Topsoil Versus Compost: The Difference and When to Use Each

Key Takeaways

• Topsoil is the mineral layer plants root into. Its solids are roughly 90% sand, silt and clay and 10% organic matter, it holds its volume once settled, and it is the material for building, filling and levelling ground.
• Compost is organic matter rotted down until nothing in it is recognisable. It feeds soil life, opens up the structure and holds water, but it keeps breaking down for around two years, so a bed filled with compost alone drops in level.
• In the UK the word compost covers two different products. Garden compost, green compost and soil improver go into the ground, while multi-purpose and John Innes composts are a blend with growing media made for pots.
• Topsoil sold to BS 3882:2015 has been tested for texture, pH, nutrients and contaminants against a published standard. A bag with no standard named can be anything from screened loam to builders’ fill.
• Compost goes onto beds at about 50 litres per square metre, a layer around 5cm deep, applied to heavy soils in autumn and to light sandy soils as winter turns to spring.

I remember pricing up a raised bed for a client one spring, and they were trying to keep the cost down by filling it with homemade compost rather than buying a bag of topsoil. It seemed reasonable to them, but when I went back later in the summer, the difference was obvious. The plants had done well enough, but the soil level had dropped several inches, leaving a strip of bare timber around the inside of the bed and some flowers lost in the plot. I’ve seen the same thing plenty of times: compost is useful for improving and feeding soil, but it isn’t for providing the bulk in a garden bed. In this case, we topped the bed back up with some decent topsoil. It settled at the right level for the following season.

What Is the Difference Between Topsoil and Compost?

Topsoil is the mineral soil plants root into and compost is rotted organic matter added to it: one is the bulk, the other the feed. Topsoil is the top layer of natural ground, about 30cm deep in a good garden, and most of it is rock ground down to sand, silt and clay. Compost is what is left when plant material, manure or green waste has been broken down by bacteria, fungi and worms. It looks like dark soil, and that is where the confusion starts.

The line I keep coming back to is the organic matter figure. That 10% is the part of topsoil that makes it work, and it is the part compost adds.

“Compost” Means Two Different Things

Compost in the UK means either garden compost that goes into the ground, or bagged growing compost mixed with soil made for pots, and the bag does not always say which. Both are right about a different product.

Garden compost is the first kind: home-made heap compost, bagged soil improver, green compost from council garden waste and composted manure. Its job is to go into or onto existing soil. Outdoor Aggregates buys its green compost in from council green waste schemes, which run hot enough to kill weed seed, then screens and grades it. Manure-based products are the one group I read the listing twice on, because aminopyralid, a lawn weedkiller that passes through a horse and survives the heap, stunts tomatoes, beans and potatoes the following year.

Growing compost is the second kind, and it is what Americans call potting soil. Multi-purpose compost is adequate for most jobs in a pot and perfect for none, and it is low in food, so liquid feeding starts within a few weeks. The John Innes composts are loam based, which means they contain real soil, and they run from JI Seed through No. 1, 2 and 3 in rising fertility. John Innes No. 3 is the one for tubs and planters that stay put. So on soil improver versus compost: a soil improver is garden compost sold under a clearer name. It goes in the ground.

Should You Use Topsoil or Compost? Job by Job

Topsoil is the right choice wherever ground has to be built, filled or levelled, and compost is the right choice wherever existing soil needs feeding. A few jobs want both, in a set order. This is the table I would have pinned in the van.

Two things before you order. Look for BS 3882:2015 on the topsoil listing, because a bag with no standard can be screened subsoil under a better name. Once it arrives, anything grey, sticky, rubbly or sour smelling goes back on the pallet. More on what makes good quality topsoil.

Can I Use Compost Instead of Topsoil?

This is an argument that runs and runs, and both sides have a point. The no-dig growers fill beds with nothing but compost and get very good crops. The growers who say a bed wants mineral soil for bulk are also right. What separates them is what the bed is sitting on.

Compost can stand in for topsoil in a raised bed for a season or two, and then the level drops and the structure goes with it. That is the best case, where the compost is properly finished. Part-rotted compost takes nitrogen back out of the soil to finish the job, so the plants go pale and stall in their first season, it carries live weed seed if the heap never got hot, and it has more shrinking left to do.

A raised bed on open ground can take a compost fill, because the roots go down into the soil beneath and the compost is really a very deep mulch. A bed stood on slabs has no soil beneath it, so it wants a topsoil and sand mix instead, which I cover in raised beds on a patio. The same goes for a bed on a concrete yard or an old garage base, which has its own guide: raised bed on concrete. Any bed you want to keep level wants topsoil for bulk and compost on top. Straight compost in a deep bed packs down as it rots, the air goes out of it, and roots and water struggle to get through.

