The short answer: tomatoes grow well in coco coir slabs and grow bags when the coir is washed to a low EC, saturated with nutrient solution before planting, and then fed in many small drip shots that leave 15–30% drain. Feed EC runs from about 1.5–2.0 mS/cm after transplanting up to 3.0 mS/cm under heavy fruit load, with root-zone pH kept at roughly 5.5–6.2. Two things differ from rockwool: coir holds potassium and sodium on its exchange sites, so calcium and magnesium need extra attention in the first weeks, and coir buffers EC, so drain EC moves more slowly and needs a wider tolerance.
Key facts
- In a 33-week trial, tomatoes on coir slabs yielded 84.9 t/ha against 67.5 t/ha on rockwool and 76.9 t/ha on peat-vermiculite (Xiong et al., Frontiers in Plant Science, 2017).
- Typical high-wire density is 2.2–3 plants/m²; 2.5 plants/m² is the Dutch reference layout (University of Kentucky; Wageningen UR).
- Drain target: 15–30% (Wageningen UR); 10–15% in a US extension guide (NC State); consultant Godfrey Dol scales it from 0–10% on dark days to 30–40% above 1,500 J/cm².
- A mature tomato plant uses 1.9–3.0 L of water a day in hot summer conditions (University of Kentucky).
- In coir, feed EC of 1.5 mS/cm gave the highest fruit number and yield to the sixth truss; 3.0 mS/cm gave smaller fruit and more blossom-end rot (Korean RDA trial, 2017).
Why coir suits high-wire tomatoes
Coco coir (coco peat or pith, usually blended with husk chips) is one of the two main substrates for long-season greenhouse tomatoes, alongside stone wool.
- Yield is at least equal to rockwool. In the 2017 Beijing trial by Xiong and colleagues, coir gave 84.9 t/ha versus 67.5 t/ha on rockwool with the same feed, the same 2.4 plants/m² and the same slab size. Fruit weight averaged 74.0 g on coir and 66.8 g on rockwool, with no real difference in quality. Blossom-end rot was 19.5% of fruit on coir and 21.2% on rockwool, so coir does not remove that problem by itself.
- It forgives irrigation mistakes. Kagome, the Japanese tomato grower that moved from rockwool to coco slabs, told HortiDaily in 2013 that coco was "more flexible than rockwool" and that plants survive an irrigation error. The flip side: slabs of pure fine peat produced "excessive vegetative growth". In the manager's words, "The more peat we use, the more vegetative growth we create." Tomato slabs are therefore blends of pith and chips.
- Physical properties are close to rockwool. Xiong et al. measured 85.6% total porosity and 5.6% air porosity for coir against 89.2% and 5.1% for rockwool.
- Disposal and re-use are simpler. Coir is organic, ships dry and compressed, and can be used a second time or composted.
The main difference to manage is chemistry. Washed, unbuffered coir still carries potassium and sodium on its exchange sites, which is why the calcium section below matters. If you are deciding which grade to buy, our article on buffered vs unbuffered coco peat covers what washing and buffering do and how EC should be specified.
Slab and grow-bag sizes and plant density
There is no single standard, but the sizes used in trials and trade reports cluster closely:
| Source | Slab / bag | Volume | Plants per unit | Density |
|---|---|---|---|---|
| Xiong et al. 2017 (coir vs rockwool) | 100 × 20 × 7.5 cm slab, 30 cm spacing | about 15 L | 3 | 2.4 plants/m² |
| Korean RDA 2022 (50:50 pith and chips) | 100 × 15 × 10 cm slab | 15 L | 4 | hanging gutters, 1 dripper/plant |
| Vinković et al. 2007 (coconut husk) | grow bag | 12.5 L | 3 | 3 plants/m² |
| Sai Substrates, HortiDaily 2026 (Spanish cherry tomato) | flat bag | 16–18 L yearly; 27 L for multi-year use | – | – |
| University of Kentucky 2026 | containers | 3–5 US gal (11–19 L) | 1 | 2.2–3 plants/m² |
| Wageningen UR tomato sheet | coir or stone-wool slabs in troughs | – | – | 2.5 plants/m², 25 cm in-row |
A 1 m slab or bag of 12–20 L carrying three or four plants is normal, giving roughly 4–6 L of coir per plant. A Brazilian trial (Pires et al., Scientia Agricola, 2011) tested 5, 7.5 and 10 L of coconut fibre per plant and found no yield effect from volume, but the 5 L volume irrigated once a day produced more blossom-end rot. Small volumes work when irrigation is frequent; they punish long gaps.
