Coco coir and parboiled rice hulls do opposite jobs. Coir holds water and stands in for peat. Rice hulls hold very little water, create large air pores and stand in for perlite. So the useful question isn't which one to use. It's whether you can replace both peat and perlite with coir plus rice hulls, and at what ratio. For most greenhouse pot and bedding crops you can, with a few adjustments.
What each one is
Coco coir is the ground pith of coconut husks, washed and dried. It's sponge-like, holds many times its weight in water and has moderate cation exchange capacity, so it holds onto nutrients.
Parboiled rice hulls (PBH) are the husks left after milling rice, steamed and dried during processing. Steaming kills weed and rice seed, so the hulls arrive clean and sterile. Fresh, unsteamed hulls are usually avoided for that reason. Hulls make up about 20% of rice grain at harvest, so supply is large wherever rice is milled. They're mostly lignin, cutin and insoluble silica, which is why they break down slowly.
Don't confuse PBH with carbonised rice hulls, which are charred and strongly alkaline. One measurement put them at pH 11.2. They need careful handling and aren't a like-for-like perlite replacement.
Air and water compared
These figures from one laboratory put peat, perlite and rice hulls side by side, with coir dust from separate tests on five commercial sources.
| Material | Total pore space, % vol | Air-filled pore space, % vol | Water holding, % vol | Bulk density, g/cm³ |
|---|---|---|---|---|
| Sphagnum peat | 82.0 | 12.5 | 69.5 | 0.10 |
| Perlite | 61.8 | 29.8 | 31.9 | 0.17 |
| Parboiled rice hulls | 88.6 | 68.1 | 20.5 | 0.10 |
| Coco coir dust | 85.5 to 89.5 | 9.5 to 12.6 | 73 to 80 (water-filled pores) | 0.04 to 0.08 |
Read it in pairs. Coir looks like peat: lots of water, little air. Rice hulls look like a more extreme perlite: even more air, even less water, and lighter.
In a peat mix, the difference shows up as you raise the rate:
| Aggregate in peat (by volume) | Air space with perlite, % | Air space with rice hulls, % | Water held with perlite, % | Water held with rice hulls, % |
|---|---|---|---|---|
| 20% | 15.4 | 22.8 | 66.4 | 61.8 |
| 30% | 18.5 | 26.5 | 64.7 | 57.9 |
At 20% the two give similar results, and the difference wasn't significant in earlier work. From around 40% upward, rice hulls give clearly more air than the same volume of perlite. So you need less rice hull than perlite to reach a given air space once you go past about 20%. Vinca, impatiens and geranium grew equally well with 10 to 30% of either in a peat base.
Coir plus rice hulls
Because coir behaves like peat, the peat results are a fair guide for coir mixes, with two differences:
- Coir is a little wetter and lighter than peat, so it pairs well with an aggregate that adds lots of air.
- Coir doesn't need liming, while peat does. Rice hulls don't shift pH much either way.
A sensible starting point for greenhouse pots is coir with 20 to 30% rice hulls by volume, the range covered by most published work. Raise the hull share for crops that hate wet feet, or for large, deep pots. Cut it for plugs and seed trays, where fine texture matters more. Check the mix on the bench before committing a whole crop. Coir and perlite mixes, for comparison, are in coco coir, perlite and vermiculite mixes.
Coir and rice hull mixes have been grown side by side with standard mixes in hydroponic strawberry and cherry tomato, and held their own. Data is still thinner than for coir and perlite, so trial on a small scale first.
Rice hulls as a peat replacement
Rice hulls can also replace part of the peat itself, though not all of it. Ground parboiled hulls have replaced up to 40% of the peat in greenhouse substrates. Whole hulls hold too little water to be the main body of a mix. That's the job coir does better: replace the peat with coir, and the perlite with rice hulls.
Rice hulls in bark nursery mixes
In outdoor container nursery stock, pine bark mixes amended with up to 40% rice hulls grew spirea as well as straight bark, with no extra irrigation, over one and two seasons. Above 40%, growth dropped. The physical properties of the hull mixes actually improved with time. Coir plays a different role in bark, adding water rather than air, which is covered in coco coir vs pine bark.
