How to specify Carbon Gold enriched biochar (NBS Q31)

This page is for landscape architects, arborists and project managers writing a biochar line into an NBS specification for the first time. It covers what to specify, the dose rates from our verified application documents, the clause wording you can paste straight into NBS Q31, and the common substitution traps to lock down before the spec reaches cost review.

If you are defending a line that is already in spec, see our Specification Defence Brief instead.

Why specify enriched biochar, not biochar

The biggest mistake on a biochar line at spec stage is writing ‘biochar’ with no remit. A specification that says only biochar gives the QS permission to source the cheapest plain biochar on the market and call it compliant. That is a problem.

Plain biochar is raw, uninoculated carbon. On its own it can lock up nutrients and moisture in the short term before any biology colonises it. On a live planting project that is detrimental, not beneficial.

Biochar has no brain. It will suck up everything, good or bad, initially, and by the time it is doing anything good the tree, turf or plants have died.

Carbon Gold enriched biochar arrives on site with all the beneficial work already done. The carbon has been pre-charged at the point of manufacture with the Carbon Gold proprietary blend, which includes Trichoderma and Mycorrhizal Fungi. The microbiology is in place before it ever goes near the soil.

So when it lands in the planting pit or rootzone, it is already biologically active and ready to work with the planting from day one.

The specifier’s job is to make sure the clause calls for enriched biochar, not biochar, and names the biological blend.

NBS Q31: what it covers

NBS Q31 is the Planting / Tree planting soil amendment clause section in the National Building Specification library. It is the standard reference for landscape architects and tree contractors writing tree planting and amendment specifications on urban and structural projects.

If you have an NBS subscription, you have the full clause text in front of you. Use this guide to set up the clause to specify Carbon Gold enriched biochar properly.

If you do not have an NBS subscription, get in touch with us. We will point you at the right reference.

Ready-to-paste clause wording for specifiers

This is the bit specifiers tend to bookmark. The wording below tells the QS that the Tree Soil Improver line is a specified product with a documented biological blend, and removes the substitution route at cost review.

“Tree Soil Improver shall be an enriched biochar product with a documented biological blend including Trichoderma and Mycorrhizal Fungi. Application in accordance with Carbon Gold Tree Soil Improver Application Guide v1.0 and Application Calculator v1.0. No equivalent substitution permitted.”

Why each bit matters:

• ‘Enriched biochar’ rules out plain biochar substitution.
• ‘Documented biological blend including Trichoderma and Mycorrhizal Fungi’ names what makes the product effective.
• ‘Application in accordance with…’ points the contractor at our verified dose documents.
• ‘No equivalent substitution permitted’ is the wording that holds the line at cost review.

Paste it as-is or adapt to your project’s house style.

Dose rates at planting

Carbon Gold dose rates come from our verified Tree Soil Improver Application Guide v1.0.

• Planting hole baseline: approximately 2.5 litres mixed into backfill, scaled to hole size.
• Topsoil mixing: approximately 150 litres per cubic metre (10% by soil volume).
• Established trees: approximately 9 litres per square metre to 10 cm depth, or 18 litres per square metre to 20 cm (5% by soil volume).
• Top dressing: approximately 2.25 litres per square metre at 25 mm depth, raked in and covered with mulch to avoid wind dissipation.

These are working baselines, not project-specific rates. For per-tree-size detail, see the next section.

Per-tree-size rates

For per-tree-size detail by container size, bare-root whip height, or standard tree girth, our Tree Soil Improver Application Calculator v1.0 has the full table.

Per tree, the range runs from approximately 570 millilitres for a bare-root whip under 30 centimetres, up to approximately 22 litres or more for a semi-mature 18 to 20 centimetre girth tree.

If you are specifying a large urban tree pit or an avenue of standards, our Calculator is the right document to reference. Link the Calculator PDF in your spec notes so the contractor has it to hand.

The evidence behind the specification

Specifiers like to know what is behind a spec before they put their name to it. The evidence base for Carbon Gold biochar is independent and published.

Field trials by Bartlett Tree Research Laboratory under Dr Glynn Percival at the University of Reading cover European beech transplant survival, urban tree establishment, apple-scab field observation, and Chalara-affected ash on a Woodland Trust site in Essex. Findings have been reported in Pro Arb Magazine, HortWeek and the Guardian.

The peer-reviewed industry review is Schaffert, Lukac, Percival and Rose 2022, Arboriculture & Urban Forestry 48(3). For mechanism: Joseph 2021 in GCB Bioenergy. For water retention: Razzaghi 2020 meta-analysis in Geoderma.

Findings reported here are the researchers’ observations.

For the full evidence-led defence of the line at cost review, see our Specification Defence Brief.

Common substitution traps at cost review

Three substitution patterns turn up at cost review more often than any others.

1. The QS swaps Carbon Gold biochar for biochar or plain biochar on cost grounds. The clause wording above blocks this.
2. The line is removed entirely on the day-one cost saving. The Specification Defence Brief gives you the lifecycle exposure arguments for this conversation.
3. The compost line is kept and the biochar removed. Where a generic compost line sits alongside the biochar line, suggest substituting the compost line for Carbon Gold Soil Improver. Approximately cost-neutral, delivers the resilience benefit.

For the full defence playbook, including BNG ecologist sign-off rules and the project-by-project exposure table, see our Specification Defence Brief.

About Carbon Gold

Carbon Gold Ltd is a B Corp certified company supplying enriched biochar for NBS Q31 tree planting specifications across the UK. Its Managing Director Sue Rawlings is the named Grantee of the company’s Royal Warrant by Royal Appointment to HM The King. Carbon Gold has been featured in industry press covering arboriculture, landscape and tree care. See our Biochar in Practice hub and the frequently asked questions for more.

Speak to the Carbon Gold team

Specification queries, datasheets, NBS Q31 clause wording support, technical questions on cost-review.

Call 0117 244 0032 or use the contact form.

Written by Sue Rawlings, Managing Director, Carbon Gold Ltd