Introduction

In ‘Core Components‘ we explored the five core base components that form the foundation of most Multipurpose Composts. Each base — whether peat, coir, wood fibre, digestate, or PAS100 compost — provides structural and physical properties essential for moisture management and aeration.

However, these bases alone rarely deliver a complete growing medium. To achieve the required balance of nutrients, structure, water-holding, aeration, and pH stability, suppliers add a range of modifiers and improvers. These additives are carefully selected to fine-tune the compost’s performance for specific uses — from seed starting to potting, containers, and grow bags.

Five key parameters modified by additives:

• Nutrient content and release rate (via NPK fertilisers or organic sources).
• pH buffering (through lime, gypsum, or mineral dusts).
• Moisture-holding capacity (coir, vermiculite, perlite, pumice).
• Aeration and structure (wood fibre, perlite, pumice).
• Biological activity and stability (microbial inoculants, biochar, compost humification agents).

Suppliers formulate composts to ensure they are “fit for purpose” — the right physical, chemical, and biological balance for their target market. This deep dive explores the common additives used, why they are included, and what this means for gardeners choosing between products.

Understanding the Lens: How Values and Preferences Shape Choices

While suppliers add these materials to meet growing specifications, your own values and preferences will shape how you interpret them.

• Organic gardeners may prefer composts certified to PAS100 or organic standards, avoiding synthetic NPK additives.
• Regenerative growers may seek products supporting long-term soil carbon and microbial health (e.g., biochar, humification agents).
• Soil Food Web (SFW) enthusiasts will note that most retail MPCs are microbially inactive — nearly sterile by design. They may prefer composts that have been inoculated naturally (e.g., PAS100 compost) or choose to reintroduce biology via separate inoculants.
• Conventional users may focus on predictable, clean, and consistent performance, valuing nutrient control over biological complexity.

The sections below summarise what’s added and why — not to judge, but to help you decide which compost fits your own growing philosophy.

Mineral & Chemical Additives

Lime and pH Adjusters

• Buffer acidity and maintain neutral pH.
• Help optimise nutrient availability.
• May increase nitrogen loss if overused.

Gypsum and Rock Dusts

• Supply calcium and trace minerals.
• Improve structure in wet, clay-heavy composts.
• Effects are often subtle and site-specific.

Organic & Fibre Additives

Coir (Coconut Fibre)

• Improves structure and water-holding.
• Renewable but imported; footprint considerations apply.

Wood Pulp, Sawdust and Straw

• Add bulk, stabilise moisture, and aid aeration.
• Decompose slowly, maintaining structure.

Paper & Cardboard

• Readily available bulking carbon source.
• Overuse may reduce airflow.

Biological Improvers

Biochar

• Enhances nutrient retention and microbe habitat.
• Adds long-term carbon stability.
• May slightly darken or densify texture.

Compost Humification Agents

• Stimulate humus formation for longer nutrient availability.
• May improve yield and soil aggregation.

Microbial Inoculants

• Add fungal or bacterial cultures to jumpstart microbial life.
• Useful when base composts are biologically inactive.

Nutrient Additives: NPK Fertilisers

Controlled-Release Granules

• Deliver consistent nutrient supply during growth.
• Inorganic forms common in mainstream MPCs.
• Organic-approved alternatives exist but release slower.

Liquid Nutrient Additions

• Boost growth when feedstock nutrient release is slow.
• Can leach if over-applied.

Key point: Nutrient additives improve short-term fertility but rarely persist. Pairing them with structural improvers (like biochar) enhances nutrient efficiency.

Physical Improvers: Moisture & Aeration

Vermiculite, Perlite and Pumice

• Lightweight minerals improving porosity, drainage, and water retention balance.
• Used in high-value or propagation blends.
• Less common in bulk composts due to cost.

Practical Takeaways

• Check labels: look for whether additives are synthetic or natural.
• Match your values: organic, regenerative, or conventional.
• Remember that most retail composts are microbially stable (not biologically active).
• Reintroduce life where needed using inoculants or PAS100 compost blends.

Linking Across the ‘What’s in the Bag’ Series

• WIB-P1 – Core Components: the foundational materials every compost begins with.
• WIB-P3 (Coming soon..) – Environmental Impact: how materials and additives affect sustainability and carbon balance.
• WIB-P4 (Coming soon..) – Market and Labelling Trends: how composition and additives influence marketing claims and standards.
  
  See ‘Multi-Purpose Compost (MCP) – Market and Trends‘ for more market info!

Together, these pillars will provide a complete view of what’s inside modern Multipurpose Composts — from raw materials to formulation intent.

How This Insight Shapes Our Unique Reviews

Understanding additives and improvers lets us evaluate composts by measurable parameters rather than vague impressions. Our review framework analyses structure, nutrient stability, microbial activity, and carbon balance — offering evidence-based ratings rather than generic stars. This gives gardeners clear insight into how well each compost meets its claimed purpose.

Summary

Compost additives aren’t inherently good or bad — they’re tools for achieving specific physical, chemical, and biological goals. Knowing what’s added and why helps you choose compost that aligns with your growing method and values.