TL;DR
- Retail MCPs are engineered substrates, not raw composts.
- Particle size: screened ≤ 10 mm (often ≤ 6 mm) → uniform texture.
- Air-Filled Porosity (AFP) / Free Air Space (FAS): 20–30% at container capacity.
- Container capacity (Water-Holding Capacity – WHC after drainage): 65–75% water fraction.
- Bulk Density: 0.3–0.6 g cm⁻³ (moisture-dependent).
- Goal: balanced air ↔ water ↔ structure for consistent rooting and watering performance.
How this specification supports our product reviews
The technical data and benchmarks in this section underpin the MultipurposeCompost.co.uk review and “What to Buy” framework. When analysing a retail compost, we compare its measurable physical performance indicators — Particle-Size Distribution, AFP, WHC, Bulk Density, and structural stability — against target ranges. These values inform our internal MCP Quality Index, which drives the star-ratings and advice on the Where to Buy and Product Reviews pages.
Summary
Fine screening (≤ 10 mm, often ≤ 6 mm), balanced AFP (~20–30%), controlled WHC (~65–75%), and suitable Bulk Density (~0.3–0.6 g cm⁻³) deliver consistent moisture and oxygen profiles. These are industry-known metrics, tuned by each blender through fibre selection, fines balance, and moisture conditioning. The result: a stable, easy-to-use medium that behaves the same bag after bag — and the benchmark against which all MCPs on this site are reviewed.
Footnote — Scope Clarification
The specifications above are used almost exclusively for retail Multi-Purpose Compost (MCP) formulation. They are not part of the PAS100 compost standard, which focuses on stability, maturity, and contamination limits rather than air-water physics. PAS100 grades define only upper particle sizes (≤ 15 mm or ≤ 25 mm) and impurity thresholds for bulk soil improvers. For background, see our PAS100 Standards Cluster.
