The short answer: keep the root zone of coco coir at pH 5.5 to 6.2 and keep the EC in a range that matches your crop and your test method. Purdue Extension states that all nutrients are readily available in soilless substrates at pH 5.4 to 6.2; Canna's 1:1.5 method targets pH 5.3 to 6.2 for coco, and Botanicoir recommends 5.8 to 6.5. For EC there is no single number: a "normal" reading is 0.76 to 1.25 mS/cm in a 1:2 extract but 2.7 to 4.6 mS/cm by pour-through on the same pot (Purdue), and fruiting vegetables in slabs run higher than bedding plants. The rest of this article gives the tables, the test procedures and the corrections.
This is a guide to managing pH and EC while plants are growing in coir. The raw-material side (why fresh coir is salty, washing, buffering and how suppliers quote EC) is covered in our article on buffered vs unbuffered coco peat and is not repeated here.
Why coir has a lower pH target than garden soil
Coir is an organic, soilless substrate, and soilless pH targets are set by micronutrient chemistry rather than by what field crops like. Michigan State University Extension puts the optimum for most greenhouse and nursery crops at 5.4 to 6.4. Above roughly 6.5, iron, manganese and boron become less soluble and iron deficiency (interveinal chlorosis on new leaves) is the most common result. Below about 5.0, iron and manganese become so soluble that sensitive crops take up toxic amounts (MSU Extension; Greenhouse Grower, 2026). Purdue's HO-242-W splits crops into three groups:
| Crop group (Purdue HO-242-W, after Fisher) | Target substrate pH | Examples |
|---|---|---|
| Iron-inefficient | 5.4 to 6.2 | petunia, calibrachoa, pansy, snapdragon, bacopa |
| General | 5.8 to 6.4 | chrysanthemum, impatiens, poinsettia, ivy geranium |
| Iron-efficient | 6.0 to 6.6 | zonal and seed geranium, marigold, New Guinea impatiens |
Coir makes this easier than peat. Sphagnum peat has a natural pH of 3.0 to 4.0 and must be limed into range (MSU Extension). Coir dust from 12 sources tested by Evans, Konduru and Stamps (HortScience, 1996) had a pH of about 5.6 to 6.9; Kekkilä-BVB quotes 4.9 to 6.8. Most washed coir therefore starts inside or just above the target and needs little or no lime. The flip side is that it has less carbonate buffering than a limed peat mix, so its pH follows whatever you pour on it: water, fertiliser and the roots themselves.
Measuring root-zone pH and EC in coir
The first rule, from every extension source: EC targets depend on the extraction method, so pick one method and keep it for the whole crop. Pour-through readings are higher than saturated media extract (SME) readings for the same pot, by about 1.4 to 1.6 times for EC, while pH is the same by both methods (Altland, Horticulturae, 2021).
| Method | How it works | Where it is used | Notes |
|---|---|---|---|
| 1:1.5 volume extract | 150 ml demineralised water in a 250 ml jug, add moist coir to the 250 ml mark, mix, settle at least two hours; pH on the slurry, EC on the filtered liquid (Canna) | Dutch standard (Sonneveld et al., 1979); behind most coir datasheets and lab reports | In-crop targets for coco: EC 1.1 to 1.3 mS/cm, pH 5.3 to 6.2 (Canna). Destructive. |
| 1:2 dilution | 1 part substrate to 2 parts distilled water, stand 30 minutes, filter (Purdue HO-237-W) | North American labs and growers | Defined water volume, so repeatable. Disturbs roots. |
| Saturated media extract (SME) | Wet 200 to 300 ml of substrate to saturation, stand 30 minutes, filter (Purdue) | Commercial labs | Accurate, but "saturation" is a judgement call. |
| Pour-through | Irrigate as normal, wait 30 to 60 minutes, pour distilled water on the surface, collect about 50 ml of leachate (Purdue) | Pots, bags and slabs in the greenhouse | Non-destructive, samples the whole root zone. More variable than lab methods. |
| Drain (run-off) water | Measure what drips from the slab or bag during normal irrigation | Fertigated slabs and grow bags | Fast, but coir "is able to retain and release elements", so drain pH and EC "generally deviate from the actual root situation" (Canna). |
Pour-through, step by step (Purdue HO-237-W)
- Irrigate or fertigate as usual.
