A good raised bed fill is mostly real soil (a loamy garden soil or topsoil) blended with finished compost, so it drains well but still holds water and nutrients. Two common starting points are about 70 per cent topsoil with 30 per cent compost, or equal parts topsoil, organic matter and coarse sand.

Filling a bed with only one ingredient, whether potting mix or compost, is where most problems begin.

What goes into a raised bed soil mix?

Three components do the work.

• Garden soil or topsoil. This supplies the mineral part of the soil that holds water and nutrients. Iowa State University Extension’s raised bed mix advice suggests about 70 per cent topsoil and 30 per cent compost, or equal parts topsoil, organic matter and coarse sand.
• Compost. It adds organic matter and slow-release nutrients. Use finished, well-rotted compost. If yours is still lumpy or smelly, our guide to compost that refuses to break down explains why.
• Other organic matter. Aged manure, leaf mould or similar material can stand in for part of the compost. Manure-based material is richer, so use less of it.

These are US extension figures, used here for principles rather than a fixed recipe. Australian soils range from heavy clay to coastal sand, so the right balance depends on what you start with.

Why not fill a bed with pure potting mix or pure compost?

Both are tempting because they are easy. Both have drawbacks.

Pure compost. South Dakota State University Extension notes in its guidance on compost in raised beds that many gardeners find plants do not grow well in 100 per cent compost. It can pack down too tightly for good aeration, and if it dries out completely it can repel water, leaving a wet-looking surface over a dry core. Its nitrogen is also used up over time, and phosphorus can build up. The same source says gardeners typically use 10 to 50 per cent compost, with about 25 per cent the most common.

Pure potting mix. Potting mix is made for containers and is usually light and open. Used alone in a large bed it is expensive and tends to shrink and settle a lot as the organic material breaks down. This part is editorial advice rather than a cited finding, but the logic is the same: a bed needs some mineral soil to give it body. For a closer comparison, see how potting mix differs from garden soil.

Potting mix can still play a role as a minor ingredient, particularly in a lawn-site or compacted-ground build where no good topsoil is available. The University of Maryland Extension lists a 1:1 compost and soilless mix for such sites.

Which fill suits your site?

The best mix depends on what is under the bed. The University of Maryland Extension’s guide to filling a raised bed splits it by situation:

Open-bottom beds on workable soil let roots grow into the ground beneath. Beds on paving cannot, so the depth you fill matters more.

How should you layer and fill it?

1. Loosen the base. On bare soil, fork it over so water and roots can move down.
2. Add the mix in stages. Add a few centimetres of blend, mix it into the layer below, then repeat. Iowa State advises this over stacking distinct layers, because sharp layers can impede water movement and discourage root growth.
3. Check the soil you buy. Good topsoil is dark, crumbly and earthy-smelling. Avoid anything that smells foul, looks grey or mottled, or feels chalky, as Maryland’s guide notes. Iowa State also advises checking imported soil for contaminants.
4. Keep the surface level or slightly domed. Grade it gently so water does not pool in the middle.
5. Water it in. Soak the bed, let it settle, then check the level.

Some gardeners put coarse woody material at the base of deep beds to save on soil volume. We have not verified an Australian source for that practice, so we leave out claims about hugelkultur-style layering and its benefits.

Why does the soil sink, and how do you top it up?

New beds settle. Air pockets collapse after watering, and the organic part of the mix breaks down. A compost-heavy mix sinks more than a soil-heavy one.

Plan for it. Buy or make a little more than the measured volume, and expect to top the bed up after the first few waterings and again before each new season. Top up with finished compost or a fresh blend of the same proportions, rather than a thick layer of pure compost. A level that has dropped by a few centimetres is normal. A bed that keeps sinking quickly suggests the mix is too compost-heavy or the compost is still breaking down.

How do you check pH and drainage?

pH. The Queensland Government’s soil pH page explains that pH affects which nutrients are available. It gives 5.5 to 7.5 as the workable range. Below that, strongly acid soils can cause aluminium and manganese toxicity and shortages of calcium and magnesium. Above it, alkaline soils can lack zinc, copper, boron and manganese.

A home kit gives a rough guide. A lab test is more informative, and the University of Maryland Extension notes labs report pH and nutrient levels. Readings vary by method, so compare like with like. If you are unsure, test the topsoil before you mix it.

Drainage. Water the bed thoroughly and watch. It should soak in without a puddle lingering for long. If water pools, the mix may be too fine or compacted; fork in coarse organic matter or sand and retest. Our overview of soil and watering basics covers how to read moisture in the root zone.

How can you fill a bed cheaply?

• Use your own garden soil if it is loamy and free of contaminants, mixed with homemade compost.
• Smother lawn or weeds in place under cardboard before filling, as Maryland describes, so those materials rot down in the bed.
• Fill the bottom with the cheapest suitable soil and put the best blend where roots will be most active. This is editorial advice, not a sourced rule.
• Order or collect one measured batch rather than several small ones. Work out length x width x depth in metres to get cubic metres.

Common mistakes and what is uncertain

Common mistakes: filling with one ingredient only, using unfinished compost, stacking sharp layers of different materials, skipping the allowance for settling, and assuming a pH inside 5.5 to 7.5 suits every crop.

The proportions above come from US extension services. They are sound principles, but Australian soils, climates and compost quality vary. Tropical and subtropical beds break down organic matter faster, so top up more often. Cooler, drier regions may settle more slowly. Where you are unsure, start with a blend near 70 per cent soil and 30 per cent compost, then adjust after a season of watching how it behaves.