How do I choose compost for pots versus beds?

Compost bags mostly look the same and are formulated for quite different jobs. Choosing wrongly usually shows up weeks later as poor growth rather than immediate failure.

Safety

Step by step

1. Understand that compost is not soil.Bagged compost is a growing medium engineered for drainage, air and nutrition in a container. Garden soil compacts in a pot, holds water badly and brings weed seeds and disease with it.
2. Use multipurpose for most potting.Bedding, containers, potting on. It is the general-purpose option and it is perfectly adequate for anything that will be planted out or replaced within a season.
3. Use seed compost for sowing.Fine, low in nutrients and free-draining. High-nutrient compost actually harms germinating seeds, which is why sowing into multipurpose sometimes gives poor results.
4. Use John Innes for anything long term.Loam-based and heavier, so it holds structure and nutrients for years. Shrubs, trees, perennials in pots. Peat and coir-based composts break down within a season and collapse.
5. Know the John Innes numbers.No 1 for seedlings and young plants, No 2 for general potting on, No 3 for mature plants and long-term containers. The number is the nutrient level, not the quality.
6. Use ericaceous only where it is needed.Blueberries, rhododendrons, azaleas, camellias, pieris. It is acidic. Using it for anything else locks up nutrients and produces yellowing.
7. Understand peat-free behaviour.Peat-free composts, generally wood fibre, coir and bark, dry out faster at the surface while staying wet lower down. Judge watering by weight and by feel below the surface rather than by the top.
8. Feed peat-free sooner.Wood fibre products can tie up nitrogen as they break down, and their built-in feed often runs out within four to six weeks. Start liquid feeding earlier than you would with peat-based.
9. Check whether feed is included.Most multipurpose contains four to six weeks of nutrients. After that the plant depends entirely on you, and this is the single commonest reason container plants stall in midsummer.
10. Improve drainage for anything that hates wet feet.A third of grit or perlite mixed in for Mediterranean herbs, succulents and alpines. Crocks in the bottom of a pot do not improve drainage — that is a persistent myth.
11. Buy fresh and store it dry.Compost degrades in the bag. An old bag that has been open and rained on is compacted, anaerobic and low in nutrients. Buy what you need for the season.
12. Do not use spent compost for new plants.Last year's container compost is exhausted and structurally collapsed. Spread it on borders as a soil improver instead, where it still has value.
13. Improve beds rather than replacing soil.In the ground, add well-rotted organic matter as a mulch on top and let the worms take it down. Digging in bagged compost year after year is expensive and less effective.
14. Be wary of unknown manure.Aminopyralid and related weedkiller residues survive composting and pass through animals. Affected manure causes twisted, fern-like growth in tomatoes, beans and potatoes and cannot be corrected.
15. Check bag weight against volume.Compost is sold by litres. A cheap bag that is heavy for its volume is usually wet and dense rather than good value.

Tips

• Never fill a pot with garden soil. It compacts, drains badly and brings weeds and disease.
• Multipurpose contains about four to six weeks of feed. After that it is down to you.
• Crocks in the bottom of a pot do not improve drainage. The compost mix does.

Common mistakes

• Sowing seed into multipurpose compost — Its nutrient level is too high for germinating seeds and can scorch fine new roots. Seed compost is deliberately low in nutrients for exactly this reason.
• Using multipurpose for a permanent container plant — Peat and coir-based composts break down and collapse within a season, so the plant ends up in a compacted airless medium. Loam-based John Innes holds structure for years.

If it doesn't work

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