Skip to content
Grow In Backyard
Composting

Can You Use Compost as Topsoil? Not on Its Own

Pure compost is too rich and too loose to work as topsoil. Blended at roughly one part compost to three parts soil, it does the job properly.

By Zakaria 3 min read Save to Pinterest — opens Pinterest in a new tab
A hand dripping water over a small seedling growing in dark crumbly soil rich in organic matter
Photo: thophilong, Pixabay Content License, via Pixabay
On this page

No — not on its own. Compost is a soil amendment, not a soil. Mixed at roughly one part compost to three parts soil it is one of the best things you can do to a bed. Used neat, it causes three predictable problems.

What each one actually is

This table answers the question on its own
TopsoilCompost
Made ofSand, silt, clay + some organic matterAlmost entirely organic matter
Organic fractionCommonly 2–10%Close to 100%
StructureStableContinues to decompose and shrink
NutrientsModerateHigh — often more than plants need
Holds waterYes, steadilyHolds a lot, then hard to rewet
Use it asThe body of the bedAn amendment or dressing

The three problems with using it neat

It is too rich. Nutrient concentrations in good compost can be high enough to scorch seedling roots, and plants that do grow often come up lush, soft and weak — plenty of leaf, poor structure, attractive to aphids.

It shrinks. The organic matter is still decomposing. A raised bed filled mostly with compost can drop several inches in the first season, which is not a failure, just physics.

It dries badly. Once a high-organic mix dries out it becomes genuinely difficult to rewet — water runs off or channels through rather than soaking in evenly.

The mineral fraction is the part doing the structural work

Sand, silt and clay are what give soil its stable volume, its structure and its ability to hold on to nutrients. Compost brings biology and organic matter. A bed wants both.

The ratios that work

  • General bed preparation: about 2–3 inches of compost forked into the top 8–12 inches — call it one part compost to three parts soil
  • New raised bed: roughly 60% topsoil, 30% compost, 10% drainage material such as coarse sand or bark fines
  • Annual maintenance: a 1-inch surface dressing, left for worms to incorporate
  • Lawn top dressing: ¼–½ inch of fine screened compost, brushed into the sward after aerating

On a lawn, you must still see the grass afterwards

A thick compost layer smothers turf. If the grass has disappeared under the dressing, there is too much on. Brush more of it in or take some off.

The one place neat compost is right

As mulch. A layer spread on the surface of a bed, not dug in — weed suppression, moisture retention, and worms carrying it down into the soil for you over the course of a year.

That is compost doing what it is good at: improving soil that already exists, rather than pretending to be soil itself.

Common questions

Can you use compost as topsoil?

Not on its own, for most purposes. Compost is a soil amendment, not a soil — it holds far more nutrients than plants need, has almost no mineral content to give it structure, and shrinks substantially as it continues to decompose. Plants grown in pure compost often grow lush and weak, and the level in a raised bed can drop several inches in the first season. Blended at roughly one part compost to three parts soil it is excellent, which is the normal way to use it.

What is the difference between compost and topsoil?

Topsoil is the mineral-rich upper layer of natural soil, made mostly of sand, silt and clay with a relatively small fraction of organic matter — commonly around 2 to 10 percent. Compost is almost entirely decomposed organic matter with very little mineral content. The mineral fraction is what gives soil its structure, its ability to hold on to nutrients and its stable volume, which is precisely what compost lacks on its own.

Can you plant directly into compost?

Seedlings and young plants often struggle in pure compost. The nutrient concentration can be high enough to scorch roots, the material drains too freely once it dries, and it is difficult to rewet properly after drying out. Established vigorous plants cope better. The reliable approach is to plant into a blend, or to use compost as a surface mulch on top of soil where worms will incorporate it for you.

How much compost should you mix with soil?

About one part compost to three parts soil for general bed preparation, which works out at roughly 2 to 3 inches of compost forked into the top 8 to 12 inches. For a new raised bed, a common and reliable mix is around 60 percent topsoil, 30 percent compost and 10 percent something for drainage such as coarse sand or bark fines. Repeat applications are better done as an annual surface dressing than as a one-off heavy mix.

Why does the soil level in my raised bed keep dropping?

Because the organic fraction is still decomposing, and the more compost went in, the more it will drop. A bed filled largely with compost can lose several inches in the first year as the material breaks down and the remaining particles settle. This is normal and not a failure — top up annually with a mix rather than pure compost, which simply repeats the cycle on a larger scale.

Is compost good for lawns?

Yes, as a thin top dressing rather than a layer. A quarter to half an inch of fine screened compost brushed into the sward after aerating feeds the grass, improves the soil underneath and does not smother the plant. A thick layer of compost over a lawn kills it, so the limit is that you should still be able to see the grass through the dressing when you have finished.

Backyard Weekly

Know what to plant, week by week

Join the weekly email for seasonal jobs, fixes for what is going wrong, and gear worth buying.

Not ready to subscribe? The planting calendar is free and needs no email.

By subscribing you agree to our privacy policy. Grow In Backyard will never sell your address.

Keep reading