- Let the container drain for 30 to 60 minutes. Studies reviewed by Altland found pH and EC stable between 20 and 120 minutes after saturation; 60 minutes is the recommended wait.
- Put a saucer under the pot and pour distilled water on the surface: about 75 ml for a 4 to 6 inch pot, 100 ml for a 6.5 inch pot, 150 ml for a 1 gallon pot, 350 ml for a 3 gallon pot. Aim for close to 50 ml of leachate; more than 70 ml dilutes the sample.
- Measure pH and EC. Take five to ten pots per crop and use the average.
Purdue does not recommend pushing a probe straight into the substrate unless moisture is very consistent and the same person tests on the same day every week. For slabs, Grodan's routine is to draw about 100 ml of solution from several slabs with a syringe, measure it, and plot the difference from the drip EC, sampling at the same time each day.
Target EC by test method
This is Purdue's interpretation table for soilless substrates (adapted from North Carolina State University's PourThru leaflet). The same pot gives very different numbers in each column, which is why a datasheet or forum EC figure without a method is meaningless.
| Pour-through (mS/cm) | 1:2 extract | SME | Status | Meaning |
|---|---|---|---|---|
| 0 to 0.9 | 0 to 0.25 | 0 to 0.75 | Very low | May not sustain rapid growth |
| 1.0 to 2.6 | 0.26 to 0.75 | 0.76 to 2.0 | Low | Suitable for seedlings, bedding plants and salt-sensitive plants |
| 2.7 to 4.6 | 0.76 to 1.25 | 2.1 to 3.5 | Normal | Standard root-zone range for most established plants |
| 4.7 to 6.5 | 1.26 to 1.75 | 3.6 to 5.0 | High | Reduced vigour, especially in hot weather |
| 6.6 to 7.8 | 1.76 to 2.25 | 5.1 to 6.0 | Very high | Salt injury, marginal leaf burn, wilting |
| above 7.8 | above 2.25 | above 6.0 | Extreme | Immediate leaching required |
Source: Purdue Extension HO-237-W (Camberato, Lopez and Mickelbart, 2009), after Cavins et al., NCSU HIL 590 (2000). The 1:1.5 method sits between the 1:2 and SME columns; Canna's in-crop target of 1.1 to 1.3 mS/cm by 1:1.5 corresponds to the "normal" band.
Target EC by crop and stage
Ornamentals are grouped by feeding level in the NCSU/Purdue system: light feeders (plugs, impatiens, pansy, rooting cuttings) at pour-through EC 1.0 to 2.6 mS/cm, medium feeders (petunia, geranium, lily, rose) at 2.0 to 3.5, heavy feeders (chrysanthemum, poinsettia) at 2.6 to 4.6.
For fertigated fruit vegetables the published numbers are mostly feed and slab-solution values, and many come from stone wool. Treat them as reference points, not recipes:
| Crop and source | Feed (drip) EC, mS/cm | Root-zone or drain EC | Comment |
|---|---|---|---|
| Tomato in coir, Korean trial (Protected Horticulture and Plant Factory, 2017) | 1.0, 1.5, 2.0 and 3.0 tested | At 2.0 or higher, all ions except P accumulated in the drain | Fruit number and yield to the sixth truss were highest at 1.5; blossom-end rot early in the crop occurred only at 3.0 |
| Tomato in coir vs stone wool vs peat (Xiong et al., Frontiers in Plant Science, 2017) | 2.3 reservoir, 20 to 50 % drain | Drain EC rose for 21 weeks, then held | Coir gave the highest yield (84.9 t/ha) and the highest K:Ca ratio in the root zone (2.3) |
| Pepper in stone wool, Grodan 6-phase advice | 2.6 to 3.5 by phase | Slab 3.0 to 4.5; 24-hour drain 15 to 30 % in production | Slab EC runs 0.2 to 1.0 above drip |
| Fruiting plants in coco, Canna grow schedule (2025) | 2.0 rooting, 2.2 vegetative and early flower, 2.7 peak, 2.0 late, including 0.4 from water | not stated | General schedule for a flowering or fruiting plant from a one-week-old seedling |
Stone-wool figures are not coir figures: coir releases potassium for months and binds calcium early (see below), so a stone-wool recipe needs adjusting first. Grodan itself says the right slab-minus-drip difference "differs for each crop type and time of year". For tomatoes in coir, the Korean trial is the most relevant data point: a 1.5 mS/cm feed outperformed 2.0 and 3.0 for the first six trusses.
