The short answer: in the UK and Ireland, coir compost is a peat-free potting medium made mainly from coconut husk pith, sold either as a ready-to-use bag or as a compressed block you soak at home. It holds water well, rewets easily, sits at a near-neutral pH and carries almost no plant food of its own, so it needs feeding sooner than a peat compost. It is not decomposed garden waste, and it is not what an American gardener means by "compost".
Key facts
- A 5 kg compressed block gives about 60 to 75 litres once soaked, though some retailers quote 80.
- Washed coir pith sits at pH 5 to 7, most lots between 5.8 and 6.5, so it needs little or no lime. Peat sits around 4.4 and is always limed.
- Ready-bagged peat-free multipurpose composts typically carry 4 to 6 weeks of feed. A plain coir block carries none.
- Coir holds six to eight times its dry weight in water and rewets readily. Dry peat repels water.
"Compost" means different things on each side of the Atlantic
On UK and Irish garden-centre labels, "compost" is the stuff you fill pots with: seed compost, multipurpose, ericaceous, John Innes No. 2. None of it has to involve anything rotting down. "Coir compost" follows the same convention. It's a potting medium with coir as the base.
In the US, "compost" is decomposed organic matter and the pot-filler is "potting mix". So an American searching for coir compost usually means coir as an ingredient in a mix, or coir pith that has actually been composted. In India, where most pith is produced, composted coir pith is a real product: raw pith is layered with urea and Pleurotus mushroom spawn (about 5 kg of urea per tonne) and left for a month, until the lignin breaks down and it turns dark and crumbly. That goes onto fields as a soil improver.
This article is about the UK meaning. For the raw material itself, start with what coco peat is.
What's in a bag of coir compost
A plain coir block is one ingredient: coconut pith, washed, dried and pressed. A bagged "coir compost" or "coir-based compost" is a recipe. Percentages are rarely printed, but these are the usual parts and why they're there.
| Ingredient | Why it's in the mix |
|---|---|
| Coir pith | The base. Water holding, air, easy rewetting, slow to break down |
| Coir fibre or husk chips | Opens the mix up and raises drainage in container grades |
| Composted bark or wood fibre | Cheaper bulk, more air, and it stops the coir slumping in large pots |
| Green compost (composted garden waste) | Some nutrients and pH buffering, lower cost |
| Perlite or grit | Drainage and weight. One common retail block is 80% coir and 20% perlite |
| Base fertiliser | The starter feed. A balanced NPK with trace elements, usually sized for 4 to 6 weeks |
| Lime | A little, if any. Coir doesn't need the pH correction peat does, but dolomitic lime supplies the calcium and magnesium coir lacks |
| Wetting agent | Standard in peat composts. Less needed in coir, though some blends include one for the bark fraction |
Two things follow. "Coir compost" from two brands can behave quite differently, because bark, green compost and fertiliser all change how it waters and feeds. And a 100% coir product with nothing added is a growing medium, not a finished compost, so you supply all the nutrition yourself.
How coir compost behaves compared with peat compost
If you've grown in peat for years, the material in the pot looks familiar and acts differently.
| Coir compost | Peat compost | |
|---|---|---|
| Surface | Dries pale on top while still wet underneath | Colour tracks moisture more honestly |
| Watering | Fewer, lighter waterings. Check by weight or finger | Regular, and once dry it's hard to rewet |
| Rewetting | Takes water readily even from bone dry | Dry peat shrinks from the pot and repels water, hence wetting agents |
| pH | About 5.8 to 6.5 as supplied, little lime needed | Around 4.4 raw, limed up to 5.5 to 6 |
| Native nutrients | High potassium, some sodium and chloride, low calcium and magnesium | Low across the board |
| Feeding | Start early, use a feed with calcium, go easier on potassium | Standard feeds work as designed |
| Nitrogen | Some early tie-up as microbes work on the high-carbon pith, more if bark is present | Little tie-up |
| Lifespan in the pot | Holds structure two seasons or more | Shrinks and compacts in a season |
Watering is where most people slip. Coir can look parched on the surface with the root zone sodden an inch down. Lift the pot. If it's heavy, leave it. Overwatered coir is cold, slows germination, rots cuttings, and every extra litre through the pot washes out feed you just paid for.
