The short answer: coco peat works in paddy mat nurseries as part of the mix, not as the whole of it. Blend roughly one part washed coco peat with two to four parts sieved soil, fill trays 2 cm deep, and add nitrogen, because coir carries almost none. You get a lighter tray that holds water well, and in field trials grain yield from coir-based mats matched soil mats.
Key facts
- Standard transplanter trays measure 58 × 28 cm for self-propelled machines and 45 × 21 cm for engine-operated ones. The medium layer is 1.5–2.5 cm.
- A soil-filled seedling box weighs about 6 kg. Guides call for about 200 boxes per hectare in Japan and 100–150 trays per acre in South Asia.
- Soil plus raw coir pith at 2:1 and a rice-husk-charcoal mix with 20 % coir pith compost were both picked as cost-effective, light media. Grain yield did not differ from soil-based mats.
- Seed rate decides how well the mat holds together. Going from 40 to 100 g of seed per tray more than doubled root entwining force and cut missing hills from about 18 % to 5 %.
- Seedlings go to the transplanter at anything from 8 to 30 days, mostly 12–25 days, and 12–20 cm tall, depending on season and variety.
Why a mat nursery needs a different medium
A transplanter doesn't pull individual seedlings. Its fingers slice small blocks off a continuous mat of roots and medium and push them into puddled soil. So the mat has to do three things at once. It has to be thin and even so the fingers pick up the same number of seedlings every time. It has to hold together when you roll it or slide it out of the tray. And it has to be light enough to carry from the nursery to the field.
Soil alone does the first two jobs well, but it's heavy and often poor in nutrients. A soil-only bed gave the weakest seedlings in a comparison of bedding mixes, with a seedling vigour index of 1,901 against 2,180–2,480 when 30 % of the soil was replaced with an organic material. That's a 15–30 % gain in vigour from changing the mix alone.
The catch is that organic material loosens the mat. In trials with six organic additives, rolling quality fell as the organic share went up, because the medium bonds less tightly than clay. Clay loam soils gave better mats than sandy loams. Mixes with 10–30 % organic material were the workable range. This is the main reason pure coco peat isn't the default for transplanter trays, and why the ratio matters more here than in a vegetable seed germination tray.
Soil, soil-coco peat and coco peat mixes compared
| Medium | Typical ratio (by volume unless noted) | Mat strength | Weight | Nutrients | Notes |
|---|---|---|---|---|---|
| Sieved field soil | 100 % | Good on clay loam, weaker on sandy loam | Heaviest | Often low | Weakest seedling vigour in comparisons |
| Soil + compost or manure | 4:1 or 70:30 | Good up to about 30 % organic | Heavy | Better | Standard extension recommendation |
| Soil + raw coir pith | 2:1 | Workable | Lighter than soil | Low, needs N | Chosen as cost-effective for dry-bed mats |
| Soil + coir pith compost | 2:1 | Workable | Lighter than soil | Moderate | Shorter seedlings than soil + cow dung |
| Rice husk charcoal + coir pith compost | 80:20 | Among the strongest mats tested | Among the lightest | Moderate | Fewer missing hills, yield equal to soil mixes |
| Coir alone with nutrient solution | 100 % | Not tested for rolling | Light | Only what you add | Seedlings fell behind soil-grown ones after about 19 days |
A few points stand out. In a mat nursery trial with sixteen media, grain yield averaged 4,344–4,441 kg/ha and didn't change with the medium. So the choice comes down to cost, weight and how well the mat handles. Soil plus raw coir pith at 2:1 under a dry-bed system was one of the cheapest options that worked.
In a later trial with eighteen tray media, the 80 % rice husk charcoal plus 20 % coir pith compost mix gave light, strong mats with few missing hills. Field yield didn't differ significantly among the ten best media.
Coir on its own, fed with a nutrient solution, kept pace with soil-grown seedlings for about 11 days. After 19 days the coir seedlings had less shoot and root weight, fewer branch roots and less chlorophyll. If you push coco peat above about a third of the mix, plan on feeding, and don't hold seedlings long.
Seed rate per tray and root binding
Seed rate is what really ties the mat together. More seed means more roots per square centimetre and a stronger mat, but crowded seedlings are thinner and more get damaged in the machine.
| Seed per 58 × 28 cm tray | Root entwining force | Missing hills | Damaged seedlings | Grain yield |
|---|---|---|---|---|
| 40 g | 9.1–12.3 kg | 17.8–19.4 % | 2.8–4.4 % | 10.6–10.8 t/ha |
| 60 g | 12.3–17.7 kg | 12.8–13.9 % | 3.9–5.6 % | 10.8 t/ha |
| 80 g | 17.4–21.6 kg | 6.7–7.2 % | 4.4–8.9 % | 10.2–10.5 t/ha |
| 100 g | 26.2–30.2 kg | 4.4–5.0 % | 12.2–12.8 % | 9.4–9.8 t/ha |
Those figures are for a hybrid rice over two seasons, where 60 g per tray gave the best return. Recommendations for inbred varieties run higher. General extension guidance often runs to 100–150 g per tray. Field trials on growing media have used 140 g of pre-germinated seed per tray.
With a coco peat mix, lean toward the middle of your local range rather than the bottom. A lighter, looser medium relies more on roots to hold it, and a thin sowing leaves you with a mat that tears.
Filling the tray and mat thickness
- Sieve everything. Trial media were air-dried and passed through a 2 mm sieve. Stones and fibre clumps jam the transplanter fingers. For tray work, pick a fine particle grade of coco peat with little long fibre.
- Hydrate and mix. Break up and wet the coco peat first, as described in the coco peat blocks guide, then blend it evenly with the sieved soil. Dry pockets of coir make patchy mats.
