Coco peat particle size is set by screening. After the husk is milled, the loose pith passes over vibrating screens that take fibre and chips off the top and, if the factory runs a fine screen, let dust fall through underneath. What's left is sold as a grade: a fine propagation pith of roughly 0 to 3 mm, a standard pith of 0 to 6 mm, coarser "fibrous" pith of 2 to 8 or 6 to 12 mm, and husk chips above that. No international standard fixes these bands, so one supplier's "fine" isn't another's. The sieve test near the end of this article is how you pin it down.
The single most useful number is the share of material under 1 mm. That fraction, the fines, decides how wet a mix sits, how fast it drains, how well it wicks in a plug tray, and how easily algae and fungus gnats get going on the surface.
How pith is screened into grades
Coir comes off the husk as long fibre, short fibre, spongy pith and husk chunks. Screening sorts them by size, and the order of screens defines the product:
- A coarse screen (6 to 12 mm, depending on the factory) holds back husk chips and long fibre. The oversize is sold as chips or re-milled.
- A mid screen (3 to 6 mm) takes out short fibre and pith lumps. What passes is standard pith.
- An optional fine screen (1 mm, sometimes 2 mm) pulls dust out of the standard pith. The oversize becomes a "low fines" or "sieved" grade. The dust goes into very fine grades, briquettes and discs.
Grading happens on dry, loose pith before pressing. Compression at around 5:1 packs particles together without breaking them, so a block of fine grade expands back to fine grade. Handling afterwards is what changes the size, and that's covered below.
Sieve fractions used in the trade
Researchers, standards bodies and factories all draw their lines in different places, which is why grade names are slippery.
| Scheme | Sieve apertures | What it separates |
|---|---|---|
| European growing-media sieve stack | 31.5, 16, 8, 4, 2, 1 mm and pan | Full distribution by mass, one figure per fraction |
| US horticultural peat practice | 2.36 and 0.85 mm | Coarse (on 2.36), medium (0.85 to 2.36), fine (passes 0.85) |
| Nursery research on coco peat | 0.5, 3 and 4 mm | Fine (under 0.5), medium (0.5 to 3), coarse (over 4) |
| Berry growers' rule of thumb | 1 mm and about 0.2 mm | Over 1 mm holds water, under 1 mm is dust that cuts aeration, under 0.2 mm gets sieved out |
| Common commercial grades | 6 mm top cut, or 2 to 8, 6 to 12 mm | "Pith grade", "fibrous", "coarse" |
On a sieve shaker, a typical pith grade has its biggest fraction near 0.7 mm and a median of 0.7 to 0.85 mm, so more than half of ordinary coco peat is under 1 mm by mass. That's normal. The question is how much of it is true dust and how much is useful 0.5 to 1 mm sponge.
For practical work, four bands cover nearly every decision:
| Band | Trade name | Behaviour |
|---|---|---|
| Under 1 mm | Fines, dust | Fills gaps between larger particles. Raises bulk density and water holding, lowers air. Wicks well. |
| 1 to 3 mm | Fine pith | The main water-holding sponge in propagation and potting grades |
| 3 to 6 mm | Medium or standard pith | Balanced water and air. The bulk of blocks and grow bags |
| Over 6 mm | Coarse pith, crush, chips | Opens the mix. Air and drainage. Short fibre sits here too |
What fines do to water, air and capillarity
Within pure pith, particle size drives nearly everything physical. Across 13 coir dusts from Asia, the Americas and Africa, air content ran from 24% to 89% by volume and total water holding from 137 to 786 litres per cubic metre, and the spread followed particle size. The 0.125 to 1 mm fraction had the largest effect. Powder below 0.125 mm and particles above 1 mm mattered far less. So the fines that change a mix are the 0.1 to 1 mm material, not just the dust you see in the air.
The mechanism is simple. Large particles leave large pores. Fine particles sit in those pores and split them into many small ones. Small pores hold water against gravity and pull it sideways and upward by capillarity, but they don't drain to let air in. Whole-material ranges are in the article on water holding and air porosity. The particle-size view:
| Property | More fines | Fewer fines |
|---|---|---|
| Water holding (volume) | Up | Down |
| Air-filled porosity | Down | Up |
| Bulk density | Up | Down |
| Capillary rise and sideways wicking | Faster, further | Slower, shorter |
| Drying between irrigations | Slower | Faster |
| Surface staying wet | Longer | Shorter |
| Shrinkage over a long crop | More | Less |
How much air you need depends on the container, because depth sets how much of the profile stays saturated after drainage.
