Coco coir has a moderate cation exchange capacity. Published figures mostly fall between about 30 and 100 meq per 100 g, lower than sphagnum peat, higher than pine bark on a weight basis, and far above perlite or rockwool, which hold almost nothing. Because coir is so light, though, a litre of it holds far fewer exchangeable nutrients than a litre of soil. In practice that means coir buffers your feed a little, not a lot, and what sits on its exchange sites at the start of a crop matters more than the headline number.
What cation exchange capacity is
Coir, peat, bark, clay and humus carry negative charges on their surfaces. Positively charged nutrient ions (cations) such as calcium, magnesium, potassium, ammonium and sodium cling to those charges. They aren't locked in. They swap places with other cations in the water around them, and roots can take them up. CEC is the total number of those charged sites, the size of the nutrient "pantry" the material can hold on its surfaces.
Negatively charged nutrients such as nitrate, sulphate and phosphate aren't held this way. They stay in the water and wash out with drainage. CEC tells you nothing about them.
Two units are used and they're numerically identical: meq/100 g (milliequivalents per 100 grams) and cmol/kg. A coir sample at 60 meq/100 g is also 60 cmol/kg.
In organic materials like coir and peat, the charge comes from organic acids, so CEC rises with pH. A substrate measured at pH 5 will usually show a lower CEC than the same material at pH 7. That's one reason lab figures move around.
How much CEC does coir have?
Less agreement than you'd like. Across 13 coir dust samples from Asia, the Americas and Africa, CEC ranged from 31.7 to 95.4 cmol/kg, and properties differed between and within sources. Improved coir-based media have tested at 73 to 117 meq/100 g. Some grower-facing material quotes 10 to 30 meq/100 g, while one comparison of commercial media put peat's CEC at three to four times that of coir.
The spread comes from three things:
- Raw material and processing. Fibre content, particle size and how long the pith aged before pressing all change the surface area and chemistry.
- pH during the test. A higher test pH reads a higher CEC in organic materials.
- Method. Different extraction methods give different answers on the same sample.
So treat any single CEC number for coir as a property of that lot and that lab, not of coir in general.
Coir compared with peat, bark, soil and inerts
| Material | CEC per weight (meq/100 g) | Typical dry bulk density (g/cm³) | Rough CEC per litre (meq/L) |
|---|---|---|---|
| Coco coir | 30–100 (most figures) | 0.04–0.08 | about 13–76 |
| Sphagnum peat | 77–106 in one set of measurements, 100–180 quoted elsewhere | about 0.08 | about 62–83 measured |
| Pine bark | 21–53 | 0.14–0.17 | 30–74 measured |
| Vermiculite | 82–150 | light | high for its weight |
| Perlite | about 1.5–3.5 | light | close to zero |
| Rockwool | effectively none | – | close to zero |
| Sandy soil low in organic matter | under 3 | about 1.5 | under about 45 |
| Heavier soil or soil high in organic matter | over 20 | about 1.2–1.4 | about 240–280 and up |
The per-litre column is the one most comparisons leave out. CEC in meq/L = CEC in meq/100 g × bulk density in g/cm³ × 10. Coir at 60 meq/100 g and 0.06 g/cm³ works out to 36 meq/L. A loam at 15 meq/100 g and 1.3 g/cm³ works out to 195 meq/L. The soil holds about five times more exchangeable nutrients in every litre, despite a much lower per-gram figure.
Pine bark shows the same effect from the other side. Bark is denser than peat, so bark batches with half the per-gram CEC of peat can match it per litre. Fine bark particles below 0.71 mm reached 217 meq/L in one set of measurements, against 83 meq/L for the finest peat fraction.
Roots live in a pot, not in a gram, so per-litre CEC is the better guide to how much buffering your substrate gives you. On that basis coir sits in roughly the same band as peat and bark, at the lower end if it's a coarse, light blend, and well below any mineral soil.
What sits on the sites matters more than the number
Straight from the husk, coir's exchange sites are loaded mainly with potassium and sodium, with little calcium or magnesium. When you irrigate with a feed containing calcium and magnesium, the coir takes them up in exchange and pushes potassium and sodium back into the root zone. The plant sees less calcium and magnesium than you mixed, plus some sodium it didn't ask for.