Mix matters as much as size. Sai Substrates described to HortiDaily how recipes differ by climate: about 70:30 coco peat to chips in warm regions such as Mexico and southern Spain, a more open, chip-heavy mix in northern Europe for faster dry-back, and 100% chips over a fibre drainage layer for long Canadian crops. Bags mixed unevenly, pith at one end and chips at the other, gave visibly uneven roots and vigour.
Wageningen UR's tomato sheet gives the standard Dutch geometry: a two-row V system with 160 cm between rows at planting, later 90 cm, 25 cm between plants, troughs at a 0.5% slope, pressure-compensated drippers of about 2 L/h on spaghetti tubes, and black/white film over the slab against algae, heat and evaporation.
For the formats themselves, see our grow bags and open-top slabs, and the overview of coir for tomato crops.
Preparing the slab: flushing, saturation, EC and pH before planting
The slab's starting chemistry sets the first four to six weeks of the crop. Wageningen UR gives these limits for any substrate before purchase: total salt EC below 0.5 dS/m, pH 4.5–6.5, sodium chloride below 1 mmol/L, sulphate and bicarbonate below 2 mmol/L, and more than 15% air-filled pores when saturated. Dr Hugh Poole, quoted by Hort Americas, says coir with EC below 0.5 mS/cm "shouldn't have to be buffered for most crops", and below 1.0 mS/cm should not need leaching on the farm.
A practical sequence, drawn from the Korean RDA protocol (2022) and the Quebec CIDES grow-bag trials:
- Lay slabs on the gutter, cut dripper holes in the top film and place drippers. Do not cut drain slits yet.
- Fill with water or dilute solution until the coir expands and the bag is firm. For a 1 m, 15 L slab this is on the order of 20 L; the Quebec trials needed about 9 L for a smaller bag. Leave for 24 hours.
- Flush if the coir or raw water EC is in doubt. The Korean trial ran 40 L of water per slab per day for at least a week before planting to remove salts and even out ions. Well-washed coir needs far less; test the drain.
- Cut drain slits low in the side of the bag after saturation, so the slab keeps a reservoir but cannot waterlog.
- The day before planting, saturate with nutrient solution at the EC you will feed the young crop. The Korean trial used 20 L per slab at EC 2.0 mS/cm; Canna's slab instructions use about 1.5 mS/cm until some drainage appears.
- Check the propagation block. Wageningen UR warns that the block's EC must be higher than the slab's, otherwise roots will not grow out, and that the block must not hold water more tightly than the slab.
Target before the plant goes on: slab pH 5.5–6.2 and slab EC within a few tenths of a mS/cm of your feed EC. If the drain EC is well above the feed at this point, the coir was not washed enough; flush again.
Drip irrigation strategy
Tomato irrigation in coir is steered by light, and the published rules of thumb agree:
- Wageningen UR: 2–3 ml of solution per m² for every J/cm² of radiation, in the smallest possible shots at the highest possible frequency, "at least more than 10x per day".
- Godfrey Dol (Glasshouse Consultancy, HortiDaily 2020): a mature plant takes up about 1.7 ml per m² per J/cm², so a dark 300 J/cm² day needs only about 510 ml/m².
- Dr Mohyuddin Mirza (Greenhouse Canada, 2021): 300 ml/m² for each 100 J/cm²; a 2,000 J day means 6 L/m², or 2 L per plant at 3 plants/m².
- NC State (Mary Peet): 120–130 ml per plant per MJ/m² for computer-controlled watering.
