Potting soil holds moisture and nutrients around your plantís roots, acting as a reservoir for these critical elements of life in your container garden. It also provides enough air for growing roots to be able to breath and not rot once you plant your container. Finally, it supports your plant, providing anchorage for the roots. A soil mix needs to settle around the roots of your plant and help hold it in place, so that it doesnít blow over from the first wind. However, it also needs to be light enough to allow water and air to always be present under the soil surface so your plantís roots have a balanced atmosphere in which to grow.