Biochar by the Tonne Is Failing Science: Why Function, Not Mass, Should Set the Dose

Biochar has become one of the most heavily researched soil amendments of the past two decades, celebrated for its potential to lock away carbon, immobilise toxic contaminants, retain nutrients, curb greenhouse gas emissions and revive degraded land. Yet a striking problem has haunted the field: even under seemingly similar experimental conditions, reported biochar effects remain wildly variable and often flatly contradictory. A new perspective published in Discover Soil by Chisom Ejileugha of Imo State Polytechnic argues that the discipline has been measuring the wrong thing all along. The culprit, according to the paper, is not merely the familiar suspects of feedstock and pyrolysis conditions, but a deeper, structural flaw in how biochar is dosed: nearly every study applies it by mass, when what actually matters is the functional capacity that each gram delivers.

The argument rests on a simple but consequential observation. Biochar is not a single material but an enormously heterogeneous family of carbonaceous solids produced by heating biomass in the absence of oxygen. Its specific surface area, porosity, ash content, alkalinity, mineral composition, nutrient content and cation exchange capacity all swing dramatically depending on what biomass was used, how hot the pyrolysis ran, and whether the material was treated before or after charring. These physicochemical properties are the machinery behind every environmental effect biochar can produce, from sorbing polycyclic aromatic hydrocarbons to liming acidic soils. Two biochars applied at the same rate of, say, two percent by weight, or twenty tonnes per hectare, can therefore deliver profoundly different amounts of the property that actually drives the outcome under study.

Ejileugha formalises this insight as the Principle of Functional Delivery, or PoFuD. The principle asserts that biochar effects in soil are governed by the type and amount of functional capacity delivered, not by the mass of material applied. In this framing, equal masses of two biochars simply do not represent equivalent treatments unless they deliver equivalent functional capacity. Functional capacity is defined operationally as the magnitude of a given property delivered per unit mass of biochar, using quantifiable proxies such as surface area, cation exchange capacity, ash content or alkalinity, because many of the ultimate functions themselves are difficult to measure directly. The proposal does not deny that biochar is heterogeneous; rather, it makes that heterogeneity analytically visible by separating the controlled functional input from everything else that arrives along with it.

The consequences of the mass-based habit are far-reaching. Because the dose is expressed as applied mass rather than delivered function, dose-response relationships become murky, and studies using identical application rates can legitimately report opposite results. In sorption experiments on polycyclic aromatic hydrocarbons and heavy metals, contrasting biochars are routinely compared at the same mass, which effectively under-doses the sorption capacity of the weaker material. Differences are then attributed generically to feedstock or pyrolysis conditions, even though the mechanistic explanations offered in such papers are usually assumptions rather than experimentally tested causes. Mass-based application, the paper contends, confounds material quantity with functional capacity, obscuring causal mechanisms and making results difficult to compare across sites, contaminants and regulatory contexts.

The mass-based convention appears to have been inherited uncritically from older practice with compost, manure and other solid organic amendments, materials whose variability, while real, is less extreme. Biochar research has implicitly recognised the importance of function for years; properties are characterised in tables and invoked in discussion sections to explain observed effects. But experimental design remains overwhelmingly mass-based, and even studies of engineered or functionalised biochars, modified with metals, nutrients or microbes, still compare treatments by mass rather than by the magnitude of the engineered function delivered. No general principle has previously articulated the explicit link between application and functional capacity, which is the gap PoFuD is designed to fill.

The operational framework that accompanies the principle is deliberately disciplined. First, the intended application objective must be clearly defined, whether containment, biodegradation, nutrient retention, pH control or vegetation establishment. Next, the dominant limiting process preventing the desired outcome is identified, for example low pH driving metal mobilisation, or poor soil structure restricting oxygen diffusion. That limiting process then dictates which target biochar property should be optimised and quantified, and the biochar is applied at a rate normalised to that property, converted to a per-unit-mass-of-soil basis for comparability. Crucially, only one functional capacity is standardised per experiment, aligned with the study hypothesis, a rule the paper calls the principle of capacity control. All co-delivered properties are measured and reported transparently but treated as contextual background rather than controlled variables, in line with the principle of co-delivery transparency.

