What is the difference between compost, manure and topsoil?

These three arrive in similar-looking bags and bulk bags and get used interchangeably, which produces two predictable failures: raised beds filled entirely with compost that sink by a third and dry out, and borders topped with topsoil that adds weight and no fertility at all.

Safety

Step by step

1. Understand topsoil: mineral bulk.Sand, silt and clay particles with a small amount of organic matter. It has body and weight, it holds its volume, and it is the correct material for filling a raised bed, levelling ground or backfilling. Screened topsoil is graded; as-dug is cheaper and may hold stones and weed seed.
2. Understand compost: decomposed plant matter.Garden compost, green waste compost, leaf mould, mushroom compost. It is light, it holds water, it opens up soil structure and feeds soil life. It carries modest nutrition. It also shrinks substantially as it continues to break down, which is why it cannot be used to fill volume.
3. Understand manure: animal waste plus bedding.Horse, cow, chicken or sheep, mixed with straw or wood shavings. Well-rotted, it is a rich soil improver with more nitrogen than compost. Fresh, it is far too strong and will scorch roots and foliage.
4. Choose by the job, not the price.Filling volume: topsoil, with perhaps a quarter to a third compost mixed in. Improving an existing bed: compost or manure, spread on the surface. Feeding hungry crops: well-rotted manure. Mulching a border: compost or leaf mould.
5. Never fill a raised bed with compost alone.It looks generous and it fails: it slumps by a quarter or more in the first year, dries out fast because it has no mineral content to hold water, and gives roots nothing to anchor into. Around 60 to 70 per cent topsoil to 30 to 40 per cent compost is the working mix.
6. Check manure is genuinely well rotted.Dark, crumbly, no ammonia smell and no recognisable straw. If it smells sharp or you can identify the bedding, stack it under cover for six months to a year first.
7. Be careful about where manure comes from.Aminopyralid and related hormone weedkillers used on grazing land survive the animal and the composting process, and cause twisted, fern-like growth in tomatoes, potatoes, beans and many other plants. There is no remedy. Ask the supplier what the pasture was treated with, or test a sample by sowing beans in it.
8. Know what green waste compost is.Municipal composted garden waste, processed hot so weed seed and pathogens are dealt with. It is cheap in bulk, excellent as a mulch and soil improver, and often slightly alkaline — worth knowing if you garden on chalk or grow acid-lovers.
9. Treat mushroom compost as a special case.Spent mushroom compost usually contains chalk, so it is useful on acid soil and on brassica ground, and a poor choice around rhododendrons, camellias and blueberries.
10. Apply improvers on the surface, not dug in.Worms take them down more thoroughly than a spade does, and the soil structure survives the process. Digging organic matter in each year burns it off faster and damages the fungal networks that make it work.

Tips

• The rule of thumb for a raised bed: two thirds topsoil, one third compost, and expect to top it up next year regardless.
• A sample of manure with a few broad bean seeds sown in it will show weedkiller contamination within three weeks, before you spread a whole load.
• Bulk bags are much better value than small bags for anything over about 200 litres, but check the supplier will get the delivery lorry to the site.

Common mistakes

• Filling a new raised bed with bagged compost — It sinks by a third, dries out in every hot spell and gives roots nothing solid to hold. Topsoil provides the mineral body that compost cannot.
• Spreading topsoil as a mulch to improve a border — It adds weight and no organic matter, and if it is heavier than your existing soil it makes drainage worse. Compost is the improver.
• Using fresh manure — It scorches roots and leaves, and while it is breaking down it takes nitrogen out of the soil rather than adding it. Six months to a year stacked under cover first.

Questions people ask