Guide
Is potting mix the same as potting soil?
Mostly yes, and usually neither contains soil. Read the ingredients.
Short answer
For practical purposes, yes. Both names are used for container growing media, and most contain no soil at all: peat moss or coir with perlite or vermiculite, sometimes bark fines, plus lime, a wetting agent and a little fertilizer. A bag sold as potting soil is more likely to include compost or soil and be denser. What is not the same is garden soil or topsoil, which packs down in a pot. Read the ingredient list rather than the name.
What is in the bag
The University of Saskatchewan’s gardening program notes that potting soil “often does not contain soil” and is generally denser than potting mix. Plant Nebraska describes the usual base as peat or coir, lightened with perlite or vermiculite. Proven Winners suggests looking for “soilless potting mix” on the label.
| Ingredient | What it does |
|---|---|
| Sphagnum peat moss | The usual base: light, holds water |
| Coir (coconut fibre) | An alternative base to peat |
| Perlite | Opens air space and drainage |
| Vermiculite | Holds water and nutrients |
| Bark fines | Bulk and structure |
| Lime | Raises the pH of acidic peat |
| Wetting agent | Helps dry peat take up water |
| Starter or slow-release fertilizer | Feeds the first weeks |
Why not garden soil
Garden soil in a container gets dense or mucky, Proven Winners warns, because a pot has none of the worms, roots and freeze and thaw that keep soil open in the ground. It is also heavy. A 16 in planter, 12 in across the base and 14 in tall, takes 39.8 L of mix to fill once settling is allowed for:
| Fill | Planter, lb | 2 cu ft bag, lb |
|---|---|---|
| potting mix (peat based) | 40 | 56 |
| potting mix (coir based) | 44 | 62 |
| Mel’s Mix (1:1:1) | 42 | 60 |
| topsoil and compost blend | 97 | 137 |
Filled with topsoil and compost, the planter weighs 97 lb before the pot and the plant, against 40 lb with peat potting mix. On a balcony or a deck that difference matters. The container soil calculator gives the volume for any pot.
Does potting mix go bad?
Not in the way food does, but it changes. Premier Tech Horticulture, a peat mix manufacturer, says growing media “does not necessarily go bad, but the pH, nutrient status and wetting ability can change”, and recommends using peat-based mixes within nine months of manufacture and bark-based mixes within 2 to 6 months.
- It dries out and repels water. Dry peat sheds water rather than soaking it up. Proven Winners suggests soaking it overnight before use. Heat and sun also break down the wetting agent, according to University of Massachusetts greenhouse specialist Tina Smith.
- The fertilizer runs out or releases early. A starter charge is used up within 2 to 3 waterings and a slow-release one in about a month, per Proven Winners. A bag that has sat too long or got wet can release fertilizer early, which can burn young plants.
- The pH can rise. A wet bag can activate the lime, raising pH.
- Moisture gels weaken over a season.
- Slime mould on the surface looks alarming but is harmless.
Store bags cool, dry and out of the sun, and buy close to when you will use them. Last season’s mix is usable once it is rewetted and fed, since its fertilizer is long gone.