Shot size. Dol's examples use 0.2 L/m² per cycle, about 100 ml per dripper at 2.5 plants/m²; the Korean RDA trial also gave about 100 ml per plant per shot. Xiong et al. used 1–2 L shots two to four times a day in a recirculating system and still had 20–50% drain, which shows how much coir tolerates.
Timing. The Korean trial started 2–2.5 hours after sunrise, stopped 2 hours before sunset, allowed no shot within 30 minutes of the last, and forced one shot every 3 hours if the light sum was not reached. Dol adjusts frequency between 11:00 and 14:00 according to drain EC and aims for an overnight dry-down of 10–15% of slab water content, with 10% as a minimum so roots get oxygen; around 10% steers vegetative, 15% or more generative.
Drain targets. Wageningen UR's rule is 15–30%; below 15% the root-zone EC climbs, so increase frequency. Dol scales it to light, assuming feed EC 3.0 and target drain EC 4.5:
| Daily light sum (J/cm²) | Drain target |
|---|---|
| under 500 | 0–10% |
| 500–1,000 | 10–20% |
| 1,000–1,500 | 20–30% |
| over 1,500 | 30–40% |
NC State's figure for bag culture is 10–15%, with drain EC and pH within 5–10% of the feed. That tight band comes from peat and perlite bags and is not realistic in coir. The Korean RDA team found that applying the usual 20–30% drain rule to coir slabs let drain EC rise well above the feed, because the organic substrate adsorbs and releases ions, and concluded that the allowable drain EC range should be set wider in coir than in mineral substrates. Their highest total yield came from the second-highest water volume, with 32–39% drain in winter. The lowest volume saw drain EC reach 6.6 mS/cm in March and the earliest, heaviest blossom-end rot; the highest volume (44–46% drain) cost fruit set on trusses 12–15 during the low-light period. Both extremes hurt.
EC and pH targets through the crop
Feed EC rises as the crop carries more fruit, then eases back as light and transpiration climb in spring. The University of Kentucky's 2026 high-wire fact sheet gives a stage table that matches Dutch practice:
| Stage | Target N (ppm) | Feed EC (mS/cm) | Note |
|---|---|---|---|
| Seedling and transplant | 80–150 | 0.8–2.0 | short, frequent shots |
| Vegetative growth after transplanting | 150–200 | 1.5–2.0 | raise frequency and strength gradually |
| Flowering and early harvest | 180–200 | 2.0–3.0 | keep growth balanced |
| Heavy fruit load | 200–220 | 3.0–3.5 | steer with irrigation |
For coir, run the lower half of those ranges. Grower2Grower's Stefan Vogrincic wrote in 2019: "I would not run EC levels as high in coco fibre substrates as I would in stone-wool." The Korean RDA 2017 trial compared feed EC of 1.0, 1.5, 2.0 and 3.0 mS/cm on coir: yield to the sixth truss was highest at 1.5, fruit weight fell with EC from the fourth truss onward, and blossom-end rot appeared first and most at 3.0. The 2022 trial started at 2.5 mS/cm after planting, cut to 2.1 in March and 1.6 from mid-April as light increased. Wageningen UR's Wim Voogt gives the Dutch open-system standard for tomato as feed EC 2.6 mS/cm with a root-zone target of 3.7 mS/cm, which shows the normal gap between feed and drain in a well-run crop.
pH. NC State recommends 5.5–6.0 in the slab for rockwool and 5.6–5.8 for peat bags, and notes that above 6.5 phosphorus and micronutrients become less available, with a 25% yield loss recorded at pH 7.0. Dr Mirza gives 5.8–6.2 for the feed solution and warns that calcium binds with phosphate above pH 6.4. In the Korean coir trials drain pH stayed within 5.5–6.5 as long as drain EC stayed at or below 4.0 mS/cm; once salts accumulated, drain pH fell to 4.0–4.2. A falling drain pH in coir is usually a salt problem, not an acid-dosing problem.
Measure feed at the dripper and drain at a tray at least daily; NC State suggests a spare dripper into a bucket at plant height for a clean feed sample.
