The short answer: a compressed coco peat block should carry roughly 15 to 20% water by weight when it leaves the press. Below that it crumbles at the corners and wets slowly. Above it you pay freight on water, get fewer litres per tonne, and the block can mould in a container. Every point of moisture on the label is a point of coir you're not buying, so the figure belongs in your purchase order with the test method and the basis (wet or dry) written next to it.
Key facts
- Moisture is quoted two ways. 20% on a wet basis is 25% on a dry basis. Same block, different number.
- A 5 kg block at 15% moisture holds 4.25 kg of coir. At 25% it holds 3.75 kg, about 12% less medium for the same money.
- Air in equilibrium with dry coconut fibre sits near 65% relative humidity. Microbes need free water, and almost nothing grows below about 60% equilibrium humidity. Wet blocks move above that line.
- Inside a closed steel container in the sun, air temperature reaches about 50°C when it's 25°C outside. A drop of about 5°C overnight is enough to start condensation on the roof.
Why the pressing moisture is 15 to 20%
Coir pith is dried in the open before it goes into the press, and the moisture at pressing decides how the block behaves afterwards.
Too dry and the pith has no plasticity. The block comes out of the mould with soft edges, sheds dust in the wrap and can split along the pressing planes in handling. Over-dried pith also rewets slowly: coir dried hard to around 15% doesn't recover its full water capacity, while coir at 30% climbs back over several waterings. A very dry block needs more water and more time to break open, as covered in our guide to hydrating coco peat blocks.
Too wet and you're shipping water, the block settles under the pallet load so the bottom tiers deform, and the fibre carries enough moisture to feed mould once the container warms up.
Most specifications put block moisture between 10 and 20% by weight, and 15 to 20% is where a press runs comfortably. A figure above 20% on a delivery note is the one to question.
Wet basis and dry basis
This is where most arguments between buyer and supplier start. Moisture can be expressed against the weight of the moist sample (wet basis) or against the weight of the dry matter (dry basis). Soil science, timber and geotechnics use the dry basis. Bulk trade, food and most coir specifications use the wet basis. Neither is wrong, but a number without the basis stated can't be checked.
Wet basis: water divided by total (wet) weight. Dry basis: water divided by dry weight.
To convert: dry basis = wet basis ÷ (1 − wet basis). Wet basis = dry basis ÷ (1 + dry basis).
| Wet basis | Dry basis | What it means for a 5 kg block |
|---|---|---|
| 10% | 11.1% | 0.50 kg water, 4.50 kg coir |
| 15% | 17.6% | 0.75 kg water, 4.25 kg coir |
| 20% | 25.0% | 1.00 kg water, 4.00 kg coir |
| 25% | 33.3% | 1.25 kg water, 3.75 kg coir |
| 30% | 42.9% | 1.50 kg water, 3.50 kg coir |
| 50% | 100% | 2.50 kg water, 2.50 kg coir |
Dry-basis figures run higher and can pass 100%, because hydrated coir holds several times its own dry weight in water. If a lab report says "moisture 120%", it's dry basis and it's about hydrated material, not a block. If a supplier quotes "under 20%" and your lab reports 24% on a dry basis, the two may agree. Ask which basis before you raise a claim. Everything below uses the wet basis.
What moisture does to the weight you pay for
You buy coir by the kilogram or the tonne, and water is part of that kilogram. Only the dry matter expands into growing medium, so read a moisture figure as the share of the tonne that isn't coir. A 5 kg block at 15% versus the same block at 25%:
| Block at 15% | Block at 25% | Difference | |
|---|---|---|---|
| Gross weight | 5.00 kg | 5.00 kg | none |
| Water | 0.75 kg | 1.25 kg | +0.50 kg water |
| Dry coir | 4.25 kg | 3.75 kg | −0.50 kg coir (−11.8%) |
| Expanded volume at about 15 L per kg dry coir | about 64 L | about 56 L | about 8 L less |
| Per tonne of blocks: dry coir | 850 kg | 750 kg | −100 kg |
| Per tonne of blocks: water carried | 150 kg | 250 kg | +100 kg |
| Per tonne of blocks: expanded volume | about 12,750 L | about 11,250 L | about 1,500 L less |
The litres column uses the common rule of thumb that a kilogram of dry coco peat expands to about 15 litres. You'll see anything from 12 to 20 quoted, because expansion depends on the material and on how the volume is measured, so run your own hydration test and put your own number in. The arithmetic doesn't change: a 10-point rise in moisture takes roughly 12% of the coir out of each tonne. A 24-tonne container at 15% lands about 20.4 tonnes of coir. The same container at 25% lands 18 tonnes, for the same freight bill.
