What’s in Potting Soil? It’s More Than Just Dirt

If you’re surprised to learn that potting soil (or potting mix) doesn’t contain any real dirt, you're not alone. This confusion likely arises because gardening experts use the word ‘soil’ differently than most people. As Paul Cunningham, hard goods buyer for Pike Nurseries & Armstrong Garden Centers, explains in an email interview, ‘Soil is a broad term often used interchangeably with planting medium. Soil typically consists of minerals and organic matter (decomposing plant, animal, or microbial material).’ He adds, ‘A planting medium refers to various materials used for planting in containers or the ground. Potting soil can be considered soil when it contains the ingredients I mentioned.’

Potting soil is typically made from a mix of Canadian sphagnum peat moss, aged bark, compost, earthworm castings, horticultural grade perlite, pumice or cinders, and controlled-release fertilizer, according to Cunningham. ‘While single ingredients like peat moss or bark aren’t technically soil because they lack the complete mix, they can serve as planting mediums for potted plants,’ he notes.

Why is 'soil' excluded from potting mix? Actual soil is much denser and less airy than potting soil. ‘Excess water drains away quickly in potting soil, allowing air and roots to move freely, promoting better plant growth and reducing stem and root diseases,’ says Jon Traunfeld, extension specialist, director, and state master gardener coordinator at the Home & Garden Information Center at the University of Maryland Extension. In contrast, ‘topsoil in containers retains moisture and limits air circulation. The smaller the container, the more significant the negative impact,’ he adds.

Potting soil must also be light in weight to keep shipping costs affordable and to ease the strain on passionate gardeners, explains horticulturist and gardening expert Melinda Myers. ‘By leaving soil out of the mix, you lower the chances of encountering soil-borne insects and diseases,’ she adds.

However, budget-conscious gardeners shouldn’t replace potting mix with regular dirt. ‘Don’t take soil from your garden and use it in pots. Even well-drained garden soil won’t drain properly when confined to a small container,’ Myers advises via email.

So when should you choose potting soil? ‘We usually recommend potting soil for all types of container gardening, whether indoors or outdoors – including glazed pots, hanging baskets, window boxes, and even raised beds,’ says Cunningham. ‘Your local garden center can help you find the best potting soil for your specific plants.’