The short answer: yes, coco coir makes good soil blocks. Swap it straight in for the peat in a standard blocking recipe, cut the lime right back, sieve out long fibre, and make the mix wetter than you think. Most coir blocks that crumble are too dry or too coarse, not the wrong recipe. Coir blocks grow seedlings about as well as peat blocks, and they rewet more easily if you forget to water.
Key facts
- The classic blocking mix is 3 parts peat, 2 parts coarse sand or perlite, 2 parts mature compost and 1 part garden soil, plus lime and a base fertiliser. Coir replaces the peat part by part.
- The mix needs about 1 part warm water to 3 parts mix. It should feel like soft putty or wet cement, with a little water squeezing out of the blocker as you press.
- Blockers come in sizes from about 1.5–2 cm (¾ in) mini blocks to 5 cm (2 in) standard blocks and 10 cm (4 in) blocks for potting on.
- Fast crops like lettuce leave the 5 cm block at about four weeks. Peppers and other slow crops go into larger blocks or pots after six weeks or more.
- Bottom-water blocks and let the top millimetre dry before the next watering.
What a blocking mix has to do
A soil block is a cube of growing medium pressed hard enough to hold its shape with no pot around it. Roots grow to the edge, meet air and stop, so the seedling builds a dense root ball instead of circling a cell. At transplant you set the whole block in the ground with almost no root disturbance.
For that to work the mix needs three things a normal seed compost doesn't:
- Fibre to bind it. Something has to hold the block together once it's pressed. In the original recipe that's peat. Coir pith does the same job.
- Fine texture. Lumps, stones and long fibres leave voids, and blocks break along them.
- Enough water to make it plastic. Dry mix pressed into a blocker comes out as a pile of crumbs.
Compost and soil add nutrients, weight and some clay-like cohesion. Sand or perlite keeps the block from going solid and airless.
Coir blocking recipes
All quantities are by volume. Use a bucket as your "part".
| Recipe | Coir | Sand or perlite | Compost | Garden soil | Other | Best for |
|---|---|---|---|---|---|---|
| Classic, coir in place of peat | 3 | 2 | 2 | 1 | Base fertiliser, a little lime only if compost is acidic | General vegetable and flower transplants |
| Compost-rich, no soil | 2 | 1 | 2 | 0 | Organic seedling fertiliser | Growers who don't want to bring garden soil in |
| Peat-free market garden style | 2 | 2 | 3 | 1 | None, compost carries the feed | Fertile compost, short crops |
| Mini block mix | 3 | 1 fine perlite or fine sand | 1 sieved | 0 | None | ¾ in blocks for germination only |
The original recipe uses about half a cup of lime per two bushels of mix, there to neutralise acidic peat. Washed coir is close to neutral, so in a coir mix you can usually leave the lime out or cut it to a pinch. If you do add lime and grow under organic rules, use ground agricultural limestone, not hydrated lime.
The base fertiliser in the classic recipe is equal parts blood meal, colloidal phosphate and greensand, at about 3 cups per two bushels. In a coir mix the fertiliser matters more than in a peat mix, because coir has almost no nitrogen or phosphorus. If your compost is weak, keep the base fertiliser in.
Preparing coir for blocking
Blocking wants a finer coir than a pot does.
- Hydrate fully. A dry core in a block of coir becomes a weak spot in the block. Our article on rewetting dry coco coir covers the method.
- Sieve it. Push hydrated coir through a 6 mm screen. For mini blocks, use a 3 to 4 mm screen. Long fibre wraps around the blocker's pins and leaves tails sticking out of the blocks.
- Check the salt. Germinating seed is sensitive. Washed, low-EC coir needs no further treatment. For the testing side, see our seed germination guide.
A 650 g briquette is a convenient size for home blocking batches. It hydrates in a bucket and gives enough coir for several trays. Our briquette propagation guide has the volumes.
Mixing and wetting
- Mix the dry or damp ingredients thoroughly. Lime, if you use it, goes in with the coir first, then sand or perlite, then fertiliser, then soil and compost.
- Add warm water, roughly 1 part to 3 parts mix, and work it in by hand.
- Keep adding water until a squeezed handful doesn't quite drip, or drips slightly. Blocking mix is far wetter than potting mix.
- Let it stand for 15 to 30 minutes. Coir and compost keep absorbing water, so the mix often firms up and needs a splash more.
Making the blocks
- Pile the wet mix into a mound or tray at least 3 to 4 cm (1¼ to 1½ in) deeper than the blocker.
- Push the blocker straight down until it hits the bottom. Press two or three times to pack it.
- Twist a quarter turn and lift. Scrape the base flat against the edge of the tray.
