The Best Potting Mix for Containers (and Why Garden Soil Fails)
Container plants live or die by what is below the surface. Potting mix is not soil, and understanding why changes how you buy, mix and reuse it.
Why garden soil does not work in pots
Dig up garden soil, put it in a container, and three things go wrong:
- Structure collapses. Soil relies on worms, frost and gravity to maintain air pockets. In a pot there is none of that, so it compacts into a dense mass within weeks.
- Drainage fails. Compacted soil holds water at the bottom while the surface looks dry. Roots sitting in that layer rot.
- It carries problems. Weed seeds, fungal spores and soil pests come along for free.
Potting mix avoids all three because it is built from materials chosen to stay open and light: peat or coir for water retention, bark or compost for nutrients, perlite or grit for permanent air space.
The base recipe
For general vegetable containers, mix by volume:
- 5 parts peat moss or coir — holds water and some nutrients
- 3 parts screened compost or worm castings — the nutrient engine
- 2 parts perlite, pumice or coarse sand — permanent drainage and air
Add per 10 litres of finished mix: one generous handful of slow-release granular fertiliser, and one tablespoon of lime if your water is very soft and the plants are brassicas or beans.
Three variations worth knowing
For seedlings and small cells
Double the fine material and drop the compost: 6 parts coir, 2 parts perlite, 2 parts fine compost. Seedlings need moisture retention and near-zero fertility — rich mix burns young roots.
For Mediterranean herbs
Basil, rosemary, thyme and oregano evolved on poor, sharp-draining ground. Use 4 parts coir, 3 parts perlite or grit, 3 parts compost, and skip the slow-release fertiliser. Rosemary in particular will rot in a rich, wet mix faster than it will starve in a lean one.
For tomatoes and heavy feeders
Start with the base recipe and add one part extra compost plus a handful of well-rotted manure per 10 litres. These plants will be fed weekly anyway, but a richer start carries them through the flowering surge.
Coir or peat?
Coir (coconut fibre) is the more sustainable choice and re-wets more easily once dry — a real advantage, because bone-dry peat is hydrophobic and repels water. Peat holds slightly more water and is usually cheaper. If you use coir, note that it contains almost no nutrients, so the compost portion of your mix matters more.
How often to replace mix
Container mix degrades: the organic fraction decomposes, structure collapses, and salt from fertiliser accumulates. Practical cycle:
- Leafy crops and herbs: replace the top 5 cm each season, full change every 2 years.
- Tomatoes and heavy feeders: full replacement every season.
- Perennial herbs: repot into fresh mix every 2 years.
Signs mix has had it: it settles into a hard block, water pools on the surface instead of soaking in, or plants stall despite regular feeding. Old mix is not waste — spread it on garden beds or blend 50:50 with fresh mix for non-fussy crops.
One 50–60 L bag of multi-purpose compost plus a 10 L bag of perlite covers roughly four 15 L containers and costs far less than pre-mixed "container" blends. Mix it in a bucket or on a tarp, wearing a mask if you are handling dry coir or perlite dust.
Common questions
Can I reuse last year's potting mix?
Partly. Remove the top 5 cm and replace it, then blend the remainder 50:50 with fresh mix for undemanding crops. For tomatoes and other heavy feeders, replace it entirely each season.
Is coir better than peat?
Coir is more sustainable and re-wets far more easily once dry, which matters in containers. It contains almost no nutrients, so the compost fraction of your mix does more of the work.
Why has my mix gone hard and crusty?
The organic fraction has decomposed and the structure has collapsed, often helped by repeated drying. Water will pool on the surface instead of soaking in. Replace it, or break it up and blend with fresh mix and perlite.
Does fresh mix already contain fertiliser?
Most multi-purpose mixes carry enough for roughly four to six weeks. Adding slow-release granules at planting extends that; after six weeks you need a liquid feeding routine regardless.
Can I use pure compost from my garden in containers?
Not on its own. Garden compost is a good ingredient at 20 to 30% of a mix, but by itself it is too dense, holds too much water and lacks the structure that container roots need. Blend it with coir and perlite rather than using it neat.