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Biochar vs compost: which one you actually need

Updated: October 2026

You hoped one bag would feed the bed and fix the soil structure, and the listing says soil improver and fertiliser in the same breath. Compost and biochar do different jobs. Compost is organic matter that breaks down and supplies a flush of nutrients. Biochar is a stable, porous carbon that holds water and nutrients and does not rot away on a compost timetable. If the bed is short of organic matter, buy compost first. Add char when you still want that stable carbon, at the rate on the bag, charged if it is plain. This page is that choice. The bags themselves are on the [biochar comparison](/hub/fertiliser-soil-and-bark-mulch/biochar). A qualifying purchase can earn a commission at no extra cost to you.

You hoped one bag would feed the bed and fix the soil structure, and the listing says soil improver and fertiliser in the same breath. Compost and biochar do different jobs.

Our top picks

McMillan McMillan Biochar 10L

Best overall

McMillan McMillan Biochar 10L
5.4/10about £26.95
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Invigortex Black Gold Activated Biochar Soil Amendment (1 Litre)

Rank 2

Invigortex Black Gold Activated Biochar Soil Amendment (1 Litre)
7.6/10about £7.99
View offer
Invigortex EarthCraft Blends Bio Char Soil Improver for Plants (40 Litre)

Preis-Tipp

Invigortex EarthCraft Blends Bio Char Soil Improver for Plants (40 Litre)
7.1/10about £33.98
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On this page
  1. 1.What each material is for
  2. 2.When compost is the right purchase
  3. 3.When biochar earns a place beside it
  4. 4.The fertiliser word on a listing

What each material is for

The Royal Horticultural Society describes biochar as organic material carbonised with little oxygen, leaving a solid that is at least about 80% elemental carbon and does not readily decompose. It classes straight char as a physical conditioner. Compost and manures are the materials that rot and actually add the organic matter gardens lose every year. Char can help a soil hold water and nutrients. It does not, on the evidence printed for these amazon.co.uk bags, replace nitrogen, phosphorus and potassium. No kept listing shows an NPK. That includes McMillan, which calls its 10 litre bag a slow-release organic fertiliser. The phrase is the seller's language. The analysis field is empty. If you are trying to feed hungry plants this season, compost or a fertiliser with a stated analysis is the purchase.

When compost is the right purchase

Buy compost when the soil is tired, pale, and short of organic matter, when you are filling a new raised bed, or when you need a mulch bulk that will slowly disappear into the surface. The RHS organic-matter guidance uses a usual soil-improver rate for composts and manures of about 5–10 kg per square metre. That figure is for those materials. It is not a biochar rate. Five to ten kilograms of compost on a square metre is a barrow job. Five to ten percent biochar in a pot is a jug of char in a bucket of compost. Naming a crop on a listing is not an analysis.
McMillan McMillan Biochar 10L

Testsieger

McMillan McMillan Biochar 10L
5.4/10about £26.95
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When biochar earns a place beside it

Add biochar when you already have compost or a feeding plan and you want the extra physical effect. On sandy soil, the RHS notes the holding of water and nutrients. On heavy soil, the claim that matters is aeration. In both cases the char should be charged if it is raw, or you risk the sponge taking up the feed you just spread. How to mix them is how to use biochar. Blend plain char at about 5–10% by volume if your bag is silent. Ten litres of char in 100 litres of compost is the 10% reading of McMillan's '100 litres of soil' sentence. It is not the other sentence, which spreads one 10 litre bag over 20 square metres.

The fertiliser word on a listing

McMillan is the sentence to read twice. 'Slow-release organic fertiliser', pure wood, 4 kg, 10 litres, £26.95. There is still no NPK. Calling it a fertiliser on the page does not create nitrogen. Barbecue charcoal is not the other half of this choice. Buy a biochar listing whose litres, weight and feedstock line you can read, when you want stable carbon as well. What to check on a listing is the biochar buying guide.

Common questions

Is biochar a fertiliser?
Not in the way a bag with an NPK is a fertiliser. The RHS treats straight biochar as a physical conditioner. McMillan's listing uses the words slow-release organic fertiliser and prints no analysis. Black Gold says organic additive. None of the eight ASINs show nitrogen, phosphorus and potassium figures. If you need a feed, buy a feed. Char can sit beside it.
Does biochar rot away like compost?
The RHS describes the solid as mostly elemental carbon that does not readily decompose. Compost is there to break down. That is why char can still be in the soil in later years and why it does not refresh a bed the way a fresh layer of compost does. You may still add compost next year. You do not need to repeat a heavy char dose on the same square of soil every spring unless a stated rate says so.
Will it feed tomatoes on its own?
Do not plan on it. McMillan names tomatoes on the listing and still shows no NPK. A tomato in a pot or a bed wants compost and a feed you can dose. Char in the mix, already charged, is a structural extra. Raw char tipped into the planting hole can hold the nutrients you meant for the plant until the pores are loaded.
Is barbecue charcoal the same choice?
No. Fuel charcoal is sold to burn, and it can carry binders or lighting residues that a soil improver listing is at least claiming not to. Looking like black lumps does not make it biochar as the RHS defines it. If a garden bag arrives as unexplained dust, that is a returns question with the seller, not a reason to empty the barbecue bag onto the salad bed.
Why does McMillan say fertiliser if the RHS does not?
The listing is using seller language. The RHS is describing the material class: a conditioner, not a measured feed. Both can be true as sentences and still leave you without an analysis. Treat 'slow-release organic fertiliser' as a claim to verify, not as nitrogen you can count. The useful facts on that page are the pure-wood line, the 4 kg weight and the two different coverage rates, which you should not merge.

Related pages

Compost soil comparison

Biochar

Best biochar for a garden, by the job you are doing

What to check before you buy biochar bags in the UK

Biochar for pots and houseplants, and how much to mix

How to use biochar in compost, pots and garden beds

Sources

Public UK sources from the research digest, used for this page in October 2026. Prices, stars and review counts are from the eight amazon.co.uk listings in this set, captured for the 19 October 2026 scoring pass. No kept bag shows an EBC class, a Soil Association number, a pH or an NPK. RPS 366 is an England position for waste-code biochar, not a household rule for Scotland, Wales or Northern Ireland.

  1. RHS, biochar (retrieved 19 October 2026)
  2. GOV.UK, RPS 366 storing and spreading biochar (retrieved 19 October 2026)
  3. GOV.UK, LRWP 60 making biochar from waste (retrieved 19 October 2026)
  4. GOV.UK, check if your material is waste (retrieved 19 October 2026)
  5. legislation.gov.uk, PFC 3 soil improver (retrieved 19 October 2026)
  6. European Biochar Certificate (retrieved 19 October 2026)
  7. Carbon Gold soil improver (retrieved 19 October 2026)
  8. SoilFixer, biochar application rates (retrieved 19 October 2026)