On feeding, the awkward combination is that coir holds nutrients less well than peat, and its exchange sites arrive loaded with potassium rather than calcium. So plants in coir compost want feeding sooner and more often than you're used to, and the feed should contain calcium and magnesium. A cheap tomato feed (high potassium, no calcium) is the wrong choice for a long-term container in pure coir. The full comparison, including cost and sustainability, is on our coir vs peat page.
Reading the bag
Peat-free labelling in the UK is voluntary, so the words on the bag carry the weight.
- "Peat-free" has to be printed. If it isn't, assume the bag contains peat. "Environmentally friendly", "organic", "sustainable" and "reduced peat" all leave room for it. Some "coir-based" bags are peat-reduced blends.
- Nutrient charge. Look for "feeds for X weeks". Four to six is typical for a multipurpose. Seed and cutting grades are deliberately low in feed and expect a liquid feed from the first true leaves. A plain block has none unless a sachet is in the box.
- John Innes on a coir bag. John Innes is a recipe, not a brand: 7 parts sterilised loam, 3 parts peat, 2 parts sharp sand, with base fertiliser at 3, 6 or 9 g per litre for No. 1, 2 and 3 and limestone to match. Traditional John Innes contains peat. Peat-free versions swap the peat for coir, wood fibre or bark and cut the lime because those materials aren't acidic. A coir compost "with added John Innes" has loam added for weight, buffering and a longer feed, which suits containers planted for two or three years. Check that bag for "peat-free" too.
- Volume. Bags are sold by litre, loosely filled. A block is sold by weight and the litres are a claim, so look for the yield per kilogram and how it was measured.
- Responsible Sourcing Scheme. Some bags carry an RSS traffic-light rating for ingredient sourcing. A useful tiebreaker between two peat-free products.
- Use this season. Peat-free blends keep breaking down in the bag.
Compressed blocks or ready bags?
Both are coir compost. They suit different buyers.
| Compressed block or briquette | Ready bag | |
|---|---|---|
| What you get | 100% coir pith, dry, pressed to about a fifth of its volume | A blended, moistened, fertilised recipe |
| Storage and carriage | Light and compact. A 5 kg block makes a wheelbarrow of compost | Bulky and heavy, much of the weight is water |
| Nutrients | None unless a sachet is included | Starter feed for 4 to 6 weeks |
| Consistency | Varies by source and grade. Check EC and expansion | Blended to a spec |
| Effort | Soak, break up, add fertiliser, mix | Open and use |
| Suits | Seed sowing, cuttings, hydroponics, blenders, anyone short on space | Potting on, baskets, containers, people who want it done |
A 5 kg block wants roughly 18 to 20 litres of water and twenty minutes or so to swell, quicker with warm water. Expect 60 to 75 litres of loose compost, and treat anything quoted above 80 with a question about how it was measured. A 650 g briquette makes about 8 to 10 litres, enough for a seed tray or two. Grades and sizes are on the blocks page.
Making your own coir compost from a block
For seed sowing, plain hydrated coir is already usable. For anything living in the pot more than a few weeks, you're building a compost, so add what the bag manufacturer would have added.
- Hydrate in a tub with room to spare. Add water in stages and break the block apart as it softens, so you don't get a wet skin over a dry core. Fluff until loose and even.
- Check the salts. Washed coir has had most of its potassium, sodium and chloride rinsed out. If the run-off tests salty, flush once more and drain.
- Set the structure. Pure pith is fine for trays and cuttings. For containers, open it up with perlite, husk chips, bark or grit so it doesn't slump. Ratios are in coir, perlite and vermiculite mixes.
