Healthy Soil

FAQ & Knowledge Base

Explore our FAQ to get straightforward answers about soil health, maintenance, and sustainable practices.

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  • Why soil & compost respiration tests cannot prove stable carbon

    Short answer

    Compost respiration tests measure biological activity, not carbon persistence. Low respiration shows that easily decomposed material has already been consumed or is temporarily unavailable. It does not demonstrate that remaining carbon will persist long-term in soil.

    What this page helps you understand or avoid

    This page helps readers avoid equating compost maturity or low biological activity with long-term carbon stability.

    Why this matters in practice

    Respiration tests are widely used and easy to interpret incorrectly. Over-reliance can lead to overstated claims about durability or permanence.

    What this question actually is

    The real question is not “is this material biologically quiet?” It is “what does low biological activity actually tell us?”

    What it is often confused with

    • Chemical recalcitrance
    • Physical protection
    • Long-term persistence

    Correcting common misinterpretations

    • Low respiration does not equal stable carbon
    • Finished compost does not equal long-term storage
    • Biological quietness does not equal permanence

    What the evidence and constraints show

    Respiration tests are rate-based, short-term assays. They respond to substrate availability and conditions, not to long-term behaviour across soils and climates.

    Summary

    Respiration tests are valuable for what they measure. They should not be asked to prove what they cannot.

  • Why PAS100 compost composition limits long-term soil persistence

    Short answer

    Standard PAS100 compost performs well for sanitation and organic matter recycling, but its material composition limits how long its soil benefits persist. This is not a failure of composting, but a consequence of what PAS100 is designed to optimise. Understanding this boundary helps avoid unrealistic expectations and misuse.

    What this page helps the reader understand or avoid

    This page helps readers understand why PAS100 compost often delivers short- to medium-term benefits, and why it should not be expected to provide long-term structural or carbon persistence in soil without further material transformation.

    Why this matters in practice

    Misunderstanding the limits of compost persistence leads to:

    • repeated re-application to maintain soil structure
    • disappointment when benefits decline over time
    • confusion between compost performance and soil degradation processes

    Clear boundaries improve decision-making and material choice.

    What this issue actually is

    PAS100 compost windrows are designed around throughput, aeration, and sanitisation. As a result, their finished material is typically dominated by:

    • woody bulking fractions
    • partially decomposed organic matter (PdOM)
    • only a small proportion of true humus formed late in the process

    These fractions perform useful functions but are not inherently persistent once applied to soil.

    What it is often confused with

    • The assumption that all organic matter fractions behave like humus
    • The belief that compost stability automatically implies soil persistence
    • Confusing screening size with biological stability

    Passing a fine screen does not mean a material has become long-lived humus.

    What the evidence and constraints show

    Compost-derived PdOM continues to mineralise under normal soil conditions. While it supports biological activity and nutrient cycling, it does not provide enduring aggregation or long-term carbon retention on its own.

    This is a structural constraint of compost systems optimised for waste processing rather than material persistence.

    Summary

    PAS100 compost excels at recycling organic matter but is structurally limited in how long its benefits persist in soil. Recognising this boundary prevents misuse and sets realistic expectations.

  • Healthy soil is often discussed in terms of how well crops grow. Yield, vigour, and visible plant performance dominate most conversations about soil quality. However, this growth‑centred view captures only a fraction of what soil actually does.

    Soil is not merely a growth medium. It is a living, multifunctional system that underpins environmental stability, long‑term productivity, and human wellbeing. Focusing on plant growth alone risks misunderstanding soil behaviour and, over time, accelerating degradation rather than preventing it.

    This article explains why healthy soil matters beyond plant growth, how broader soil functions operate, and how HealthySoil approaches this complexity without over‑claiming outcomes.

    Soil is a multifunctional system, not a single‑purpose input

    Soil performs many functions simultaneously. These functions interact, reinforce one another, and sometimes compete. Together, they allow soil to support ecosystems, agriculture, and human societies over long timeframes.

    Five core soil functions are widely recognised as central:

    • Biomass production – supporting crops, pasture, forests, and natural vegetation
    • Carbon regulation – storing, stabilising, and cycling organic carbon
    • Habitat provision – supporting diverse soil organisms and food webs
    • Nutrient cycling – transforming, retaining, and releasing nutrients
    • Water cycling – storing, transmitting, and regulating water movement

    Healthy soil is not defined by excellence in just one of these functions. Instead, it is defined by the soil’s ability to perform all of them to a reasonable degree, given its climate, texture, and landscape position.

