Coco coir settles a little after the first few irrigations and shrinks slowly over a long crop, but it holds its volume better than peat and far better than wood-based substrates. Most of the loss comes from fine particles packing together, roots filling the big pores, hard drying and pressure at filling, not from the coir rotting away. Choose a grade with some fibre or chip, fill loosely, avoid letting it dry out hard, and air space stays where you need it.
What "shrinkage" covers
People use the word for several different things. It helps to keep them apart because each has a different fix.
| Type | When it happens | Main cause | Reversible? |
|---|---|---|---|
| Settling | First few irrigations | Particles slide into gaps as water moves through | No, but predictable |
| Drying shrinkage | Whenever the coir dries hard | Fine particles pull together as water leaves | Partly, on rewetting |
| Compaction | At filling, handling, or under weight | Pressure crushes large pores | No |
| Long-term volume loss | Over months | Slow decomposition, particle breakdown, root growth | No |
How coir compares with other substrates
Coir is one of the most stable organic substrates you can grow in. Its pith is about 30 to 35% lignin, and lignin resists both microbial attack and physical collapse.
Published volume losses give a sense of scale, though the trials differ in length, container and material:
| Substrate | Volume loss reported |
|---|---|
| Peat-based substrates | About 8% over 31 weeks, and 12% after 14 weeks in another container crop |
| Wood-based substrates | From 36% over 15 months up to 50% over 51 weeks |
| Coir | Less shrinkage than peat in direct comparisons. Composted coir stayed relatively stable over 21 months |
Adding coir to peat also reduces shrinkage of the mix in the first irrigations, which is one reason it's used as a stabiliser in peat-reduced blends. Wood fibres broke down faster than peat in long-term tests, so blends heavy in wood fibre lose height faster than coir-based ones.
Why coir settles and shrinks
Fines
Fine particles are the main driver of both settling and drying shrinkage. When peat dries, the finest fractions shrink while fractions above 2 mm barely shrink at all. Coir follows the same pattern. A coir grade with a lot of dust settles more, shrinks more on drying and loses more air space than a coarser grade. The sieve fractions and what they do are covered in coco peat particle size grades.
Hard drying
When coir dries well past its normal range, fine particles pull together and the whole mass contracts from the pot wall. You see a gap down the side and a slab that has dropped in height. Rewetting brings much of the volume back, but the gaps at the edge become channels that carry water straight past the roots. That's why dried-out coir often seems to drain fast and stay dry in the middle. The fix for that is in rewetting dry coco coir.
Compaction at filling
Pressing coir into a pot or tray is the fastest way to lose air space. In propagation trays, pressing the mix down can cut its air space to a fraction of what it was, and fine grades lose the most. That loss happens in seconds, and no later irrigation recovers it.
Roots
Over a long crop, roots grow into the large pores. That reduces air-filled porosity even if the coir itself hasn't changed. In a dense tomato or cucumber slab late in the season, much of the change in drainage comes from roots, not from coir breakdown.
Slow decomposition
Coir does break down eventually, and pith goes faster than fibre or chip. Particles get smaller and the mix drifts toward the fine end. Over one season this is minor. Over several seasons, or in reused coir, it adds up. How long each grade lasts is set out in how long does coco coir last.
What shrinkage does to air and water
Shrinkage matters because it changes the balance between air and water in the root zone. As the coir settles, large pores close up and become small pores. Small pores hold water. Large pores drain and hold air.
So a shrinking substrate:
- holds more water after each irrigation,
- has less air-filled porosity,
- drains more slowly, and
- stays wet longer, especially at the bottom of the container.
A working target for a professional substrate is roughly 10 to 20% air-filled porosity after drainage. Coir usually starts above that, so a little settling doesn't hurt. Trouble starts when fine coir, compacted at filling, is irrigated heavily through a long crop. The background on these numbers is in coco coir water holding and air porosity.
You'll notice it as:
- the substrate surface dropping well below the rim,
- a gap between coir and pot wall when dry,
- slabs that feel heavier after irrigation as the crop goes on,
- slower dry-back overnight, and
- more algae and fungus gnats on a surface that stays wet.
How to limit shrinkage and compaction
Choose the right grade
Match particle size to crop length. Fine pith suits propagation and short crops. For anything that runs for months, a blend with fibre or husk chips keeps a structure that resists settling. Coarse particles don't shrink much on drying and hold pores open as roots fill in. Our husk chips are used for exactly this in long-crop blends.
Fill loosely
- Expand blocks fully and fluff the coir before filling.
- Fill to the top, tap the container once or twice to settle it, then top up. Don't press.
- Fill trays by dropping substrate across the whole tray and brushing off the excess, rather than pressing it into each cell.
- Don't stack filled pots or trays on top of each other.
Wet before planting
Coir settles on the first watering. Wet slabs, bags and pots thoroughly before planting so the settling happens before the plant goes in. Top up pots after the first irrigation if they've dropped.
Don't let it dry out hard
Steady moisture keeps fine particles from pulling together. Irrigate before the coir dries to the point where it pulls away from the wall. If it does, rewet slowly with several small irrigations rather than one flood.
Avoid heavy irrigation late in the crop
As roots fill the slab and air space falls, cut shot size or frequency so the root zone can still breathe. Longer dry-backs help.
Handle bags and slabs carefully
Bags that are dropped, stood on, or stacked high in storage arrive compacted. Store them flat, at a modest stack height, and lift rather than drag them.
Reusing coir
Coir that's been through a crop is finer, denser and full of old roots. It holds more water and less air than when it was new. If you plan to reuse coir, add fresh coarse material such as chip or fibre to restore structure, and expect the reused blend to settle more than new coir.
About our coir
We make coco peat blocks, briquettes, grow bags, open-top slabs, husk chips and custom pith and chip blends in Kanyakumari, Tamil Nadu, India. Our coco peat is washed with water to reduce EC, with no calcium nitrate or other chemicals added, and it isn't buffered. If you want a grade that keeps its structure through a long crop, tell us what you're growing.
FAQ
Does coco coir shrink?
A little. It settles in the first few irrigations and loses some volume over a long crop, mainly through fine particles packing together and roots filling pores. It shrinks less than peat and much less than wood-based substrates.
Why has my coco coir pulled away from the sides of the pot?
It has dried out too far. Fine coir particles contract as they lose water, leaving a gap at the wall. Rewet with several small irrigations so water soaks into the middle instead of running down the gap.
How much air space should coco coir have?
Around 10 to 20% air-filled porosity after drainage is a common target for professional substrates. Coir usually starts above that range. Shrinkage, compaction and root growth bring it down over a crop, so start with a grade that leaves you some margin.
Should I pack coco coir firmly into pots?
No. Pressing coir crushes the large pores that hold air. Fill loosely, tap the pot to settle it, and top up after the first watering.
Does adding perlite stop coir from compacting?
Perlite adds rigid particles that hold pores open and don't shrink, so it helps. Husk chips and coir fibre do a similar job and are organic. The best choice depends on crop length, cost and whether you want to compost the substrate afterwards.
Does coir lose volume faster when reused?
Yes. Used coir is finer and contains old roots, so it settles more and holds less air. Mix in fresh coarse material before reusing it and expect more shrinkage than in the first crop.