Two Different Materials, Two Different Jobs
Topsoil and compost look similar in a bag, and garden centres sometimes display them side by side as if they're interchangeable. They're not. Understanding the difference saves you money and gives you better results.
Topsoil is natural soil — the top layer of earth, typically screened to remove stones, roots, and debris. It provides the mineral structure (sand, silt, clay particles) that plants root into. Good topsoil holds moisture without waterlogging and provides physical support for root systems.
Compost is decomposed organic matter — garden waste, food waste, bark, or manure that has been broken down by microorganisms. It's rich in nutrients and beneficial microbes, but it has almost no mineral structure.
When to Use Topsoil
Topsoil is the right choice when you need volume and structure:
- Filling raised beds (the main bulk of the fill)
- Creating new borders or beds from scratch
- Laying a base for a new lawn
- Levelling uneven ground
- Replacing soil removed during construction
- Building up levels in a garden
For any project where you're adding more than 50mm of material, topsoil is what you need. It's stable, it won't shrink significantly over time, and it provides the physical matrix that roots need.
If you're starting a new lawn, topsoil is essential — see our full guide on preparing topsoil for a new lawn.
When to Use Compost
Compost is the right choice when you need nutrients and soil improvement:
- Top-dressing existing flower beds (25-50mm layer)
- Mulching around shrubs and trees
- Improving the organic content of tired soil
- Adding to planting holes when putting in new plants
- Feeding a vegetable patch between seasons
Compost is a soil improver, not a soil replacement. It breaks down over time (which is the whole point — that decomposition releases nutrients), so a bed filled purely with compost will sink dramatically within a year.
The Common Mistakes
Filling Raised Beds with Pure Compost
This is the most expensive mistake people make. A raised bed filled with peat-free compost at garden centre prices costs roughly three times more than one filled with quality topsoil — and within 12 months, the level will drop by 30-40% as the compost decomposes. You'll need to keep topping it up.
The better approach: fill raised beds with quality topsoil and mix in 10-20% compost by volume. This gives you the structure of topsoil with the nutrient boost of compost.
Using Topsoil as a Mulch
Spreading a thin layer of topsoil over existing beds doesn't add much value. Topsoil doesn't contain enough organic matter to significantly improve what's already there when applied thinly. If you're top-dressing beds, compost or well-rotted manure is the better choice.
Confusing Multi-Purpose Compost with Soil
Multi-purpose compost from garden centres is designed for pots and containers. It's too light and too nutrient-rich for use as a bulk growing medium in the ground. It dries out quickly, blows away in wind, and shrinks as it decomposes.
The Ideal Blend
For most garden projects, the best growing medium is a blend of topsoil and organic matter:
- New lawns: Quality screened topsoil with no more than 10% organic matter content. Too much organic matter in lawn soil causes the surface to become spongy and uneven. See how deep topsoil should be for lawns
- Raised beds for vegetables: 80% topsoil, 20% compost by volume
- Flower borders: Quality topsoil, then mulch the surface with 50mm of compost annually
- Tree and shrub planting: Backfill the hole with the excavated soil mixed 50/50 with topsoil. Avoid adding compost to planting holes — research shows it can discourage roots from growing beyond the hole
What About Cost?
Good quality topsoil delivered in bulk bags typically costs £60-90 per bag (0.6m³). Multi-purpose compost from a garden centre costs roughly £6-8 per 50-litre bag — which works out to about £70-95 per 0.6m³. They're similar in price, but you get far more lasting value from topsoil because it doesn't decompose.
For large volumes, ordering topsoil for delivery is always more cost-effective than buying bags from a garden centre. A loose load of screened topsoil can cost as little as £25-35 per cubic metre — far less than any bagged product.
How to Tell Quality Topsoil from Quality Compost
Good Topsoil Should:
- Feel gritty and crumbly, not sticky or smooth
- Be dark brown, not black
- Smell earthy, not sour or ammonia-like
- Be free of large stones, roots, and debris (if screened)
- Ideally be BS3882 certified
Good Compost Should:
- Be dark and crumbly with visible organic structure
- Smell sweet and earthy
- Be moist but not wet
- Have no visible uncomposted material (twigs, food scraps)
- Be PAS 100 certified if commercially produced
The Bottom Line
Buy topsoil for volume. Buy compost for nutrition. Blend them for the best results. And never fill a raised bed with pure compost unless you enjoy topping it up every year.