Lesson 3
Compost and feeding: the container difference
Standard multipurpose compost fails in containers because it is designed for short-term propagation, not sustained cropping. It compacts, sheds water, and runs out of nutrients within six weeks. For edibles, use a peat-free vegetable compost blended with 20–30% perlite or fine bark to maintain structure. John Innes No.3 — a loam-based mix — is heavier but holds nutrients longer and suits perennials.
Ericaceous compost is essential for blueberries, cranberries, and any acid-loving crop — they need pH 4.5–5.5 and will yellow and fail in alkaline mixes. Citrus want a free-draining, slightly acidic blend; specialist citrus compost exists, but John Innes No.2 with added grit works well.
Feeding becomes critical because container compost is a closed system. Roots cannot forage; they rely entirely on what you provide. For leafy crops, use a high-nitrogen liquid feed (NPK roughly 10-5-5) weekly once established. For fruiting crops — tomatoes, peppers, aubergines — switch to a high-potassium feed (NPK 4-5-8 or similar) from the first flower. Dilute to the rate on the bottle; more is not better and causes salt buildup.
Common mistakes
- Using garden soil in pots — it compacts, drains poorly, and harbours pests
- Forgetting to feed after the first month — crops stall and yellow
- Overfeeding in winter when plants are dormant — roots burn
- Reusing compost year after year without refreshing — nutrients deplete
The spring refresh for permanent pots
- 1.In March, remove the top 5–8cm of old compost without disturbing roots.
- 2.Scratch the exposed surface lightly with a hand fork.
- 3.Top up with fresh vegetable compost mixed with a handful of slow-release fertiliser granules.
- 4.Water thoroughly and mulch with 2cm of bark or gravel to retain moisture.
- 5.Resume weekly feeding once growth restarts in April.
If you can only afford one bottle of feed, buy a tomato feed — it works for everything once plants flower.