My own answer is the middle one. I would fill with a fertile screened topsoil, which is what Outdoor Aggregates’ vegetable topsoil is, and treat the compost as the annual feed. Compost does its best work as a catalyst. A small amount, well rotted, mixed in at planting or spread on top, inoculates the soil with the organisms that made it, with none of the overfeeding risk of a chemical fertiliser. I don’t use it for bulk.

Can I Mix Compost and Topsoil Together?

Compost and topsoil mix well together, and a blend of the two is what most bagged vegetable and beds and borders topsoils already are. So the first question is whether you need to mix anything. For a new bed I would buy it pre-blended, because the mixing is the worst job of the lot, on a tarpaulin, in the rain, with a shovel.

There is no universal ratio, and I am wary of anyone who gives you one. A vegetable bed wants more organic matter than a shrub border, a sandy soil takes more than a loam, and a bed topped up every autumn needs less in the first fill. The number that does hold is the annual one: around 5cm of compost over the surface each year, which is about 15 wheelbarrow loads for 25 square metres, dug in or left as a mulch.

Mix or layer depends on the bed. A new bed gets the compost turned through before planting. An established bed gets it spread on top, and the worms take it down over the season, which is easier on your back and on the roots. Either way, spring compost must be well rotted. Compost still breaking down draws nitrogen out of the soil to finish the job, and the plants go pale and sulk. Autumn compost can be rougher, because the organisms have until spring to catch up.

When Not to Add Compost

Compost is the wrong addition on waterlogged ground, in a wildflower area, around the roots of a newly planted tree and in a bed about to be sown with carrots. Four exceptions, and each one is a job I have seen go wrong because someone reached for the compost on reflex.

1. Waterlogged soil: Organic matter in standing water builds up methane, and the tell is a rotten egg smell when you dig. The answer is drainage plus sharp sand or gravel, and compost once the water has somewhere to go.
2. Wildflower areas: Fertile ground favours the vigorous grasses and weeds, and the wildflowers lose. Sow them into a low fertility soil and keep the compost for the vegetable patch.
3. A large tree going in: Compost in the planting hole keeps the roots in the enriched pocket instead of spreading out. Ring the tree instead, from 60cm out from the trunk to 30cm beyond the drip line, and rake it in.
4. A bed for carrots and parsnips: Root crops fork in ground that has just had organic matter worked in. Grow beans or brassicas in the fresh bed this year and sow the roots next year.

One more check. Bagged improvers are often alkaline and spent mushroom compost contains lime, so test the pH before a large application, keep mushroom compost away from rhododendrons, camellias and blueberries, and never put lime on in the same season.

Next Steps

Decide what the ground needs before the material. Ground to be built, filled or levelled wants topsoil, and existing ground that needs feeding wants compost. We stock both. Outdoor Aggregates grades its vegetable topsoil to BS 3882:2015 Multi Purpose, screens it to 10mm and makes it without manure, so it is aminopyralid free and ready to plant the day it lands. The green compost, soil improvers and composted manures are in the compost and soil conditioners range, and a wildflower patch wants the low fertility topsoil instead. Measure the bed in litres, put the figures into our calculator, and order once.

If you are still unsure whether your job needs topsoil or compost and would rather talk it through with a person, call Outdoor Aggregates on 0330 043 7585. The office is open 7am to 7pm, seven days a week.

Have you ever filled a bed with topsoil, compost or a mix of the two? Which did you use, and how far did the level drop by the end of the first year? Love to hear how it went in the comments.

FAQs

Should I use topsoil or compost first?

Topsoil goes in first and compost goes on top because topsoil is the bulk and compost is the feed that worms take down. On a new bed, turn the two together before planting and after that the compost only goes on the surface.

Is topsoil cheaper than compost?

Topsoil is cheaper than compost for filling a bed because compost shrinks and the bulk has to be bought again the following year. Price per bag is the wrong comparison between the two because they do different jobs.

Can you just lay compost on top of soil?

Compost laid on top of soil at around 5cm works well as a mulch, and the worms take it down over the season. That is how to feed an established border, and it is easier on the plant roots than forking it in.

Does compost eventually turn into soil?

Compost does not turn into soil because soil is mostly mineral and compost has almost none. Compost breaks down into humus which binds the sand, silt and clay into crumbs and then it is used up, which is why beds need topping up every year.

Is it okay to fill raised beds with topsoil?

Topsoil is a good fill for a raised bed, provided it is a fertile screened topsoil laid 200 to 300mm deep. It is the ideal fill with 50mm of compost spread on top each year: in spring on a vegetable bed, or in autumn on a bed of shrubs and flowers or one left empty over winter.