Healthy Soil

Healthy Soil – Canonical Positions

Section 1 — Materials and behaviour

1.1 Humus

Humus refers to the fraction of soil organic matter that exhibits long mean residence time (MRT) as a result of stabilisation within the soil system.

Within HealthySoil, humus is defined by persistence and system behaviour, not by colour, origin, degree of decomposition, or analytical fraction alone.

Humus forms through the transformation of organic inputs (including plant residues, compost-derived material, and microbial products) followed by selective stabilisation pathways operating in soil. These pathways may include:

  • association with mineral surfaces and metal oxides;
  • incorporation of microbial residues and metal–organic complexes;
  • physical or spatial protection within aggregates or fine pore structures.

The defining characteristic of humus is persistence, expressed as long MRT under prevailing soil conditions. The pathway by which persistence arises is context-dependent and varies with soil mineralogy, structure, management regime, and disturbance history.

Within HealthySoil:

  • humus is not a material input or manufactured product;
  • humus is not equivalent to total organic matter or soil organic matter;
  • humus is not guaranteed by the addition of compost, manure, or biochar.

Humus formation is a system outcome, not an inherent property of organic materials. Only a fraction of organic inputs is transformed into humus, and this occurs over time through interaction with soil minerals, biology, structure, and management.

Where organic materials contain pre-stabilised or mineral-associated fractions prior to soil application, these may contribute more rapidly to humus pools once incorporated. However, long-term persistence is ultimately expressed and maintained in soil, not at source.

For boundary clarity:

  • humus must not be substituted with the term “persistent carbon” without explicit reference to MRT and soil context;
  • humus must not be treated as interchangeable with compost, biochar, or soil itself;
  • claims relating to humus accumulation or permanence must declare context, pathway, and time horizon.

1.2 Compost

Compost is not a single material. Within HealthySoil, compost is defined as a heterogeneous organic material produced through controlled biological decomposition of organic feedstocks. It contains multiple organic fractions with different properties, behaviours, and fates once applied to soil.

Compost typically contains, in varying proportions:

  • Partially degraded organic matter (PdOM): biologically processed material that has not reached long-term stabilisation. This includes carbon that may be physically nested or chemically protected (for example within lignified or hemicellulosic structures) and becomes available only as those protective matrices are progressively broken down.
  • Mineral-associated or pre-stabilised organic fractions: organic compounds that have interacted with mineral surfaces or ash-derived cations during processing and may persist longer once incorporated into soil.
  • Coarse woody or fibrous residues: structural components that decompose slowly but are not assumed to form humus without further processing and suitable soil context.

The relative balance of these fractions governs compost behaviour. As a result, composts with similar names or standards may behave very differently in soil.

Within HealthySoil, compost is treated as:

  • an organic material input, not soil;
  • a source of nutrients and biological feedstock influencing short- to medium-term processes;
  • a material whose contribution to aggregation, available water capacity (AWC), and long-term soil stability is conditional, not guaranteed.

Compost does not automatically create humus. Only a portion of compost-derived material may contribute to humus formation, and only after biological processing, physical integration, and selective stabilisation within the soil system over time.

Compost performance depends on feedstock composition, processing history, degree of decomposition, soil mineralogy and structure, post-application management, and time horizon.

For regulatory and interpretive clarity:

  • compost is not equivalent to topsoil;
  • compost is not equivalent to humus;
  • compost presence must not be used to imply long-term soil improvement without stated conditions and timescales.

1.3 Partially degraded organic matter (PdOM)

Partially degraded organic matter (PdOM) refers to organic material that has undergone biological decomposition and transformation but has not reached long-term physical or chemical stabilisation.

Within HealthySoil, PdOM is defined by state and behaviour, not by particle size or analytical fraction.

PdOM typically arises from:

  • controlled composting and curing processes;
  • early-stage soil integration of organic inputs;
  • biological processing that alters chemistry and accessibility without conferring persistence.

Key properties include altered chemistry, reduced recognisability of original structures, continued biological accessibility once protective matrices break down, and lack of inherent persistence unless further stabilised.

Within soil systems, PdOM:

  • acts as a transitional organic fraction;
  • contributes to nutrient release and biological activity over short- to medium-term timescales;
  • may influence aggregation and water behaviour indirectly;
  • represents a potential precursor to humus formation, not humus itself.

PdOM is explicitly distinguished from fresh residues, humus, and analytical particulate organic matter (POM).

1.4 Multipurpose compost (MPC)

Multipurpose compost (MPC) is a manufactured growing medium designed for containerised horticultural use. Within HealthySoil, MPC is treated as a product category, not a soil material.

MPC is formulated to provide predictable short-term plant performance in containers, including water retention, aeration, nutrient availability, and physical stability over a limited growing period.

MPC commonly includes composted organics, wood fibre or bark, coir or peat substitutes, mineral additives, and fertilisers.

Within HealthySoil:

  • MPC is not soil;
  • MPC behaviour must not be extrapolated to field soil systems;
  • MPC performance metrics are not indicators of soil health;
  • MPC must not be used as evidence for humus formation or long-term carbon persistence.

Where MPC materials are incorporated into soil, their behaviour follows the canonical rules for compost and PdOM and remains context-dependent.

1.5 Biochar

Biochar is a carbon-rich solid material produced by heating biomass under limited oxygen conditions (pyrolysis). Within HealthySoil, biochar is defined as a material class, not a performance claim.

Biochar properties vary widely depending on feedstock, processing conditions, and post-processing. Behaviour must not be assumed to be uniform or beneficial by default.

Within HealthySoil, biochar is treated as:

  • a material used with soil, not soil itself;
  • a material whose interaction with soil is mediated by surface chemistry, pore structure, and mineral context;
  • a material whose effects are context-dependent, not intrinsic.