    When management focuses narrowly on maximising biomass production, other functions often decline. Over time, this loss of multifunctionality reduces resilience, increases risk, and undermines future productivity.

    Why plant growth alone is an incomplete measure of soil health

    Plant growth is a visible outcome, not a direct measure of soil condition. In the short term, crops can perform well even as soil structure, organic matter, and biological function deteriorate.

    Examples include:

    • High yields maintained through increasing fertiliser inputs while soil organic matter declines
    • Strong early‑season growth followed by late‑season water stress due to poor structure
    • Productive systems that become increasingly vulnerable to erosion, compaction, or drought

    These systems appear leaving healthy, but their underlying soil functions are weakening. When stress increases — through climate variability, reduced inputs, or management change — performance often collapses rapidly.

    HealthySoil treats plant growth as one signal among many, not as a definitive indicator of soil health.

    Soil functions underpin outcomes beyond agriculture

    The importance of soil extends far beyond crop production.

    Carbon regulation

    Soils store more carbon than the atmosphere and vegetation combined. The stability of this carbon depends on soil structure, biological processing, and mineral associations. Loss of soil organic matter releases carbon to the atmosphere and reduces long‑term soil resilience.

    Water regulation

    Soil is the largest terrestrial reservoir of freshwater. Its structure controls infiltration, storage, drainage, and runoff. Degraded soils shed water quickly, increasing flood risk, erosion, and pollution, while failing to supply crops during dry periods.

    Biodiversity and disease regulation

    Soil supports a vast proportion of terrestrial biodiversity. Complex soil food webs help regulate pests and pathogens, buffer nutrient flows, and stabilise ecosystem behaviour. Simplified soils are more prone to disease outbreaks and nutrient losses.

    Human health links

    Soil influences human health through food nutrition, contamination pathways, water quality, and even pharmaceuticals derived from soil organisms. Degraded soils increase exposure to pollutants while reducing nutritional quality of food.

    These functions emerge from soil behaviour, not from yield targets.

    Why HealthySoil focuses on behaviour, not outcomes

    HealthySoil deliberately avoids defining soil health in terms of guaranteed outcomes such as yield, climate mitigation, or biodiversity gains.

    Instead, it focuses on:

    • Soil behaviour under given conditions
    • Structural stability and aggregation
    • Effective plant‑available water capacity
    • Nutrient buffering and release dynamics
    • Biological processing of organic matter

    These are the variables that land managers can realistically influence.

    Outcomes such as yield stability, carbon storage, or ecosystem services emerge conditionally from these behaviours. They depend on climate, cropping system, management intensity, and time. Treating them as direct design targets risks oversimplification and unintended consequences.

    Multifunctionality explains why degradation is often delayed

    One reason soil degradation is difficult to detect early is that soils can temporarily compensate for lost functions.

    For example:

    • Nutrient inputs can offset declining nutrient cycling
    • Irrigation can mask poor water‑holding capacity
    • Tillage can temporarily relieve compaction while worsening structure long term

    These compensations allow production to continue while soil multifunctionality erodes. Eventually, thresholds are crossed and systems fail abruptly.

    HealthySoil emphasises early warning signs and structural indicators precisely because visible failure often comes too late.

    Healthy soil is contextual, not idealised

    Not all soils can perform all functions equally. Sandy soils in dry climates will never match the water‑holding capacity of deep clays in humid regions. This does not make them inherently unhealthy.

    Healthy soil is best understood relative to a reference condition — how a comparable soil would function under minimal disturbance. Degradation is measured by loss of function relative to that baseline, not against a universal ideal.

    HealthySoil adopts this contextual view to avoid unrealistic targets and misleading comparisons.

    What this means for land managers

    Looking beyond plant growth changes how soil is managed:

    • Short‑term performance is balanced against long‑term stability
    • Structural integrity becomes as important as nutrient supply
    • Organic matter quality matters more than simple quantity
    • Water behaviour becomes a primary design constraint

    The goal shifts from maximising output to maintaining system capacity.

    Summary

    Healthy soil matters beyond plant growth because soil is a multifunctional system that underpins environmental stability, resilience, and human wellbeing.