Calcium and magnesium in unbuffered coir
Washed, unbuffered coir arrives with potassium and sodium on its exchange sites. When the first feed hits the slab, calcium and magnesium are pulled out of solution and potassium is released. Hort Americas quotes Dr Poole: "Most of the coir's exchange sites are tied up with sodium and potassium." His example is a feed with 50 ppm calcium leaving only 10 ppm in the leachate at first, and his advice is to start "with higher calcium and magnesium levels" for the first two to four weeks until the exchange sites have charged.
The trial data show the same mechanism. Xiong et al. fed a solution with a K:Ca ratio of 2.36 and measured an average root-zone K:Ca of 2.3 in coir against 1.6 in rockwool. The Korean RDA 2017 paper reported that early drainage from coir carried less calcium, magnesium, nitrate and phosphate than the feed at EC 1.0–1.5, while potassium, which coir contains in high concentration, was released. Both effects fade as the crop runs.
In practice:
- For the first four weeks, raise calcium and magnesium above your normal recipe and trim potassium, then return to the standard ratio. Voogt's Dutch standard tomato feed is 5.4 mmol/L Ca, 2.4 mmol/L Mg and 9.5 mmol/L K; the root-zone targets are 10.0 Ca, 4.5 Mg and 8.0 K, which shows how much calcium the root zone is meant to hold relative to the feed.
- Keep ammonium low. NC State and Yara both say no more than 10% of total N; NC State reports blossom-end rot at 20% ammonium.
- Watch sodium in the drain. Wageningen UR's substrate limit is under 1 mmol/L NaCl. If drain sodium rises steadily, increase drain rather than feed EC.
- Yara's figures for blossom-end rot prevention: calcium above 150 ppm in the substrate solution (400 ppm in rockwool), magnesium not above 150 ppm, and night root temperature of 18–20 °C to help calcium move into fruit.
- If you buy buffered coir, the exchange step has been done at the factory, but Poole notes that calcium-nitrate buffering also needs magnesium supplied, and buffered coir is not accepted in organic production.
Common problems and fixes
| Problem | What you see | Likely cause in coir | Fix |
|---|---|---|---|
| Blossom-end rot | Sunken black patch at the fruit tip on trusses set during a stress period | Water stress between shots, high root-zone EC, high ammonium, low early calcium, hot or humid days cutting transpiration to the fruit | Smaller, more frequent shots; drain 20–30%; feed EC at the lower end; extra Ca and Mg early; ammonium under 10% of N; root zone moist through daylight (Mirza; UNH Extension; Korean RDA; Pires et al.) |
| Salt build-up | Drain EC more than about 1.5 mS/cm above feed and rising; drain pH sliding towards 4–5; smaller fruit | Too little drain for the light level; coir releasing stored ions; sodium in raw water | Raise drain first, then lower feed EC; adjust feed EC pre-emptively in low-light months before spring light drives EC up (Korean RDA 2022) |
| Waterlogging, soft vegetative plants | Thick stems, large leaves, slow roots, root disease | All-pith slab, drain slits too high or missing, shots too large, no overnight dry-down | Pith-chip blend; slits low; 10–15% dry-down overnight; stop irrigating 2 hours before sunset (Kagome; Dol; Korean RDA) |
| Roots stay in the block | Plant stalls after transplanting | Block EC lower than slab EC, or block wetter than slab | Saturate slab at or slightly below block EC (Wageningen UR) |
| Uneven growth along the row | Some plants vigorous, others weak | Uneven pith/chip mixing in bags; uneven dripper output | Specify mixing in the purchase spec; pressure-compensated drippers; check output per bag (Sai Substrates; Wageningen UR) |
| Drain EC too low | Pale, "hungry" plants | Over-irrigating on dull days | Follow the light-sum drain table; one or two shots can be enough under 500 J/cm² (Dol; NC State) |
Re-using coir slabs
Coir slabs can carry a second tomato crop. Warsaw University of Life Sciences grew four cherry tomato cultivars on cocofibre and rockwool mats in their second year of use and found no significant yield difference: 17.9 kg/m² on reused cocofibre against 18.9 kg/m² on reused rockwool over a February-to-December crop at 2.7 plants/m². Fruit from the coir had slightly lower dry matter and vitamin C, and the authors noted that the organic material decomposes and settles, which worsens the air-water balance over time.
