Pine bark gives a container mix air, weight and stability. Coco coir gives it water. Aged pine bark on its own sits close to the ideal air and water range for outdoor nursery pots, but it's acid, ties up some nitrogen and is hard to rewet once it dries. Coir holds far more water, rewets easily and needs no lime, but it's very light and too wet for big pots on its own. In most nurseries the real question isn't which one, but how much coir to put in the bark. Which one forms the base mostly comes down to what's available locally.
The numbers side by side
| Property | Aged pine bark (screened, 13 mm) | Fresh pine bark | Coco coir dust | Suggested range for nursery mixes |
|---|---|---|---|---|
| Total porosity, % vol | 84 | Similar to aged | 85.5 to 89.5 | 50 to 85 |
| Air space after drainage, % vol | 19 to 25 | 39 | 9.5 to 12.6 | 10 to 30 |
| Water at container capacity, % vol | 61 to 65 | 49 | 73 to 80 | 45 to 65 |
| Plant-available water, % vol | 26 to 33 | 10 | Not directly comparable | 25 to 35 |
| Dry bulk density, g/cm³ | 0.19 | Differs from aged | 0.04 to 0.08 | 0.19 to 0.70 |
| pH as supplied | 4.0 to 5.0 | 4.0 to 5.0 | 5.6 to 6.9 | Depends on crop |
| Cation exchange capacity, meq/100 g | Moderate | Moderate | 38.9 to 60 |
Bark figures come from standard porometer tests on screened bark, and coir figures from five commercial coir dust sources. Coarser coir with fibre or chips has more air and less water than the dust measured here. The point stands either way: bark sits near the middle of the recommended range, while coir sits at the wet, light end.
Air and water
Bark particles hold water mostly between them, not inside them. That makes bark naturally airy. Fresh bark is too airy. In one comparison, fresh bark had 39% air space and only 10% available water, against 25% and 26% for bark aged for a year. Plants in fresh bark needed small, frequent irrigations to avoid drought stress. That's why nurseries buy aged or composted bark.
Coir holds water inside each particle as well as between them, so it holds much more. Fine coir in a tall pot drains well enough. In a shallow tray or a short, wide pot the same coir can stay waterlogged. The details of how grade and pot height change this are in coco coir water holding and air porosity.
That's why the two blend so well. Bark gives the mix its air and coarse structure. Coir fills in the water reserve, so pots don't dry out on a hot afternoon.
Weight and wind
Dry coir weighs well under half what dry bark does per litre. On a greenhouse bench that's an advantage. Outdoors, in 3 to 20 litre pots of shrubs, it's a problem. Light pots blow over, and staff spend hours standing them back up. Coir also falls below the bulk density range usually recommended for nursery mixes. Bark-based mixes, or coir with sand or grit, keep pots standing.
Rewetting
Air-dry pine bark is very hard to wet again. In one classic test, milled bark took 48 days of wetting to reach 58 to 70% of saturation, while a peat-vermiculite mix reached 70 to 78% in 5 days. In a nursery that means a pot that dries out in a heatwave sheds water down the sides for days.
Coir rewets readily without a wetting agent. That alone is a good reason to include some coir in a bark mix in hot, dry regions. For blocks that have dried hard before use, see rewetting dry coco coir.
Shrinkage and stability
Pine bark is generally considered stable over the production time of most container crops. It does break down slowly. In one 17-month test the largest particle fractions shrank and the fine fractions grew, so the mix held more water and less air as time went on.
Coir is also slow to break down because it's high in lignin. Most coir volume loss comes from fines packing down and roots filling pores, not decomposition. See coco coir shrinkage and compaction.
For a two- to three-year container tree crop, both hold up. A fine coir grade in the bottom of a deep pot is more of a risk than either material breaking down.
pH and lime
Pine bark is acid, usually pH 4.0 to 5.0. Most nurseries add dolomitic lime, typically 4 to 15 lb per cubic yard (roughly 2.4 to 8.9 kg/m³), which also supplies calcium and magnesium. Dolomitic lime can bring bark up to about pH 6.5, after which extra lime makes little difference.
Coir usually arrives between pH 5.6 and 6.9, close to where most crops want to be. Coir-heavy mixes need little or no lime for pH. They still need calcium and magnesium in the feed, because coir holds onto both from the first irrigations.