    Plant performance is an outcome, not a definition.

    HealthySoil focuses on soil behaviour and constraints that land managers can influence directly. These behaviours enable wider ecosystem and planetary functions, but they do not guarantee specific outcomes. By maintaining soil multifunctionality at the behavioural level, the conditions for long‑term productivity and environmental health are preserved.

    This distinction — between enabling conditions and claimed outcomes — is central to understanding what healthy soil really means.

  • Even within the same garden centre, two bags labelled “compost” can
    behave very differently. Compost can differ hugely between brands and batches.
    Here are the six most common reasons.

    1️⃣ The wrong product for the job

    A key distinction is between [[PAS100]] compost and
    [[Multipurpose Compost (MPC)]].

    • PAS100 compost is a recycled soil improver, not a precision
      growing medium.
    • MPC, in contrast, is formulated for pots and seed trays.
      Confusing these two markets — or mixing bulk PAS100 feedstock into
      retail blends — often leads to unpredictable results.

    2️⃣ Input material quality

    Most UK compost is based on green waste from councils or
    landscapers. The feedstock mix (grass, hedge trimmings, leaves) changes
    seasonally, altering texture, nutrient balance, and smell.
    Some suppliers supplement with food waste, bark, or loam — each shifts
    performance. Poorly sorted inputs can also carry plastic, glass, or metal fragments.

    3️⃣ Incomplete processing

    Even certified material can vary in stability and humification.
    Compost leaving a site too early may still contain woody fragments
    or active decay, leading to nitrogen drawdown or later heating in
    bags.
    See also “Wood-Rich Composts: The Hidden Variable” in our [[What’s
    in the Bag]] section.

    4️⃣ Contamination and screening issues

    When trommels or magnets fail in production, whole runs can escape QA checks — sometimes up to 10,000 bags before detection.
    These faults introduce oversize wood, stones, or plastic, which buyers
    understandably interpret as “poor quality compost.”

    5️⃣ Storage and re-bagging

    Moisture content, fungal growth, and odour can change if bags sit
    outdoors for months.
    Bulk material is sometimes re-bagged under contract, meaning the
    brand on the front may not match the site that made it.

    6️⃣ Inconsistent formulations

    Retail MPCs are lightly standardised at best. Each brand tweaks
    ingredients (peat-free fibres, digestate, coir, bark fines, composted
    green waste) to hit price or performance targets.
    This “recipe drift” explains why the same product name can look or feel
    different from season to season.

    🌱 Summary

    Variation in compost isn’t random — it reflects different goals, feedstocks, and quality-control systems.
    To get predictable results:

    • Use [[Multipurpose Compost]] for containers and pots.
    • Use [[PAS100 compost]] only as a soil improver.
    • Buy from reputable suppliers who publish feedstock details and batch
      QA data.

  • Why biochar replaces wood chip as a bulking agent at compost scale

    Short answer

    At compost scale, bulking agents are structural tools, not disposable inputs. Biochar can replace wood chip in this role while maintaining airflow and process control. This substitution improves both material performance and value capture.

    What this page helps the reader understand or avoid

    This page explains why replacing wood chip with biochar does not reduce composting performance, and why concerns about throughput loss are misplaced when bulking is viewed as a system function rather than a feedstock.

    Why this matters in practice

    Operators often assume wood chip is essential to maintain porosity and aeration. This assumption can block innovation and prevent higher-value use of woody resources.

    Understanding bulking as a functional role opens new system design options.

    What this issue actually is

    Bulking agents are used to:

    • maintain air-filled pore space
    • prevent compaction
    • stabilise moisture distribution

    Wood chip performs these roles within the compsoting windrow, but when woody compost is later added to soil, it degrades slowly over time and contributes little to long-term soil structure.

    What it is often confused with

    • Treating bulking agents as expendable waste material
    • Assuming biological degradability is desirable for structural components
    • Believing airflow depends on fibre volume rather than pore architecture

    What the evidence and constraints show

    Granular biochar provides persistent porosity, resists collapse, and maintains structure through wet–dry cycles. These properties allow it to function as an effective bulking agent at scale without compromising composting control.

    At the same time, woody feedstocks can be upgraded before composting rather than consumed as low-value bulking material.

    Where this sits within the wider BHC framework

    This clarification supports production and stabilisation logic by explaining how compost system design can evolve without sacrificing operational performance.

