The short answer: for most containers, 70% coco coir and 30% perlite by volume is the standard starting point. Michigan State University trials (Argo and Biernbaum, 1996) measured a 70/30 coir/perlite mix at 22% air space and 59% total water-holding capacity in a 6-inch pot, which sits inside the ranges normally recommended for container media. Move towards 50/50 for propagation, hydroponic drain-to-waste and anything that is watered often; move towards 80/20 or swap perlite for vermiculite when you want more water held, such as seed trays and hanging baskets; and for cacti and succulents make coir a minority (around one part in five) in a grit-based mix. The rest of this article explains what each amendment does, what the published trials say happens to coir when you add them, and how to mix without the usual problems.
Coir pith on its own already holds a lot of water and a fair amount of air, so the question is rarely "does it need amending?" and more often "what am I correcting for?" If you are not yet sure what coir pith is and how it differs from chips and fibre, start with what coco peat is.
What each amendment does to coir
| Amendment | What it adds | What it costs you | Key figures |
|---|---|---|---|
| Perlite | Large, rigid, non-decaying particles. More drainage and air after watering. Lighter mix. | Holds little water in coarse grades, floats, dusty when dry, no nutrient holding (zero CEC). | Expanded at about 900 °C, up to 15 times its original volume (Perlite Institute). pH 7.0 to 7.5, 6 to 9 lb per cubic foot (Cornell). Water held by grade: coarse 20 to 40%, medium 30 to 50%, fine 40 to 60%, extra fine 60 to 90% by volume (Perlite Institute). |
| Vermiculite | Water and nutrient holding, buffering. Supplies some K and Mg. | Compressible, "less durable than sand and perlite" (Texas A&M), so air space falls over long crops. Slightly alkaline. | Exfoliated at about 745 °C (Texas A&M). pH 6 to 9, CEC 50 to 100 meq/100 g, holds 220 to 325% of its weight in water, 20 to 50% by volume (The Vermiculite Association). |
| Sphagnum peat | Fine, fibrous water holding; buffered pH when limed; improves "flow" of lumpy coir through filling lines. | Needs lime (pH about 4.4 in Rose and Haase's sample), re-wets poorly when dry, peat-free policies in many markets. | Peat moss can absorb 10 to 20 times its weight in water and has some fungistatic effect (Texas A&M). Peat in a 1:1 blend with coir "can improve the consistency of coir so that there are fewer clumps" (Newman, 2007). |
| Compost | Nutrients, microbial activity, some structure. | Variable EC and particle size; too much makes a heavy, airless mix. Screen it. | University of Arizona Extension's coir recipe uses 1.5 parts screened compost to 7 parts coir (Cook). |
| Pine bark | Coarse structure, cheap bulk, slow to break down. | Ties up nitrogen as it decomposes; needs the fines screened out. | 100% pine bark: 30% air space but only 39% container capacity, below the 45% recommended minimum; adding coir fixes that (Basiri Jahromi et al., 2020). |
| Coir chips | Air space without leaving the coir system, same pH and chemistry as the pith. | Unwashed chips carry salt like pith does; shorter-lived than perlite. | Chips raise the air content of a coir mix by 10 to 40% depending on quantity, size and shape (Klasmann-Deilmann technical note, 2024). |
Two points from that table matter more than the rest. Perlite is the only amendment that is both inert and permanent: it does not shrink, rot or hold nutrients, so a coir-perlite mix behaves the same way in month six as in week one, and all the chemistry comes from the coir and the feed. Vermiculite does the opposite job, holding water and cations, which is why the two are often combined rather than chosen between.
How coir's air and water change with each amendment
Published trial data for coir blends are scattered across extension articles and research papers. The figures below are the ones we could verify.
Coir with perlite or bark at 70/30
Argo and Biernbaum at Michigan State University grew impatiens for 16 weeks in media with coir at 70% of the volume and either perlite or bark as the other 30%, alongside peat-based equivalents. After 24 hours of saturation in a 6-inch pot:
| Medium (70/30 by volume) | Air space | Total water-holding capacity | Total porosity |
|---|---|---|---|
| Coir/perlite or coir/bark | 22% | 59% | 81% |
| Peat/perlite or peat/bark | 24% | 52% | 73% |
Coir media also settled less (8% loss of volume after planting, against 14% for peat-based and 15% for commercial mixes), and dry coir media absorbed more water than any other media tested when top-watered, drip-irrigated or sub-irrigated. The authors noted that the percentage of water-holding capacity regained on re-wetting was similar for coir and peat, so coir absorbs more because it holds more, not because it re-wets more efficiently.