- Set the blocker in your tray and press the plunger as you lift. The blocks should stand with crisp edges.
- Dip the blocker in a bucket of water between pressings. Coir sticks less to a wet blocker.
Leave a finger's width between rows if you want air pruning on all sides. Blocks packed tight together still work but roots will cross between them.
Why coir blocks crumble, and the fixes
| Symptom | Likely cause | Fix |
|---|---|---|
| Blocks break apart as you release them | Mix too dry | Add water until it's like wet cement and let it stand |
| Blocks slump and spread | Mix too wet or too little fibre | Add a little more sieved coir, press firmer |
| Cracks along the side | Lumps, long fibre or large perlite | Sieve the coir and compost, use fine perlite or sand |
| Blocks break when moved a day later | Not enough fine material or soil to bind | Add soil or more fine compost, press three times |
| Corners crumble as blocks dry | Blocks drying out | Bottom-water sooner, cover until germination |
| Mini blocks fall out of the blocker | Mix too coarse for the small cells | Use a finer screen and a mini-block recipe |
Coir on its own binds a little less than peat because its particles are less felted. A share of garden soil or a fine, mature compost makes up for it. If you've dropped soil from the recipe and blocks fall apart, putting a part of soil back in is the quickest fix.
Watering and growing on
Bottom-water. Pour water into the tray and let the blocks soak it up for a few minutes, then tip off the rest. Overhead watering erodes the edges and washes seed out.
Let the top millimetre of the blocks dry before you water again. Coir blocks rewet well even when the surface has gone pale, which makes them more forgiving than peat blocks if you miss a day. Small blocks dry out much faster than large ones, so check mini blocks twice a day in warm weather.
Sow one or two seeds per block. Cover them lightly or not at all, depending on the crop, and keep a clear cover over the tray until they germinate.
When to move them on:
- Mini blocks: as soon as the seedling is up and roots reach the edge, set the mini block into the matching hole of a 5 cm block.
- 5 cm blocks: lettuce, brassicas and quick flowers like zinnia are ready to harden off and plant at around four weeks. Don't hold them much longer, because the block runs out of food and room.
- Slow crops: peppers, aubergines and tomatoes held six weeks or more should go into 10 cm blocks or pots.
Feed blocks with a dilute liquid feed from the first true leaf if they'll be in the block more than three weeks, or if your compost is weak. Coir-heavy blocks go pale sooner than soil-rich ones.
Coir or peat for blocks?
Growers who have run both side by side report no real difference in seedling growth. Coir rewets more easily and needs no lime. Peat binds slightly better and holds a touch more water. If you're moving away from peat, coir is the straightforward swap, and the only adjustments are sieving, extra water and less lime.
Where the coir comes from
CVM Fiber and Industries makes coco peat in Kanyakumari, Tamil Nadu, India, washed with water to reduce EC and supplied unbuffered, with no calcium nitrate or other chemicals. We supply 5 kg blocks, 650 g briquettes and custom blends. If you're mixing blocking compost at farm scale, contact us.
FAQ
Can I use coco coir instead of peat in soil blocks?
Yes. Replace the peat part for part. Sieve the coir to remove long fibre, add more water than a peat mix needs, and reduce or leave out the lime, because coir is not acidic like peat.
Why do my coir soil blocks fall apart?
Almost always because the mix is too dry. It should be like wet cement, with a little water coming out of the blocker. If wetter mix still crumbles, sieve out lumps and fibre and add a part of garden soil or fine compost to help it bind.
Do I need lime in a coir blocking mix?
Usually not. Lime is in the classic recipe to neutralise acidic peat. Washed coir sits close to neutral, so lime can push the pH too high. Add a pinch only if your compost or soil is acidic, and use agricultural limestone.
Can I make soil blocks from 100% coco coir?
You can press pure, fine, well-wetted coir into blocks, but they bind weakly and carry no nutrients. They are fine for germinating in mini blocks. For larger blocks add compost and some soil so the block holds together and feeds the seedling.
How wet should soil block mix be?
Much wetter than potting mix. A squeezed handful should nearly drip or drip slightly. Start with about 1 part warm water to 3 parts mix, let it stand 15 to 30 minutes and adjust, since coir keeps absorbing water.
How do you water soil blocks made with coir?
From below. Pour water into the tray, let the blocks absorb it for a few minutes, then pour off the excess. Let the very top of the blocks dry before watering again, and check small blocks often because they dry out fast.
How long can seedlings stay in a 2 inch soil block?
About four weeks for quick crops like lettuce, brassicas and zinnias. Slow crops such as peppers and tomatoes kept longer should move into a 4 inch block or pot. Feed lightly if they stay more than three weeks.