Soil for Raised Beds: What to Use and How to Maintain It

Learn what soil mix to use in raised beds in Ireland, how to prevent settling, when to top up, and how to maintain fertility and drainage over time.

Soil for Raised Beds: What to Use and How to Maintain It

Raised beds are popular in Irish gardens because they warm faster in spring, drain better than heavy clay, and make gardening easier on your back. But filling them properly matters. Get the soil mix wrong and you’ll waste money, watch plants struggle, or spend every year refilling as the soil sinks.

This guide walks through the best approach for Irish raised beds: what to fill them with, how to avoid common mistakes, how to manage drainage and drought, and how to maintain beds year after year.

Quick navigation:

What’s the best fill for raised beds?

The best raised bed fill is a mix of topsoil and compost. This gives you the bulk and structure of soil with the fertility and drainage benefits of compost.

Standard mix:

  • 60–70% good-quality topsoil
  • 30–40% compost or well-rotted manure

Mix thoroughly or layer (topsoil base, compost on top). Either works.

Why this mix?

  • Topsoil provides structure and doesn’t shrink much over time
  • Compost improves drainage, adds nutrients, and supports soil life
  • The mix drains well but holds enough moisture for plant roots
  • It’s stable and won’t sink dramatically in the first year

How much do I need? Calculate bed volume: length × width × depth (in metres), then multiply by 1000 to get litres.

Example: 3 m × 1 m × 0.4 m = 1.2 cubic metres = 1,200 litres. You’d need roughly 800 litres topsoil and 400 litres compost.

Can I use bagged soil? Yes, but it’s expensive for larger beds. Bagged “raised bed soil” or “vegetable bed soil” is often a similar mix of topsoil and compost. Bulk delivery is cheaper if you’re filling multiple beds.

See our guide to topsoil vs compost vs mulch for more on materials.

Don’t fill with 100% compost

A common mistake is filling raised beds entirely with compost. It seems logical-compost is great for plants-but it causes problems.

Why not 100% compost?

  • Compost breaks down and shrinks. A bed filled with compost can sink by a third in the first year.
  • Pure compost drains very fast and dries out quickly in summer.
  • It’s expensive. A cubic metre of compost costs far more than a mix of topsoil and compost.
  • Some plants struggle in pure compost (too rich, inconsistent structure).

What happens if you do it anyway? The bed will sink over the first season as compost decomposes. You’ll need to refill repeatedly. Plants may grow lush but produce less (too much nitrogen). In dry spells, the bed dries out rapidly.

Better approach: Use topsoil as the base, compost as the amendment. Top up with compost each year rather than refilling the whole bed.

What does good topsoil look like?

Not all topsoil is equal. Much sold as “topsoil” is poor-quality subsoil, clay-heavy fill, or builder’s rubble.

Good topsoil should:

  • Be dark brown or rich in colour (not pale yellow or grey)
  • Smell earthy, not sour or chemical
  • Feel crumbly when dry, hold together lightly when moist
  • Contain some organic matter (visible dark specks, roots)
  • Be free of large stones, plastic, and rubble

Check before buying:

  • Ask the supplier what the source is (scraped farmland, screened topsoil, etc.)
  • Look at reviews or ask for a sample
  • Avoid “economy topsoil” unless you’ve checked quality
  • Visit the supplier’s yard if possible

Red flags:

  • Very cheap bulk topsoil (often subsoil or clay)
  • Grey or yellow colour (subsoil, not topsoil)
  • Strong chemical or sewage smell
  • Visible plastic, glass, or builder’s waste

If topsoil quality is poor, you’re better off using more compost in the mix or sourcing better soil, even if it costs more upfront.

How deep should a raised bed be?

Deeper is better, but there’s a practical limit.

Minimum depth: 20–25 cm Suitable for lettuce, radish, shallow-rooted herbs, and flowers. Cheap to fill, but dries out quickly and limits crop choice.

Ideal depth: 30–40 cm Suits most vegetables (tomatoes, beans, carrots, potatoes, courgettes). Holds moisture better, gives roots room, and balances cost with performance.

Deep beds: 50 cm+ Excellent for deep-rooted crops (parsnips, asparagus, perennials). Holds moisture well, but expensive to fill and requires more materials.

For most Irish gardeners: 30–40 cm is the sweet spot. Deep enough for good growth, shallow enough to fill affordably. If your bed is shallower, just stick to shallow-rooted crops and water more often.

Sitting on soil vs hard surface: If the bed sits on soil, plant roots can grow down into the native soil below, so you can get away with slightly shallower beds. On concrete or paving, you need the full rooting depth in the bed itself.

Drainage vs drought: finding the balance

Raised beds drain better than ground-level beds, which is great for Irish heavy clay. But too much drainage creates a drought risk in dry spells.

Drainage benefits:

  • Beds warm faster in spring
  • No waterlogging in wet winters
  • Easier to work soil in all weathers

Drought risks:

  • Raised beds dry out faster than ground-level soil
  • Wind exposure increases evaporation
  • Shallow beds (under 30 cm) dry out very quickly

How to balance:

Use a good topsoil/compost mix: Compost helps retain moisture. Pure sand or very light mixes drain too fast.

