Shop-bought composts and when to use them

Not all bagged compost is equal. Multi-purpose compost is a blend of peat (or peat substitute), composted bark, and sometimes green waste. It is light, cheap, and fine for short-term container planting — bedding, salads, annuals. It has little structure and breaks down fast, so it is a poor choice for long-term pots or soil improvement. For that, you want a soil-based compost like John Innes, which contains sterilised loam, peat or coir, and grit. John Innes No. 2 is the general-purpose grade; No. 3 is for permanent pots and hungry feeders like tomatoes.

Peat-free composts have improved sharply in the last five years. Early versions were inconsistent and dried out hard, but modern coir- and bark-based blends perform well. They often need more frequent feeding because coir holds fewer nutrients than peat, and they can be slightly more alkaline (pH 6.5–7.0 versus peat's 5.5–6.0). For ericaceous plants — blueberries, rhododendrons, camellias — you still need a specialist acid compost, whether peat or peat-free.

When buying compost for soil improvement, choose something coarse and fibrous — a soil conditioner or farmyard manure product, not a seed compost. Spread 5 cm over beds in autumn and let worms pull it down, or dig it into the top 15 cm if you are preparing a new bed. Avoid anything labelled 'topsoil' unless you have seen it — the quality is wildly variable, and cheap topsoil is often subsoil with no structure or life.