The short answer: a 70/30 coco coir blend is 70% coir pith and 30% husk chips by volume, and it's the most common stock mix for grow bags and slabs because it holds plenty of water but drains and re-aerates faster than pure pith. Go towards 50/50 or 30/70 when you want a drier, faster-cycling root zone (hot climates, high-frequency drip, blueberries), and towards 80/20 or 100% pith when irrigation is infrequent or the plants are small. One warning before anything else: "70/30" is written both ways. Some makers mean 70% pith, others mean 70% chips. Always write the ratio as pith:chip on the order.
Key facts
- Sieved coir pith on its own has an air content of about 20% at saturation. Adding chips raises that by 10 to 40 percentage points depending on how many you add and how big they are.
- A 70:30 pith:chip blend measured 40% air-filled porosity and held 0.28 cm³ of water per cm³ at 50 cm suction. An 80:20 pith:short-fibre blend held 0.66.
- Commercial coir bags span roughly 20 to 45% air and 50 to 70% water by volume at full saturation. The pith:chip ratio is the main lever that moves a bag along that range.
- Blueberry substrates are usually built for more than 30% air-filled porosity. Nursery mixes target 10 to 20%.
- Chips are sold in bands such as 3–6, 6–12 and 12–18 mm, or as crush screened 0–7 or 0–15 mm. The same ratio with a different chip grade behaves differently.
What the ratio actually changes
Coir pith is the fine, spongy material between the fibres of the husk. Husk chips are cut pieces of the husk with fibre and pith still bonded. Pith packs into small pores and holds water. Chips prop the mix open and create large pores that drain and fill with air. Short fibre does a milder version of what chips do and is always blended, never used alone.
Swapping pith for chips does five things at once:
| Property | More pith | More chips |
|---|---|---|
| Water holding | Higher. Pith on its own holds the most | Lower. The 70:30 blend had the lowest water retention of four blends tested |
| Air-filled porosity | Lower. Around 20% for sieved pith | Higher. 40% for 70:30, 44% for a 100% chip bag |
| Drainage speed and nutrient breakthrough | Slow. The fibre-pith blend didn't show nutrient breakthrough until 0.3 to 0.4 pore volumes had passed | Fast. The 70:30 chip blend broke through at 0.1 pore volumes |
| Dry-back | Slow. Stays wet longer, fewer or smaller irrigations | Fast. Needs more daytime irrigation but dries down quickly for the night |
| Life and shrinkage | Pith is the fraction that decomposes first and gets finer and wetter with time | Chips keep their structure longer and hold the pore network open |
The numbers above are for the blends they were measured on. Yours will differ with chip size, how fine the pith is and how hard the block was pressed. For the measurement methods behind these figures, see water holding and air porosity in coco coir.
Two things people get wrong. First, chips still hold water. Each chip is a small sponge, so a 50/50 bag is nothing like a bag of bark or perlite. Second, the ratio isn't the only variable. A finer pith or a load of dust under 1 mm will make a 70/30 behave like an 80/20, which is why good suppliers sieve out material below about 0.2 mm.
The common blends and where they fit
Ratios below are pith:chip by volume, which is how the industry quotes them.
| Blend | Character | Typical use |
|---|---|---|
| 100:0 | Wettest. About 20% air when the pith is sieved to around 6 mm. Slow dry-back | Propagation plugs and cubes, small pots, strawberries under low-frequency irrigation, cool climates |
| 80:20 | Slightly more open, still pith-dominated. 20 to 30% chips is the usual advice for "opening up" a mix | Potted plants, first step up from pure pith, growers switching from peat |
| 70:30 | The industry default. Around 40% air in the blend tested, fast re-aeration, earliest nutrient breakthrough | Tomato, cucumber and pepper slabs, strawberry bags, blueberry bags, potted ornamentals |
| 60:40 | A step drier. One commercial slab at this ratio lists about 25% air at saturation, which shows how much chip grade and pressing matter | Long-season fruiting crops, bags that will run two or three crops |
| 50:50 | Very open, dries fast. One maker's "most common type", recommended for roses and strawberries and for hotter climates | High-frequency drip, hot climates, roses, strawberries where root oxygen is the worry |
| 30:70 or 100% chips | Highest air (44% for a pure chip bag), lowest buffer. Needs tight irrigation management | Moderate climates with heavy, consistent irrigation, blueberries in pots, growers who steer hard |
Fibre additions sit alongside this. A few percent of short fibre stops a slab slumping and spreads water sideways through the bag. One gerbera disc is built around the maximum fibre content for structural stability. Fibre never replaces chips as the aeration fraction, though.
For bag formats, sizes and film, see coco coir grow bags explained.