Plugs and seed trays. Cells are 20 to 40 mm deep and nearly the whole depth sits in the saturated zone. Plug media are designed around 10 to 15% air after drainage, and they rely on fine particles for two jobs: holding enough water that the cell survives an afternoon, and wicking so a seed on the surface stays in contact with moisture. Coarse mixes in small cells dry fast and germinate unevenly. In side-by-side seedling trials of coco peat fractions, 0.5 to 3 mm material gave the best tomato, pepper and cucumber seedlings (95% germination for tomato), while sub-0.5 mm material won only for cabbage germination at 94%. Fine, but not pure dust, is the target.
Pots. A 10 to 15 cm pot has enough height that the top half drains even with a fairly fine mix. Standard 0 to 6 mm pith on its own typically measures 60 to 70% water and 20 to 30% air by volume after drainage. In a hot climate you'll lean finer. In a cool, dull winter you want fewer fines so the pot dries.
Slabs and grow bags. Slabs are 7 to 10 cm high, irrigated many times a day for months, and drain through a few slits. There's no deep profile to drain into, so the air has to come from the particles. That's why slab blends include husk chips or crush, and why fines content is the first thing to check when a slab stays soggy. Ratios are in pith-to-chip blend ratios.
Why fines matter for gnats and algae
Fungus gnats need a surface that stays moist, and the larvae feed on the algae and fungi growing in it. At about 24 °C a generation takes roughly 17 days: eggs hatch in about 3 days, larvae feed for about 10, and adults emerge a few days after pupating. The standard advice is to let the surface dry between waterings. A fine, dusty top layer makes that hard. Capillarity keeps pulling water to the surface, the surface stays dark and wet, algae colonise it under greenhouse light, and gnats follow.
A coarser top centimetre breaks the cycle. Propagators who need a fine medium for germination often top-dress with coarse vermiculite or a coarser coir grade for exactly this reason. If you already have an outbreak, see mould, algae and fungus gnats in coco coir.
How fines content changes with handling and age
The grade on the label is the grade at the screen. Three things change it afterwards.
Mechanical handling. Coir is friable. Shake it on a sieve for longer and the distribution keeps shifting finer, which is a polite way of saying the particles break. Expansion machines, screw conveyors, paddle mixers and bag-filling lines do the same. Every transfer adds fines.
Wetting and drying cycles. Pith swells wet and shrinks dry. Repeated cycles in a slab loosen the structure and rub fines off every particle.
Decomposition. Pith is the fastest-decaying coir fraction. Over a long crop it gets finer, water holding rises, air falls and the slab shrinks. Growers treat more than about 5% shrinkage as the point where root-zone air starts to suffer. Pre-aged pith is more stable than fresh pith because the easily decomposed material has already gone.
So specify fines at the point of delivery, and go a grade coarser than you think you need for crops longer than six months.
What "low fines" and "dust-free" claims mean
Neither term has a standard definition. "Dust-free" usually means the factory ran a fine screen or an air classifier, and the result has less sub-1 mm material than ordinary pith, not none. Truly dust-free coco peat isn't possible, because pith sheds fines whenever it's handled.
When you see the claim, ask for three things: the aperture of the fine screen, the percentage passing 1 mm by mass on a dried sample, and the method. A supplier who can't name the mesh hasn't measured it. A claim backed by a 1 mm sieve report and a figure of, say, under 15% passing means something. "Low dust" without a number describes the colour of the pile.
How to check fines with a sieve
You don't need a laboratory. The full method with moisture correction and sample volumes is in the sieving section of the blend-ratio article linked above. The short version for fines:
- Expand a sample from at least three blocks or bags, mix, and air-dry at 40 °C until it's crumbly and below about 15% moisture. Wet coir won't sieve.
- Weigh 100 to 150 g, or measure about 400 cm³.
- Pass it through a 1 mm sieve over a pan. Shake about 5 minutes by machine, or by hand until almost nothing more falls through. Don't go on much longer: the longer you shake coir, the more it breaks, and you'll be measuring fines you made.
- Weigh the pan contents. That's your fines percentage by mass.
- For the coarse end, add a 4 or 6 mm sieve on top and weigh what stays on it.
Do it the same way every time so deliveries can be compared. Mass fractions aren't volume fractions: dust is denser per litre than fluffy pith, so 15% by mass is less than 15% by volume.
Quick field check: rub a handful of dry pith between your palms over a dark surface. A fine-screened grade leaves noticeably less dust than standard pith. The wider acceptance routine is in how to test a coco peat sample.