Mulch the surface: A 2–5 cm layer of compost, leaf mould, or straw reduces evaporation. Refresh mulch each year.

Add organic matter annually: Compost improves water-holding capacity over time.

Consider deeper beds in dry areas: If your garden is exposed or free-draining, aim for 40 cm depth rather than 25 cm.

Water consistently in dry spells: Raised beds need more frequent watering than ground beds in summer. A drip irrigation line or soaker hose makes this easier.

In most Irish gardens, drainage is more of a problem than drought. But in free-draining areas (sandy soil, coastal gardens), be mindful of moisture retention. See improving sandy soil for more.

Settling over time: why beds sink

All raised beds sink slightly over time. This is normal.

Why beds settle:

  • Compost breaks down and compresses
  • Soil particles settle as you water and work the bed
  • Worms and microbes incorporate organic matter deeper into the soil
  • Roots grow and decay, creating spaces that collapse

How much settling? A well-made bed (topsoil + compost mix) might sink 5–10 cm in the first year, then slow to 2–3 cm per year after that. A bed filled with pure compost can sink 30% or more in the first season.

How to manage settling:

  • Expect it and plan for it. Don’t fill beds right to the brim initially.
  • Top up each year with fresh compost (see below).
  • Avoid compacting the soil by walking on beds.
  • After a few years, beds stabilise and need less frequent topping up.

Settling isn’t a failure-it’s part of how soil matures.

How to top up each year

Raised beds need regular feeding. Compost and organic matter break down, nutrients are used by plants, and the soil level drops.

Annual top-up routine:

Late winter or early spring (February–March): Add 3–5 cm of compost or well-rotted manure across the bed surface. Spread evenly.

Why this timing? Gives the compost time to settle before planting. Worms incorporate it into the topsoil. Nutrients are available when plants need them.

How much? For a 3 m × 1 m bed, a 5 cm layer needs roughly 150 litres (about 2–3 large compost bags). Scale up or down based on bed size.

Can I use unfinished compost? Yes, if it’s mostly broken down. Rough compost works as a surface mulch and finishes breaking down in place. Just avoid it around seedlings.

What if I run out of compost? Use what you have on the beds you’re actively planting. Leave unused beds unmulched or use leaf mould, well-rotted manure, or other organic mulch.

Every few years: If the bed has sunk significantly, you might add a mix of topsoil and compost to restore the level. This isn’t needed annually-just when the bed feels shallow.

Topping up isn’t optional. Without it, fertility drops, structure degrades, and beds become hard and unproductive. See composting in Ireland to make your own supply.

Quick checklist

  • Best fill: 60–70% topsoil, 30–40% compost
  • Don’t fill with 100% compost (it shrinks and dries out)
  • Good topsoil = dark, crumbly, earthy smell, no rubble
  • Check topsoil quality before buying in bulk
  • Ideal bed depth: 30–40 cm for most vegetables
  • Deeper beds hold moisture better; shallow beds dry out faster
  • Raised beds drain well but can dry out in summer
  • Mulch the surface to retain moisture
  • Beds will settle 5–10 cm in the first year (normal)
  • Top up with 3–5 cm compost each late winter/spring
  • Mix topsoil and compost thoroughly or layer (both work)
  • Calculate volume: length × width × depth (m) × 1000 = litres
  • On soil, roots grow down; on hard surfaces, need full depth in bed
  • Add organic matter annually to maintain structure and fertility
  • Use well-rotted manure or compost as top-up material

FAQ

Can I use my native soil in raised beds? Only if it’s good quality. Heavy clay or poor subsoil won’t work well even in a raised bed. It’s better to fill with good topsoil and compost, or improve your native soil over time with amendments.

How often do I need to replace the soil? You don’t. Just top up with compost each year. Over time, the original topsoil and compost blend together into rich, living soil. Beds improve with age if maintained.

Should I put anything at the bottom of the bed? Not necessary. If the bed sits on grass, the grass will die and break down. Some people add cardboard to suppress weeds, but it’s optional. Don’t add gravel or drainage layers-they don’t improve drainage and waste depth.

Can I plant immediately after filling? Yes. If using well-rotted compost and good topsoil, you can plant straight away. Give it a good watering first to settle the soil and eliminate air pockets.

What’s the difference between raised beds and no-dig beds? Raised beds are physical structures (wood, metal, brick) that contain soil above ground level. No-dig is a method of managing soil (adding compost on top, not digging). You can use no-dig methods in raised beds by topping up with compost instead of turning the soil. See no-dig gardening in Ireland.

Do I need to line the bed? Not for vegetables. Lining with landscape fabric blocks worms and drainage. If using treated timber, some people line the inside to avoid contact between soil and chemicals, but modern treated timber is generally safe. Cedar or untreated larch need no lining.

Raised beds are a long-term investment. Fill them well from the start, top up with compost each year, and they’ll produce healthy crops for decades. If you’re curious about your soil’s baseline condition, soil testing can help, but most beds thrive with regular compost and common-sense management.