    Summary

    Replacing wood chip with biochar as a bulking agent preserves composting performance while enabling higher-value material outcomes. The shift is functional, not cosmetic.

  • Using water holding capacity (WHC) as a proxy for humus-like carbon

    Short answer

    There is no routine laboratory test that directly measures humus or long-lived humus-like carbon. Water-holding capacity (WHC) is sometimes used as a practical proxy because it reflects functional properties associated with humus behaviour in soil. However, WHC is indicative only and cannot, by itself, demonstrate long-term carbon stability. Used carefully, it supports comparison and screening rather than proof.

    What this page helps you understand or avoid

    This page helps readers avoid treating simple physical tests as proof of long-term soil carbon persistence. It clarifies what WHC can indicate, and where its limits lie.

    Why this matters in practice

    WHC is easy to measure and often improves as organic materials mature. Without clear boundaries, it is sometimes misused to imply stability or permanence that it cannot support.

    What this question actually is

    The real question is not “does this material hold water?” It is “can water retention be used as a signal of humus-like behaviour?”

    WHC reflects properties such as surface area, functional groups, and colloidal behaviour. These are associated with humus-like material, but they are not the same thing as persistence over time.

    What it is often confused with

    • Direct measurement of humus
    • Proof of carbon permanence
    • Evidence of long-term sequestration

    Correcting common misinterpretations

    • High WHC does not prove long-term stability
    • Improved water retention does not equal certified persistence
    • WHC reflects function, not residence time

    What the evidence and constraints show

    WHC can increase as materials become more functionally complex and less biologically reactive. However, WHC alone cannot distinguish between biological processing, mineral protection, or chemical recalcitrance.

    Where this sits within the wider BHC framework

    WHC is used as an early-stage, operational indicator to support formulation consistency and comparison. It is complemented by stronger evidence when claims move toward durability, standards, or external scrutiny.

    Summary

    Water-holding capacity is a useful functional signal, not a proof of stability. It supports comparison and development, but it must remain bounded by its limitations.

  • STM Management pathways & soil transition

    quick orientation


    This page does not tell you what to apply or which system to follow.
    It explains what can realistically change in soils once you know where you are in the STM, and what usually cannot.
    Use this to avoid false expectations, misapplied inputs, and short‑term fixes that fail to shift soil state.

    Purpose and scope of this document within the STM

    This document defines how soils move between states within the Soil Transition Model (STM). It does not re‑diagnose soil condition or restate mechanisms covered elsewhere. Instead, it sets out the management pathways available once a soil has been placed within the STM framework, clarifying which levers can realistically shift soil state, over what timescales, and within what limits.

    The scope is deliberately constrained to controllable management actions. Climate, inherent texture, and parent material are treated as fixed boundary conditions. Outcomes are framed in terms of physical state, water buffering, nutrient flux behaviour, and carbon persistence, consistent with STM assumptions.

    The soil transition model: from diagnosis to pathway choice

    STM diagnosis places a soil within a bounded state space defined by effective plant‑available water capacity and nutrient flux. Transition pathways describe directional movement within that space under sustained management.

    Pathway choice follows diagnosis. Management does not act uniformly across all soils; the same input or practice can produce materially different outcomes depending on starting state. Therefore, pathways are expressed relative to constraint type rather than as universal prescriptions.

    Baseline soil states recognised by the STM

    The STM recognises a limited number of baseline states sufficient to guide management decisions:

    • Structurally degraded soils with low effective water buffering.
    • Nutrient‑limited soils with adequate structure but low flux capacity.
    • Water‑limited soils where structure exists but storage is insufficient.
    • Functionally balanced soils operating near the upper bounds set by climate and texture.

    These states are not labels of quality or ideology. They are operational descriptions used to bound expectations and select appropriate pathways.

    Management levers available to change soil state

    Only a small set of levers can reliably shift soil position within the STM:

    • Physical intervention: traffic control, compaction relief, residue handling, and surface protection.
    • Carbon inputs with persistence: materials that may contribute to durable soil carbon pools only where stabilisation mechanisms are present; many organic matter inputs increase short‑term biological activity but do not result in persistent soil carbon.
    • Water management: drainage, infiltration control, and surface roughness affecting retention and loss pathways.
    • Nutrient regime control: timing, form, and placement affecting flux efficiency rather than gross input quantity.