Biochar alone does not constitute humus, and its presence must not be used to imply long-term soil improvement without defined conditions and evidence.

1.6 Soil-fit biochar

Soil-fit biochar refers to biochar materials that meet suitability criteria for intentional soil application within HealthySoil system boundaries.

A biochar may be considered soil-fit only where its origin, processing, chemistry, physical form, and contaminant profile are known and compatible with the receiving soil context and intended management objective.

Soil-fit biochar is not assumed to be beneficial by default and must not be framed as a substitute for humus or soil organic matter.

Where soil-fit biochar contributes to longer-term soil outcomes, this occurs through interaction pathways rather than inherent permanence or biological activity.

1.7 Biochar–humus composite (BHC)

Biochar–humus composite (BHC) is a composite soil amendment formed by intentional integration of soil-fit biochar with humus-rich organic fractions under controlled conditions.

BHC is defined by functional integration, not simple co-presence. The defining feature is physical, chemical, and biological association such that combined behaviour differs from either component applied independently.

BHC is not assumed to deliver guaranteed outcomes and must not be framed as a substitute for soil or soil organic matter. Performance depends on component properties, degree of integration, soil context, management regime, and time horizon.

Section 2 — Topsoil: definition and behaviour

2.1 What healthy topsoil is

Healthy topsoil exhibits functional balance rather than perfection. Accepted indicators include stable aggregation, adequate porosity, darker colour relative to subsoil, and resistance to surface sealing and erosion.

Assessment prioritises physical behaviour (structure, infiltration, stability), with chemical fertility and biological activity as supporting attributes.

2.2 What topsoil is not

For clarity and regulatory separation, topsoil is not compost, growing media, mulches, manures, or manufactured blends. These materials may be used with soil but are not soil themselves.

2.3 Texture vs. structure

Texture refers to sand, silt, and clay proportions and does not change through management. Structure refers to aggregation and can change. Healthy soil management focuses on structure, not texture.

2.4 Topsoil vs. subsoil

Topsoil has higher organic matter, biological activity, and aggregation. Subsoil is denser, lower in organic matter, and provides rooting depth and mineral reserves.

2.5 Allowed descriptions of topsoil behaviour

Topsoil behaviour may be described only in terms of physical structure, water movement, resistance to compaction and erosion, and support of biological activity without implying engineered or guaranteed effects.

Section 3 — Biological behaviour and claim boundaries

This section defines the behavioural limits and interpretive boundaries for biological and process-related descriptions within HealthySoil. It forms part of the canonical system definition and applies across all materials, soils, and management contexts described in this document.

Its purpose is to ensure that biological processes are described consistently, mechanistically, and without implied guarantees, in line with the material definitions set out earlier.

3.1 Mechanism versus outcome

Biological and process-related behaviour may be described only at the level of mechanism unless outcomes are explicitly evidenced and contextually bounded.

The presence of a process, organism, or interaction does not imply a specific or repeatable outcome. Outcomes emerge from the interaction of material properties, soil context, management regime, and time.

3.2 Biological presence and functional effect

Biological presence (for example microbial colonisation, biofilms, or root–microbe association) may be described as an observed or plausible state.

Biological presence alone must not be used to imply:

  • treatment or remediation;
  • stabilisation or persistence;
  • yield improvement or soil restoration.

Functional effects, where claimed, must be framed as context-dependent responses, not intrinsic material properties.

3.3 Humus and persistence boundary

Within HealthySoil, persistence is defined explicitly through humus, characterised by long mean residence time (MRT).

Persistence may arise through multiple pathways, including:

  • association with mineral surfaces;
  • microbial residues and metal–organic binding;
  • physical or spatial protection within aggregates or porous structures.

The pathway is context-specific. Persistence must not be implied through vague terms such as “permanent” or “persistent carbon” without reference to humus as defined.

3.4 Managed biological processes

Managed biological processes (including composting, curing, conditioning, or engineered biological systems) may influence organic matter transformation and soil behaviour.

Descriptions may explain process effects, but must not imply:

  • guaranteed transformation;
  • absolute permanence;
  • universal performance across soils or climates.

3.5 Claim strength discipline

Canonical statements relating to biology and processes must align with one of the following levels:

  • established definitions;
  • plausible mechanisms;
  • context-dependent outcomes;
  • measured results from specific settings.

Unless measured results are explicitly cited, claims must remain within the first two levels.

3.6 Language guardrail

Across HealthySoil, biological and process-related behaviour must be described using conditional language appropriate to soil systems.

Implied certainty, engineered outcomes, or guaranteed performance must not be attributed to soil materials or biological presence alone.

Section 4 — System boundaries and context validity

HealthySoil positions apply within defined system boundaries: managed topsoil, declared time horizons, known management regimes, and appropriate soil contexts.

Soilless growing media are out of boundary for soil behaviour claims. Dual use of “multipurpose compost” must be explicitly distinguished.

Claims must declare time, scale, climate, texture sensitivity, and management context.

Section 5 — UK standards context

BS 3882 defines fitness-for-use parameters for topsoil in the UK. Compliance addresses suitability and contamination limits, not long-term soil health or humus formation.

Section 6 — Boundary statement (canonical guardrail)

Topsoil is a natural system, not a formulation. HealthySoil content avoids guaranteed biological claims, soil–compost conflation, and engineered performance language.

Section 7 — Interpretation and applicability (non-canonical guidance)

Interpretive questions clarify common misunderstandings and guide correct reading without altering canonical definitions.

This document is descriptive, not prescriptive. Practical guidance is provided through separate decision-support tools and resources.