A raised bed is only as good as the soil inside it. Fill it with poor soil and you get poor plants, however well you water and feed; fill it with a good mix and vegetables grow fast from the first season, even on ground that was useless before. The right raised bed soil mix is loose enough for roots to push through, holds moisture without staying soggy, and carries enough nutrients to feed a full season of growth. This guide gives the standard recipe, explains how much you need, what to avoid, and how to keep the mix good year after year.
The classic recipe is simple: roughly 60 per cent good topsoil, 30 per cent compost and 10 per cent coarse material such as perlite, grit or composted bark by volume. That ratio works for most vegetables in most climates, and it is a starting point you can adjust — more compost on poor ground, more grit on heavy ground, more coir for lightness in containers. The rest of this guide explains why each ingredient is there and how to adapt the mix for different jobs.
What goes into a good raised bed mix
Topsoil (about 60 per cent). Topsoil gives the mix its body, its mineral content and its weight, so it holds moisture and anchors roots. Buy screened topsoil from a reputable supplier — it should be dark, crumbly and earthy-smelling, not pale, dusty or full of rubble, glass or weed roots. Avoid subsoil, which is the pale, lifeless layer from below the topsoil and grows almost nothing, and avoid topsoil from unknown sources, which can be contaminated. If you are unsure about a supplier, ask where the soil comes from and whether it has been tested.
Compost (about 30 per cent). Compost supplies the nutrients, feeds soil life, and holds moisture. Use mature, dark, crumbly compost — your own, or bagged compost from a reputable source. Well-rotted manure can replace part of the compost, but use it mature and well-rotted, never fresh. If you use your own compost, make sure it was made hot enough to kill weed seeds; otherwise you will spend the season weeding the bed. Our composting basics guide covers home composting.
Coarse material (about 10 per cent). Coarse perlite, grit, composted bark or coir fibre keeps the mix open so it drains and lets air reach the roots. Perlite is light and good for containers; grit is heavier and better for beds that need to stay put; composted bark adds structure and breaks down slowly. Do not use fine sand, which fills the gaps and makes the mix denser, and do not use fresh bark or wood chips in the mix, which temporarily rob nitrogen. See our perlite vs vermiculite guide for the difference between the two minerals.
Optional extras. A handful of garden lime per square metre if your topsoil is acidic and you are growing brassicas — but only on the basis of a soil test; see our soil pH testing guide. A small amount of biochar, up to 5–10 per cent by volume and charged first, adds long-term water and nutrient holding; see our biochar for garden guide. A slow-release organic fertiliser mixed into the top layer gives heavy feeders a good start. Do not add synthetic fertiliser in large doses to a new mix — it can burn young roots and leach away.
How much soil do you need?
Work out the volume before you order, because bulk soil is heavy and delivery is expensive. Volume is length × width × depth, all in metres, which gives cubic metres. A bed 2.4 m long, 1.2 m wide and 0.3 m deep needs 2.4 × 1.2 × 0.3 = 0.86 cubic metres, roughly 860 litres. A bed 30 cm deep is the usual minimum for vegetables; 45 cm is better for root crops such as carrots and for beds on concrete or compacted ground. The raised bed soil calculator does the maths for any bed size, and the soil volume calculator covers other shapes. Order about 10 per cent extra, because the mix settles by several centimetres in the first weeks — fill the bed slightly proud of the top and let it settle before planting.
A cubic metre of soil mix weighs roughly 800–1,000 kg, so a single bed can weigh close to a tonne. Make sure the ground underneath can take the weight, and that the bed walls are strong enough to hold it — soil pushes outwards when wet, and weak boards bow or split. If the bed is on a roof, balcony or deck, check the weight limit first and use a lighter mix with more coir and perlite and less topsoil.
Step-by-step: filling a raised bed
1. Prepare the base. If the bed sits on soil, fork the ground underneath to break any compaction, and lay cardboard over grass or weeds to smother them — the cardboard rots away and lets worms through. Do not line the base with plastic, which blocks drainage and worms. If the bed sits on concrete, drill drainage holes in the base or raise it slightly on blocks, and put a 5 cm layer of coarse gravel or broken pots at the bottom for drainage.
2. Add a base layer if you like. The hugelkultur method — logs and branches at the bottom, then leaves, then soil — adds organic matter that feeds the bed for years as it rots. It works well in deep beds (45 cm or more) but is optional; a plain mix works fine in a standard bed.
