Potting mix basics
What's in the bag, and what perlite, bark and coir actually do
Why not garden soil?
Don't fill pots with plain garden soil on its own. Dug up and packed into a container, it loses most of its air spaces, holds too much water and can bring in weeds, pests and diseases.
Potting mix is made to stay light and airy in a pot. Most bagged mixes contain little or no actual soil.
What's in the bag
Each ingredient has a job.
- Peat moss: holds water and nutrients like a sponge. It's acidic, and peat bogs take centuries to regrow.
- Coir: coconut husk fiber, used in place of peat. It holds water well but few nutrients.
- Perlite: white, popcorn-like bits of heated volcanic rock that keep the mix airy and draining.
- Vermiculite: soft, shiny flakes that hold water and some nutrients.
- Pine bark: chunky pieces that open up the mix. Orchid mixes use coarse bark.
- Coarse sand: mostly adds weight to steady a pot. It does little for drainage.
Match the mix to the plant
An all-purpose potting mix suits most houseplants. Cactus mix drains faster, with plenty of sand or grit. Orchid mix is mostly lumpy bark that holds little water, because many orchids grow on trees in the wild.
Dry peat and coir repel water, so moisten a new mix before you plant, and don't let pots dry out completely.
Read the bag
Many bagged mixes include a little starter fertilizer. Hold off on feeding a freshly potted plant for a few weeks, and follow the label when you start.
Perlite contains fluoride, so avoid perlite-heavy mixes for spider plants and dracaenas.
Freshen it up
Potting mix breaks down over time. It packs tighter, drains more slowly and holds fewer nutrients. Repot fast growers every year and slow growers every 2 or 3 years, into fresh mix.
Handle it safely
Perlite and dry mixes can be dusty, so dampen them before use and avoid breathing the dust. Bagged mix and compost are full of microbes, including, rarely, bacteria that cause lung infections. Open bags away from your face, and wash your hands before you eat or touch your face.
Related plant problems
Sources
- NC State Extension: Extension Gardener Handbook, 18: Plants Grown in Containers
- Iowa State University Extension: Potting Media: Components and Handling
- University of Maryland Extension: Growing Media (Potting Soil) for Containers
- CDC, Emerging Infectious Diseases: Risk Factors for Legionella longbeachae Legionnaires' Disease, New Zealand
General guidance, not a diagnosis. Written by PlantWiz in our own words from the sources above.