    Biological activity is treated as a consequence of these levers acting on physical habitat and carbon persistence, not as an independent control variable.

    Transition pathways by soil constraint type

    For structurally degraded soils, pathways prioritise restoring aggregation and pore continuity. Early gains typically arise from physical protection and reduced disturbance, with slower improvement linked to the conditional accumulation of persistent carbon fractions, where stabilisation processes operate.

    For nutrient‑limited soils, pathways focus on improving retention and buffering rather than increasing headline application rates. Structural stability and carbon‑mediated sorption capacity govern whether added nutrients contribute to sustained flux or rapid loss.

    For water‑limited soils, pathways emphasise increasing effective storage through aggregation, pore size distribution, and organic carbon that remains functionally persistent under the given soil conditions Irrigation or rainfall alone does not shift state without corresponding structural change.

    For functionally balanced soils, pathways are primarily protective. Management aims to avoid regression through compaction, excessive disturbance, or depletion of persistent carbon pools.

    Time horizons, expectations, and limits of intervention

    Soil transition is inherently asymmetric in time. Degradation can occur rapidly, while recovery is slower and bounded.

    • Physical damage may occur in single seasons; reversal typically requires multiple cycles.
    • Increases in effective water buffering lag behind visible surface changes.
    • Where it occurs, persistent carbon accumulation operates on multi‑year timescales; many organic inputs do not contribute to persistence.

    STM explicitly rejects claims of rapid, universal transformation. Transition is incremental and constrained by starting state and boundary conditions.

    Trade-offs, risks, and common failure pe

    Common failure modes arise when management actions are applied without reference to starting state:

    • Adding nutrients where retention capacity is low, increasing loss risk.
    • Increasing organic inputs that decompose rapidly or mineralise without contributing to persistent carbon
    • Expecting biological indicators to compensate for unresolved physical constraints.

    Trade‑offs are unavoidable. Gains in one axis may temporarily constrain another, particularly during early transition phases.

    How to use this document alongside other STM definitions

    This document should be read after STM diagnosis documents and alongside material‑specific definitions. It does not replace detailed guidance on compost, biochar, nutrients, or structure, but provides the directional logic that links those components into coherent management sequences.

    Used correctly, this article functions as the pathway layer of the STM: defining what can change, how it can change, and what cannot reasonably be expected to change under management.

    Trade‑offs are unavoidable. Gains in one axis may temporarily constrain another, particularly during early transition phases.

  • ❓ What are pharmaceuticals doing in biosolids and composts?

    Trace amounts of medicines we use (painkillers, antibiotics, antidepressants, hormones) pass through our bodies and enter wastewater. Treatment plants remove most contaminants, but some persistent molecules bind to solids and remain in sewage sludge. When sludge becomes biosolids and is applied to land, these trace residues come with it.

    In hot composting (PAS100), many pharmaceuticals are significantly reduced — far more than in wastewater treatment — but some persistent compounds can survive in very small amounts.

    ❓ Does this mean crops can absorb pharmaceuticals and pass them on to us?

    In almost all cases: no. Most pharmaceutical molecules are:

    • too large,
    • too complex,
    • hydrophobic (repelled by water),
    • strongly bound to soil particles or [[humus]], and
    • unable to cross plant root membranes.

    Plant membranes only transport small, highly soluble molecules. Very few pharmaceuticals meet those criteria.

    A handful of mobile compounds (e.g., carbamazepine) can enter plants in tiny amounts, but levels are:

    • 10,000–1,000,000× lower than a single pill dose.

    You would need to eat hundreds to thousands of kilograms of vegetables to equal one therapeutic dose.

    ❓ If pharmaceuticals persist in soil for 10 years, does that mean risk is increasing?

    No — this is one of the most common misinterpretations.

    When researchers find pharmaceuticals in soil years later, it means:

    • the molecules are persistent,
    • they are immobile,
    • they are tightly bound to soil or organic matter,
    • they are not being degraded, and
    • they are not entering plants.

    Persistence ≠ mobility ≠ danger.

    They remain as chemically stable residues, not as active contaminants moving through the ecosystem.

    ❓ Is [[PAS100]] compost safer than biosolids?

    Generally, yes.

