Topsoil vs Compost vs Garden Soil: What Goes in a Raised Bed?

Quick answer: Topsoil is the bulk, structural layer — screened natural soil for body and drainage. Compost is decomposed organic matter, rich in nutrients, used to feed plants and hold moisture. Garden soil is topsoil pre-blended with some compost for convenience. In a raised bed, use topsoil as the base and compost as the fertility, roughly 60:30 with 10% grit, rather than relying on any one of them alone.

These three terms get used interchangeably in conversation, but they’re different materials with different jobs — and buying the wrong one for the wrong layer wastes money or stunts your plants. Someone who fills a whole bed with compost pays too much and ends up with soil that slumps; someone who fills it with plain topsoil ends up with poor fertility and drainage. This guide explains what each material actually is, when to use it, and how they combine into a mix that works. Use the calculator above to work out quantities once you’ve chosen your blend.

Topsoil: the foundation

Topsoil is the upper layer of natural ground, screened to remove stones and debris. It provides the bulk and structure of a raised bed — something stable for roots to anchor in, with the mineral content that holds a soil together.

On its own, topsoil compacts and drains poorly in a raised bed, so it isn’t a complete growing medium. But as the majority ingredient in a blend, it’s essential, and it’s considerably cheaper per litre than compost. Buy screened or “grade A” topsoil, not unscreened subsoil or builder’s spoil, which can be full of stones, clay, rubble, and weed seeds. Good topsoil looks dark and crumbly, not pale, sticky, or stony.

Compost: the fertility

Compost is organic matter — plant material, manure, or green waste — broken down into a dark, crumbly, nutrient-rich material. It’s what feeds your plants and helps the soil hold moisture, and it’s the source of the living organisms that keep soil healthy.

It’s also the most expensive component per litre, which is why you use it as a proportion of the bed rather than filling the whole thing with it. Multipurpose compost suits most vegetables; specialist vegetable or peat-free composts are worth it for the growing layer. Compost breaks down over time, which is why raised beds sink a little each year and need topping up — it’s the organic matter depleting, not the topsoil.

Garden soil: the convenience blend

“Garden soil” (sometimes sold as “raised bed soil”) is usually topsoil already mixed with compost and sometimes other amendments, sold ready to use. It’s convenient — one bag, no blending — but you pay for that convenience, and you have less control over the ratio, which varies between brands.

For a small bed it’s a fine shortcut. For a large one, buying topsoil and compost separately and blending them yourself is cheaper and lets you tune the mix to your crops. Check the bag: some “garden soil” is mostly topsoil with little compost, while other blends are compost-heavy, so the value depends on what’s actually in it.

Quick comparison

How to combine them

For a raised vegetable bed, the reliable approach is topsoil as the bulk, compost as the fertility — around 60% topsoil to 30% compost, with 10% grit or composted bark for aeration. Buy the two main components separately, work out each volume from your total, and blend them in the bed as you fill, so the mix is even throughout. You get better structure than compost alone and better fertility than topsoil alone, at a lower cost than pre-blended garden soil.

If you’d rather keep it simple, a 50/50 topsoil-and-compost mix grows most vegetables well and skips the grit. The three-part mix just gives longer-lasting structure.

Which to buy for which job

Use topsoil when you need to build up the level of a bed cheaply, fill the bulk of a deep bed, or level ground under turf. Use compost as the fertile top layer, for annual top-ups, and for containers where you want a rich, moisture-holding medium. Use garden soil as a one-bag shortcut for small beds and planters where the convenience is worth the slightly higher price. Matching the material to the job is what keeps the cost down and the results good.

Common mistakes to avoid

The biggest mistake is filling a whole bed with compost — expensive and prone to slumping as it breaks down. The second is using unscreened builder’s topsoil full of stones and weed seeds. The third is assuming all “garden soil” blends are equal when the compost content varies widely between brands. And the fourth is buying pre-blended garden soil for a large bed when mixing topsoil and compost separately would be cheaper and better. Read the bag, and for anything bigger than a planter, consider blending your own.