Breakdown and crop length
Both materials are organic and will break down eventually.
- Rice hulls break down slowly thanks to their silica and lignin, which suits long nursery crops. They can soften over long crops, and extension guides class them as short-lived compared with mineral aggregates.
- Coir is high in lignin and stays stable for months to years. Fine grades lose air faster than coarse ones as fines pack down.
- Perlite doesn't break down at all, which is why some growers prefer it for multi-year crops and why it's harder to compost at the end.
Weight, dust and handling
Rice hulls and coir are both light and both compost at the end of a crop. Perlite gives off fine siliceous dust when dry, an eye and lung irritant. Dampen any dry aggregate before mixing to keep dust down. Coir is shipped compressed and expands on wetting, so it takes far less space in transport than either hulls or perlite.
Cost and supply
| Coco coir | Parboiled rice hulls | Perlite | |
|---|---|---|---|
| Source | Coconut-growing regions, shipped compressed | Rice-milling regions | Mined volcanic ore, expanded by heating |
| Cost driver | Washing, drying, freight | Freight. Bulky and uncompressed | Mining, transport and heating to about 980 °C |
| Cheapest where | Asia, and anywhere freight from Asia is reasonable | Close to rice mills | Close to perlite plants |
| End of life | Compost | Compost | Stays in soil as mineral |
Rice hulls are cheap at the mill and get dearer quickly with distance because they're bulky. Coir costs more at source but travels compressed. In rice-growing regions, hulls can undercut perlite on price. Elsewhere, it depends on local freight.
Choosing
| Situation | Better choice |
|---|---|
| You need a peat replacement | Coir |
| You need a perlite replacement and rice mills are nearby | Parboiled rice hulls |
| Fully renewable greenhouse mix | Coir with 20 to 30% rice hulls |
| Plugs and seed trays | Mostly coir, little or no hulls |
| Multi-year crops in containers | Coir with perlite or chunky coconut husk chips, or hulls with regular checks |
| Outdoor nursery stock in bark | Bark with up to 40% hulls for air, or some coir for water |
Where CVM fits
CVM Fiber and Industries, in Kanyakumari, Tamil Nadu, India, makes coco peat in 5 kg blocks and briquettes, plus husk chips and custom blends of pith and chips. The coco peat is washed with water to reduce EC. No calcium nitrate or other chemicals are added, so it isn't buffered. If you're blending your own coir and rice hull mix, talk to us about a pith grade that suits it.
FAQ
Can rice hulls replace perlite?
Yes, parboiled rice hulls are the closest organic substitute for perlite. At 20% of a mix they give about the same air space. Above that they give more air than the same volume of perlite, so you can use less.
Can rice hulls replace coco coir?
Not as the main medium. Rice hulls hold very little water, about a fifth of their volume. Coir holds most of its pore space as water. They work together, not as substitutes.
What is the best ratio of coir to rice hulls?
Start with 20 to 30% parboiled rice hulls by volume in coir for greenhouse pots. Use more for crops that dislike wet roots and less for plugs and seed trays. Test on a small batch first.
Do rice hulls cause nitrogen tie-up?
Parboiled hulls are mostly lignin, cutin and silica and break down slowly, which limits the food available to nitrogen-hungry microbes. Fresh or raw hulls were long avoided partly for fear of tie-up and because they carry weed and rice seed.
Are carbonised rice hulls the same as parboiled rice hulls?
No. Carbonised hulls are charred and strongly alkaline, with pH measured above 11 in one test. Parboiled hulls are only steamed and behave much more like perlite.
How long do rice hulls last in a mix?
Long enough for most greenhouse crops and one or two nursery seasons. They break down slowly, but they don't last indefinitely the way perlite does.
Is coir or rice hulls more sustainable?
Both are crop by-products and both compost at the end of use. Rice hulls are usually used close to the mill because they are bulky, while coir is compressed and shipped further. Either is a renewable alternative to mined perlite and dug peat.