Perlite share and grade
Rhie and Kim (HortScience, 2017) tested coir dust and perlite at 10:0, 8:2, 6:4, 4:6, 2:8 and 0:10 by volume. As the perlite share rose, mean particle size went up and both water-holding capacity and EC went down. A practical side effect: capacitance moisture sensors needed a separate calibration for each ratio, and the manufacturer's generic "soilless" calibration was out by as much as 0.18 m³/m³. If you blend your own, calibrate your sensors on the blend.
Grade matters as much as share. Matkin's peat/perlite tests for the Perlite Institute found air space after drainage of 46% with 75% coarse perlite but only 21% with 75% fine perlite, and concluded that fine perlite "has water holding capacities similar to peat moss" while coarse grades add aeration. The same report gives 15 to 25% air space as the desirable range for container media. The lesson transfers directly to coir: 30% fine perlite adds very little air; 30% coarse perlite adds a lot.
Coir as the water-holding amendment in bark
The reverse experiment, adding coir to pine bark, shows the water side of the ledger. Basiri Jahromi, Fulcher, Walker and Altland (University of Tennessee and USDA-ARS, published in Water, 2020) measured:
| Coir in pine bark (by volume) | Total porosity | Air space | Container capacity | Bulk density (g/cm³) |
|---|---|---|---|---|
| 0% | 69% | 30% | 39% | 0.18 |
| 10% | 79% | 35% | 44% | 0.18 |
| 25% | 77% | 29% | 48% | 0.16 |
| 40% | 77% | 29% | 48% | 0.14 |
| 65% | 81% | 21% | 59% | 0.11 |
They cite Bilderback and colleagues' recommended ranges for container substrates: 10 to 30% air space, 45 to 65% container capacity, 50 to 85% total porosity and 0.19 to 0.70 g/cm³ bulk density. Note that every coir-amended blend was lighter than the recommended bulk density, which is fine on a greenhouse bench and a problem for a tall nursery pot in wind. Plant-available water rose from 14% with no coir to about 17% at 40 to 65% coir, and photosynthesis under drying was highest at 65% coir.
Coir pith, chips and fibre
Coir can also be "amended" with itself. Klasmann-Deilmann's technical note puts the air content of standard quarter-inch sieved pith at about 20%, with chips adding 10 to 40% more air depending on their size and quantity. Botanicoir publishes slab and bag blends of pith, chips and short fibre spanning 48 to 62% water-holding capacity and 20 to 42% air-filled porosity for tomatoes, and 25 to 35% air-filled porosity with 65 to 75% water-holding for its propagation grade. That is the range a blender can reach without any mineral at all. Our article on coconut husk chips covers chip grades and the salt question; the chips themselves are at /products/husk-chips/.
Ratio table by purpose
All ratios are by volume, measured on moist, expanded coir. They are starting points from the sources named; adjust after a pour-through test and a look at the roots.
