There is a particular kind of confusion that only happens in the compost aisle. Three bags, three prices, three labels promising healthy growth — and no obvious clue as to which one your tomato seedlings, new border or bald patch of lawn actually need. They are not interchangeable, and choosing the wrong one is one of the most common reasons a potted plant sulks all summer. Here is what each bag is really for.
What is actually in the bag
Growing media falls into two broad families: things that are made to grow plants in, and things that are made to improve the ground plants already grow in.
Multi-purpose compost is a manufactured blend. Peat-free versions are typically built from bark, coir, wood fibre or composted green waste, with a starter fertiliser and a wetting agent mixed in. It is light, free-draining and holds moisture well.
Topsoil is exactly what it sounds like: screened soil, sometimes blended with a little compost to make it easier to work with. It is dense, mineral-rich and heavy.
Soil conditioner is composted organic matter — often green waste — sold to dig into borders or spread as a mulch. It feeds the soil rather than the plant.
Multi-purpose compost: the everyday choice
If you only keep one bag in the shed, make it this one. Multi-purpose compost is the right medium for pots, containers, hanging baskets, grow bags and trays of bedding. It is light enough for roots to push through easily, and it drains fast enough that a fortnight of rain will not drown your geraniums.
The catch is stamina. The fertiliser in a bag of multi-purpose compost typically feeds a plant for a few weeks, not a whole season. After that you need to start liquid feeding, roughly weekly through the growing season for hungry crops like tomatoes and courgettes. Plants in small pots need it sooner than those in large ones.
When multi-purpose isn't enough
Two situations call for something else. Long-term plantings — a bay tree, a shrub in a large patio pot, anything that will live in the same container for years — do better in a loam-based potting compost, which holds its structure and nutrients far longer than a light blend. And acid-lovers such as blueberries, camellias and rhododendrons need ericaceous compost, because ordinary multi-purpose is the wrong pH for them.
Topsoil: for ground-level work
Topsoil is the one to reach for when you are building volume at ground level. Filling a raised bed, topping up a border that has sunk, levelling a dip in the lawn, backfilling around a new shrub, patching a bare patch before re-seeding — all topsoil jobs.
What it is not for is pots. In a container, topsoil compacts, drains poorly and turns into something closer to brick than compost. Roots struggle, and watering becomes guesswork.
Buying topsoil is worth a moment's thought. Look for a screened, weed-free product, and if it is going into a vegetable bed, buy from a supplier who can tell you where it came from. Cheap topsoil can carry weed seeds, stones or worse, and the savings rarely feel worth it by July.
Soil conditioner, seed compost and the rest of the shelf
Soil conditioner is for improving what you already have: fork it into heavy clay to open it up, or spread it as a mulch around perennials and let the worms do the digging. It is not a growing medium in its own right, and seeds sown into it will struggle.
Seed compost is finer, lower in nutrients and made for germination and cuttings. It is worth the extra bag if you sow a lot, though a sieved multi-purpose will often do at a push.
Matching the bag to the job
- Sowing seeds and rooting cuttings: seed compost, or fine multi-purpose
- Bedding plants, baskets, grow bags: multi-purpose compost
- Shrubs and trees in long-term pots: loam-based potting compost
- Blueberries, camellias, rhododendrons: ericaceous compost
- Filling a raised bed: roughly two parts topsoil to one part multi-purpose compost
- Levelling, filling hollows, patching a lawn: topsoil
- Improving a tired border or mulching: soil conditioner
How to buy well and store the leftovers
A few habits separate a good bag from a disappointing one. Squeeze it: fresh compost should feel springy and smell earthy. If it smells sour or of ammonia, walk away. A suspiciously heavy bag usually means it has been sat out in the rain and is already waterlogged. Check the label for what is in it, and buy from stock that has been kept under cover rather than baking in the sun outside the shop.
At home, store bags off the ground, sealed and out of direct sun. An opened bag left to catch the rain will lose its air pockets and its usefulness. Aim to use a bag within a season or so rather than letting it sit for years, and never re-use compost from tomato or potato pots for next year's crop — disease carries over.
The short version
Multi-purpose compost is for anything growing in a container. Topsoil is for everything happening at ground level. Soil conditioner is for feeding and loosening soil you already have. Buy those three, keep them dry, and the compost aisle stops being a puzzle and starts being a quick stop on the way to the till.
Photo: Antranias / Pixabay