Reading feed EC against drain EC
The difference between what goes in and what comes out is the most useful daily number in a fertigated coir crop.
- Drain EC a little above feed EC is normal: plants take up water faster than salts. Grodan's pepper advice has slab EC 0.2 to 1.0 mS/cm above drip EC through the season.
- Drain EC climbing week on week means salts are accumulating faster than they are leached. In the Korean coir trial, drain EC rose sharply when irrigation volume fell short and the drain fraction dropped; restoring 20 to 30 % drain brought it down.
- Drain EC below feed EC in the first weeks is normal in coir and not a sign of underfeeding. The exchange sites hold calcium and magnesium at first: Dr Hugh Poole (via Hort Americas) gives the example of 50 ppm calcium in the feed and only 10 ppm in the leachate. In the Korean trial, nitrate, phosphorus, calcium and magnesium in the drain were below feed levels early on at 1.0 and 1.5 mS/cm, while potassium was released by the coir.
- Drain pH drifts for its own reasons. In the same trial it stayed above 6.5 for 24 weeks in the low-EC treatments and fell below 5.0 in the high-EC treatments, which the authors attribute to the balance of anions and cations taken up. Xiong et al. saw root-zone pH in coir fall gradually for 14 weeks, then stabilise.
Drain percentage is the control lever. Grodan's guidance quoted in Greenhouse Canada (2013): with slab EC in range, aim for 15 to 20 % drain per cycle; to lower it, give large morning irrigations of 4 to 6 % of substrate volume and allow 40 to 80 % drain in the following cycles; on dark days keep drain minimal.
Correcting high EC
- Confirm it with a root-zone test, not just drain, and compare against the table for your method. Purdue's "extreme" band (above 2.25 mS/cm by 1:2) calls for immediate leaching; the "high" band can be brought down more gently by increasing drain.
- Leach with pH-corrected water. Canna's advice for coco with very high EC is to rinse with water acidified to pH 5.8. For slabs, use the Grodan routine above (40 to 80 % drain in the morning cycles, then back to 15 to 20 %). In the Korean trial the researchers lowered root-zone EC simply by raising irrigation volume for four days.
- Retest with the same method once the substrate is back to normal moisture before leaching again. Avoid repeated flushing with pure reverse-osmosis water: it contains no calcium or magnesium, and coir's exchange sites will pull those out of the next feed (Poole, Hort Americas).
- Fix the cause: feed EC too high for the light level, too little drain, or dry-back between irrigations. Purdue notes that sampling dry pots also inflates EC readings.
Correcting pH drift
Water alkalinity, not water pH
The biggest cause of rising substrate pH is bicarbonate in the irrigation water. Purdue's HO-242-W puts it plainly: irrigating with high-alkalinity water has the same effect as adding lime, and alkalinity "has a greater effect on substrate pH than water acidity or pH". USDA-ARS researcher James Altland showed this experimentally and found no correlation between pH and alkalinity across 192 Ohio groundwater samples, so a water pH reading says nothing reliable about alkalinity.
| Alkalinity (as CaCO3) | Classification (Purdue HO-242-W) | What to expect |
|---|---|---|
| below 80 ppm | Low | Substrate pH may fall over time, especially with ammonium fertilisers |
| 100 to 200 ppm | Moderate | Watch trends; Purdue's rule of thumb is that 120 ppm is likely to cause high-pH problems in long crops |
| above 200 ppm | High | Acidify the water |
The Saskatchewan Ministry of Agriculture's greenhouse water guideline recommends 1 to 100 mg/L as CaCO3 (60 to 80 mg/L for plugs), treatment above 200 mg/L, and notes that 120 mg/L can already be a problem for long-term crops. To convert lab units, 1 meq/L equals 50.05 ppm CaCO3 (Purdue). Test water at least once a year; well water changes with the season.
Because coir has little lime of its own, it tracks water alkalinity closely. If in-crop pH climbs toward 6.5 and iron chlorosis appears on young leaves, test alkalinity before reaching for iron chelate; Purdue warns that chelate greens the crop without removing the cause.