    Hot, aerobic composting (55–70°C) creates conditions that:

    • destroy many pharmaceuticals,
    • degrade antibiotics,
    • break down natural hormones,
    • reduce mobile compounds,
    • and oxidise complex organics.

    Research shows:

    • Painkillers → >90% degraded
    • Hormones → rapidly degraded
    • Antibiotics → significantly reduced
    • Persistent compounds (carbamazepine, diclofenac) → partially degraded (40–80%)

    Biosolids, by contrast, are treated via:

    • wastewater processes (low temperature),
    • mesophilic or thermophilic anaerobic digestion,
    • stabilisation steps,

    …but not the intense oxidative environment needed to destroy synthetic chemicals.

    So biosolids generally retain more pharmaceutical residues than compost.

    ❓ Do pharmaceuticals in soil bioaccumulate into plants or humans?

    This is where media headlines often go wrong.

    For bioaccumulation to happen, a chemical must:

    • move into the plant,
    • move into edible tissues,
    • persist during digestion,
    • accumulate faster than it is broken down.

    Most pharmaceuticals cannot pass the first step.

    Even in scientific studies where uptake occurs:

    • concentrations in crops are nanograms per kilogram,
    • human exposure is microscopic,
    • doses are orders of magnitude below any biological effect.

    There is no credible exposure pathway for most compounds.

    ❓ So is this a real problem or a perception issue?

    Both — but more perception than risk.

    Real issues:

    • Pharmaceuticals persist in soil.
    • They can be detected many years later.
    • Biosolids carry a wider range of contaminants than compost.
    • Some regulatory monitoring is justified.

    Not real issues:

    • Pharmaceuticals entering vegetables at harmful levels.
    • Significant human exposure through food.
    • Bioaccumulation in soil organisms leading to risk.

    The key misunderstanding:

    Finding a molecule in soil ≠ it moving into plants or people.

    ❓ Why doesn’t sewage treatment break pharmaceuticals down?

    Because treatment plants are designed to:

    • remove BOD (biological oxygen demand),
    • remove solids,
    • kill pathogens,
    • treat nitrogen and phosphorus.

    They are not designed — chemically or biologically — to break down complex synthetic molecules.

    Pharmaceuticals often survive because they:

    • resist biodegradation,
    • sorb to sludge solids,
    • avoid microbial attack,
    • are stable at treatment temperatures,
    • were designed to survive digestion in the human gut.

    Hot composting, however, does provide the right conditions to degrade many of these molecules.

    Deep Dive — Technical Summary

    1. Why pharmaceuticals persist in biosolids

    Biosolids contain microbial biomass, proteins, lipids and carbohydrates, but the persistent synthetic fraction remains because:

    • wastewater systems are low‑temperature, anaerobic or semi‑aerobic,
    • oxidative enzymes are absent,
    • many pharmaceuticals strongly sorb to organic solids,
    • digestion reduces volatile solids but concentrates persistent molecules.

    Thus biosolids become a sink for immobile, long‑lasting contaminants.

    2. Why [[PAS100]] hot composting is far more effective

    Thermophilic composting provides:

    • high heat (55–70°C),
    • abundant oxygen,
    • oxidative radicals,
    • lignin‑degrading enzymes,
    • dense and dynamic microbial communities.

    These pathways are proven to degrade:

    • ibuprofen,
    • naproxen,
    • paracetamol,
    • natural hormones,
    • many antibiotics,
    • and partially degrade persistent compounds like carbamazepine.

    PFAS and other extremely stable chemicals remain — but these are not pharmaceuticals.

    3. Persistence ≠ mobility ≠ plant uptake

    Most persistent pharmaceutical residues:

    • remain bound to clays or [[SOM]],
    • become inaccessible inside micro‑aggregates and pores,
    • do not dissolve into soil water,
    • cannot cross root membranes,
    • cannot travel through mycorrhizal networks.

    A molecule stuck on clay or humus cannot bioaccumulate.

    4. Plant uptake is extremely limited

    Only a tiny number of pharmaceuticals are mobile enough to enter plants. Even then:

    • uptake coefficients are typically 0.001–0.01,
    • plant concentrations are ng/kg to µg/kg,
    • human exposure is trivial compared to therapeutic doses.

    Example: Eating 200 g of vegetables containing 0.1 µg/kg of a pharmaceutical gives an intake 1,000,000× lower than a 100 mg pill.

