Topsoil vs Compost vs Mulch: What's the Difference?

Clear guide to topsoil, compost and mulch for Irish gardens. Learn when to use each, common mistakes, and how to choose quality materials for your soil.

Topsoil vs Compost vs Mulch: What’s the Difference?

If you’ve ever stood in a garden centre wondering whether you need topsoil, compost, or mulch, you’re not alone. These three materials look similar in the bag, but they do completely different jobs in the garden. Using the wrong one wastes money and effort, especially in Irish conditions where soil drainage and structure matter year-round.

This guide breaks down what each material actually is, when to use it, and how to avoid the most common Irish gardening mistakes.

Quick navigation:

What is topsoil?

Topsoil is the upper layer of soil, usually the top 15–30 cm. It contains mineral particles (sand, silt, clay), some organic matter, and the bulk of soil life. When you buy “topsoil,” you’re buying this layer-often scraped from construction sites or fields.

Good topsoil should:

  • Be dark brown or rich in colour
  • Hold together lightly when squeezed but break apart easily
  • Smell earthy, not sour or chemical
  • Be reasonably free of weeds, stones, and plastic debris

What topsoil does:

  • Provides the main growing medium for roots
  • Holds water and nutrients
  • Supports soil structure and drainage

What topsoil doesn’t do:

  • It won’t dramatically improve existing soil fertility on its own
  • It won’t fix drainage problems if your native soil is heavy clay
  • It won’t suppress weeds or retain moisture the way mulch does

In Ireland, much sold as “topsoil” is variable quality-sometimes subsoil, clay-heavy fill, or contaminated material. Always check before bulk-buying.

What is compost?

Compost is decomposed organic matter: vegetable peelings, garden waste, leaves, and other plant material broken down by microbes. Finished compost is dark, crumbly, and smells like woodland floor.

What compost does:

  • Feeds the soil with nutrients and organic matter
  • Improves soil structure (especially in heavy clay or light sand)
  • Supports beneficial microbes and earthworms
  • Helps soil hold moisture without becoming waterlogged

What compost doesn’t do:

  • It won’t add bulk like topsoil (compost breaks down over time)
  • It won’t fix severe drainage issues alone
  • Fresh or unfinished compost can lock up nitrogen and harm plants

Compost is a soil improver, not a growing medium on its own. You dig it into soil or layer it on top, but you wouldn’t fill a raised bed with 100% compost. Learn more in our guide to composting in Ireland.

What is mulch?

Mulch is material spread on the soil surface to suppress weeds, retain moisture, and slowly feed the soil as it breaks down. Common mulches in Ireland include:

  • Wood chip or bark: long-lasting, good weed suppression, but can tie up nitrogen if dug in fresh
  • Leaf mould or composted leaves: excellent for moisture retention and gentle feeding
  • Straw or hay: quick to break down, useful in vegetable beds
  • Well-rotted compost: technically both compost and mulch when used on the surface

What mulch does:

  • Keeps soil moist during dry spells
  • Suppresses annual weeds (not established perennials)
  • Moderates soil temperature
  • Slowly feeds soil as it breaks down
  • Reduces soil compaction from rain impact

What mulch doesn’t do:

  • It won’t improve soil structure if left on the surface (needs worms to incorporate)
  • Fresh wood chip can rob nitrogen if mixed into soil
  • It won’t fix underlying drainage or pH issues

Mulch is especially valuable in Irish gardens during late spring and summer when rain becomes unpredictable. It also protects clay soils from crusting in heavy rain.

Decision guide: which one do I need?

If you want to…

Fill a new raised bed: Use a mix of good topsoil (60–70%) and compost (30–40%). See our guide to soil for raised beds.

Improve heavy clay soil: Add compost or well-rotted manure each year. Mulch on top. Avoid adding sand or poor topsoil. More in improving clay soil in Ireland.

Improve sandy or free-draining soil: Add compost regularly to hold moisture and nutrients. Mulch heavily. Details in improving sandy soil.

Top up an existing bed: Use compost if you want to feed the soil. Use mulch if you want to suppress weeds and retain moisture.

Level a bumpy area or fill a hole: Use topsoil. Compost will sink over time.

