You are in the garden centre in October holding a bag labelled "garden soil" next to one labelled "topsoil" next to one labelled "compost", and they cost different amounts for reasons nobody has explained. The bed needs topping up before winter. Buy the wrong one and you either waste money or spend next season fixing a growing medium that will not grow anything.
One is mineral, one is organic, and one is a blend with no fixed recipe. That is the whole distinction, and it decides which job each product can do.
Topsoil is mineral soil: sand, silt and clay, scraped from somewhere else. Compost is decomposed organic matter with almost no mineral fraction. Garden soil is usually topsoil with peat, bark or compost mixed in, at a ratio that varies by brand with nothing forcing consistency. Our soil and composting guide covers the wider picture.
Now the part that trips up most gardeners. Utah State University Extension notes that unlike compost, there are no standards for topsoil quality and no legal definition for the types of bagged soil. Worse, much of what is sold as pit topsoil is actually subsoil, because the real topsoil was stripped and sold first.
What this guide covers:
Key Takeaway
Topsoil is the bulk and the mineral structure. Compost is the biology and the fertility. You need both, in the right proportion, and neither one works alone as a growing medium.
| Product | What It Is | Use It For | Do Not Use It For |
| Topsoil (bulk) | Mineral soil, variable quality | Filling volume in new beds, regrading | Growing directly without amending |
| Compost | Decomposed organic matter | Top dressing, amending, up to 25% of a mix | Filling a bed on its own |
| Garden soil (bagged) | Topsoil plus organic amendments | In-ground beds, small top-ups | Containers, filling large volumes cheaply |
| Raised bed mix | Blend, often 50/50 or 60/40 | Filling raised beds in one go | Assuming the blend is consistent |
Sources: Utah State University Extension, Topsoil Quality Guidelines, UC Master Gardeners of Placer County, Bagged Soils, Oregon State University Extension EM 9308
Because compost is a fertiliser and a soil conditioner, not a soil. It lacks the mineral fraction that holds a root system in place and buffers nutrient swings.
Oregon State University Extension is explicit: do not fill raised beds with compost alone, and add up to 25 percent compost by volume when filling a new bed. Compost lacks the mineral component of earthen soil and must be mixed with it to support plant growth.
The second problem is salts. Colorado State University Extension puts a number on it: an amendment up to 10 dS/m total salt is acceptable if incorporated six to eight inches deep into low-salt garden soil below 1 dS/m. Concentrate that same amendment in a shallow layer and seedlings take the damage first.
The third is settling. A bed filled largely with compost keeps decomposing, and the surface drops. Illinois Extension treats settling as a normal fact of raised bed gardening and recommends topdressing a few inches of compost or quality topsoil each year to keep up. The more compost in your original fill, the further it sinks. Not tilling it helps, which is one of the quieter arguments for no-dig gardening.
Why This Works: Soil as a Living System
Permaculture treats soil as a habitat rather than a substrate. The mineral fraction is the architecture, the pore space, the cation exchange sites, the thing roots grip. The organic fraction is the food web living inside it. Pure compost is all food and no house, which is why it burns hot for one season and slumps the next. Building real topsoil means growing the organic fraction inside a mineral matrix, not replacing it.
Far less than most gardeners assume, once a bed is established. The generous first fill is a one-time event. After that, maintenance is thin.
University of Minnesota Extension recommends about an inch or less of compost over garden beds each year, with a quarter inch to one inch being typical for existing vegetable beds. For a brand new landscape planting area, the guidance shifts to a three to four inch layer worked into the top six to eight inches of soil.
| Situation | What to Use | How Much |
| Filling a new raised bed | Topsoil or raised bed mix plus compost | Compost at 25% of volume or less |
| Annual maintenance, existing bed | Compost only | 0.25 to 1 inch per year |
| New in-ground planting area | Compost, incorporated | 3 to 4 inches into top 6 to 8 inches |
| Bed has settled 2 to 3 inches | Topsoil or bed mix, then compost on top | Refill with mineral bulk, top with compost |
Sources: University of Minnesota Extension, Oregon State University Extension EM 9308, Oregon State University Extension EC 1561
Notice the pattern. When a bed sinks, gardeners reflexively buy more compost, because compost is what they associate with feeding soil. But settling is a loss of volume, and volume is the mineral fraction's job. Refill with topsoil, then put the inch of compost on top. Our guide to refreshing raised bed soil covers the annual sequence.
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Send Me the ChartTwo tests, both cheap, and one of them will save an entire growing season.
The first is a lab soil test, the only reliable way to judge a bulk topsoil purchase. University of Missouri Extension recommends testing topsoil when buying a large quantity, checking pH, texture and organic matter. Utah State puts the organic matter target at 3 percent or greater.