| Purpose | Suggested blend | Why | Source for the ratio |
|---|---|---|---|
| Seed starting and plug trays | 1:1 coir:fine vermiculite, or 2:1:1 coir:vermiculite:perlite | Even moisture at the surface, easy root removal; vermiculite's CEC holds the starter charge. | Cornell Peat-Lite Mix A is 1:1 peat:vermiculite for seedlings; Rose and Haase (2000) used 2:1:1 coir:vermiculite:perlite for Douglas-fir seedlings with growth equal to or better than peat. |
| Cuttings under mist | 1:1 coir:coarse perlite, up to 100% coarse perlite | Mist adds water constantly, so air after drainage is the limit. | Matkin for the Perlite Institute; Cornell Mix B is 1:1 peat:perlite. |
| General containers, bedding and pot plants | 70:30 coir:perlite (range 5 to 40% perlite) | 22% air, 59% water at 70/30 in a 6-inch pot. | Argo and Biernbaum (1996); Perlite Institute's 5 to 40% range for potting mixes. |
| Hanging baskets and patio pots | 80:20 coir:perlite, or 70:20:10 coir:perlite:vermiculite | Baskets dry fastest, so keep perlite at the low end of the potting range and add vermiculite for buffering. Coir's low settling (8%) helps baskets that run for a whole season. | Perlite Institute range; settling data from Argo and Biernbaum. |
| Vegetables in large containers and raised planters | 7:2:1.5 coir:vermiculite (or fine volcanic cinder):screened compost | Water buffer for big plants plus a modest nutrient base. | University of Arizona Cooperative Extension (Cook). |
| Hydroponic drain-to-waste pots | 70:30 or 50:50 coir:perlite; or pith with 30 to 50% coir chips | Frequent fertigation needs faster drainage and higher air; EC is easier to manage with less fine pith. | 70/30 and 50/50 pre-blends from Char Coir and FoxFarm; chip effect from Klasmann-Deilmann. |
| Slabs and grow bags for fruiting crops | Pith/chip/fibre blends at 20 to 42% air-filled porosity | Steerable water content for generative or vegetative growth. | Botanicoir tomato range. |
| Cacti and succulents | 3:1:1 grit or coarse sand:potting mix:coir | Must drain in under a minute; coir is only the organic fraction. | New York Botanical Garden Plant Information Service. Ask Extension and Fine Gardening recipes are even leaner on organics (5 parts perlite to 4 potting mix to 1 sand). |
| Peat replacement in an existing peat recipe | Replace peat 1:1 with coir, then drop the lime by most or all of it | Coir arrives at pH 5.4 to 6.4 (Argo and Biernbaum); peat needs liming up from around 4.4. | Texas A&M peat recipes (2:1:1 peat:perlite:vermiculite) as the template. |
Three cautions on the table. First, "coir" here means pith; a chip-heavy coir needs less perlite. Second, extension recipes for succulents are written around bagged potting mix, which already contains perlite, so do not add coir on top of that mix as well. Third, Rhie and Kim's result (less water and lower EC with every step up in perlite) means your fertigation EC and frequency have to move with the ratio.
How to mix evenly and pre-wet
The two failure modes are a dry, dusty mix that never wets properly and a wet, smeared mix that has lost its structure. Cornell's Peat-Lite bulletin, written for peat but equally true for coir, warns to "never add large quantities of water during mixing since this destroys structure."
- Expand the coir first, to the right moisture. Hydrate blocks or briquettes fully, then let them drain until the coir is damp and crumbly rather than dripping. Mixing dry compressed coir with perlite does not work; the coir swells afterwards and the ratio is wrong. Hydration is covered in our blocks guide and is not repeated here.
- Damp the perlite before it goes in. Perlite is "very dusty when dry" (Texas A&M) and the Perlite Institute classes it as a nuisance dust. Cornell's advice is about 1 gallon of warm water per 2 bushels of dry peat to keep dust down; a light misting of the perlite does the same job. Wear a mask when handling it dry.
- Measure by volume, not weight. Moist coir is many times heavier than the same volume of perlite, and its weight changes with every batch's moisture, so weighing gives you a different blend every time. All the ratios in this article are by volume.
- Load the mixer in the right order. Cornell: put the fibrous component in first so it cushions the perlite or vermiculite and reduces breakdown. Lime and fertiliser are sprinkled over the pile, not dumped in one place.
- Mix long enough and no longer. Two to three minutes in a mechanical mixer, or turning a pile 10 to 15 times with shovels (Cornell). Over-mixing grinds vermiculite and perlite into fines that undo the aeration you paid for.
- Screen compost and lumpy coir. Quarter-inch screening removes fur-balls of fibre and compost lumps. Newman (2007) notes that clumpy coir "may not move smoothly through automated mixing and filling equipment"; sieved pith or a share of peat solves that.
- Allow for shrinkage. Cornell found 15 to 20% volume loss when peat-lite components were mixed; coir blends behave similarly, so make 1.2 m³ of components for 1 m³ of mix.
- Wet the finished mix before filling. Fill pots with moist mix, firm the edges, water thoroughly and let it drain before sowing or planting. Never sow into a dry mix (Cornell).