3. Mix the ingredients. Mix topsoil, compost and coarse material together on a tarp or in a barrow before filling, so the blend is even. Filling in layers — a layer of topsoil, then a layer of compost — gives an uneven bed where roots hit sudden changes. Mix first, fill second.
4. Fill and water. Fill the bed to a few centimetres above the rim, water it thoroughly so it settles, and top it up after a few days. Let it settle for a week before planting if you can, so the level is stable.
5. Mulch the surface. Cover the surface with 5 cm of straw or shredded leaves, keeping mulch away from stems. Mulch keeps the mix moist, stops it crusting, and rots down into more organic matter. Our mulch types guide compares the options.
6. Plant and feed. Plant into the mix and water well. The compost in the mix feeds the first season, but heavy feeders such as tomatoes and cabbage will still need feeding through the season — see our fertilizing vegetables guide.
Maintaining the mix year after year
Raised bed soil sinks and loses nutrients every season, so maintenance is an annual job. Each autumn or spring, top up the bed with 5–8 cm of compost, forked lightly into the top 15 cm or left as a mulch. This replaces the organic matter that broke down and the nutrients the harvest removed, and it keeps the level up. Test the pH every year or two — raised bed mixes drift over time, especially if you water with hard tap water — and adjust only on the basis of results. Rotate crops so the same family is not in the same bed year after year, and grow a cover crop such as crimson clover over winter if the bed is empty; see our cover crops for soil guide. After three or four years, a bed may need a deeper refresh: fork in a fresh batch of compost and coarse material, or replace the top 15 cm entirely if the mix has become compacted and lifeless.
What not to put in a raised bed
Do not fill a bed with pure compost: it is too rich, holds too much water, and shrinks dramatically as it breaks down, leaving the bed half-empty by midsummer. Do not use pure topsoil either: it compacts and drains poorly without compost and coarse material. Do not use fresh manure, which can burn roots and carry pathogens — compost it first. Do not use soil from an unknown or industrial site, which may be contaminated; if in doubt, test it or buy from a reputable supplier. Do not use treated timber for the bed walls where it touches the soil, and do not add large amounts of fresh wood chips into the mix. And do not add lime without a test — many bought topsoils are already near neutral, and over-liming locks up iron and causes yellowing.
Common mistakes
The most common mistake is under-filling the bed, so plants have too little root depth — 30 cm is the minimum for vegetables, and deeper is better for root crops. The second is filling with layers instead of mixing, which gives uneven growing conditions. The third is using pure compost or pure topsoil instead of a blend. The fourth is forgetting that the mix settles, so the bed is half-empty by midsummer — fill it proud and top it up. The fifth is never topping up again: a bed that is not fed annually declines within two seasons. And the sixth is planting the same crop in the same bed every year, which builds up pests and diseases — rotate, even in a small garden.
Safety notes
Wear gloves when handling compost, manure and soil, and wash your hands before eating. Use only mature, well-rotted manure. If you are building beds from timber, check whether it is treated and avoid treated wood in contact with soil for food beds. If the ground under the bed is on a former industrial site, test the soil or use an impermeable barrier and imported mix, and check your local extension service for advice.
Frequently asked questions
What is the best soil mix for raised beds?
Roughly 60 per cent good topsoil, 30 per cent mature compost and 10 per cent coarse material such as perlite, grit or composted bark by volume. Adjust the ratio for the job: more compost on poor ground, more grit for drainage, more coir for lightness on a roof or balcony.
How deep should a raised bed be?
At least 30 cm for most vegetables, and 45 cm or more for root crops such as carrots and parsnips, or for beds on concrete or badly compacted ground. Deeper beds hold more moisture and give roots more room, but they cost more to fill.
Can I use garden soil in a raised bed?
Yes, if it is good topsoil and you mix it with compost and coarse material. Heavy clay or very sandy garden soil should not be used on its own — improve it first, or use bought topsoil. See our clay soil improvement and sandy soil improvement guides.
How often should I replace raised bed soil?
You should rarely need to replace it entirely. Top up with 5–8 cm of compost every year, and refresh the top 15 cm after three or four years if the mix has become compacted. A well-maintained mix improves with age rather than declining.
How much soil do I need for a raised bed?
Multiply length × width × depth in metres to get cubic metres. A bed 2.4 m × 1.2 m × 0.3 m needs about 0.86 cubic metres. Use the raised bed soil calculator for your exact bed size, and order about 10 per cent extra for settling.