The second is a herbicide bioassay. Aminopyralid, clopyralid and picloram are pyridine carboxylic acid herbicides that survive an animal's digestive tract and the composting process itself. NC State Extension reports concentrations as low as one part per billion damage sensitive vegetables including tomato, lettuce, beans and peas.
Mix a test batch and a control
Fill three small pots with two parts of the suspect compost to one part peat-based potting mix. Fill three more with potting mix alone. Label both sets clearly.
Sow peas or beans, three seeds per pot
Peas are the standard indicator because they react early and visibly. Beans and tomatoes work too. Water and keep them somewhere warm and bright.
Compare after two to three weeks
Look for cupped, twisted or fern-like new growth in the test pots that the control pots do not show. That distortion is the signature of auxinic herbicide carryover.
If it fails, do not use it on food beds
There is no quick fix. Contaminated material needs to be spread thin somewhere non-food and left to break down over one or more seasons. Tell the supplier.
Common Mistake to Avoid
Do not assume "organic" or "certified compost" means herbicide-free. Aminopyralid arrives through hay, straw and manure that were legally treated on someone else's land, and it passes through composting intact. Municipal compost and free manure from a stable are the two highest-risk sources. Test before you spread, not after your tomatoes curl.
For topsoil and bed refills, yes. For heavy compost applications, it depends on your winter.
Autumn is the practical window for structural work: refilling settled beds, regrading, mixing in mineral bulk. Soil is workable, suppliers are quieter, and the material has months to knit before spring planting.
Compost is more nuanced. A thin autumn layer left on the surface under a fall mulch behaves well. Working large quantities deep into a bed before months of winter rain risks leaching soluble nitrogen below the root zone. In a wet-winter region, top dress rather than incorporate and save heavy amending for spring.
Key Takeaway
Fall is for volume and structure. Spring is for fertility. Refill the mineral bulk now, cover with an inch of compost and a mulch layer, and let winter do the mixing for you.
The arithmetic is three steps, and getting it right is the difference between one delivery and two.
Multiply length by width by depth in feet to get cubic feet, then divide by 27 for cubic yards. A 4 by 8 foot bed filled 12 inches deep is 32 cubic feet, or about 1.2 cubic yards. Oregon State University Extension and Iowa State University Extension both walk through the same calculation. For surface applications, one cubic yard covers roughly 162 square feet at two inches deep, which works out to 6 to 7 cubic yards per 1,000 square feet.
On price, be careful with any number you read online, including this one. Bulk pricing is intensely regional and delivery minimums often dominate the cost of a single-bed order. The reliable rule: bulk beats bagged once you need more than about half a cubic yard, roughly 13 or 14 standard bags, and most yards charge for a full load whether you take one or not. Call two local suppliers and compare delivered price per cubic yard, not shelf price.
You can, and it works for a few weeks before it stops. Compost has no mineral fraction, so it holds nothing structurally, releases nutrients unevenly, can carry soluble salts that damage seedlings, and slumps as it keeps decomposing. Oregon State University Extension recommends keeping compost to 25 percent or less of a new raised bed fill by volume. Use it as an amendment and a top dressing, not the growing medium.
No. Topsoil is mineral soil taken from the surface layer of a site: sand, silt and clay with little added organic matter. Garden soil is generally topsoil that a manufacturer has blended with peat, bark, compost or fertiliser. Because there is no legal definition for either term, the actual contents vary between brands and between loads from the same supplier, which is why a lab test is worth it on any bulk purchase.
Test it, and look at it before it is dumped. University of Missouri Extension recommends a soil test on large quantities, checking pH, texture and organic matter, with Utah State putting the organic matter target at 3 percent or above. Visually, reject loads with construction debris, stones over about an inch, obvious weed root fragments, or a heavy grey clay appearance. Ask the supplier where it came from. If they cannot say, that is an answer.
Roughly three parts mineral to one part compost is the defensible starting point, matching the Oregon State University Extension guidance of up to 25 percent compost by volume in a new bed. Commercial "50/50" mixes exceed that and rely on the compost being mature and low in salts. If you buy a 50/50 blend, plan for more settling and top up with topsoil rather than more compost in year two.
They do different jobs. Compost supplies organic matter, improves structure and water holding, and feeds the soil food web, but its nutrient release is slow and imprecise. Fertiliser supplies a known quantity of a specific nutrient on a known schedule. A soil test tells you whether you need either. Many established beds that get an inch of compost yearly are already high in phosphorus and need nitrogen alone, not another general amendment.
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