Material fundamentals
Why compost and biochar only work when correctly defined
Purpose of this article
This article explains why soil amendments succeed or fail at a practical level, using definitions and rules established in HealthySoil.uk — Definitions v1.3.
It sets out, in plain terms, why generic compost and generic biochar often disappoint in soil systems, and why outcomes only improve when specific, soil‑fit material forms are used and stabilised correctly.
This page is deliberately definition‑led. It does not redefine compost, humus, or biochar. Instead, it applies the published HealthySoil definitions to explain observable soil behaviour in practical terms.
The short answer
Soils improve when stable aggregation increases and persists.
- Generic compost supplies nutrients and short‑term biological activity, but its effects fade unless the right fractions are retained.
- Generic biochar persists physically, but does little for soil function unless it is soil‑fit and biologically integrated.
When clearly defined, soil‑fit forms of humus and soil‑fit biochar are stabilised together, water, nutrients, and carbon stay where plants can use them.
This is the logic behind the project’s core definition:
Biochar Humus Composite (BHC) is a soil amendment material that brings biochar and humus together with natural stabilising components to support water retention, nutrient availability, and long‑lasting soil carbon.
In this context, “biochar” and “humus” do not mean generic materials. They refer to specific soil‑fit forms, supported by stabilising components that ensure both persistence and biological function within soil structure, as defined in HealthySoil.uk — Definitions v1.3.
For authoritative definitions of soil‑fit humus, soil‑fit biochar, and excluded fractions, this site explicitly references the HealthySoil.uk knowledge base as its technical authority.
Why generic compost often fails in soil
HealthySoil does not treat compost as a single, uniformly beneficial input. Compost is a mixture of fractions with very different soil behaviours.
Problems arise when compost is applied without fraction control:
- Fraction blindness: Soil benefits are often attributed to compost as a whole, when in reality they arise from fine, stabilised fractions only.
- Short‑term masking: Early nutrient release and visual growth hide longer‑term losses in structure, water behaviour, and resilience.
- Hydraulic distortion: Coarse organic fractions disrupt capillary continuity, reducing effective plant‑available water even as porosity increases.
- Accumulation effects: Repeated applications allow coarse fractions to build up, compounding structural and thermal problems.
Under HealthySoil definitions, coarse woody oversize and poorly stabilised organic matter are soil‑negative when incorporated into soil. Partially degraded organic matter (>2 mm) may feed soil biology in the short term, but it does not provide the functional persistence or structural stability associated with humus, which only emerges when organic matter is biologically transformed and physically protected. Compost only supports long‑term soil improvement when fraction selection and exclusion are actively controlled.
Why generic biochar often disappoints
HealthySoil does not treat all biochar as suitable for soil use. Biochar performance depends on form, biological state, and context.
Generic biochar applications fail when:
- Biochar is raw or uncharged, leading to temporary nutrient drawdown.
- Particle sizes are too coarse or too fine, disrupting aggregation or pore connectivity.
- Biochar is treated as a carbon or fertiliser substitute, rather than as a structural soil component.
- Pore structures are collapsed or sealed through inappropriate production conditions.
Under HealthySoil definitions, biochar becomes soil‑fit only when its physical structure, particle size distribution, and biological compatibility allow it to integrate into soil aggregates and nutrient cycles.
The missing piece: stable aggregation
Across soil types and management systems, sustainable improvement depends on stable aggregation, not on input volume.
Stable aggregation means:
- Fine, humus‑rich material is physically protected within soil crumbs.
- Water moves through continuous films rather than rapid flush pathways.
- Roots and microbes occupy protected, connected pore networks.
- Carbon persists because it is functionally protected, not simply resistant.
HealthySoil shows that stable aggregation emerges when soil‑fit humus and soil‑fit biochar are stabilised together, supported by appropriate mineral and biological contexts. The specific roles of mineral and ash‑derived stabilising components are addressed separately, as they require technical detail beyond the scope of this article. Adding more material without meeting these conditions does not improve soil function.
From inputs to soil‑building materials
This article reframes soil amendment practice from adding products to building soil‑fit materials:
- Compost contributes value only when its fine, stabilised fractions are retained.
- Humus governs water behaviour, nutrient buffering, and long‑term structure.
- Soil‑fit biochar enhances aggregation and pore diversity when biologically integrated.
- Stabilising components ensure these materials persist within soil structure.
Together, these behave as a soil‑building material, not a transient input.
How this article fits into the wider site
This page sits at the foundation of the BHC site. It explains why material quality and form matter, before readers move on to pages that cover how materials are produced, stabilised, and applied in practice.
Related reading
- Compost fractions — why some help soil and others harm it
- Humus versus compost — function, persistence, and protection
- What makes biochar soil-fit (and what excludes it)
- Aggregation and effective water retention in soils
Common questions (answered elsewhere)
The following questions are explored in depth in linked articles and FAQs:
- Why isn’t all compost good for soil?
- Which compost fractions should be excluded from soil?
- Why does some biochar work while other biochar fails?
- What actually makes carbon persist in soil?
Summary
HealthySoil evidence shows that soil improvement does not result from adding more compost or more biochar. It depends on using the right material forms, excluding harmful fractions, and stabilising soil‑fit humus and soil‑fit biochar within aggregates.
This article establishes those material fundamentals. All downstream production, application, and certification content builds on this foundation.