Commercial practice has partly moved the other way. Sai Substrates told HortiDaily in 2026 that Spanish cherry tomato growers switched from 27 L bags meant for several years to 16–18 L bags replaced every year, after disease outbreaks. Steam treatment may change that: SoilSteam and Proefcentrum Hoogstraten reported in 2026 that steamed, reused substrate matched new substrate for yield, quality and plant health in strawberry, with tomato and pepper trials under way.
If you reuse slabs, flush them until drain EC is close to raw-water EC, check that they still drain freely, discard any slab that carried root disease, and plan on replacement after the second crop. Spent coir still has value as a potting-mix ingredient or soil conditioner, which is what Kagome's local growers did with theirs.
What to put in the spec
- Pith-to-chip ratio matched to climate and irrigation style, and a requirement for even mixing.
- Slab or bag dimensions, volume and planting-hole layout; 1 m units of 12–20 L for three or four plants are the norm.
- EC below 0.5 mS/cm on a stated extraction method, pH 5.5–6.5, and a sodium figure.
- Whether the coir is washed only or washed and buffered, and the buffering agent.
- White outer film, pre-cut dripper holes and the position of drain slits.
CVM supplies washed, unbuffered coco peat and husk chips as grow bags, open-top slabs and compressed blocks, with blends made to the customer's ratio. If you are planning a tomato crop in coir, contact us to discuss formats and specifications.
FAQ
What EC should I run for tomatoes in coco coir?
Start around 1.5–2.0 mS/cm after transplanting and rise to 2.5–3.0 mS/cm as fruit load builds, staying at the lower end of what you would use in rockwool. In a Korean coir trial, feed at 1.5 mS/cm gave the highest early yield and 3.0 mS/cm gave smaller fruit and more blossom-end rot. Ease EC back by 0.4–0.5 mS/cm in spring as light increases.
What drain percentage is right for coco slabs?
Aim for 15–30% over the day, scaled to light: near zero to 10% on dark days under 500 J/cm², rising to 30–40% on days over 1,500 J/cm². Coir buffers ions, so judge by the drain EC trend rather than a fixed percentage. If drain EC keeps rising, add midday shots before you lower feed EC.
How do I pre-saturate a coir slab before planting?
Fill the slab with water or dilute solution until it expands fully, leave it 24 hours, flush if the drain EC is high, then cut drain slits low in the side. The day before planting, saturate with nutrient solution at about the EC you will feed the young crop, typically 1.5–2.0 mS/cm, until drainage appears. The propagation block should have an EC equal to or higher than the slab so roots grow out.
How many tomato plants per coir slab or grow bag?
Most 1 m slabs and bags of 12–20 L carry three or four plants, about 4–6 litres of coir per plant. Greenhouse density is 2.2–3 plants/m², with 2.5 plants/m² the common Dutch layout. Small volumes only work with frequent irrigation; 5 L per plant watered once a day produced more blossom-end rot in a Brazilian trial.
Why do tomatoes in unbuffered coir show calcium deficiency early?
Washed coir still holds potassium and sodium on its exchange sites. The first feeds exchange those ions for calcium and magnesium from the solution, so less calcium reaches the plant for two to four weeks. Raise calcium and magnesium in the feed during that period and keep ammonium under 10% of total nitrogen. The effect fades once the exchange sites are charged.
Why is my drain pH falling in coco coir?
In coir trials, drain pH stayed at 5.5–6.5 while drain EC was at or below 4.0 mS/cm, then dropped to around 4.0 when salts accumulated. A falling drain pH is usually a sign of too little drain and salt build-up, not of wrong acid dosing. Increase drain and reduce feed EC before touching pH control.
Can coco slabs be reused for a second tomato crop?
Yes. A two-year Warsaw trial found no significant yield difference between second-use cocofibre and second-use rockwool slabs. Flush the slabs back to a low EC, check that they still drain, and discard any that carried root disease. Some growers now prefer smaller bags replaced yearly for hygiene, and steam treatment of used substrate is being tested commercially.