Lowering a high pH
- Inject acid (sulfuric is the most common; phosphoric and nitric also supply nutrients) at a rate calculated from alkalinity, not water pH. The online Alkalinity Calculator from the Universities of New Hampshire, North Carolina State and Purdue does the arithmetic (Purdue HO-242-W).
- Shift the nitrogen form. Ammonium and urea acidify the root zone as ammonium converts to nitrate; calcium nitrate and potassium nitrate push pH up (MSU Extension; Kekkilä-BVB).
- In severe cases only: an aluminium sulfate drench, which Purdue flags as a toxicity risk.
Raising a low pH
- Change to a nitrate-based fertiliser and stop acidifying the water (Purdue).
- Flowable lime at 1 to 2 quarts per 100 gallons, or potassium bicarbonate at 2 pounds per 100 gallons, rinsing foliage and leaching afterwards (MSU Extension, Lindberg and Owen, 2018).
Low pH is less common in coir than in peat, but it happens with very low-alkalinity water (rain, RO) and an ammonium-heavy feed. Purdue advises such growers to keep iron-efficient crops at pH 6.0 to 6.6 to avoid iron and manganese toxicity.
Meters and calibration
- Use a meter with automatic temperature compensation (Purdue). Conductivity changes with temperature, so EC is referenced to a standard temperature: 20 °C in Dutch practice (Grodan), 25 °C in Canna's schedules. Readings taken at different temperatures without compensation are not comparable.
- Calibrate with standards that bracket the values you expect (HORIBA). For pH, calibrate at 4.0 and 7.0 when measuring substrate extracts (Purdue).
- Store meters and buffer solutions in a cool, dry room, never in the greenhouse (Purdue).
- Be consistent: same method, same time of day, ideally the same person. Plot the results. Purdue calls the graph "the real value of testing".
- Frequency: HORIBA suggests every two to three weeks for container crops; Kekkilä-BVB suggests weekly for most greenhouse crops and daily pH and EC for high-value fruit such as strawberries. Fertigated slab crops should check drain EC daily and slab solution weekly (Grodan).
Common mistakes
- Comparing EC numbers across methods. A 1.2 by 1:1.5 and a 3.5 by pour-through can be the same pot.
- Treating drain water as the root zone. Coir holds and releases cations, so drain lags what the roots see, especially in the first month.
- Chasing feed pH instead of alkalinity. Acidifying to pH 5.8 without measuring alkalinity leaves the bicarbonate load untouched.
- Reacting to one afternoon reading. Slab EC moves through the day; sample at the same time each day and act on the trend (Grodan).
- Running coir at stone-wool EC. Coir releases potassium for months (Korean trial; Xiong et al.), so a stone-wool recipe can over-supply K and under-supply Ca and Mg. Poole's advice is a higher Ca and Mg feed for the first two to four weeks, especially on RO water.
Washed coir for greenhouse use arrives with a known starting EC, but everything above is about what happens afterwards: the in-crop numbers come from your water, your feed and your drain strategy. CVM supplies washed, unbuffered coco peat in blocks, briquettes, grow bags and open-top slabs; to discuss a format for a fertigated crop, contact us.
FAQ
What is the ideal pH for coco coir?
For most crops, keep the root zone between 5.5 and 6.2. Purdue Extension states that all nutrients are readily available in soilless substrates at pH 5.4 to 6.2, and Canna's coco target is 5.3 to 6.2 by the 1:1.5 method. Iron-efficient crops such as geraniums and marigolds can go a little higher, to 6.0 to 6.6, where iron and manganese toxicity is less likely.
What EC should coco coir be?
It depends on the test method and the crop. In a 1:2 extract, 0.76 to 1.25 mS/cm is the normal root-zone range for established plants; the same pot would read 2.7 to 4.6 by pour-through. Canna's in-crop target by the 1:1.5 method is 1.1 to 1.3 mS/cm. Fertigated fruit vegetables run higher in the slab than bedding plants, but a coir tomato trial found a 1.5 mS/cm feed gave the best early yield.
How do I measure the EC of coco coir in the root zone?
Use one method consistently. For pots and bags, the pour-through is easiest: irrigate as normal, wait 30 to 60 minutes, pour distilled water on the surface and collect about 50 ml of leachate. For lab-style numbers, mix one part moist coir with 1.5 parts demineralised water, settle for two hours, then measure pH on the slurry and EC on the filtered liquid. Average five to ten samples per crop.
