Topsoil vs Compost: Which One Does Your Project Need?
Topsoil is soil. Compost is decomposed organic matter. They look similar in a pile, they are sold by the same yards in the same trucks, and using one where the other belongs is one of the more expensive mistakes a homeowner makes.
The short version: topsoil is what plants grow in; compost is what you add to make it better. Compost on its own is not a growing medium, however good it looks.
The difference in one table
| Topsoil | Compost | |
|---|---|---|
| What it is | Mineral soil — sand, silt and clay — with 2–10% organic matter | Fully decomposed plant and animal matter. Almost no mineral content |
| Structure | Holds its shape; supports roots | Light and fluffy; settles and shrinks |
| Nutrition | Modest, and releases slowly | High, and releases fast |
| Job | The body of the bed | The improvement you mix in |
| Use alone? | Yes | No — see below |
| Cost per yard | Lower | Usually 1.5–3× topsoil |
What happens if you plant into pure compost
It is tempting. Compost is dark, light to handle and obviously rich. Beds filled with it fail in four predictable ways.
- It shrinks. Compost is still decomposing. A bed filled to the top in April is two or three inches down by August, and down again next year.
- It dries out fast, then refuses to rewet. With no mineral content there is nothing holding water at the root zone. Once a compost bed dries hard, water runs off rather than in.
- Too much nitrogen. Rich compost pushes soft leafy growth — lush tomato plants with no fruit, lettuce that bolts, grass that grows fast and thin.
- Nothing for roots to hold. Taller plants rock in the wind because there is no mineral structure to anchor into.
What happens if you use only topsoil
Less dramatic, but it underperforms. Plain topsoil — especially the screened subsoil-ish material some yards sell cheaply — can be low in organic matter, slow to drain if it is heavy in clay, and short on the nutrition a new bed needs in its first season. It will grow things. It will not grow them well without help.
The answer is nearly always a blend
Every good bed mix is mostly soil with compost worked through it.
| What you are filling | Topsoil | Compost |
|---|---|---|
| Lawn — new or repaired | 80–90% | 10–20% |
| General planting beds | 70–75% | 25–30% |
| Raised vegetable beds | 60–70% | 30–40% |
| Topdressing an existing lawn | 70% | 30% |
| Bulk filling below the root zone | 100% | none |
Most yards sell this pre-mixed — usually as "garden mix", "bed mix" or blended topsoil. Buying it blended is normally cheaper than buying the two separately and mixing them yourself, and considerably less work. See blended vs screened topsoil for what is actually in those mixes.
When compost really is the right thing to buy
- Topdressing an established bed. An inch of compost spread on the surface each spring, left for the worms to take down. This is compost doing exactly its job.
- Improving soil you already have. Two or three inches forked into a tired bed beats replacing the soil.
- Mulching. Compost as a surface mulch feeds and covers at the same time, though bark lasts longer.
- Fixing heavy clay, slowly. Organic matter is the only real long-term answer to clay. It takes years, not a season.
Buying either one
Ask what the compost is made from. Green waste, leaf mould, mushroom compost and composted manure behave differently. Mushroom compost is salty and alkaline — excellent for many things, poor for blueberries and other acid-lovers.
Ask if it is finished. Compost that is still hot or smells of ammonia is not done, and it will rob nitrogen from your plants while it finishes. It should smell of woodland floor and nothing else.
Buy soil by volume, not weight. Compost is far lighter than topsoil — roughly 800–1,000 lb a cubic yard against 2,000–2,700 for soil. A ton of compost is a lot more material than a ton of topsoil, which is why the good yards price both by the yard.
Work out how much of each you need with the topsoil calculator: take your total volume, then split it by the ratio in the table above.
Common questions
Can I use compost instead of topsoil in a raised bed?
Not on its own. Fill with 60–70% topsoil and 30–40% compost. A bed of pure compost will sink several inches in the first season and dry out hard in summer.
Is compost the same as manure?
No. Composted manure is manure that has broken down and is safe to plant into. Fresh manure will burn roots and should be composted for six months or more first. If a yard offers "manure", ask which one they mean.
Which is better for a new lawn?
Topsoil, with 10–20% compost worked in. Grass seed wants a firm, fine surface, and pure compost is too loose to give it one.
Can I put compost straight on my lawn?
Yes, thinly — a quarter to half an inch as a topdressing, raked so the grass still shows through. It is one of the better things you can do for a tired lawn.
Which is cheaper?
Topsoil, per cubic yard, usually by a wide margin. But you need less compost, so on a single bed the difference is smaller than the price list suggests.