Biochar quality standards are not one document. They are four stacked layers, and each one draws the line in a different place. A product standard asks whether the char is safe to put in soil. A fertiliser regulation asks whether you may sell it. A carbon registry asks how much of its carbon it will credit. A test-method standard asks whether the number on the lab report means what you think it means.
The clearest sign that the layers differ is the one ratio everybody quotes, the molar ratio of hydrogen to organic carbon. The product standards and EU law cut it at 0.7. One registry cuts it at 0.5. The feed classes and the carbon-sink standard cut it at 0.4. Same material, three answers, because each layer is answering a different question.
We design gasification plants that produce biochar alongside power and heat, so this is not an academic exercise for us. Below are the thresholds, by layer, with the sources, and then the part the standards do not spell out: what happens when the char comes from an ash-rich feedstock such as sewage sludge.
Layer 1: Product Standards, IBI and EBC
IBI Biochar Standards v2.1: three test categories
The International Biochar Initiative's standard is the reference most other documents point back to. It sorts tests into three categories. Category A, basic utility, and Category B, toxicant assessment, are required for every biochar. Category C, advanced analysis, is optional. Under Category A the material needs at least 10 percent organic carbon and a molar H:Corg of 0.7 or less, with Class 1 at 60 percent Corg or more, Class 2 from 30 to 60 percent and Class 3 from 10 to 30 percent. Moisture, ash, nitrogen, pH, electrical conductivity, liming value and particle size distribution are declared rather than limited.
Category B is where biochar differs from compost. The standard separates toxicants that arrive with the feedstock, metals and PCBs, from those the process can create, PAHs and dioxins. It requires a germination inhibition assay, and sets total PAH at 6 to 300 mg per kg depending on jurisdiction with a 3 mg per kg benzo(a)pyrene toxic-equivalent cap, dioxins and furans at 17 ng per kg WHO-TEQ, and PCBs at 0.2 to 1 mg per kg. The ranges are deliberate: IBI tells producers to meet the limit of the jurisdiction where the char is made or used. Testing must happen after thermochemical processing and before soil application, in laboratories the standard expects to hold ISO/IEC 17025 accreditation.
EBC 10.3: seven certification classes
The European Biochar Certificate takes the opposite approach. Instead of one pass mark with jurisdictional ranges, it defines classes by end use and tightens the limits as the use gets closer to the food chain. Version 10.3 lists seven: EBC-FeedPlus, EBC-Feed, EBC-AgroOrganic, EBC-Agro, EBC-Urban, EBC-ConsumerMaterials and EBC-BasicMaterials. Across all of them the molar H/Corg must be below 0.7, and below 0.4 for the two feed classes, while the molar O/Corg must be below 0.4.
The metals table shows how steeply the classes diverge. Lead is capped at 10 grams per tonne for feed, 45 for AgroOrganic and 120 for Agro, Urban and the materials classes. Mercury runs 0.1, 0.4 and 1 gram per tonne across the same three tiers. The sum of 16 EPA PAHs is limited to 6 grams per tonne, with a measurement tolerance of 2.4, for the feed and agricultural classes, and only declared for the materials classes. The guideline strongly recommends EBC-endorsed laboratories for the PAH analysis because the extraction is the step most likely to go wrong.
What the two agree on
Strip away the formats and the two standards converge on the same four facts: organic carbon has to be measured separately from carbonate carbon, H/Corg below 0.7 is the entry ticket, PAHs are the process contaminant to fear, and a germination or ecotoxicity screen is non-negotiable. Where they differ is purpose. IBI describes a material. EBC certifies a production facility and a use.
Layer 2: Legal Market Access, EU CMC 14
In the European Union a biochar sold as a fertilising product needs to fit a component material category under the Fertilising Products Regulation. Delegated Regulation 2021/2088 added CMC 14 for pyrolysis and gasification materials. The thresholds track EBC closely: H/Corg below 0.7, tested on the dry and ash-free fraction when organic carbon is below 50 percent, no more than 6 mg per kg of PAH16, 20 ng WHO-TEQ per kg of dioxins and furans, 0.8 mg per kg of non-dioxin-like PCBs, and a chlorine ceiling of 30 grams per kg. Thallium is capped at 2 mg per kg where process additives exceed 5 percent of the input.
The input list matters more than the limit table for anyone in our niche. CMC 14 explicitly excludes material from mixed municipal waste, sewage sludge, industrial sludge, dredging sludge and animal by-products. A sludge-derived char can be clean on every analyte and still have no route to a CE-marked fertiliser. Its markets are the materials classes and non-soil carbon storage, which is why the registry layer below is where the sludge question actually gets decided.