Should drain EC be higher or lower than feed EC?
Slightly higher is normal once the crop is established, because plants take up water faster than salts. Grodan's pepper advice has slab EC 0.2 to 1.0 mS/cm above drip EC. In the first weeks in coir, drain EC and especially drain calcium can be below the feed because the coir's exchange sites are still filling. A steady week-on-week climb means too little drain or too strong a feed.
How do I flush coco coir with high EC?
Rinse with water acidified to about pH 5.8 rather than plain tap water, let the substrate dry back to its normal moisture, then retest with the same method. For slabs and bags, large morning irrigations with 40 to 80 percent drain over a few cycles bring the EC down, after which drain goes back to 15 to 20 percent. Do not flush repeatedly with pure RO water, because coir will then strip calcium and magnesium from the next feed.
Why does the pH in my coco keep rising?
Almost always because of bicarbonate alkalinity in the irrigation water, not the water's pH. Alkalinity acts like a slow dose of lime, and coir has little lime of its own to mask it. Test alkalinity; above about 120 ppm as calcium carbonate, Purdue suggests acidifying the water. Nitrate-heavy fertilisers also push pH up, while ammonium-based ones pull it down.
Is runoff pH a reliable guide in coco coir?
Only as a trend. Coir retains and releases ions, so drain pH and EC generally differ from the actual root-zone values, and drain pH moves with the balance of anions and cations the plant is taking up. Use drain readings daily to spot change, and confirm with a 1:1.5 or pour-through test before correcting.
How often should I test pH and EC in coir?
Fertigated slab and bag crops: drain EC daily and slab or pour-through weekly. Pot crops: weekly for most greenhouse crops, or every two to three weeks at minimum. High-value fruit such as strawberries justify daily checks. Whatever the interval, plot the numbers; the trend is worth more than any single reading.
Sources
- Purdue Extension, Camberato, Lopez and Mickelbart: pH and Electrical Conductivity Measurements in Soilless Substrates, HO-237-W (2009)
- Purdue Extension, Lopez and Mickelbart: Alkalinity Management in Soilless Substrates, HO-242-W (2010)
- North Carolina State University, Cavins, Whipker, Fonteno et al.: Monitoring and Managing pH and EC Using the PourThru Extraction Method, HIL 590 (2000)
- Altland, J.E.: The Pour-Through Procedure for Monitoring Container Substrate Chemical Properties: A Review, Horticulturae (2021)
- Altland, J.E.: Irrigation water alkalinity, not pH, affects substrate pH, ASHS Annual Conference poster and Acta Horticulturae 1212 (2017)
- Michigan State University Extension, Lindberg and Owen: Managing substrate pH of soilless substrates (2018)
- Michigan State University Extension, Splawski, in Greenhouse Grower: Finding and Maintaining the Perfect Substrate pH for Your Plants (2026)
- Government of Saskatchewan, Ministry of Agriculture: Water Quality in Greenhouses
- HORIBA: pH and Conductivity Measurements in Coconut Coir Substrate, application note (2016)
- Canna: Measuring coco with the extraction method (1:1.5); Canna Coco Grow Schedule (2025)
- Grodan: EC, learning pages; 6-phase advice, Grodan Vital, pepper
- Lee, A., Greenhouse Canada: water content and EC monitoring with Grodan substrates (2013)
- Evans, Konduru and Stamps: Source variation in physical and chemical properties of coconut coir dust, HortScience 31(6) (1996)
- Xiong, Tian, Wang, Liu and Chen: Comparison of Coconut Coir, Rockwool, and Peat Cultivations for Tomato Production, Frontiers in Plant Science 8:1327 (2017)
- Protected Horticulture and Plant Factory 26(4): Effect of EC Level of Irrigation Solution on Tomato Growth and Inorganic Ions of Root Zone in Soilless Culture Using Coir Substrate, Rural Development Administration, Korea (2017)
- Kuack, D., Hort Americas, quoting Dr Hugh Poole: Buffering coir not necessary if it is processed properly (2015)
- Kekkilä-BVB: How does pH level in growing media affect crop performance?
- HortiDaily / Botanicoir: A guide to managing coco coir nutrient and pH levels (2025)