Nitrogen tie-up
Bark ties up more nitrogen than peat. Microbes feeding on the bark take nitrogen from the fertiliser while they work, and fresher bark ties up more. In bark-based orchid media, bark that is kept wet and fed decomposes faster and holds back more nutrients. Nurseries cover this with controlled-release fertiliser rates set for bark.
Well-aged, washed coir ties up very little nitrogen because most of its carbon is in lignin that microbes break down slowly. The full explanation is in nitrogen drawdown in coco coir. In practice, adding coir to bark doesn't raise your nitrogen needs.
Salts
Bark arrives with low salt. Coir can carry sodium, potassium and chloride from the husk, especially if it wasn't washed well. Always check EC on a new coir delivery before mixing it into a nursery batch.
Availability by region
This is often the deciding factor.
| Region | Main bark source | Supply trend | Where coir fits |
|---|---|---|---|
| South-east and midwest United States | Loblolly pine bark | Tighter and dearer as bark goes to fuel, logs are debarked in the forest and harvests fall. Less than 5% of bark generated is thought to be available to horticulture | Peat replacement in bark mixes, rewetting aid |
| US West Coast | Douglas fir bark | Similar pressures | Same role |
| Southern and eastern Australia | Composted pine bark, the base of most container mixes | Restricted in some areas as bark goes to energy and trees are harvested younger | Water-holding component, and a main ingredient where bark is short |
| Netherlands and northern Europe | Small bark share | Bark is a small share of Dutch professional substrates, well behind coco | Main peat alternative |
| South and South-East Asia | Little pine bark | Bark is scarce or imported | Coir is often the base itself, with husk chips for air |
Fresh pine bark must be composted before use in Australia and elsewhere to remove compounds that harm plants. Coir needs washing instead.
When to use which
- Outdoor container nursery stock, 3 litres and up, where bark is local: bark base with some coir pith. Starting ratios by crop are in coco coir for container nursery stock.
- Where bark is short or expensive: coir pith for water, coconut husk chips for the coarse fraction bark would provide, plus grit for weight.
- Greenhouse pot plants and liners: coir or a coir and peat base, with perlite or bark fines for air.
- Plants sold into hot, dry markets: add coir to bark so buyers can rewet a dried-out pot.
Where CVM fits
CVM Fiber and Industries, in Kanyakumari, Tamil Nadu, India, makes 5 kg coco peat blocks, coconut husk chips and custom blends of pith and chips. The coco peat is washed with water to reduce EC, with no calcium nitrate or other chemicals added, so it isn't buffered. For a nursery blending coir into bark, that means you control the calcium and magnesium in your own mix. Contact us to discuss grades.
FAQ
Is coco coir better than pine bark for potting mixes?
Neither is better on its own. Pine bark provides air, weight and structure, while coir provides water holding and easy rewetting. Most nursery mixes use bark as the base with some coir, or coir as the base with chips or grit where bark is scarce.
Can coir replace pine bark completely?
It can, but plain coir pith is too wet and too light for most outdoor nursery pots. Replace the bark's coarse fraction with coconut husk chips and add grit or sand for weight, then adjust irrigation for a wetter mix.
Does pine bark need lime?
Usually. Pine bark runs about pH 4.0 to 5.0, and dolomitic lime at roughly 4 to 15 lb per cubic yard is common. Coir starts at about pH 5.6 to 6.9 and usually needs little or no lime.
Does pine bark tie up nitrogen?
Yes, more than peat does, and fresher bark more than aged bark. Aged, washed coir ties up very little nitrogen because its carbon is mostly in slow-decomposing lignin.
Why is dry pine bark hard to rewet?
Dry bark particles repel water and hold it only between particles. Milled bark can take weeks to wet back up fully. Adding coir or a wetting agent makes rewetting much easier.
Why is pine bark getting harder to buy?
More bark is burned for fuel, more logs are debarked in the forest, harvests have fallen in some regions and freight costs have risen. Growers in the United States and Australia have reported tighter supply and higher prices.
Should I use fresh or aged pine bark with coir?
Aged or composted bark. Fresh bark has too much air, too little available water and ties up more nitrogen. Adding coir improves the water side but not the nitrogen draw.