Correcting a quoted expansion figure
Suppliers quote expansion in litres per kilogram or per block, and the figure depends on the moisture of the block they tested. To compare two quotes, bring both to the same moisture:
litres at your moisture = quoted litres × (1 − your moisture) ÷ (1 − quoted moisture)
A block quoted at 70 litres "at 15% moisture" delivered to you at 22% gives 70 × 0.78 ÷ 0.85, about 64 litres. That's the number your potting line or grow bag count should use. A supplier who quotes litres per kilogram without a moisture figure is quoting a number you can't check.
For how moisture shows up in a bag or block price per litre, and how the pallet weights work out, see buying coco coir in bulk.
Measuring moisture content
There's one reference method and several shortcuts.
| Method | How it works | Good for | Limits |
|---|---|---|---|
| Oven drying | Weigh a sample, dry at 100 to 105°C to constant weight, reweigh. Moisture = (wet − dry) ÷ wet | Claims, specifications, calibrating everything else | Takes hours. Needs a 1 g scale and an oven that holds temperature |
| Microwave drying | Short bursts, weigh between each until the weight stops falling | A fast check on arrival | Easy to scorch the sample, which reads as extra "moisture" |
| Pin (resistance) meter | Two pins pushed into the block measure electrical resistance | Quick screening of many blocks | Calibrated for wood species, not coir. Reads where the pins reach. Readings shift with temperature and with salts in the material |
| Pinless (electromagnetic) meter | A pad on the surface reads to about 6 or 19 mm depth | Scanning a pallet face for wet spots | Calibrated for a set density. A damp surface or condensation can push readings 10 to 20% high. Won't see a wet core |
| Squeeze test | Squeeze a handful of hydrated coir | Working moisture on a mixing line | Useless on a block, and only a rough guide on loose coir |
Oven drying is the method to write into a specification, because everyone can reproduce it. Pin meters built for hay or timber read electrical resistance, and dissolved salts conduct as well as water does, so a block with higher EC reads wetter than it is. They also need a temperature correction when the material is far from room temperature, which is the case on a container floor in summer. Treat meters as a sorting tool: scan twenty blocks, pull the three that read high, and oven-test those. The full step-by-step is in how to test a coco peat sample.
Moisture isn't even through a pallet. The bottom tier picks up water from a damp floor or wet pallet timber, the top tier gets dripped on from a container roof, and the core lags behind the faces. Sample from at least three positions, from the centre of a block as well as the face, and record where each came from.
Mould on coco peat blocks
Mould on a block is a moisture story, not a hygiene one. Dry coconut fibre holds the air around it at roughly 65% relative humidity. Almost no microbe grows below about 60% equilibrium humidity, and most moulds need something nearer 80%. So a block that stays at its pressing moisture doesn't mould, however long it sits. A block that creeps up through 20, 25 and 30% crosses the line.
On coconut fibre you'll see green and red mould on the surface, and brownish-black mildew spots with a musty smell where bacteria have been at work. Surface fuzz on an otherwise dry block is cosmetic. A sour or ammonia smell, dark wet-looking patches or a block that's warm in the middle of the pallet mean the moisture went deep and stayed. Damp coconut fibre is very prone to self-heating, and over-moist bales in long storage tend to rot from the inside.
How to judge mouldy blocks once hydrated is covered in mould, algae and fungus gnats in coco coir. The purchasing point is simpler: mould means the lot either left the factory wet or got wet on the way, and moisture at arrival tells you which.
What happens inside a container
A closed steel box is a condensation machine. In the sun, the air inside a brown-painted container reaches about 50°C at an outside temperature of 25°C, and a white one around 38°C. The warm air picks up water vapour from the cargo, the pallets and the wooden floor. At night the roof cools fastest, the air under it drops below its dew point, and water condenses on the underside. Unlike the walls, the ceiling doesn't dry out during the day, so the water builds up and drips onto the top tier. A drop of about 5°C is enough to start it.
Coir makes this worse because it's hygroscopic. As the cargo warms it gives up water vapour, and the wetter it was loaded, the smaller the gap between air temperature and dew point. That's why a lot pressed wet moulds in transit while a dry lot in the same container doesn't. Loading as dry as possible is the single most effective measure, and the supplier controls it. Worn door seals leak too, and a container arriving at a cold port from the tropics is the worst case, because the whole shell drops below the dew point of the air inside.
What packaging does and doesn't do
Shrink wrap on single blocks keeps dust, splashes and drips off and holds a cracked block together. It isn't a vapour barrier, so a wrapped block still gains moisture slowly through the seams in a humid store. It also traps water if the block was wrapped wet, which is why mould inside intact wrap usually points to pressing moisture rather than transit damage.
