TL;DR: Most bagged “Multipurpose Composts” (MPC) are formulated growing media that give fast plant performance but are not humus. True compost (e.g., PAS100-certified green-waste compost) is biologically active and still decomposing; humus is the stable end-product that forms slowly in living soils.
Quick Answers (FAQ)
1) Are compost and humus the same?
No.
- Compost: recently decomposed organic matter; active, still breaking down.
- Humus: the stable, long-lasting fraction of soil organic matter; forms over years and resists further decay.
For the science deep-dive, see HealthySoil: Humus Explained.
2) Does bagged Multipurpose Compost contain humus?
Generally, no. Most retail MPCs are formulated growing media (coir/wood fibre/digestate, plus lime and nutrients). They are near-sterile at purchase and designed for consistent growing performance — not a source of stable humus.
Exception: mixes that include PAS100 compost will add more biological life and “fresh organic matter,” but even then only a small fraction eventually becomes humus.
Learn what’s in retail mixes: MPC-P1: Components of Multipurpose Compost
See biological life expectations: MPC-P4: Biological Activity
3) What is PAS100 and why does it matter?
PAS100 is a UK quality specification for stabilised green-waste compost. Products clearly labelled as PAS100 have passed process and quality checks (stability, contamination, etc.).
- PAS100 compost: biologically active, contributes fresh organic matter and microbes.
- Formulated MPC: primarily a growing medium; performance-tuned, often low biology at purchase.
4) Will using compost create humus in my soil?
Over time, a little. Compost feeds microbes; a small fraction of that carbon is stabilised into humus via soil processes (microbial transformation, binding to clay, mineral associations). Most carbon is respired as CO₂ during decomposition.
For mechanisms and timescales, see HealthySoil: Soil Food Web.
5) What should I buy if I want both quick growth and long-term soil quality?
- For quick, reliable plant growth: choose a quality formulated MPC appropriate to your use (seed, potting, containers).
- To build living soil and nudge humus formation: blend in PAS100 compost or your well-made home compost, mulch, and avoid over-disturbing soil.
Simple Comparison
| Compost (incl. PAS100) | Formulated MPC (retail) | Humus (in soil) | |
|---|---|---|---|
| Status | Active, decomposing | Near-sterile growing media | Stable, persistent |
| Timescale | Weeks–months | Immediate use | Years–centuries |
| Main Benefit | Feeds microbes & plants | Consistent plant performance | Water & nutrient buffering |
| Where it lives | Bag or heap → soil | Bag/container | In soil only |
Key Takeaway
Use MPC for dependable, immediate growing performance. Add PAS100 or good home compost and keep soils biologically active if your goal is to slowly increase humus over time.
