3. Use proper potting compost, and refresh it each year
Garden soil does not work in pots. It compacts into a dense block, drains badly and brings weed seeds and pests with it. Containers need a purpose-made potting mix that stays open and holds water without becoming a brick.
Buy peat-free multipurpose compost for most things, and a soil-based mix such as John Innes No. 3 in the UK or a loam-based potting soil elsewhere for anything permanent like a shrub, a fruit bush or a bay tree. Soil-based mixes are heavier and hold nutrients longer; multipurpose is lighter and cheaper for annual crops.
Skip the cheapest bags. Bargain compost is often coarse, poorly sieved and low in nutrients, and the money saved reappears as failed seedlings. Mid-range peat-free stock from a garden centre is the right level, and a 50-litre bag runs about $12 to $18, CA$16 to CA$24 or £7 to £12.
Improve it as you fill. Mix in a handful of slow-release granular fertiliser per large pot, and stir in a couple of handfuls of perlite or vermiculite if your balcony is windy and dries fast, or fine bark if it stays damp. Leave 1 to 2 inches of gap below the rim so water does not run straight over the edge.
Fresh compost holds roughly six weeks of nutrition and no more. After that you feed, which is step five. At the end of each season, tip spent compost out, break it up and reuse it for the bottom third of next year’s pots with fresh mix on top — full replacement every year is wasteful, and full reuse invites disease.