A pot is a closed box. Roots cannot escape into native ground, and extra water has nowhere to go except out the drainage hole — if the mix stays open enough to let it. Garden soil is built for the opposite situation: mineral particles that settle, hold water, and sit in a landscape that drains sideways and down.
What to use where
| Place | Use | Skip |
|---|---|---|
| Plastic, ceramic, or fabric pots | Soilless potting mix (compost + peat or coco + perlite + a little vermiculite) | Garden soil, topsoil, and straight compost alone |
| Open-bottom raised bed on soil | Mineral soil plus compost — see raised bed mix | Filling the whole frame with bagged potting mix (costly and often too light) |
| Raised bed on a patio or driveway (closed bottom) | A lighter bed mix with compost and perlite so it drains | Native clay dumped into a sealed box |
What potting mix is
Bagged potting mix is mostly organic particles and air. Typical ingredients are peat moss or coconut coir, composted bark or compost, and perlite or pumice. Some bags include a starter fertilizer charge; that runs out in weeks, not all season. The point is drainage and air around roots in a container, not cheap bulk.
- “Potting mix” or “container mix” is the right aisle for grow bags and patio pots.
- “Garden soil” and “topsoil” are for in-ground or raised beds. In a pot they pack, stay wet, and cut off oxygen.
- “Raised bed mix” is a middle ground: more mineral than potting mix, still not a good closed-pot fill.
- “Moisture control” mixes hold water longer. That can help plastic on a windy balcony; it can drown plants in a pot with a slow drain.
DIY potting mix recipes
Bagged mix is fine. If you blend your own, stay soilless — compost, coco peat or peat moss, perlite, and a little vermiculite. These are the same Classic and Budget recipes in the potting mix calculator, by volume:
- Classic: 30% finished compost, 30% coco peat or peat moss, 30% horticultural perlite, 10% coarse vermiculite
- Budget: 50% finished compost, 20% coco peat or peat moss, 20% perlite, 10% vermiculite — more compost when you already have it
How much mix to buy
Potting mix is sold in quarts and cubic feet, not gallons of pot. A 5-gallon grow bag is about two-thirds of a cubic foot before you leave a watering rim. Eight-quart bags are for a couple of herb pots; 1.0 and 2.0 cubic-foot bags are the usual vegetable sizes. Use the potting mix calculator for bag counts and, if you DIY, for ingredient volumes in cubic feet and dry quarts.
After the first season
Potting mix shrinks and breaks down. You do not have to throw it all out every year. Pull roots, dump the mix into a tub, blend in a third or so fresh mix plus compost, and refill. If last year’s plants stayed soggy or gnats took over, replace more of it. Do not reuse mix that had a serious soil-borne disease on tomatoes or peppers.
Frequently asked questions
Can I mix garden soil into potting mix to save money?
No — that is how pots stop draining. To save money, use the Budget DIY recipe (more compost, smaller shares of the other bags), buy compost in bulk, or use fewer larger pots. Keep garden soil for raised beds.
Is compost the same as potting mix?
No. Finished compost is an ingredient. Straight compost in a pot can stay too wet, shrink, and be too rich for seedlings. Blend it with coco or peat, perlite, and a little vermiculite (Classic or Budget); do not fill the pot with compost alone.
Can I fill a raised bed with potting mix?
You can, and some patio beds do, but it is expensive at 4×8 scale and the bed can be too fluffy for tall tomatoes unless you add mineral bulk. Use a raised-bed recipe (Backyard or Budget) for frames, and soilless potting mix for pots.