Is coco coir better than rockwool for tomatoes?
Yields are at least comparable. A 2017 trial reported 84.9 t/ha on coir against 67.5 t/ha on rockwool at the same density and feed, with similar fruit quality. Coir is more forgiving of irrigation errors and easier to dispose of, but needs lower EC settings and early attention to calcium and magnesium. Blossom-end rot incidence was similar on both.
Sources
- Xiong, J., Tian, Y., Wang, J., Liu, W., Chen, Q.: Comparison of Coconut Coir, Rockwool, and Peat Cultivations for Tomato Production: Nutrient Balance, Plant Growth and Fruit Quality. Frontiers in Plant Science 8:1327 (2017)
- Blok, C., van Os, E., Kromwijk, A. (Wageningen UR Greenhouse Horticulture) with EcoConsult: Cultivation of Tomato, Technical information sheet No. 8 (undated)
- Choi, G.L., Lim, M.Y., Kim, S.H., Rho, M.Y. (Rural Development Administration, Korea): Effect of Difference in Irrigation Amount on Growth and Yield of Tomato Plant in Long-term Cultivation of Hydroponics. Journal of Bio-Environment Control 31(4) (2022)
- Protected Horticulture Research Institute, RDA, and Kongju National University: Effect of EC Level of Irrigation Solution on Tomato Growth and Inorganic Ions of Root Zone in Soilless Culture of Tomato Plant Using Coir Substrate. Protected Horticulture and Plant Factory 26(4) (2017)
- Sharma, A., Leonberger, K., Gauthier, N., Ying, Q.: High-wire Tomato Production in the Greenhouse, CCD-SP-21, University of Kentucky Center for Crop Diversification (2026)
- Peet, M.M. (North Carolina State University): Irrigation and Fertilization, greenhouse tomato presentation (undated)
- Dol, G. (Glasshouse Consultancy): Irrigation de-mystified: follow the rules of thumb and the plant performs as required. HortiDaily (2020)
- Mirza, M.: Blossom end rot: is it a rot of blossoms or something more? Greenhouse Canada (2021)
- Kuack, D., quoting Poole, H.: Buffering coir not necessary if it's processed properly. Hort Americas blog (2015)
- Voogt, W., van Winkel, A., Bustamante, M., Xiong, J., Eveleens, B.: Maximizing Nitrogen Use Efficiency in Long Term High Yielding Soilless Grown Tomato, Wageningen University & Research presentation (2019)
- Abukhovich, A., Kobryń, J. (Warsaw University of Life Sciences): Yield and changes in the fruit quality of cherry tomato grown on the cocofibre and rockwool slabs used for the second time. Acta Scientiarum Polonorum, Hortorum Cultus 9(4) (2010)
- Pires, R.C.M., Furlani, P.R., Ribeiro, R.V., et al.: Irrigation frequency and substrate volume effects in the growth and yield of tomato plants under greenhouse conditions. Scientia Agricola 68(4) (2011)
- Vinković, T., Iljkić, D., Paradjiković, N.: Tomato cultivation on coconut husks substrate. Acta Agriculturae Serbica 12(23) (2007)
- Vogrincic, S.: Controlling the EC levels for young tomato plants growing in Coco Fibre. Grower2Grower (2019)
- University of New Hampshire Cooperative Extension: Growing Vegetables: Managing Blossom End-Rot (undated)
- Yara: Preventing Blossom End Rot in Tomato, crop nutrition guidance (undated)
- HortiDaily: Japan: Kagome optimizes coco mix to achieve balanced growth (2013)
- HortiDaily: The right coir recipe is now part of the cherry tomato root-zone equation (Sai Substrates) (2026)
- HortiDaily: New substrate steam machine in development (SoilSteam, Proefcentrum Hoogstraten) (2026)
- Bunch, R.J.: Lambert E.B. Blok grow-bag trials, Rivière-Ouelle and CIDES, Quebec (2002–2004)
- Canna: Coco Slabs product instructions (undated)