Nutrient charge and pH for coir-based mixes
A coir blend behaves differently from a peat blend in three ways, and your starter charge should reflect them.
- Less or no lime for pH. Argo and Biernbaum put unamended coir at pH 5.4 to 6.4, already at or near the soilless target, so "media containing coir require less lime." Vermiculite nudges pH up a little (6 to 9 in water, with a normally alkaline reaction from carbonates), perlite not at all. Test the finished blend rather than assuming.
- But do not drop calcium and magnesium. The same MSU work found that dolomitic lime was still a significant source of long-term Ca and Mg buffering, and that gypsum or Epsom salts washed out within a few weeks. Rose and Haase (2000) measured two coir sources at roughly 4,400 and 20,000 ppm potassium against 99 ppm in peat, with 9 to 16 ppm calcium against 127 ppm, and warned that calcium deficiency "is inevitable" without supplementation. If you add little lime, supply Ca and Mg through the feed. See calcium and feeding in coir for the in-crop detail.
- Account for the coir's own salts. Unbuffered, washed coir carries potassium, sodium and chloride; Argo's coir read 0.9 mS/cm EC with 65 ppm K, 85 ppm Na and 250 ppm Cl in a saturated media extract. Perlite dilutes that by volume; vermiculite and compost add their own. Test the EC of the blend before choosing a starter charge. Our coir is washed with water to lower EC and is not buffered, so a calcium-containing starter charge or feed is the normal pairing.
- A less acidic feed. Because coir does not need the acidifying push that limed peat does, MSU suggested a less acidic water-soluble fertiliser to stop the medium drifting below 5.8.
Cornell's standard charge for a peat-vermiculite seedling mix (per cubic yard: 10 lb dolomitic limestone, 1 to 2 lb superphosphate, 1 lb calcium or potassium nitrate, plus trace elements) is a useful template; for coir, cut the limestone to what your pH test calls for and keep a calcium source.
Mistakes we see most often
- Perlite floating to the top. Texas A&M lists it as a known drawback: perlite "tends to float to the top during irrigation." It is worst with coarse perlite, high perlite shares and heavy overhead watering, and the surface then dries out faster while the base holds water. Use a medium grade, keep perlite at or below 30% in overhead-watered pots, and drip-irrigate where you can.
- Fine perlite that adds nothing. Matkin's 21% versus 46% air space result shows that grade can matter more than ratio. If a 70/30 mix still feels wet, check the perlite grade before changing the ratio.
- Too much vermiculite in long crops. It compresses and loses air over time; keep it for short crops, plugs and baskets, or pair it with perlite.
- Dust. Dry perlite and dry coir both produce it. Pre-wetting both, and masks, are the fix; horticultural-grade vermiculite only (Cornell), given the history of asbestos contamination at one former mine.
- Uneven moisture from poor mixing. Pockets of pure coir stay wet and pockets of pure perlite stay dry, and your sensors and pour-through readings become meaningless. Klasmann-Deilmann's note is blunt that uniform mixing is what gives even distribution of air and water and therefore even rooting.
- Weighing instead of measuring. See step 3 above.
- Forgetting that coir is light. Every coir-bark blend in the Tennessee study fell below the 0.19 g/cm³ minimum bulk density. For tall nursery stock outdoors, use coarse sand or less perlite for ballast.
- Changing the ratio and not the feed. Lower water holding means more frequent, smaller fertigations; Argo's team found more water per irrigation also delivers more fertiliser, so the concentration does not have to rise.
Buying coir for blending
For blending, the raw material matters more than the recipe. A consistent, sieved pith with known EC removes most batch-to-batch adjustment. Compressed 5 kg blocks suit blenders who hydrate on site; if you want a defined pith, chip and fibre mix, or a specific sieve fraction, our custom blends are made to the ratio you specify. Tell us the crop and the system and we will suggest a starting blend: contact us.
FAQ
What is the best ratio of coco coir to perlite?
For general container growing, 70% coir to 30% perlite by volume is the most widely used blend and measured 22% air space and 59% water-holding capacity in Michigan State University trials. Use 50/50 for cuttings and fast-draining hydroponic pots, and 80/20 where you want more water held. The grade of perlite changes the result as much as the ratio.
Should I use perlite or vermiculite with coco coir?
