Defending Carbon Gold biochar at cost review

There are two problems with the way biochar gets specified on landscape, BNG, sports turf and tree-planting projects. First, the spec often just says ‘biochar’ with no remit as to what that is, which gives the QS permission to substitute the cheapest plain biochar at cost review. Second, even where Carbon Gold enriched biochar is specified by name, the line can be removed entirely on the day-one cost saving. Both routes leave the project exposed. This page is for the specifier writing a tighter clause in the first place, and for the arborist, ecologist, project manager or contractor defending the inclusion at cost review.

Enriched biochar gets flagged because the cost looks like it can come out without consequence. It can’t. The work biochar does is establishment-phase survival of trees and turf, water-holding capacity in dry conditions, biological habitat for soil microbes and mycorrhizae, soil structure and aggregation, and carbon-stable contribution to durable soil quality over project lifecycles. That work doesn’t get done if the line is removed. It also doesn’t get done if Carbon Gold enriched biochar is swapped for plain biochar. The line item changes on paper. The exposure stays on the project.

Plain biochar is not a like-for-like substitute

This is the single most important point on this page. Enriched biochar and plain biochar are not the same product.

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. Specifying it as a like-for-like swap for Carbon Gold enriched biochar can be detrimental to soil and plant health rather than 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.

That is plain biochar’s problem on a live planting project. The high-surface-area carbon adsorbs nutrients and moisture during the exact window the new planting needs them. By the time microbiology has colonised it (months later), establishment has already failed.

Carbon Gold enriched biochar arrives on site with all the beneficial bits 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 works with the planting from day one instead of competing with it.

It is the reason NBS Q31 states that no equivalent substitution is permitted for Tree Soil Improver. The carbon is not the active part. The pre-charged biology is.

There is another layer to this. Not all biochars are the same. Different feedstocks, different particle sizes and different production methods produce different end products, and some perform better than others on a planting project. Carbon Gold has done that selection work. We have done the research on which feedstock to use, which biological blend to load it with, and what particle size to ship. The specifier, contractor or QS does not have to run that experiment on a live project.

If the specification says biochar generically, the safe specification is enriched biochar from a supplier whose biological blend is documented. Carbon Gold is that supplier.

If the spec just says ‘biochar’, the QS can substitute anything

Most spec defence problems begin at the specification writing stage, not at the cost-review meeting. A spec that says only ‘biochar’ gives the QS permission to source the cheapest product on the market and call it compliant. To prevent that, the clause needs to do four things.

• Name the product as enriched biochar, not biochar. The two are not the same.
• Require a documented biological blend that includes Trichoderma and Mycorrhizal Fungi.
• Cite NBS Q31 where Tree Soil Improver is being specified, with the ‘no equivalent substitution permitted’ clause carried through.
• Set application rate and particle size: 10% v/v of backfill (NBS Q31), 0 to 16 mm particle size, minimum 1kg per pit for bare-root, 2 to 5kg for standard and heavy-standard trees.

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.

That clause tells the QS that the line is a Tree Soil Improver line, not a generic biochar line, and removes the substitution route.

BNG critical note: read before any substitution

Where Carbon Gold biochar has been specified into a Biodiversity Net Gain habitat management plan, removal or substitution requires written approval from the ecologist of record and may necessitate revision of the BNG habitat unit calculations. The QS does not have authority to make this change unilaterally.

Framework reference: POST PN 0728 (UK Parliament, September 2024).

What Carbon Gold biochar actually contributes

Carbon Gold is a longstanding hardwood-source enriched biochar product range, Soil Association and Biodynamic Association approved. The Carbon Gold proprietary blend, which includes Trichoderma and Mycorrhizal Fungi, supports:

• Establishment-phase survival of trees and turf
• Water-holding capacity in dry conditions
• Habitat for soil microbes and mycorrhizae
• Soil structure and aggregation
• Carbon-stable contribution to durable soil quality over project lifecycles

This is resilience, not cure. Carbon Gold biochar is a soil amendment under the Fertiliser Regulations, not a plant protection product. The framing matters at cost review.

Specifying the rate

Tree pits are commonly specified under NBS Q31 (Planting / Tree planting soil amendment).

For Carbon Gold dose rates, see:

• Tree Soil Improver Application Guide v1.0 — at-planting baseline approximately 2.5 litres per planting hole, scaled to hole size; 10% by soil volume for topsoil mixing; 5% by soil volume for established trees treatment.
• Tree Soil Improver Application Calculator v1.0 — per-tree-size rates by container, whip height or tree girth (range from ~570ml for whips under 30cm, up to ~22L+ for semi-mature 18-20cm girth).

What’s at risk if it’s removed

Day-one saving against downstream exposure, by project type.

Three options if cost truly has to change

Option 1. Concentrate. Keep Carbon Gold enriched biochar in the highest-risk positions (street trees, BNG enhancement areas, contaminated or compacted sites). Reduce or remove in lower-risk areas. Preserves most of the resilience benefit at a fraction of the cost.

Note: not applicable inside a BNG plan without ecologist approval.

Option 2. Moderate the rate. Published research (Frenkel and Graber on dose response) supports inclusion at around 5% v/v. Reducing to 3% or 4% in lower-risk areas reduces material cost while staying inside the effective range. Use the full 5%, or NBS Q31’s 10% v/v for tree pits, where it matters most.

Option 3. Substitute the compost line. Where the spec already includes a generic compost or soil conditioner, replace that line with Carbon Gold Soil Improver. It is enriched biochar with the Carbon Gold proprietary blend, which includes Trichoderma and Mycorrhizal Fungi. The swap is approximately cost-neutral and delivers the resilience benefit without a separate line item.

The published evidence base

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

Schaffert, Lukac, Percival and Rose (2022), peer-reviewed industry review in Arboriculture & Urban Forestry 48(3), pulls the arboricultural evidence together.

For mechanism, see Joseph (2021), GCB Bioenergy. For water retention, see Razzaghi (2020) meta-analysis in Geoderma.

Findings reported here are the researchers’ observations.

If you are writing a turf rootzone specification with biochar in it, our NBS Q31 specifier guidance lays out the clause wording for the contract drawings.

Download the one-page PDF brief

The full Specification Defence Brief is available as a PDF. Useful at cost-review meetings, BNG handovers, or any conversation where the line is being defended.

For Sports Turf rootzone specific specification see our Sports Turf Specifier Guide

About Carbon Gold. Carbon Gold Ltd is a B Corp certified company. 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 including HortWeek, Pro Arb and Arboriculture Today. For more on the practical application of enriched biochar, see our Biochar in Practice hub and the frequently asked questions.

Speak to the Carbon Gold team

Specification queries, datasheets, NBS Q31 clause text, technical support on cost-review questions. We cover them all.

Call 0117 244 0032 or use the contact form.

Datasheets, application-rate guidance, and the published research summary are available on request.

Written by Sue Rawlings, Managing Director, Carbon Gold Ltd