Pallet stretch film holds the stack and sheds roof drips to the floor. Cover the top and leave the bottom open so the pallet can breathe, or you've built a tent that collects condensation inside. Kraft paper or cardboard over the top tier soaks up drips before they reach the blocks.
Desiccants pull water out of the container air. Calcium chloride container desiccants absorb up to about three times their own weight under shipping conditions. They can't dry a wet lot, but they keep a dry lot dry through a few condensation cycles. Hang them along the top rails, not on the floor.
Ventilation cuts both ways. Ventilated containers are used on tropical-to-Europe routes to let cargo moisture escape, and a hold of damp fibre is given around ten air changes an hour. For a dry cargo, taping the vents slows the container's breathing and keeps humid port air out. Which is right depends on how dry the lot went in.
Pallets and floors are hygroscopic too. Kiln-dried pallet timber under 18% moisture is the usual target, because a wet pallet wicks into the bottom tier.
What to specify in a purchase order
A specification that says "low moisture" can't be enforced. Write the figure, the method and the basis.
- Maximum moisture at delivery, wet basis, by oven drying at 100 to 105°C to constant weight. Most buyers write 20%, some 18%.
- Sampling: blocks per container, which tiers, face or core.
- Expansion in litres per kilogram at the stated moisture, by a named method, so you can correct it with the formula above.
- Price basis: per tonne as delivered, or corrected to a reference moisture. A moisture-corrected price removes the argument about water entirely.
- Packaging: wrapped or bare blocks, pallet film, top-tier protection, desiccants per container, kiln-dried pallets.
- Loading record: moisture tested at the factory before stuffing, with the figure on the packing list and photographs of the stuffed container.
- Arrival protocol: photos before unloading, moisture samples from top, middle and bottom tiers, and a retained sample from each lot.
Potting-mix producers need the figure to dose coir by volume. How moisture at delivery feeds a blending line is covered in coco peat for potting mix producers.
CVM Fiber and Industries makes 5 kg coco peat blocks and 650 g briquettes in Kanyakumari, Tamil Nadu, India. The pith is washed with water to lower EC, without calcium nitrate or any other chemical treatment, and it's not buffered. If you want a specification drafted with moisture, basis and test method spelled out, contact us and we'll work through it with you.
FAQ
What is the ideal moisture content for a coco peat block?
Between about 15 and 20% by weight on a wet basis at pressing, and under 20% at delivery. In that window the block holds together, ships without mould risk and still wets through reasonably quickly. Blocks pressed much drier crumble and wet slowly, and blocks above 20% carry water you pay for.
How do I convert coco peat moisture from wet basis to dry basis?
Divide the wet-basis figure by one minus itself. 20% wet basis is 0.20 divided by 0.80, which is 25% dry basis. Going the other way, divide the dry-basis figure by one plus itself. Always ask which basis a report uses before comparing it with your specification.
How much coir is actually in a 5 kg block?
Subtract the water. At 15% moisture a 5 kg block contains 4.25 kg of dry coir and 0.75 kg of water. At 25% it contains 3.75 kg of coir and 1.25 kg of water. That half kilogram difference is about 12% of the medium, and at roughly 15 litres per kilogram of dry coir it's around 8 litres per block.
Can I use a hay or timber moisture meter on coco peat?
As a screening tool, yes. Pin meters measure electrical resistance and are calibrated for wood or hay, so on coir they read relative rather than absolute moisture, and salts in the coir make them read high. Use one to find the wettest blocks on a pallet, then confirm those by oven drying.
Why did my coco peat blocks arrive mouldy when the supplier said they were under 20%?
Either they weren't, or they got wet in transit. Oven-test blocks from the top, middle and bottom tiers. Mould under intact shrink wrap with high moisture through the whole pallet points to wet pressing. Mould on the top tier only, with water marks on the film, points to condensation dripping from the container roof.
Does a mouldy coco peat block have to be thrown away?
Not always. Light surface fuzz on a block that's otherwise dry is cosmetic and disappears on hydration. Blocks that smell sour or of ammonia, have dark wet patches through the core or are warm inside the stack have been decomposing and should be rejected. Check EC and pH on anything you decide to use.
Do desiccants stop coco peat blocks moulding in a container?
They help a dry lot stay dry. Calcium chloride container desiccants absorb up to about three times their weight in water and take the edge off the condensation cycle. They can't remove the water from a lot that was pressed or loaded wet, so the moisture figure at stuffing matters more than the number of desiccant bags.
Should I buy coco peat on a moisture-corrected price?
If you buy regularly, yes. Agree a reference moisture, usually 15 or 20% wet basis, and adjust the invoiced tonnage to it from oven tests on arrival. It removes the incentive to ship wet, and it makes quotes from different suppliers comparable on the coir you actually receive.