Perlite adds air and drainage and stays stable for years; vermiculite adds water and nutrient holding but compresses over time. Use perlite for containers and hydroponics, vermiculite for seed starting and short crops, and both together (for example 2:1:1 coir:vermiculite:perlite) when you want a balance.
Can I mix coco coir with sphagnum peat moss?
Yes, and nurseries often do. A 1:1 blend keeps some of peat's fine texture and makes coir flow better through filling machines, while the coir improves re-wetting. Remember the blend still needs lime for the peat half; test the pH of the finished mix.
Do I need to add lime to a coir and perlite mix?
Usually little or none for pH, because washed coir typically sits around pH 5.4 to 6.4 and perlite is neutral. You still need a calcium and magnesium source, since coir is low in both and high in potassium. Many growers add a small amount of dolomitic lime or gypsum and supply the rest through a calcium-containing feed.
Why does perlite float to the top of my coir mix?
Perlite is far lighter than wet coir, so heavy overhead watering lifts it to the surface, especially coarse grades at high shares. Use a medium grade, keep perlite at or below about 30% in overhead-watered pots, and water gently or by drip.
What ratio of coco coir to perlite for seed starting?
Seed trays want even surface moisture rather than fast drainage, so use coir with vermiculite at about 1:1, or coir, vermiculite and perlite at 2:1:1. Use fine grades of both minerals in small cells. Pre-wet the mix before sowing.
How much perlite for succulents and cacti in coir?
Treat coir as the minority organic fraction, not the base. Botanical garden and extension recipes run roughly three parts grit or coarse sand to one part potting mix and one part coir or similar organic, and the mix should drain within a minute of watering.
Can I use coco coir and perlite for hydroponics?
Yes; 70/30 and 50/50 coir-perlite blends are sold specifically for drain-to-waste pots, and coir pith blended with 30 to 50% coir chips does a similar job without any mineral. Higher perlite means lower water holding and lower EC in the pot, so feed more often at a steady concentration.
Sources
- Argo, W. R. and Biernbaum, J. A. (Michigan State University): Component comparisons: coconut coir. GrowerTalks (1996)
- Basiri Jahromi, N., Fulcher, A., Walker, F. and Altland, J.: Optimizing Substrate Available Water and Coir Amendment Rate in Pine Bark Substrates. Water 12:362 (2020)
- Rhie, Y. H. and Kim, J.: Changes in Physical Properties of Various Coir Dust and Perlite Mixes and Their Capacitance Sensor Volumetric Water Content Calibrations. HortScience 52(1) (2017)
- Rose, R. and Haase, D. L.: The Use of Coir as a Containerized Growing Medium for Douglas-fir Seedlings. Native Plants Journal (2000)
- Boodley, J. W. and Sheldrake, R.: Cornell Peat-Lite Mixes for Commercial Plant Growing. Cornell Cooperative Extension Information Bulletin 43
- Perlite Institute: The Water-Holding Capacity of Perlite (2019); Why Perlite Works (2009); Perlite: The Mineral for Healthy Plants (2018); Horticulture and Health and Safety pages
- Matkin, O. A. (Soil and Plant Laboratory): Perlite Gradation and Peat/Perlite Mixtures. Perlite Institute research report
- The Vermiculite Association: Vermiculite Data sheet; Horticultural Vermiculite brochure
- Texas A&M AgriLife Extension, Aggie Horticulture: Growing Media (Ornamental Production)
- Klasmann-Deilmann: Technical Note, Coir Physically: Fine tuning with raw materials (2024)
- Botanicoir: Tomato and Propagation product pages
- Newman, J.: Core facts about coir. Greenhouse Management and Production 27(2) (2007)
- Cook, B. (University of Arizona Cooperative Extension, Greenlee County): Home made potting soils
- New York Botanical Garden, Plant Information Service: What kind of potting mix should I use for my cactus? (2025)
- Ask Extension (Oregon): Succulent soil recipe answers (2018, 2022)
- Moore, J.: Potting Soil Recipe for Cacti and Succulents. Fine Gardening, issue 125
- Char Coir and FoxFarm: 70/30 coco-perlite and Cloud Coir 50/50 product listings
- US Environmental Protection Agency: Protect Your Family from Asbestos-Contaminated Vermiculite Insulation