Layer recipes by bed depth
Plants feed in the top 20–30 cm. Everything below that is there for water storage, root anchorage and drainage, and does not need to be premium. That is the whole logic of layering: pay for quality where the roots feed, and buy volume cheaply underneath.
| Bed depth | Bottom | Middle | Top (where the roots feed) |
|---|---|---|---|
| 20–25 cm | – | – | All vegetable mix (AS 4419) |
| 30 cm on open soil | Fork the ground underneath | 5–10 cm compost or soil conditioner | 20–25 cm vegetable mix |
| 40 cm on open soil | Fork the ground underneath | 15 cm compost / manure blend | 25 cm vegetable mix |
| 45–60 cm on concrete or paving | 3–5 cm scoria or 10 mm gravel in geotextile | 15–25 cm compost / soil-conditioner blend | 25–30 cm vegetable mix |
| Wicking bed | Reservoir (scoria + pipe) with geotextile over it – see how to build a wicking bed | – | 30–35 cm vegetable mix with extra compost |
The raised bed calculator lets you enter each layer's depth and prices it separately, so you can see exactly how much of the bill is premium mix.
What goes in the top layer
A good raised-bed mix is soil-based. It should hold water but drain, hold nutrients, and stay put for years. The proportions that suit Australian and New Zealand vegetable beds:
- About 60 % screened loam or sandy loam – the mineral fraction that gives the mix body and holds nutrients. This is what pure compost and potting mix lack.
- About 30 % mature compost (AS 4454 if bagged) – biology, slow-release nutrients and water holding. Composted manure can be part of this.
- About 10 % coarse sand or fine scoria – keeps the mix open so it does not slump into a brick. Skip this if the loam is already sandy (much of Perth, coastal Queensland, Canterbury plains).
- Optional extras: a handful of rock minerals or blood-and-bone per square metre, and a light dusting of lime or dolomite if the compost is acidic. Mushroom compost is already alkaline – do not lime it. Test first: the soil pH guide explains how.
You do not have to mix this yourself. Every decent landscape yard sells a "vegetable mix", "veggie blend" or "organic garden mix" that is close to this recipe, and the major bagged brands sell an equivalent as "raised bed mix" or "vegetable and herb mix". The label check in the next section tells you whether it is any good.
Reading the label: AS 4419, AS 4454 and AS 3743
Australia has three standards that matter here, and their logos on a bag are worth more than any marketing word:
| Standard | Covers | What it tells you |
|---|---|---|
| AS 4419 Soils for landscaping and garden use | Garden mixes, topsoil blends, vegetable mixes | Tested for pH, salinity, organic matter, drainage, wettability and contaminants. This is the mark to look for on raised-bed fill. |
| AS 4454 Composts, soil conditioners and mulches | Compost, soil conditioner, manure blends, organic mulch | Tested for maturity (no ammonia burn), weed seeds, pathogens and contaminants. Look for it on anything you will dig in. |
| AS 3743 Potting mixes | Potting and container mixes | Red ticks = regular, black ticks = premium (contains starter fertiliser, tighter air/water limits). For pots, not for filling beds. |
Bulk products cannot carry a bag logo, but good yards will tell you their mixes are "made to AS 4419" and can show a test certificate. If a product carries none of these and the price is very low, assume it is fill, not growing medium.
NZ does not have compulsory equivalents. Many NZ producers reference the Australian standards or their own published specifications; the practical checks below apply either way.
Practical checks that work anywhere: dark and crumbly, earthy smell (not sour or ammonia), no glass or plastic, no large woody chunks in a "soil" product, and it wets easily when you pour water on a handful rather than beading and running off.
What not to fill a raised bed with
- Potting mix on its own. It is bark-based, designed for pots, and in a bed it slumps by a third within a year, dries out fast and needs constant feeding. Use it only in genuinely container-sized beds under 100 L.
- Straight compost or manure. No mineral fraction, shrinks dramatically, can be too rich (nitrogen burn, salty) and often too acidic or too alkaline. Compost is an ingredient, at 20–30 %.
- Cheap "garden soil" or "topsoil" of unknown origin. Often subsoil, clay or sand with dyed bark. Test a bag before buying a pallet.
- Fill, builder's sand or excavated clay. Fine under a path, useless for roots.
- A thick gravel layer "for drainage" on open ground. It does not drain – it perches water in the soil above it – and it wastes 5–10 cm of depth you paid for. Only use a thin drainage layer on sealed bases.
- Untreated fresh wood chips mixed in. They tie up nitrogen as they rot. Wood chips belong on top as mulch, not in the mix.
Keeping the cost down
- Only buy premium for the top 25 cm. On a 50 cm bed that halves the premium volume.
- Buy bulk for anything over a dozen bags – see bagged vs bulk. Yards will often deliver a vegetable mix and a compost on one truck, separated by a board.
- Make your own compost for the middle layer and for the seasonal top-up. It costs nothing and is better than most of what you can buy.
- Do not over-build. A 30 cm bed on forked open soil grows almost everything. Read how deep should a raised bed be? before you decide on 60 cm.
For free bottom layers (logs, turf, straw and site soil) and a worked cost comparison, see how to fill a raised bed cheaply.
Year two and beyond
Every raised bed sinks. Compost breaks down, the mix settles and roots and worms rearrange it; expect to lose 10–15 % of the depth in the first year and a few centimetres a year after that. The fix is cheap: each season, before planting, spread 3–5 cm of compost on top, fork it lightly into the surface and re-mulch. Every few years, if the mix has become dense, fork through a bag or two of coarse sand or a barrow of fresh vegetable mix. You should never need to empty and refill a bed unless it has been contaminated or has become diseased.
Questions
Can I use potting mix in a raised garden bed?
Not as the main fill. It is designed for containers, shrinks fast and dries out. Mixing up to a third of premium potting mix into a heavy soil-based blend is fine; filling the bed with it is expensive and disappointing.
What is the best soil for a raised vegetable garden?
A soil-based vegetable mix made to AS 4419 – roughly 60 % loam, 30 % compost, 10 % coarse sand – in the top 25 cm, with cheaper compost or soil conditioner underneath on deeper beds.
Should I put cardboard or newspaper at the bottom?
Yes, on open ground, to smother grass and weeds. Plain cardboard with the tape removed rots away within months and roots go straight through. It is not a drainage layer and it is not a root barrier.
How much compost should I add to raised bed soil?
20–30 % by volume in the mix, then 3–5 cm on top each season. Use the compost calculator for the seasonal top-up.
Sources
- Standards Australia, AS 4419:2018 Soils for landscaping and garden use; AS 4454:2012 Composts, soil conditioners and mulches; AS 3743:2003 Potting mixes. standards.org.au
- ABC Gardening Australia fact sheets on building and filling raised beds, wicking beds and soil improvement.
- Perched-water-table effect of coarse layers under finer media: standard container-media soil physics as summarised in horticultural extension literature.
Last reviewed 23 September 2026 · Report an error