- Fill to 1.5–2.5 cm. Most guides settle on 2 cm. Thicker layers add weight without a better mat.
- Sow evenly and cover lightly. Spread pre-germinated seed (soaked about 24 hours, then incubated about 24 hours) and cover with 3–4 mm of the same mix.
- Water gently. Sprinkle until the medium is saturated without washing seed into heaps.
Watering and seedling age at transplanting
For the first 3–4 days, cover the trays with leaves or sheeting against sun and heavy rain, and moisten them at least twice a day. Take the cover off once seedlings emerge. Be careful with the first two or three irrigations, as a hard stream breaks a young mat. Coco peat helps here because it holds water between irrigations, but don't keep the trays waterlogged. Over-wet nursery soil encourages seedling disease.
Drain the water 12–18 hours before transplanting so the mat firms up.
Guides disagree on age, mostly because of season and variety:
| Source type | Seedling age | Height | Leaves |
|---|---|---|---|
| Tropical extension guide | 8–16 days, best at 12 | 15–20 cm | – |
| Northeast India factsheet | 14–18 days | 18–20 cm | 2–3 |
| Mechanisation research | 15–25 days | 12–15 cm | about 3 |
| Northwest India (basmati) | 25–30 days | – | – |
Younger mats grown in a richer mix can reach transplanting height quickly. Soil plus a 30 % organic amendment gave 14.6–14.9 cm shoots at 16 days, against 11.7 cm in soil alone.
Tray weight and soil hauling
This is where coco peat earns its place. A soil-filled seedling box weighs about 6 kg, and 10 hectares of paddy in Japan need about 2,000 of them. Extension guides put the count at 100–150 trays per acre in South Asia, which at that weight is 600–900 kg of filled trays to lift on and off a trailer for every acre.
A 58 × 28 cm tray filled 2 cm deep holds about 3.2 litres of medium. Dry coir dust has a bulk density of about 0.04–0.08 g/cm³, so the coir in a full tray weighs only 130–260 g before wetting. Coir holds a lot of water, so a wet tray won't fall to a fraction of a soil tray's weight. Still, every litre of soil you replace with coco peat is mineral weight you don't carry, and topsoil you don't have to dig, sieve and haul to the nursery.
A specification for the coco peat
| Property | What to ask for | Why |
|---|---|---|
| Washing | Washed with fresh water to lower EC | Seedlings at 2–3 leaves are salt-sensitive |
| Particle size | Fine, passes a 2 mm sieve, low long-fibre content | Even depth, clean cuts by transplanter fingers |
| pH | Expect roughly 5.6–6.9 in coir | Rice box soils are often adjusted to pH 4.5–5.0 against seedling disease, so test your final mix |
| Contaminants | Free of sand, stones and husk pieces | Stones damage fingers and leave gaps |
| Nutrients | Assume almost none | Add compost, vermicompost or a starter N feed |
| Share of the mix | 20–33 % for most soils | Higher shares weaken the mat unless seed rate and feeding go up |
CVM Fiber and Industries makes coco peat in Kanyakumari, Tamil Nadu, India, from husk sourced across several districts. It's washed with water to reduce EC, with no calcium nitrate or other chemicals, so it's unbuffered. For tray work, 5 kg coco peat blocks are the practical format to break up and blend with soil at the nursery.
If you're setting up mat nurseries for a transplanter service or a seedling business and want to talk through grades or blends, get in touch.
FAQ
Can I use 100 % coco peat for paddy mat nursery trays?
You can grow rice seedlings in pure coir, but they need a strong nutrient solution and still fall behind soil-grown seedlings after about two and a half weeks. Pure coir also gives a looser mat. Most growers get better results blending it with soil at about 1:2 to 1:4.
What is the best soil to coco peat ratio for rice tray nursery?
Soil plus raw coir pith at 2:1 has been used successfully and was one of the cheapest workable options. Keeping the organic share at 10–30 % protects mat strength, so 1 part coco peat to 2–4 parts sieved soil is a sensible range. Test a few trays on your own soil first.
How much seed should I sow per tray for a mechanical transplanter?
For a 58 × 28 cm tray, guidance runs from about 60 g for hybrid rice to 90–150 g for inbred varieties. Higher rates make a stronger mat and fewer missing hills but more damaged seedlings. With lighter coco peat mixes, stay in the middle of your local range.
How thick should the medium be in a mat nursery tray?
Guides recommend 1.5–2.5 cm, and 2 cm is the most common choice. That's enough for roots to knit into a mat that rolls and feeds evenly. Thicker layers just add weight.
At what age should mat seedlings go to the transplanter?
Most guides use 12–25 days, at about 12–20 cm tall with two or three leaves. Tropical guides lean toward 12–16 days, and cooler or basmati regions sometimes wait 25–30 days. Height and mat strength matter more than the day count.
Does coco peat in the mat change rice yield?
In field trials comparing many mat media, including soil plus coir pith and rice husk charcoal plus coir pith compost, grain yield did not differ significantly from soil-based mats. The gains are in tray weight, handling and cost rather than yield.
Do I need to buffer coco peat for rice nurseries?
Washed, low-EC coco peat is usually enough for a mix that is mostly soil, because the soil supplies calcium and magnesium. Check the EC and pH of your final blend. If you use a high share of coir, feed the seedlings from the second leaf.
Why are my coco peat mats breaking when I roll them?
The usual causes are too much coco peat in the mix, too little seed per tray, a sandy soil, or rolling the mat while it's still wet. Cut the coir share, raise the seed rate a little, and drain the trays 12–18 hours before lifting.