TL;DR (summary)
- Multi-purpose Compost (MCP) is engineered for consistency, safety, and nutrient reliability, not biological liveliness.
- Most MCPs use inert bases – peat, coir, bark, wood fibre – plus added NPK; they are biologically subdued by design.
- Home/DIY compost and PAS100 green-waste composts are the main biologically active materials available to gardeners.
- MCP improves structure and water dynamics but does not “prime” soil life the way many assume.
- When mixed into soil, existing microbes will slowly colonise it – a mild, secondary effect.
Introduction – How This Deep Dive Fits
This is the fourth deep dive in the What’s in the Bag series:
- P1 – Core Components – Base materials such as peat, coir, wood fibre, and digestate.
- P2 – Additives & Improvers – Nutrients, limes, wetting agents, biochar, and more.
- P3 – Physical Properties – Structure, porosity, aeration, and water dynamics.
- P4 – Biological Activity (this page) – What life exists in MCP, and what doesn’t.
Together these pages reveal how formulation choices shape real-world performance and help explain the results you’ll later see in our MCP Reviews Hub.
Why “Biological Activity” Needs Careful Definition
Compost and biology are often used interchangeably, yet commercial MCPs are not living composts. They are designed to stay stable in storage, pathogen-free, and uniform between batches — qualities that limit microbial life.
By contrast:
- Home/DIY composts teem with microbes but vary widely in texture and nutrient content.
- PAS100 green-waste composts are biologically active on an industrial scale but can be inconsistent or contain plastic/glass fragments.
Recognising these differences avoids unrealistic expectations when MCP is used in soil improvement or container culture.
The Biological Spectrum – How the Types Compare
| Compost Type | Biological Status | Typical Use | Strengths | Limitations |
|---|---|---|---|---|
| Home/DIY compost | Highly active | Garden soil improver | Microbial diversity, local, low cost | Variable, may contain weeds/pathogens |
| PAS100 / Green Waste Compost | Active | Landscaping, soil conditioner | High microbial content | Contamination and variability risks |
| MCP | Biologically subdued | Pots, raised beds, general use | Clean, safe, consistent, nutrient-charged | Low microbial life, minimal soil “priming” |
(See P1 – Core Components for base-material detail.)
Wood-Rich Composts: The Hidden Variable
Even certified composts vary dramatically in texture. PAS100 sets limits on contaminants and stability but does not guarantee fineness or full humification. Typical grades are ≤ 25 mm or ≤ 15 mm, meaning many still contain 25–50% woody bulking material from the composting process.
- act like slow-release carbon (good for structure, poor for feeding plants),
- tie up nitrogen temporarily during further decay,
- and can reduce water-holding if the chip fraction is high.
This explains why some PAS100 or council composts perform inconsistently in pots and mixes — their biological activity is genuine, but the chemistry and physics are still in flux.
What’s Inside MCP – and What Isn’t
Inert bases such as coir, bark, peat, and wood fibre contain little labile carbon and high lignin content. They give MCP its stability — but they do not feed microbes readily.
- Soil biology gains little “fuel.”
- Growth relies on added NPK, not decomposition.
- Microbial colonisation after use is slow and surface-limited.
Raised Beds & Soil Improvement
- Short term: rapid growth from fertiliser charge and porosity.
- Medium term: microbial activity remains low; resistant organic matter decomposes slowly.
- Long term: beds settle and require top-ups of biologically active composts (Home/DIY compost, PAS100, or manure) to maintain fertility.
For those following Soil Food Web or organic principles, MCP alone won’t restore soil biology — it functions mainly as a structural substrate.
Labile vs Resistant Carbon – the Science
Microbes thrive on labile carbon (sugars, cellulose, proteins). MCP ingredients offer resistant lignin and cellulose, requiring specialist fungi found mainly in surface litter layers. Thus, MCP’s biological “priming” effect is weak compared with fresh or home-made composts rich in labile fractions.
Market Perspectives
- Conventional / Retail: Focus on safety and repeatability. Avoid live inputs to prevent spoilage or disease.
- Organic / Soil Food Web: Value microbial diversity; often blend MCP with home composts.
- Regenerative: Use MCP as bulk organic matter but add inoculants or local composts for life.
- Technology-Driven: Experiment with biochar carriers, microbial inoculants (Trichoderma, mycorrhizae), and nanobubble hydration.
These remain specialist but signal future evolution of MCP.
Biological Activity – Setting Expectations Clearly
| Material | Primary Function | Biological Role |
|---|---|---|
| MCP | Structural substrate + nutrient carrier | Minimal active life; colonisable over time |
| Home/DIY compost | Soil improver | High microbial diversity; energises soil food web |
| PAS100 compost | Landscape improver | Active biology but variable consistency |
MCP and living composts are complementary, not competing, materials: MCP delivers reliability, while living composts deliver biology.
How We Use This Knowledge in Our Reviews
The detailed understanding from the What’s in the Bag series isn’t just academic — it underpins our product reviews. By comparing each brand’s biological profile with its base materials, additives, and physical traits (see P1–P3), we identify which MCPs are:
- Balanced – good structure + nutrient control,
- Nutrient-driven only, or
- Beginning to integrate biological potential.
This lets our reviews go beyond star ratings. They translate deep technical data into clear buying guidance — helping gardeners choose sterile blends for precision pots or biologically richer options for soil enhancement.
Summary – Clarity for Gardeners and Experts
Modern MCPs are biologically subdued by design. They provide the dependable base most gardeners need. For living biology, supplement with Home/DIY compost or PAS100 material. Used together, these create both the structure and the life required for sustainable soil health.
Understanding where MCP fits ensures informed choices and avoids false expectations — the cornerstone of credible reviews and responsible gardening advice.
