Two ways to approach mixing.
Straightforward:
Combine Multipurpose Compost (MPC) with soil simply to improve drainage and structure, then feed plants as needed through the season.
Soil-building:
View the mix as the foundation of a living bed — adding PAS100 compost, home compost, or biochar builds microbial life and organic matter that enhance fertility with every season.
Explore Soil-Building Approaches →
Why Mix Compost into a Soil Bed?.
Raised beds thrive on air and moisture. A few bags of MPC stirred in give the soil fresh pores to breathe and food for microbes to wake up. Unlike filling a bed entirely with compost, blending it with soil keeps the structure stable and slows nutrient depletion.
If you’re thinking of using 100% MPC to fill a new raised bed, see our related guide:
Can I (or Should I) Fill a Raised Bed Using Only Multipurpose Compost?
If you’re improving ground-level garden soil rather than a raised bed, read:
Can I Mix Multipurpose Compost with My Garden Soil?
These subtle differences matter — raised beds warm and drain faster, so decomposition and water loss happen sooner than in in-ground soil.
How Much to Add.
| Bed Type | Recommended Mix (by volume) | Notes |
|---|---|---|
| New or sandy beds | 1 : 1 (50%) MPC : soil | Maximises structure and water-holding. |
| Established or heavier beds | 1 : 2 (33%) MPC : soil | Maintains fertility without excessive nitrogen drawdown. |
Anything richer than 50% compost works short-term but collapses faster and needs more feeding.
What to Expect Through the Season.
Early season (0–2 months) – The compost’s starter fertiliser drives strong growth.
Mid-season (3–6 months) – Microbes begin breaking down the carbon-rich fibres and temporarily lock up nitrogen.
Later (6 months +) – Structure settles; a light feed or new compost keeps things in balance.
Compost-rich beds dry out more quickly than plain soil, so expect to water about half again as often, especially in warm spells.
Maintenance Tips.
- Feed lightly – use a general organic fertiliser or liquid feed every few weeks.
- Top up each spring – new compost restores bulk and microbial life.
- Mulch or cover in winter – reduces drying and nutrient leaching.
- Watch moisture – peat-free fibres can become hydrophobic when dry; re-wet thoroughly.
Expert Summary Table — What Happens When Compost Meets Soil.
| Concept | What’s Going On | How It Affects the Bed | Summary (for Gardeners) |
|---|---|---|---|
| Nutrient Flow | Compost carbon fuels microbes; they need nitrogen to digest it. In MPC-rich mixes (C:N ≈ 80–300 : 1) microbes temporarily immobilise N before releasing it again. | Early nutrient flush → mid-season dip → stabilised fertility after renewal. | Compost doesn’t lose nutrients — they’re locked in biology. Feed lightly to keep plants supplied. |
| Water Dynamics | Compost particles create porosity but poor water movement between them. Once dry, many fibres turn water-repellent. | Beds with > 40% compost drain fast and dry quickly at the surface. | Expect to water around 50% more often; keep moisture even to avoid slump and dry pockets. |
| Physical Structure | Large organic particles improve aggregation; as they decay, fine humus forms and volume shrinks. | Structure improves for one season, then consolidates; annual top-ups rebuild texture. | Compost adds short-term structure and long-term humus — renew yearly to keep beds friable. |
Not all composts behave the same once they meet real soil. Multipurpose compost acts differently from home-made or PAS100 green-waste mixes.
- DIY composts are richer in active microbes and nutrients, forming part of a Soil Food Web-driven system that cycles nitrogen more efficiently.
- PAS100 composts often contain a higher proportion of 6–25 mm woody bulking fragments, which decompose slowly and influence both aeration and moisture dynamics.
These differences explain why feeding and watering tips vary. For deeper soil science, explore our HealthySoil guides:
In Summary.
Just remember the three rules — blend it (33–50%), feed it, and water it — and your raised bed will repay you with richer soil and steadier growth all year.