Crop by crop
Published recommendations are supplier practice, not standards, and they disagree. The table gives the range you'll actually see quoted, plus the reason behind it.
| Crop or system | Range quoted (pith:chip) | Why |
|---|---|---|
| Tomato, cucumber, pepper slabs | 70:30 to 50:50 | Long crops under drip. Growers steer generatively by drying down overnight, which needs air. Northern European tomato growers run bags at roughly 30 to 33% air and 48 to 51% water |
| Strawberry (table-top bags, troughs) | 100:0 to 70:30, 50:50 in hot sites | Shallow root system, sensitive to low root oxygen but also to drying out. Pith-heavy where irrigation is infrequent, chip-heavy under frequent pulses |
| Raspberry and other cane fruit | Chip-containing blends, usually 70:30 or more open | Long-cane plants in pots dislike a wet base. Few published ratios, so trial before committing |
| Blueberry in pots | 70:30 to 30:70, built for more than 30% air | Fine roots that rot in saturated media. Some bags are layered with fine material on top and coarse chips below. Pots of 5 to 6 gallons (19 to 23 litres) with plenty of drainage are now preferred over larger ones |
| Roses, gerbera | 50:50 for roses, fibre-rich for gerbera | Multi-year crops that need structure to last. Roses are a named use for 50:50 bags |
| Propagation | 100:0, fine sieved pith | Small plugs need water close to the seed or cutting and dry out fast anyway |
| Hot climates, high-frequency drip | 50:50 or more chip | Water is applied often, so the bag doesn't need to store it. Air and fast drainage matter more |
| Northern Europe, low light, infrequent irrigation | 70:30 to 100:0 | Fewer, larger irrigations. The bag has to carry the plant through long gaps and short winter days |
The pattern is simpler than the table looks. The right ratio depends more on your irrigation than on your crop. Drippers every twenty minutes keep a pith-heavy bag saturated. Two irrigations a day leave a chip-heavy bag dry by mid-afternoon. Pick the blend for the dry-back you want, then set irrigation to match.
Chip size matters as much as the ratio
"30% chips" tells you very little until you know how big the chips are. Three grades turn up in slab and bag production:
- Crush 0–7 mm. Fine chips with some pith attached. Blends evenly and gives a modest air increase. Usual for strawberry bags and finer slabs.
- Crush 0–15 mm or chips 6–12 mm. The standard grade for vegetable slabs. Noticeably more air than 0–7 mm at the same ratio.
- Chips 12–18 mm and up. Blueberry pots, orchid mixes and the coarse bottom layer of layered bags. At 30% these give a very open, fast-draining mix.
Two suppliers' "70/30" can therefore behave quite differently if one uses 0–7 mm crush and the other 12–18 mm chips. Size bands are covered in the coconut husk chips guide.
One more wrinkle: the ratio is by volume of the loose, expanded fractions, not by weight of the compressed block. A 5 kg pith block expands to roughly 70 to 75 litres while a chip block of similar size gives 50 to 60 litres. If a factory blends by weight, the volume ratio you receive won't be the one on the label.
Dry-back, re-wetting and slab life
Dry-back. A higher chip share means the bag loses water faster after the last irrigation of the day. That's what growers want for generative steering at night, and it's why chip-heavy bags need more irrigation by day but are easier to steer. A pith-heavy bag stays wet, which suits a vegetative push or a grower who can't irrigate often.
Re-wetting. Pith that dries hard can be slow to take water again. An open blend re-wets more evenly after a deliberate dry-down, because water moves through the large pores and each chip soaks up from the outside in.
Life. Pith is the fraction that breaks down. Young, cellulose-rich coir gets finer and wetter during a crop and its air content falls. Chips hold their shape longer, so a 50/50 bag loses less air over two or three crops than a 100% pith bag. Shrinkage above about 5% is the usual warning sign. See how long coco coir lasts.
How to write the blend into a purchase order
A ratio alone is not a specification. Put these lines on the order so the factory and your QC team are reading the same thing:
- Fractions and ratio, by volume. "70% coir pith : 30% husk chips : 0% fibre, by expanded volume." State the order of the numbers.
- Grade of each fraction. Pith sieve size (for example under 6 mm), chip band (6–12 mm) or crush screen (0–15 mm), fibre cut length if any.
- Fines limit. Maximum share below 1 mm, and whether material below about 0.2 mm has been removed.
- Compressed and hydrated dimensions, and expansion volume per bag or block.
- Moisture at pressing, since it affects how the block expands.
- Washing and buffering status, and the EC test method (1:1.5 volume, 1:5 or saturated extract).
- A retained counter-sample from each batch, kept by the supplier.
