are some plants you would want potting mix for, especially cacti and for seed-starting purposes.