If plants in your pots look stunted, yellow or permanently soggy, the problem is very often what they’re planted in. Container growing turns soil physics upside down, and the dirt that works beautifully in a garden bed becomes a dense, airless brick in a pot. Here’s why, and what to use instead.
Why garden soil fails in containers
In the ground, water drains away through deep soil, and worms and roots keep it open. In a container, gravity pulls water down only as far as the bottom of the pot, where it collects in a saturated layer called the perched water table. Fine-textured garden soil holds that water tightly and collapses into small pores, so roots run out of oxygen. It may also carry weed seeds, pests and diseases. The fix is a mix with larger, stable particles that keep air spaces open even when wet.
What a good container mix contains
- A moisture-holding base — coir (coconut fibre), composted bark or, less sustainably, peat. This makes up the bulk of most mixes.
- Aeration — perlite, pumice or fine bark chips create air pockets and improve drainage.
- Organic matter and nutrients — well-made compost or worm castings supply slow nutrients and beneficial microbes.
- Optional: a slow-release organic fertiliser and a little lime to balance acidity when using peat or coir.
Three simple DIY recipes
All-purpose vegetable and flower mix
Combine roughly 2 parts coir or composted bark, 1 part compost and 1 part perlite or pumice, by volume. Add a slow-release fertiliser at the rate on the label. Moisten coir before mixing.
Herb and Mediterranean mix
Use 2 parts base, 1 part compost and 2 parts perlite, pumice or grit. Rosemary, thyme, oregano and lavender prefer it lean and fast-draining.
Succulent and rooftop sedum mix
Use 1 part base, 1 part compost and 3 parts pumice or other mineral grit for excellent drainage and low weight.
Do I need gravel in the bottom?
No. A layer of gravel or broken pots does not improve drainage — it actually raises the perched water table closer to the roots. Use a uniform mix throughout and make sure the container has drainage holes. A piece of mesh over the hole stops mix washing out.
How much mix will I need?
Containers swallow more than people expect: a 120 × 60 × 30 cm raised planter holds over 200 litres. Use our soil volume calculator to get litres, cubic feet and bag counts.
Refreshing old mix
Mix shrinks and loses structure over time. Each spring, remove the top third, loosen the rest, mix in fresh compost and a little new perlite, then top up. Replace mix entirely if plants had soil-borne disease, or every few years for long-term containers. Spent mix can go into compost or garden beds.
Next: put your new mix to work with our balcony vegetable guide.