    5. What this means for soil stewardship

    • Biosolids provide N, P and C but carry chemical noise, not a major biological risk.
    • PAS100 compost is a near‑closed risk loop for pharmaceuticals.
    • Most headlines exaggerate human exposure pathways.
    • Real concerns relate to public perception, regulation, and monitoring, not crop safety.

    If you’d like, I can now convert this into:

    • a Helpie FAQ accordion block,
    • an SEO‑optimised WordPress article,
    • a printable PDF,
    • or an infographic summarising persistence vs mobility vs exposure.

  • Fertilisers and their role in soil

    Short answer

    Soil fertility and fertilisers are not the same thing. Regulated NPK fertilisers supply nutrients, but soil fertility depends on how nutrients interact with soil structure, retention processes, and biological activity under specific conditions. Different farming systems use the term fertiliser in different ways, but fertility ultimately reflects soil function rather than inputs alone. Biochar is not a fertiliser under regulation, yet it can influence how applied nutrients behave in soil.

    Introduction

    The word fertiliser is widely used but rarely well defined. In conventional agriculture, it typically refers to regulated products supplying nitrogen, phosphorus, and potassium (NPK) with strict labelling requirements. In organic and regenerative systems, fertiliser is often understood more broadly as any input that improves soil fertility, even if it does not directly supply soluble nutrients. Technology-driven approaches sit somewhere between these views, seeking precision nutrient delivery while improving efficiency and reducing losses.

    This article clarifies how fertilisers function within soil systems, how different definitions arise, and why fertiliser alone does not equal fertile soil.

    NPK fertilisers: regulation and strict labelling

    NPK fertilisers are tightly regulated in the UK and EU. Labels specify nutrient form, concentration, and application guidance, allowing accurate nutrient budgeting and compliance with environmental rules. This clarity underpins conventional nutrient management strategies and crop yield optimisation.

    However, these same regulations mean that materials which do not declare nutrient content — even if they strongly influence nutrient behaviour in soil — are not classified as fertilisers. Biochar, compost structure materials, and biological carriers fall into this category.

    The broader, functional definition of fertilisers

    Outside formal regulation, fertiliser is often used to describe anything that increases plant growth or soil fertility over time. This may include:

    • Organic matter inputs such as composts and manures
    • Mineral amendments that alter pH or nutrient availability
    • Biological inputs that influence nutrient cycling

    From this functional perspective, fertility is not just about adding nutrients but about enabling soils to retain, cycle, and supply them when plants need them.

    How fertilisers interact with soil systems

    Fertilisers — whether mineral or organic — do not act in isolation. Their effectiveness depends on several interacting soil properties:

    • Nutrient supply: the amount and form of N, P, K, and micronutrients added
    • Nutrient retention: soil organic matter, clay content, and cation exchange capacity
    • Biological processing: microbial immobilisation, mineralisation, and root–microbe interactions
    • Soil structure: aggregation, porosity, and water–air balance affecting root access

    Soils with poor structure or low biological activity may lose applied nutrients rapidly, while healthy soils often deliver greater returns from lower fertiliser inputs.

    Efficiency, losses, and unintended consequences

    Poorly matched fertiliser use can lead to leaching, volatilisation, runoff, and reduced biological function. Excess soluble nutrients may temporarily boost yields while degrading long-term soil performance.

    More integrated approaches aim to reduce losses by improving soil structure, increasing organic matter, and aligning nutrient release with plant demand.

    Biochar as a supporting soil amendment (cross-reference)

    Biochar does not meet the regulatory definition of a fertiliser because it does not supply declared amounts of N, P, or K. However, it strongly influences fertiliser performance by:

    • Retaining applied nutrients and reducing leaching
    • Providing habitat for nutrient-cycling microbes
    • Buffering soil pH
    • Improving aggregation and porosity

    In some formulations, biochar is blended with nutrient sources to create biochar compound fertilisers that do meet NPK labelling requirements.

    → See dedicated biochar content: Biochar in healthy soil (internal link).

    Conclusion

    Fertilisers are essential tools, but they are not synonymous with soil fertility. Regulated NPK products supply nutrients, while soil fertility emerges from how those nutrients interact with soil structure, organic matter, and biology.

    Understanding fertilisers as part of a wider soil system helps explain why similar inputs can produce very different outcomes across soils — and why building soil function is central to nutrient efficiency and long-term resilience.