This discipline matters because biochar properties are tightly coupled. Raising ash content to increase pH will simultaneously alter mineral sorption sites, dissolved organic carbon release, porosity, microbial colonisation and contaminant partitioning. Delivering an equivalent functional capacity with a different biochar may require a much larger mass, smuggling in a heavy load of co-delivered properties that can shift the outcome. The framework therefore demands that trade-offs be evaluated explicitly: whether collateral effects are detrimental, whether the benefits of the target property outweigh them, and whether co-delivered effects interact synergistically, additively or complementarily with the optimised function. The best biochar, the paper notes, may not be the one with the highest sorption or immobilisation figures, but the one that maximises net ecological recovery and long-term soil functionality.

The perspective is candid about limitations. Isolating a single property in a material whose characteristics co-vary during pyrolysis is genuinely difficult, and not all functional capacities are readily quantifiable, particularly in low- and middle-income countries where analytical resources are constrained. Elemental ratios such as O/C and H/C can serve as proxies for carbonisation and aromaticity, but as dimensionless values they cannot easily be expressed per unit mass, limiting their use in functional dosing. Interlaboratory variability poses another challenge: a well-known comparison across twenty-two laboratories found poor reproducibility in biochar physicochemical characterisation, which could propagate uncertainty into reported functional capacities. Initiatives such as the European Biochar Certificate and the International Biochar Initiative have advanced standardised characterisation, and PoFuD aims to build on that foundation by standardising application itself. Soil texture, mineralogy, organic matter, baseline pH, redox conditions and climate will still modulate outcomes, but the author argues this is a strength rather than a weakness, because the framework separates what is controlled by the biochar from what is governed by the environment.

The paper also grapples with messy real-world cases. In soils co-contaminated with heavy metals and hydrocarbons, the properties ideal for sorbing organic pollutants are rarely those best suited to immobilising metals, so single-property optimisation may be insufficient. The proposed remedy is hierarchical functional prioritisation: rank objectives, optimise the primary target property, and treat the rest as co-delivered functions, as might be done at a mining site where metal immobilisation comes first, microbial recovery and hydrocarbon degradation second, and plant growth later. Where several properties contribute to one function, systematic biochar engineering should disentangle the dominant driver. And since maximum sorption capacity alone can mislead, because fine-particle biochars that sorb more also desorb more phenanthrene, ammonium and phosphorus, sorption-desorption behaviour and hysteresis may matter as much as capacity itself. Pairing biochar with compost or digestate, as demonstrated in work showing combined immobilisation and microbial degradation of polycyclic aromatic hydrocarbons, offers another route to multi-functionality.

The broader stakes extend well into policy. Function-based standardisation would sharpen the translational value of biochar research, support risk assessment and regulatory evaluation, and give carbon-credit schemes and greenhouse gas mitigation programmes a defensible basis for comparing materials. The paper’s closing claim is bold but measured: biochar research has matured to the point where future progress depends less on inventing new biochars than on fixing the conceptual foundations of how they are applied. If the field adopts functional delivery as its dosing logic, the contradictory literature that has accumulated over decades may finally begin to converge, and the promise of biochar for soil remediation, biogeochemistry and climate mitigation could be tested on terms that actually reflect how the material works.

Subject of Research: Function-based standardisation of biochar soil application through the Principle of Functional Delivery

Article Title: Reframing biochar soil application from mass-based to function-based through the principle of functional delivery

Article References: Reframing biochar soil application from mass-based to function-based through the principle of functional delivery. (n.d.). https://doi.org/10.1007/s44378-026-00293-y

Image Credits: AI Generated

DOI: 10.1007/s44378-026-00293-y

Keywords: biochar, soil amendment, functional delivery, standardisation, soil remediation, contaminant sorption, greenhouse gas mitigation, pyrolysis, cation exchange capacity, dose-response, soil biogeochemistry, carbon sequestration