Start a new lawn: Use topsoil to level and build depth. Work in compost if the soil is poor. See lawn soil preparation.

Start a no-dig bed: Layer compost or well-rotted manure on top. Mulch with cardboard and more compost. Learn more in no-dig gardening in Ireland.

Feed existing plants: Use compost worked in lightly, or apply compost as a mulch around plants.

Common Irish mistakes

Buying poor-quality topsoil

Not all topsoil is equal. Cheap bulk topsoil often contains subsoil (yellow-grey and lifeless), clay lumps, or builder’s rubble. Always ask what the source is, check reviews, or buy a small sample first.

Using fresh wood chip as compost

Fresh wood chip (especially softwood) looks like compost but behaves differently. When dug into soil, it uses up nitrogen as it breaks down, starving your plants. Use it as surface mulch instead, or let it age for 6–12 months before mixing in.

Thinking compost fixes drainage instantly

Compost improves drainage over time by building soil structure, but it won’t drain a waterlogged clay bed in one season. Heavy clay needs repeated additions, reduced digging, and patience. See our clay soil guide.

Filling raised beds with 100% compost

Compost shrinks as it breaks down. A bed filled entirely with compost will sink by a third in the first year and cost a fortune to refill. Use topsoil as the base, compost as the amendment.

Applying mulch too thickly in wet seasons

A 2–5 cm mulch layer is ideal. Any thicker in Irish wet winters can keep the soil surface too wet, encourage slugs, and prevent the soil from warming in spring. Adjust seasonally.

Buying “topsoil” without checking what’s in it

Some suppliers sell anything brown. If it smells sour, has bits of plastic, or is pure clay, walk away. Good topsoil should feel and smell alive.

Quick checklist

  • Topsoil = the main growing medium; use for filling, levelling, building depth
  • Compost = decomposed organic matter; use to feed and improve soil
  • Mulch = surface layer; use to suppress weeds, retain moisture, protect soil
  • Good topsoil: dark, crumbly, earthy smell, not clay-heavy
  • Compost should be well-rotted: dark, crumbly, no recognisable chunks
  • Fresh wood chip = mulch only; don’t dig in or use as compost
  • Raised beds: 60–70% topsoil, 30–40% compost
  • Clay soil: add compost, not sand or poor topsoil
  • Sandy soil: add compost regularly, mulch heavily
  • Don’t fill raised beds with 100% compost (it will shrink)
  • Check topsoil quality before bulk-buying (ask for samples)
  • Mulch 2–5 cm deep, not thicker in wet Irish winters
  • Use compost to improve fertility; use mulch to protect and suppress weeds
  • Don’t expect instant results-soil improvement takes time

FAQ

Can I use compost instead of topsoil in a raised bed? Not on its own. Compost breaks down and sinks over time. Use a mix of topsoil (for bulk and structure) and compost (for fertility). A 70:30 or 60:40 split works well. See our raised bed guide.

Is bagged compost the same as homemade compost? Both are decomposed organic matter, but quality varies. Homemade compost can be excellent if made properly. Bagged compost is more consistent but sometimes pasteurised (fewer microbes). Either works-just make sure it’s well-rotted.

Can I make my own topsoil? Not really. Topsoil takes decades to form naturally. You can improve poor soil with compost, but you can’t “create” topsoil from scratch. If you need depth, buy good-quality topsoil and amend it with compost.

What’s the difference between mulch and compost if I use compost on the surface? When you spread compost on the surface, it acts as both a mulch (protecting soil, suppressing weeds) and a slow-release feed. Proper mulch (like wood chip) lasts longer and suppresses weeds better but feeds the soil more slowly.

Can I use fresh grass clippings as mulch? Yes, in thin layers (1–2 cm). Thick layers can mat down, smell, and block air. Let clippings dry slightly first, or mix with straw. Better yet, compost them first for more stable material.

Should I remove old mulch before adding new? No need. Old mulch breaks down into the soil. Just top up when the layer gets thin. If using fresh wood chip, avoid piling it up too thick-2–5 cm is plenty in Irish conditions.

If you’re curious about your soil’s current condition, soil testing can give you a baseline. But most garden soil improves simply by adding compost regularly, mulching well, and avoiding unnecessary digging.