Compost vs topsoil vs mulch

Compost, topsoil and mulch are often sold in similar bags, but they are not the same material and they are not for the same job.

Compost is a soil improver, and it can also be used as a mulch. Topsoil is soil, used to make or raise a bed or a lawn where the ground is poor or missing. Mulch is a layer on the surface, not a substitute for soil.

This page keeps those three jobs distinct. How to build and run a heap is in starting a compost heap, compost bays and greens and browns.

Compost

The RHS describes home-made compost as a way to recycle garden waste into a valuable soil improver and mulch. Finished compost is organic matter that has broken down. It is dug into soil to improve it, or spread on the surface as a mulch, once it is ready.

Compost is not a complete substitute for topsoil when you are building a new bed from scratch or laying a lawn on poor or missing ground. It can improve the soil you already have, including heavier or lighter ground.

Follow your allotment's rules on what may be composted and where bays can stand.

Topsoil

The RHS buying guide says topsoil is widely available in bags or in bulk, and that it can be used for making new beds, borders, raised beds, or as a base for lawns where the natural soil is poor or non-existent. For laying lawns, it says you will need at least a 10 cm (4 in) layer. New beds will need about 20 cm (8 in) or more.

It groups bought topsoil into three grades: premium, general-purpose and economy, with economy often supplied unscreened, for building up volume rather than for a fine finish. Check the load before you buy: look for high stone content, thick fibrous roots, weeds, and contaminants such as glass. The same guidance notes that imported topsoil can sometimes introduce invasive plants, so inspect it.

If you are filling raised beds, the volume of material is a planning question as well as a product question. A growing mix is not automatically the same as straight topsoil; see raised-bed growing mix before you order a single material for every bed.

Mulch

The RHS describes a mulch as a layer of material, at least 5 cm (2 in) thick, applied to the soil surface. It is used to help the soil retain moisture in summer, let rain in during winter, suppress weeds, protect roots in winter, and give the bed a tidy finish.

Some mulches also add nutrients or organic matter as they break down. Well-rotted garden compost and manure, chipped bark, gravel, grit and slate chippings are among the examples it gives.

A mulch sits on top. It does not replace the 10 cm or 20 cm of topsoil the RHS quotes for new lawns and beds. You can mulch soil that is already in place, including a no-dig bed, once the surface is the one you mean to keep.

Choose the material for the job: a compost mulch will feed the soil as it breaks down; a mineral mulch will not, but it will not disappear into the bed as quickly either. Follow the RHS timing of late autumn to late winter for the organic mulches it discusses, unless a particular product's own instructions say otherwise.

A short chooser

• Need to improve soil you already have, or to spread a surface layer of well-rotted organic matter? Use compost.
• Need to create or raise a bed, border or lawn where the soil is poor or missing? Buy suitable topsoil, in the depth the job requires, and check the load before it is tipped.
• Need to cover the surface to hold moisture, reduce weeds or tidy the bed? Apply a mulch at least 5 cm thick.
• Need more than one of those jobs? That is normal. A new raised bed might take topsoil or a growing mix, then a compost mulch on the surface. The three words still name three jobs.

Check the current price of bagged or bulk material once you know which job you are buying for. We have not listed a price band here, because we did not open a two-maker sample of compost, topsoil and mulch listings for this page.