Note the phrase "dry and ash-free". A char with 30 percent organic carbon and 60 percent ash gives a different H/Corg on a whole-sample basis than on the organic fraction alone. The regulation chose the organic fraction. Ask your lab which basis it reported.
Layer 3: What Carbon Registries Add
Registries do not replace the product standards. They sit on top of them and add three things: a tighter stability cut-off, a definition of what a batch is, and rules about keeping physical samples.
Tighter H/Corg cut-offs
Isometric's module for crediting under the EU Carbon Removals and Carbon Farming framework requires H/Corg to be measured for every production batch and to be below 0.5; batches at or above 0.5 are excluded from crediting. The same module lists the property set per batch, organic carbon by EN 15936 or ISO 10694, hydrogen by EN 15407, ash by EN 14775, random reflectance by ISO 7404-5 with at least 500 measurements, and PAHs by DIN EN 17503 or EPA 8270E.
Puro's CRCF biochar methodology keeps 0.7 as the ceiling for its decay-function route but asks for more per batch. Operators must test every production batch for organic carbon, H/Corg, energy density on a lower heating value basis, the fraction of the char with a random reflectance of 2 percent or more where that route is used, and compliance with the limited-substance thresholds. The decay function itself is parameterised by H/Corg and the temperature at the site of application and estimates carbon remaining after 200 years.
The Global Biochar C-Sink Standard is stricter still, and it makes the product layer a prerequisite. Only EBC or WBC certified biochar is eligible; a char with H/Corg below 0.4 is registered with 75 percent persistent aromatic carbon and 25 percent semi-persistent carbon, while a char at or above 0.4 is registered as 100 percent semi-persistent. A soil sink needs at least EBC-Agro, and BasicMaterials char may never be used to establish a sink in soil. Read with the earlier layers, the C-Sink rule means the 0.7 line gets a char certified and the 0.4 line gets it paid in full. The science behind those fractions is covered in our piece on how long biochar stores carbon.
Per-batch testing and batch definition
The registries also define the unit of quality. Isometric caps a production batch at under one month and 500 tonnes dry mass, requires a consistent feedstock and consistent pyrolysis conditions within it, and states that any excursion outside the documented moisture or peak-temperature range ends the batch and starts a new one. The quality claim therefore belongs to the batch, not the plant.
Carbon Standards International's EU biochar carbon removal standard goes further on sampling. It requires a reference analysis on a representative sample from the first week of the initial batch, at least three samples per batch for the permanence fraction, and certifies permanence exclusively by random reflectance, leaving the decay-function route to other schemes. Which route a plant chooses decides what it must measure, and we walk through that choice in our guide to CRCF permanent carbon removal credits.
Retention samples and reference analyses
Puro requires a one-litre retention sample for each production batch on every day biochar is produced, stored for at least two years and available to the certification body on request. The CS-BCR standard mirrors the daily one-litre rule and draws later batch samples from those retained jars. A plant that cannot produce the jar from a given day has, in effect, an untested batch. Verra's VM0044, version 1.2, active since 27 June 2025, covers soil and non-soil uses under its own monitoring rules. How those verified tonnes turn into a tradable unit is in how biochar carbon credits work.
Layer 4: Test Methods, the Part Lab Reports Get Wrong
Two labs can test the same char and report different H/Corg values, and both can be "correct", if they ashed at different temperatures or did not separate carbonate carbon. The US biochar guidelines on reading a lab report put the warning plainly: most reports separate total carbon into organic carbon and carbonates, both values matter, and you should confirm that the laboratory separates them. The same guide notes there is no federal biochar standard in the United States and that the NRCS conservation practice standard aligns with IBI.
The 2026 American National Standard on biochar test methods fixes the mechanics. Ash is determined per ISO 18122 at 550 C, chosen because it preserves carbonate carbon in the ash so inorganic carbon can be measured and subtracted to give organic carbon and then H:Corg. Samples that stay black at 550 C are re-ashed at 815 C and the report must say so. pH follows ISO 10390 and must state the extraction solution, and electrical conductivity follows ISO 11265 at a 1:10 ratio because biochar absorbs water.