CVM Fiber and Industries in Kanyakumari, Tamil Nadu, India, produces custom pith, chip and fibre blends to a written specification like this, in blocks, grow bags and open-top slabs. The coco peat is washed with water to reduce EC and is unbuffered, so if you want a buffered product you'll be doing the calcium treatment yourself.
How to check a delivered blend by sieving
You can verify the ratio and the fines content with a stack of sieves and a kitchen-quality balance. The method below follows the European growing-media standard for particle size.
- Take a representative sample. Pull from at least three bags or blocks across the pallet, expand with water, mix, and split down to three sub-samples.
- Air-dry. Spread each sub-sample on a tray and dry at 40 °C for at least 16 hours. Material should be below 15% moisture or it won't pass the sieves. Don't oven-dry hotter, because it changes the material.
- Measure the volume. For a 200 mm sieve stack use 375 ml of dried sample (125 ml if more than half of it is under 8 mm). For a 300 mm stack, 750 ml or 250 ml.
- Stack the sieves largest aperture on top: 31.5, 16, 8, 4, 2 and 1 mm, with a pan underneath. Shake for 7 minutes on a machine, or a few minutes by hand until nothing more falls through.
- Weigh each fraction and express it as a percentage of the total dry mass, rounded to whole numbers. Average the three sub-samples.
- Read the result. The material retained on the 4 mm and coarser sieves is, for practical purposes, your chip fraction. What passes 1 mm is dust. Compare against the order, remembering that sieve results are by mass while the order is by volume. Chips are denser per litre than fluffy pith, so a 30% volume share of chips will typically weigh more than 30% of the sample.
For a rough field check, expand a handful from a bag, pass it through a 4 mm garden sieve and compare the two piles by volume in identical buckets. It won't give you a number for the file, but it will show straight away if a 70/30 has arrived as a 90/10.
Which blend should you start with?
With no trial data, start at 70:30 pith:chip with 0–15 mm crush for a fruiting vegetable slab, 100% pith or 80:20 for propagation and small pots, and 50:50 or more open for blueberries, roses and hot climates under frequent drip. Run one row a step up and one a step down, log water content and dry-back, and you'll know within a crop which way to move.
Tell us the crop, the container and the irrigation regime and we'll quote a blend to match. Contact CVM to get started.
FAQ
Does 70/30 mean 70% pith or 70% chips?
It depends on the maker. Most suppliers mean 70% pith and 30% chips, but at least one major brand sells a "70/30" bag that is 70% husk chips and 30% pith. Always write the ratio as pith:chip on your order and ask the supplier to confirm which fraction comes first.
Is the ratio by weight or by volume?
By volume of the loose, expanded fractions. A 5 kg pith block expands to around 70 to 75 litres, while a similar chip block gives about 50 to 60 litres, so the two fractions have different densities. A blend mixed by weight will not give you the volume ratio on the label.
What is the best coco coir blend for tomatoes?
Most tomato slabs are 70:30 or 50:50 pith:chip. Growers who steer generatively by drying the slab overnight go for the more open end, and commercial tomato bags typically sit around 30 to 33% air and 48 to 51% water at saturation. If you irrigate infrequently, stay closer to 70:30.
Which blend do blueberries need?
Blueberry substrates are built for more than 30% air-filled porosity, so chip-heavy blends from 70:30 up to 30:70 are used. Many growers layer finer material on top and coarse chips at the bottom of the pot. Pots of 19 to 23 litres with plenty of drainage have replaced the larger pots once used.
Can I add chips to a block of pure pith myself?
Yes. Expand the pith block and a chip block separately, measure the volumes, and mix to the ratio you want. Twenty to thirty percent chips by volume is the usual advice for opening up a pith-based mix. Mix thoroughly, because uneven blending gives wet and dry pockets in the same bag.
Does a chip-heavy blend need different irrigation?
Yes. It holds less water and lets nutrients pass through sooner, so it needs more frequent, smaller irrigations and dries down faster at night. A pith-heavy blend needs fewer, larger irrigations and longer pauses so the root zone can re-aerate. Never carry an irrigation schedule across from one blend to another without checking.
Do chips make the slab last longer?
Usually. Pith is the fraction that decomposes first and gets finer and wetter over time, which lowers the air content. Chips hold their structure longer, so a blend with more chips keeps its pore space better over two or three crops. Shrinkage above about 5% is a sign the structure is going.
How do I check a delivered blend without a lab?
Expand a sample, air-dry it, and pass it through a stack of sieves from 16 mm down to 1 mm. The material on the 4 mm and coarser sieves is roughly your chip fraction and what passes 1 mm is dust. For a quick field test, a 4 mm garden sieve and two identical buckets will show whether the volume split is anywhere near the ratio you ordered.