Matching grade to use
| Use | Grade to ask for | Fines under 1 mm | Why |
|---|---|---|---|
| Seed trays, plugs, discs | Fine pith, 0 to 3 mm, fibre removed | Moderate to high | Capillarity and water holding in a 25 mm cell |
| Cuttings | Fine to medium pith, often with 20 to 30% perlite or chip | Moderate | Water at the stem base, air for callusing |
| Potting mixes, bedding plants | Standard pith, 0 to 6 mm | Moderate | Balanced in 9 to 15 cm pots |
| Long-term containers | Standard pith plus 20 to 30% chips or crush | Lower | Drainage as roots fill the pores |
| Slabs and grow bags, vegetables | Medium pith plus about 30% chips or 0 to 15 mm crush | Low | Air in a shallow, heavily irrigated profile |
| Strawberry bags | Fine to medium pith plus 0 to 7 mm crush | Low to moderate | Shallow bags, frequent fertigation |
| Blueberries, orchids | Chip-heavy blends, 12 to 18 mm chips | Very low | Open and fast-draining |
| Mulch, soil improvement | Coarse pith, crush, fibre | Not critical | Cover and structure, not capillarity |
| Animal bedding | Coarse, low-dust pith or fibre | Low | Dust control in the barn |
How to specify grade in an order
Write it in sieve terms, not adjectives. A workable line in a purchase order:
Coco pith, screened through 6 mm. Fibre and foreign matter under 3% by mass. Material passing 1 mm not more than X% by mass on an air-dried sample, European sieve method. Sieve report with each lot.
Fill in X for your use. For a slab blend you might set 10 to 15%. For propagation pith you might set no fines limit at all, but a 3 mm top cut with the fibre out. For chip blends, add the chip size band and the ratio, and say the ratio is by expanded volume. Test the first delivery with the sieve method above and keep the result as your reference.
CVM Fiber and Industries in Kanyakumari, Tamil Nadu, India produces coco peat blocks, briquettes, grow bags, open-top slabs and husk chips, and screens and blends to a specified grade as a custom blend. The pith is washed with water to bring EC down, with no calcium nitrate or other chemicals, and it isn't buffered. If you have a sieve spec, or want help writing one for your crop, send us the details and we'll quote against it.
FAQ
What particle size is coco peat?
Ordinary coco peat has most of its mass between about 0.25 and 3 mm, with a median around 0.7 to 0.85 mm. Commercial grades are sold by top cut, such as 0 to 6 mm for standard pith or 0 to 3 mm for propagation pith. The share under 1 mm varies widely between factories.
What counts as "fines" in coco coir?
In the trade, fines usually means everything passing a 1 mm sieve. The fraction that changes water and air behaviour most is 0.125 to 1 mm, while powder below 0.125 mm has little extra effect. Some berry growers ask for anything under 0.2 mm to be screened out.
Is fine coco peat better for seed starting?
A fine grade beats a coarse one because a plug cell needs water holding and capillary wicking. Pure dust, though, holds too much water and starves roots of air. In trials, 0.5 to 3 mm material grew the best tomato, pepper and cucumber seedlings, and sub-0.5 mm material only won for cabbage germination.
Do fines cause fungus gnats?
Not directly, but they keep the surface wet, and a wet surface grows the algae and fungi that gnat larvae feed on. A generation takes around 17 days at 24 °C, so a permanently moist fine surface builds a population fast. A coarser top layer that dries between waterings is the usual fix.
What does "dust-free coco peat" mean?
It means the factory removed some sub-1 mm material with a fine screen or air classifier. It never means zero dust, because pith sheds fines whenever it is handled. Ask for the screen aperture and the percentage passing 1 mm by mass.
Does compressing coco peat into blocks change the particle size?
No. Pressing at around 5:1 packs the particles together without breaking them, and the block expands back to the grade that went in. Mixing, conveying and bag filling after expansion are what add fines.
Why does my coco slab get wetter over the season?
Pith decomposes slowly and particles break down with wetting and drying, so the fines fraction rises, water holding goes up and air goes down. Roots also fill the large pores. Shrinkage above about 5% is the usual warning that the slab has drifted too fine.
What sieve do I need to check fines at home?
A 1 mm sieve and a pan is enough for the fines figure, with a 4 or 6 mm sieve on top if you want the coarse fraction too. Air-dry the sample first, shake for about 5 minutes, and weigh what falls into the pan as a percentage of the total.