- Add a base fertiliser. A balanced granular or controlled-release product with trace elements, at the rate on its pack. The John Innes benchmark of 3 g per litre for young plants and 6 g per litre for established ones is a sense check if the pack is vague. Include a calcium and magnesium source, usually a light dusting of dolomitic lime rather than the heavy liming peat needs. Test the pH afterwards and stay below about 6.5.
- Mix on a tarp or in a cement mixer, leave it a day so the fertiliser starts dissolving, then pot up.
- Feed from week four to six with a liquid feed that contains calcium, weekly to begin with.
Our own coir is washed with water to lower the EC and is not buffered, so the calcium step is the normal pairing rather than an optional extra.
Where coir compost suits, and where it doesn't
Seeds. Good. Fine, even texture, no weed seeds, moist yet free-draining, and the pH suits most vegetables and bedding. The one caution is overwatering, because the surface lies to you.
Cuttings. Good, often better than peat. Coir stays aerated at high moisture, which is what a cutting wants, and roots lift out cleanly. Many commercial propagators root into pure coir or coir and perlite.
Containers and baskets. Good, with conditions. Pure coir is light and dries fast in a sunny basket, so blend in something with body, and feed for the whole season because the starter charge runs out in six weeks. For a two or three year planting, a coir compost with loam holds up better than pure pith.
Where it's weaker. Acid-lovers such as blueberries and rhododendrons want a lower pH than coir gives, and coir's potassium works against them, so use a dedicated peat-free ericaceous mix. Tall, top-heavy plants in pure coir blow over. Anything you plan to neglect will suffer, because the medium holds little nutrient reserve.
Where peat-free policy stands
As of late 2026 there is still no law banning peat compost sales in the UK: the government has committed to legislate when parliamentary time allows, while the big retailers and the RHS have gone peat-free on their own. The wider picture is in our peat-free growing media guide.
If you blend or bag coir compost and want to talk about pith grades, fibre content or washed, unbuffered supply, get in touch and tell us what you're making.
FAQ
Is coir compost peat-free?
A 100% coir block is peat-free by definition. A bagged coir compost is peat-free only if the bag says so, because some coir-based blends are peat-reduced rather than peat-free. If the words "peat-free" aren't printed on it, assume it contains some peat.
Is coir compost the same as coco peat or coco coir?
The raw material is the same: the pith from coconut husks. "Coco peat", "coir pith" and "coco coir" all describe that material. "Coir compost" is the UK retail term for it packaged as a potting medium, sometimes blended with bark, green compost and fertiliser.
Does coir compost contain any nutrients?
A plain coir block contains almost none that plants can use, apart from potassium and a little sodium left from the husk. Bagged coir composts have a starter fertiliser added, usually enough for four to six weeks. After that, everything in the pot comes from your feed.
How much water does a coir compost block need?
A 5 kg block takes roughly 18 to 20 litres of water and about twenty minutes to swell, quicker with warm water. Add the water in stages and break up the block as it softens. Expect 60 to 75 litres of loose compost from a good block.
Can I sow seeds straight into coir compost?
Yes. Hydrated coir is fine-textured, weed-free and holds moisture evenly, which suits most seeds. Keep it moist rather than wet, because a saturated tray is cold and slows germination. Start a liquid feed once the seedlings show their first true leaves.
Do I need to add lime to coir compost?
Not for pH. Coir sits at roughly 5.8 to 6.5 already, which is where most potting composts aim. A little dolomitic lime is still worth adding to a home mix because it supplies calcium and magnesium, both of which coir lacks. Test the pH afterwards and keep it under about 6.5.
Is coir compost better than peat compost?
Different rather than better. Coir rewets easily, lasts longer in the pot and doesn't involve digging up peatland. Peat holds nutrients better and is more forgiving of careless feeding. Most growers who switch find plants do as well once they've adjusted watering and started feeding earlier.