Two ways to approach raised beds
Straightforward:
Use MPC as a convenient filler, but expect to feed more often and replace material regularly as structure and nutrients decline.
Soil-building:
Start with a mix of MPC, topsoil and compost to build a dynamic soil ecosystem that supports root–microbe partnerships, water retention and long-term fertility.
Explore Soil-Building Approaches →
TL;DR
You can fill a raised bed with just multipurpose compost — it’ll grow plants fast to begin with. But think of it as a big pot, not a self-sustaining soil. It dries quicker, slumps sooner, and soon needs extra feed. For lasting results, mix one part MPC to one or two parts soil (33–50%).
Why Gardeners Try MPC Beds
It’s easy to see why people do it — a few bags of MPC look clean, light, and full of promise. And for the first few weeks, they usually deliver.
But a raised bed isn’t just a bigger pot. Once compost meets air and weather, its texture, water behaviour, and nutrients change fast.
What Happens in an MPC-Filled Bed
| Timeframe | Structure | Nitrogen & Nutrient Behaviour | Water Behaviour | Outcome |
|---|---|---|---|---|
| 0–2 months | Open and fluffy, easy to work with. | The starter fertiliser gives a quick growth boost. | Holds moisture well if kept evenly damp. | Fast, lush early growth. |
| 3–6 months | Begins to settle; air spaces close. | Microbes feed on woody bits, borrowing nitrogen. | Top dries quickly; needs more watering. | Growth slows unless fed again. |
| 6–12 months+ | Shrinks 30–40%; fine and compact. | Most fertiliser used; mild nitrogen lock-up continues. | Hard to re-wet; forms dry patches. | Plants fade unless compost and feed are renewed. |
Why It Happens
Modern peat-free MPCs are made mostly from wood fibre, coir, or peat, plus a short-term feed. They’re high in carbon and low in nitrogen, so they start rich but don’t stay that way.
| Material | Typical C : N Ratio |
|---|---|
| Wood fibre / bark | 200 – 350 : 1 |
| Coir | 80 – 150 : 1 |
| Peat | 50 – 120 : 1 |
When soil life moves in, it needs nitrogen to break those materials down. It grabs what’s available — leaving less for your plants. That’s the “nitrogen lock-up” gardeners notice.
Water Matters Too
- Big air gaps mean quick drainage but poor upward water movement.
- When peat-free fibres dry, they often become water-repellent.
- Light materials heat up and cool down quickly, speeding evaporation.
You’ll likely water about 50% more than with a soil mix.
A Better Mix for Long-Term Beds
| Goal | Best Mix (by volume) |
|---|---|
| New or sandy beds | 1 : 1 (50% MPC : 50% soil) |
| Established beds or heavier soil | 1 : 2 (33% MPC : 67% soil) |
Anything above 50% MPC will work — just expect to feed and water more often.
See also: Can I Add Multipurpose Compost to Raised Beds?
Keeping Beds Healthy After Year 1
- Feed regularly. A little fertiliser every few weeks, or a slow-release source each season.
- Water consistently. MPC needs about 50% more water than a soil mix.
- Top up yearly. Add fresh compost to rebuild structure and moisture-holding power.
Quick Summary – Structure, Nitrogen & Water
| Phase | Structure | Nitrogen | Water | Key Action |
|---|---|---|---|---|
| 0–2 months | Light, open, free-draining. | Fertiliser available. | Moist if kept watered. | Enjoy fast growth. |
| 3–6 months | Settling, less air. | Nitrogen tied up by microbes. | Dries faster. | Feed and water. |
| 6–12 months+ | Compact, heavy. | Fertiliser gone; mild deficit. | Hard to wet. | Renew compost and feed. |
Expert Note: Why MPC Beds Behave Differently
A bed filled only with MPC behaves like a large open container, not like living soil. By the time compost reaches the bag, it’s nearly sterile — so the first months are ruled by physics, not biology.
| Aspect | MPC Bed | Soil–MPC Mix |
|---|---|---|
| Microbes | Sparse at first; colonisation takes months. | Active immediately. |
| Nitrogen | Early flush, then microbial lock-up. | Steadier release and balance. |
| Water | Fast drainage, little upward movement. | Better retention and balance. |
| Temperature | Heats and cools fast; loses moisture. | Thermal mass keeps it stable. |
| Structure | Shrinks 30–40% in year one. | Holds shape longer. |
In short: MPC starts rich but biologically empty. As microbes move in, it loses structure and available nitrogen unless managed like a container. Add soil, and it becomes a living, self-balancing bed.
For practical ratios, see Can I Add Multipurpose Compost to Raised Beds?
For deeper soil science, visit HealthySoil.co.uk → Raised Beds – What Works and Why