How that's handled before the coir reaches you, by washing, buffering with calcium nitrate, or both, is explained in buffered vs unbuffered coco peat. The short version: a bigger CEC means more sites to load or reload, so CEC and the starting chemistry together decide how much your first weeks of feeding are distorted.
What coir's CEC means for feeding
1. Coir is a small buffer, not a store. With a few dozen meq per litre, coir can soften a single missed feed or a slightly off mix. It can't carry a crop for weeks the way a soil bed can. Most growers on coir feed at every irrigation for that reason.
2. Expect a calcium and magnesium pull early on. If the coir is washed but not buffered, your feed does the loading in the first weeks. Running calcium and magnesium a bit higher and potassium a bit lower at the start is normal. Watch young leaves for calcium problems and older leaves for magnesium yellowing. The recipe side is covered in our nutrient solution guide for coco coir.
3. Runoff readings lag behind what the roots see. While the exchange sites are still swapping ions, the drain can carry more potassium and sodium than your feed contained, and less calcium. Don't chase a drain EC in the first week by changing the recipe every day. Watch the trend over several irrigations.
4. Ammonium gets held too. Ammonium is a cation, so some of it sits on exchange sites rather than reaching the roots straight away. Nitrate isn't held at all, which is why coir drains carry nitrate readily.
5. Mixing changes the buffer. Coir blended with perlite has less CEC per litre than coir alone, because perlite adds volume and almost no sites. Coir blended with compost, vermiculite or garden soil has more. A raised bed of soil and coir behaves far more like soil than like a hydroponic slab. Ratios are in our guide to coco coir, perlite and vermiculite mixes.
6. pH and CEC interact. Coir usually sits around pH 4.9 to 6.8, higher than sphagnum peat, so it needs less lime than peat to reach a growing pH. Because its CEC is pH-dependent, a coir mix held at the low end of that range offers a little less exchange capacity than the same mix nearer neutral. Day-to-day pH and EC control is a separate topic, covered in coco coir pH and EC management.
Where CEC helps and where it doesn't
Coir's moderate CEC is one reason it's easier to manage than rockwool for growers who occasionally miss a feed or run a slightly imprecise mix. It's also one reason it's less forgiving than peat-based compost bought with a starter charge, or soil in a raised bed.
CEC won't fix salt. Sodium and chloride that arrive with the coir or with your water still have to be washed through. And CEC doesn't retain nitrate, sulphate or phosphate at all, so a high CEC never means you can feed less often in a free-draining system.
CVM Fiber and Industries, in Kanyakumari, Tamil Nadu, India, washes its coco peat with water to bring EC down. It isn't buffered, and no calcium nitrate or other chemical is added, so plan your first weeks of feeding with that in mind. If you'd like to talk through how a washed coir fits your feeding program, get in touch.
FAQ
What is the CEC of coco coir?
Most published figures sit between about 30 and 100 meq per 100 g, with some coir-based media testing higher and some grower sources quoting as low as 10 to 30. The number varies with raw material, particle size, the pH of the test and the method. Treat any single figure as a property of that lot.
Does coco coir have a higher CEC than peat moss?
Usually not. Sphagnum peat generally measures around 100 meq per 100 g or more, and one comparison found peat at three to four times the CEC of coir. Per litre, the gap is smaller because both materials are similarly light.
Is coir CEC higher than soil?
Per gram, coir is higher than most mineral soils. Per litre, it's much lower, because soil is 15 to 40 times denser. A litre of loam can hold around five times more exchangeable nutrients than a litre of coir.
Why does coco coir cause calcium and magnesium deficiency?
Raw coir's exchange sites carry mostly potassium and sodium. When you feed calcium and magnesium, coir binds them in exchange and releases potassium and sodium, so the plant gets less calcium and magnesium than you mixed. Buffering beforehand or a stronger calcium and magnesium feed in the first weeks deals with it.
Does CEC mean I can feed coir less often?
Not much less. Coir's CEC per litre is modest, and it holds no nitrate, sulphate or phosphate at all. Most coir crops are fed at every irrigation.
Does perlite add CEC to a coir mix?
No. Perlite has almost no exchange capacity, so it dilutes the CEC per litre of a coir mix while adding air. Vermiculite or compost raise it.
Does CEC change as coir ages?
It can. As coir breaks down, particles get finer and surface area rises, and finer particles generally show a higher CEC in organic substrates. The sites also fill with whatever you've been feeding, which is why reused coir starts with a different chemistry from new coir.