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Terra preta raised bed: litres, depth, and a one-time rate

Updated: October 2026

You are looking at a terra preta raised bed on an allotment or in a back garden, and you want to know how much mix it actually needs. A bed is a volume. Separate a full fill from a blend worked into the top, price a trial rate before you copy it across the whole plot, and leave a CANNA 50 litre peat bag out of that sum. The [Terra preta comparison](/hub/diy-supplies/terra-preta) has no biochar sack among its twelve listings, so the litres below come from named outside examples and from geometry. A qualifying purchase can earn a commission at no extra cost to you.

You are looking at a terra preta raised bed on an allotment or in a back garden, and you want to know how much mix it actually needs. A bed is a volume.

Our top picks

Books on Demand Home Terra Preta, Kindle

Best overall

Books on Demand Home Terra Preta, Kindle
6.8/10about £2.99
View offer
CANNA CANNA Terra Professional Plus, 50 L

Alternative

CANNA CANNA Terra Professional Plus, 50 L
6.4/10about £21.48
View offer
Acres U.S.A. Modern Grower's Guide to Terra Preta

Rank 3

Acres U.S.A. Modern Grower's Guide to Terra Preta
7.2/10about £18.65
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On this page
  1. 1.A bed is a volume, not a keyword
  2. 2.What the published figures refer to
  3. 3.Why a 50 litre peat bag is a different sum
  4. 4.Once through the topsoil, then stop

A bed is a volume, not a keyword

A common timber bed of 1.2 m by 2.4 m, filled 30 cm deep, has a floor area of 2.88 square metres. The empty volume is 1.2 × 2.4 × 0.3 m, which is 0.864 cubic metres, about 864 litres. That figure is geometry for an empty frame, not a sack size. SoilFixer states about 85 litres of their SF40 blend for a bed that size at 30 cm, as their terra-preta-type compost, not as raw char and not as a CANNA sack. Eighty-five litres is about a tenth of 864, which fits a blend worked into soil you already have.

What the published figures refer to

Keep each number tied to its source. SoilFixer's 85 litres is their SF40 blend for that 1.2 m by 2.4 m bed at 30 cm. It is not a rule for every biochar, and it is not a full fill of the 864 litre shell. Garden Organic cites trials around 1–2 kg per square metre, and a 10 kg sack can typically cost around £20. That cost is their note, not an amazon.co.uk price. Marshalls Garden lists Carbon Gold peat-free biochar compost at £12.99 for 20 litres. Twenty litres is one retail unit. It is not an 85 litre SoilFixer bed, and it is not 864 litres.
Books on Demand Home Terra Preta, Kindle

Testsieger

Books on Demand Home Terra Preta, Kindle
6.8/10about £2.99
View offer

Why a 50 litre peat bag is a different sum

The only about-50-litre rows in this folder are CANNA Terra Professional at £18.99 and Professional Plus at £21.48, with a 47.5 litre cell on Plus. Both are peat mixes. Fifty litres matches neither the 85 litre blend example nor an 864 litre fill. A peat bag can suit pots on a CANNA feed schedule. It is a mismatch for a terra preta bed.

Once through the topsoil, then stop

The RHS says to follow the manufacturer's rate and treats char as something you do not reapply like a feed. The Aberystwyth factsheet says not to apply biochar with every crop, because the char persists. For this 1.2 m by 2.4 m bed, work the mix through the top 30 cm once, using the rate on the bag you bought, then stop. Next spring you top up compost or feed. The how to make terra preta page is the charge and the kiln stop, the best biochar compost page is the bag test, and the terra preta buying guide is the check order before you multiply litres by the number of beds.

Common questions

How much do I need for a standard raised bed?
For a 1.2 m by 2.4 m bed at 30 cm, start from the product you bought. SoilFixer states about 85 litres of their SF40 blend for a bed that size. The empty frame is about 864 litres by geometry, so 85 litres is a blend worked in, not a solid fill of char. If your product prints a different rate, use that rate. This folder has no biochar volume to substitute.
Is 85 litres a rule for every biochar?
Eighty-five litres is SoilFixer's figure for their SF40 compost in a 1.2 m × 2.4 m bed at 30 cm. It is not a national rate for raw char, for a 20 litre retail bag, or for a CANNA peat mix. Garden Organic's separate trial note is 1–2 kg per square metre, which on 2.88 square metres is about 2.9–5.8 kg, not 85 litres. Keep the litres with the blend and the kilograms with the trial.
Can I fill the bed with CANNA Terra Professional?
CANNA Terra Professional at £18.99 and Professional Plus at £21.48 are peat potting mixes, about 50 litres, and Plus has a volume cell of 47.5 litres. Filling a raised bed with either bag gives you a peat growing medium, which is a reasonable choice for pots on a CANNA feed schedule and a mismatch for a terra preta bed. Fifty litres is also well short of an 864 litre shell and is a different claim from SoilFixer's 85 litre blend.
Do I mix it through 30 cm or top-dress?
SoilFixer's 85 litre example is tied to a 30 cm bed depth for their blend, which means working that compost through the bed rather than painting a skin on top. A top-dress is a smaller job and suits a product that says top-dress, such as the way some 30 litre veg blends are sold. Straight char, once charged, goes through the soil you are improving, at the bag rate, one time. Follow the label if it specifies a depth.
What does 1–2 kg per square metre cost if a 10 kg sack is about £20?
Garden Organic's around £20 for 10 kg is about £2 a kilogram, as a rule of thumb. A 1.2 m by 2.4 m bed is 2.88 square metres, so 1 kg per square metre is about 2.9 kg (around £6) and 2 kg per square metre is about 5.8 kg (around £12). Those figures are not an amazon.co.uk price. Ten beds at the high rate is where a trial note stops being a small experiment.

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Sources

Public UK sources for this comparison. amazon.co.uk listings were read on 16 October 2026.

  1. RHS, Biochar (retrieved 16 October 2026)
  2. RHS, Peat-free choices (retrieved 16 October 2026)
  3. Defra, sale of horticultural peat to be banned (retrieved 16 October 2026)
  4. DEP2026-0072, letter of 29 January 2026 (retrieved 16 October 2026)
  5. Environment Agency, RPS 366 (retrieved 16 October 2026)
  6. Garden Organic, biochar (retrieved 16 October 2026)
  7. IBERS, biochar factsheet (retrieved 16 October 2026)
  8. CANNA UK, Terra Professional Plus (retrieved 16 October 2026)