Why H/Corg rather than the oxygen ratio? A 2025 review in Sustainability notes that classification on H/C is preferred because hydrogen is measured directly while oxygen is calculated by difference, and summarises the Spokas bands for O/C: below 0.2 a half-life above 1,000 years, 0.2 to 0.6 between 100 and 1,000 years, above 0.6 under 100 years. EBC keeps O/Corg as a second gate for the same reason it keeps H/Corg as the first: one measured, one inferred.
The Gasification Angle: Ash, Class and Consistency
Everything above was written with pyrolysis of wood in mind. Gasification char, and especially char from sewage sludge, breaks two of the silent assumptions.
Ash-rich char lands in a lower class
Gasification runs hotter than pyrolysis, so its char tends to carry a low H/Corg, which is good. But the same feedstock that gives a stable char often gives little carbon per tonne. A sludge char with 25 percent organic carbon is IBI Class 3, falls under the CMC 14 rule that H/Corg is tested dry and ash-free, and is excluded from the CE-fertiliser route by its feedstock regardless of the analysis. The honest position is that its credit volume is set by organic carbon content, its stability by H/Corg, and its market by the input list. Those are three separate numbers, and a developer should model all three before promising offset tonnes. The arithmetic for the first is in our biochar yield calculation.
Impurities move the numbers too. Puro's 2025 methodology gives a worked example: at a 20 percent dry-mass yield, a feedstock with 2 percent glass and metal can produce a biochar that is 10 percent impurities, and a lab measuring organic carbon will screen those out while the weighbridge counts them. Intentionally added plastics are limited to 5 percent and to polymers that volatilise. Mixed waste streams fail this before they fail any analyte limit.
Feedstock consistency is a certification variable
Because the registries tie quality to the batch, and end a batch on any excursion outside the documented feed moisture or temperature window, feedstock variability is no longer an operations problem. It is a certification problem. We have argued publicly that thermal pre-treatment is non-negotiable in gasification and pyrolysis, for moisture reduction, energy density and homogenisation of the feedstock. A wet, uneven feed gives uneven carbonisation, uneven carbonisation gives a spread of H/Corg values, and a spread of values forces either more sampling or a shorter batch. Drying has its own compliance side, which we covered in VOC emissions from sludge drying.
Our public five-step process treats biochar and verified CO2 offsets as one output of a plant whose first job is dispatchable power and heat. In practice that means the quality standard is designed into the front end of the plant, in the dryer and the feed handling, not bolted on at the lab.
A Buyer's and Builder's Checklist
- Start from the end use, then pick the standard. Soil in the EU means CMC 14 plus EBC-Agro or better. Feed means EBC-Feed and H/Corg below 0.4. Materials means the EBC materials classes or IBI, and a non-soil registry route for the carbon.
- Ask for H/Corg on the basis the standard uses. Dry, ash-free for CMC 14. Ask whether organic carbon was separated from carbonates and at what ashing temperature.
- Ask which registry and which batch definition. A quality certificate without a batch boundary and a retention-sample policy will not survive an output audit.
- Treat 0.7 as the entry line, not the target. The money sits at 0.5 and 0.4.
- Check the input list before the analyte list. Feedstock can disqualify a char that passes every limit, which is one of the biochar disadvantages nobody puts on a datasheet.
Biochar quality standards reward plants that control their feedstock and document their batches. The thresholds are public. The hard part is producing the same char on Tuesday that you produced on Monday.
FAQ
What H/Corg ratio does biochar need?
It depends on the layer. IBI, the general EBC classes and EU CMC 14 require a molar H/Corg below 0.7. Isometric excludes batches at or above 0.5 from CRCF crediting. EBC-Feed and EBC-FeedPlus require below 0.4, and the Global Biochar C-Sink Standard registers the 75 percent persistent fraction only for char below 0.4.
Can biochar made from sewage sludge be certified?
Not as an EU CE-marked fertilising product: CMC 14 lists sewage, industrial and dredging sludge among the excluded inputs. The char can still be analysed under IBI or the EBC materials classes and stored in non-soil applications where a registry's methodology allows it.
How often does biochar have to be tested for carbon credits?
Every production batch. Isometric defines a batch as under one month and at most 500 tonnes dry mass with consistent feedstock and conditions. Puro requires a one-litre retention sample for each batch on every production day, stored for at least two years.
Which laboratory can test biochar to a standard?
IBI expects laboratories with ISO/IEC 17025 accreditation using its listed methods, such as EPA 8270 for PAHs. EBC recommends its endorsed laboratories for PAH analysis. Many coal, compost and activated-carbon labs can run the full suite if they follow the specified methods and report the basis used.