If you find it shocking that there's no actual dirt in potting soil (also known as potting mix), you're not alone. Much of the confusion is probably because gardening experts don't use the word "soil" the same way that regular people do. "Soil is a broad term and is often used interchangeably with planting medium. Soil in general is composed of minerals and organic matter (decomposing plant, animal or microbial matter)," explains Paul Cunningham, hard goods buyer for Pike Nurseries & Armstrong Garden Centers in an email interview. "A planting medium refers to a variety of materials used for planting in containers or the ground. Depending on the recipe, potting soil is soil because it contains all ingredients mentioned."
Potting soil usually contains a combination of Canadian sphagnum peat moss, composted or aged bark, compost, earthworm castings, horticultural grade perlite, pumice or cinders and controlled-release fertilizer, according to Cunningham. "Single-ingredient items like peat moss or bark are not technically soil because they do not contain the full mix; however, they can be used as planting mediums for potted plants," he says.
So how come "soil" gets left out of potting mix? Actual soil is much heavier and less porous than potting soil. "Excess water drains quickly [in potting soil] and air and roots can move and grow more freely, which improves plant growth and reduces root and stem diseases," emails Jon Traunfeld, extension specialist and director and state master gardener coordinator with the Home & Garden Information Center at the University of Maryland Extension. By comparison, "topsoil placed in a container stays wet and restricts air movement. The more shallow the container the bigger the negative effect," he says.
Potting soil also needs to be lightweight to keep shipping costs down and protect the backs of avid gardeners, says horticulturist and gardening expert Melinda Myers. "Eliminating soil from the mix reduces the risk of soil borne and based insect and disease problems," she adds.
But thrifty gardeners looking to save a few bucks should never forego potting mix for readily available dirt. "Do not dig up the soil from your garden and place it in a pot. It will not drain well (even if it is well-drained garden soil) when confined to the small space of a container," Myers says via email.
So when should you use potting soil? "We generally recommend potting soil for any indoor or outdoor container gardening – glazed pots, hanging baskets, window boxes and even raised beds," Cunningham explains. "Your local garden center can confirm which potting soil is the best planting medium based on plant selection."