    Glossary (excerpt)

    • Fertiliser: A regulated product supplying declared plant nutrients.
    • Soil fertility: The soil’s capacity to supply nutrients, water, and biological support to plants.
    • NPK: Nitrogen, phosphorus, and potassium.
    • Soil amendment: Any material added to soil to improve physical, chemical, or biological properties.

    Note: Full definitions are held in the central Glossary & Definitions page (marked “Incomplete” until all pillars are finalised).

  • Quick answer

    Compost works in soils according to its particle size distribution and material fractions, not its label or certification. Very fine, humus‑associated material (often present in the <2 mm fraction) can interact strongly with water, nutrients, and microbes under appropriate conditions; intermediate particulate organic matter (a transitional organic state that commonly occurs in the ~2–6 mm range, occasionally up to ~10 mm) continues to decompose; coarse, woody fragments (>~10–15 mm) mainly affect physical structure. These properties influence soil behaviour, but they do not guarantee outcomes.

    Intro

    Compost is not a uniform material. Its behaviour in soils is governed by the distribution of particle sizes and material fractions created during composting, maturation, and screening. These fractions influence how compost interacts with water, nutrients, and soil biology, but they do not determine outcomes on their own.

    This article explains compost fractions as a material property, using HealthySoil canonical definitions. It describes how different fractions behave in soils, the limits of those effects, and why screening and grading matter. It does not prescribe management actions or soil improvement strategies.

    The three compost fractions

    Across most composted feedstocks, three broad fractions can be distinguished. These are not regulatory categories, but observable material groupings that help explain compost behaviour.

    1) Humus‑associated micro‑fraction (typically <2 mm)

    Description
    The finest fraction consists of very small particles, typically passing a dry 2 mm sieve and often much finer when wetted. This fraction often contains humified organic compounds and may include mineral‑associated organic material, depending on feedstock, processing, and maturity.

    Observed behaviour
    This humus‑associated micro‑fraction has a high surface area and strong interactions with water and ions. In soils, this fraction is commonly associated with altered water retention behaviour, nutrient buffering capacity (CEC), and provision of habitat for microbial communities, subject to soil context.

    Limits
    The presence of this humus‑associated micro‑fraction does not guarantee biological activity, nutrient availability, or long‑term carbon persistence. Their influence depends on soil texture, mineralogy, moisture regime, and system context.

    2) Intermediate particulate organic matter (PdOM)

    Description
    This fraction includes partially decomposed plant residues and fibrous material that has not yet been fully stabilised.

    Observed behaviour
    Intermediate organics continue to mineralise in soil, contributing to short‑ to medium‑term nutrient cycling and biological activity. During decomposition, they may temporarily immobilise nitrogen or alter oxygen dynamics at the microscale.

    Limits
    Rates and directions of change depend on environmental conditions and existing soil nutrient balance. Effects cannot be assumed from material appearance alone.

    3) Woody or bulking fragments

    Description
    The coarsest fraction is dominated by woody particles and structural bulking agents, typically retained when compost is screened at larger mesh sizes.

    Observed behaviour
    Coarse fragments influence physical structure by increasing macroporosity and resistance to compaction. They contribute carbon slowly and interact weakly with soil water compared with finer fractions.

    Limits
    High proportions of coarse material can dilute water‑holding capacity and may increase short‑term nitrogen immobilisation during decomposition. Structural effects are context‑dependent and not inherently beneficial.

    Why screening size matters

    Screening determines the relative proportions of these fractions. Compost screened to ≤15 mm typically contains a much higher proportion of fines than compost screened to ≤25 mm, where woody fragments may comprise a large share of the material. Screening size affects material behaviour but does not indicate degree of humification or stability on its own.

    Compost structure as a soil system property

    When incorporated into soil, compost fractions interact with existing soil structure rather than replacing it. Fine fractions tend to associate with soil minerals and aggregates, while coarse fractions act primarily as physical inclusions. The resulting behaviour reflects the combined soil–material system, not the compost alone.

    What compost structure can and cannot do

    Can:

    • Modify water distribution and pore architecture at the aggregate and bulk scale
    • Contribute organic inputs with different residence times
    • Influence biological habitat availability

    Cannot:

    Create persistent soil carbon independent of soil context

    Override soil texture or mineral constraints

    Guarantee nutrient availability or biological outcomes