A nutrient solution for coco coir looks like a normal hydroponic feed with two changes: more calcium and magnesium, and less potassium, at least for the first few weeks. That's because fresh coir arrives with its exchange sites loaded with potassium and sodium, and it swaps them for the calcium and magnesium in your feed. Once the coir is saturated, you can drift back towards a standard recipe. The tables below give you a starting point by crop stage, then water quality, drain targets and deficiency symptoms.
This article is about the feed itself. Day-to-day pH and EC control during the crop is covered in our guide to coco coir pH and EC management, and the raw-material side (what buffering does and whether you need it) is in buffered vs unbuffered coco peat.
Why coir needs a different feed
Coir carries a negative charge, so it holds positively charged ions. Published figures for its cation exchange capacity vary a lot, from 10 to 30 meq per 100 g in some sources to 40 to 100 meq per 100 g in others, but every source agrees on the direction of the effect. Straight from the husk, those sites are mostly occupied by potassium and sodium, with very little calcium or magnesium. One lab comparison found coir carrying around 1,560 mg/kg of potassium against 178 mg/kg in rockwool, with a quarter of rockwool's calcium and magnesium.
When you irrigate with a feed containing calcium and magnesium, the coir prefers these divalent ions and binds them, kicking potassium and sodium back into the root zone. The plant sees less calcium and magnesium than you mixed, and more potassium. The effect is strongest in the first two to four weeks. In a tomato trial on coir, the drain from plants fed at EC 1.0 and 1.5 mS/cm contained less nitrate-nitrogen, phosphorus, calcium and magnesium than the feed going in during the early stage. At EC 2.0 and above, the pattern reversed and most ions concentrated in the drain. In another trial, the potassium-to-calcium ratio in the coir root zone sat around 2.3 compared with 1.6 in rockwool fed the same solution.
Buffered coir has had this done in advance with a calcium nitrate soak. Washed, unbuffered coir hasn't, so your feed does the job. CVM Fiber and Industries, in Kanyakumari, Tamil Nadu, India, washes its coco peat with water to bring EC down and doesn't add calcium nitrate or any other chemical. With washed-only coir, your feed does the loading. Excess potassium also depresses magnesium uptake, which is why magnesium deficiency is the most common thing growers notice in young coir crops.
Target EC by crop stage
Feed (inflow) EC in mS/cm. Nutrient makers and research groups quote slightly different figures, so the ranges are wide.
| Stage | Feed EC (mS/cm) | Notes |
|---|---|---|
| Rooted cuttings, seedlings, first days in coir | 0.8 to 1.2 | Pre-wet dry coir with this solution, never plain water |
| Establishment, first 2 to 4 weeks | 1.2 to 1.8 | Research trials often hold 1.5 for the first four weeks before splitting treatments |
| Vegetative growth | 1.4 to 2.0 | Coco-specific nutrient brands quote 1.2 to 1.8 for this stage |
| Fruiting or flowering | 1.8 to 2.6 | Dutch standard tomato supply is 2.6 at a root-zone target of 3.7 |
| Heavy fruit load, low light, winter | 2.4 to 3.0 | Raise EC to hold fruit quality, lower it again when light and transpiration rise |
Feed pH for coir sits in the 5.4 to 6.2 range. Don't go lower to "fix" a reading: in the coir tomato trial, the drain from low-EC plots stayed above pH 6.5 for 24 weeks, while high-EC plots dipped below 5.0, so the EC you choose moves root-zone pH too.
Element ranges by stage (ppm)
Ranges for fruiting crops such as tomato, cucumber and pepper, drawn from published coir and soilless recipes. Leafy crops and herbs sit at the low end or below. N is total nitrogen, P and K as the elements.
| Element | Seedling / first days (EC ~1.0 to 1.5) | Establishment, weeks 1 to 4 in coir (EC ~1.5 to 2.0) | Production (EC ~2.0 to 2.6) |
|---|---|---|---|
| N | 100 to 145 | 145 to 185 | 185 to 240 |
| P | 15 to 25 | 25 to 35 | 35 to 46 |
| K | 140 to 200 | 200 to 285, aim for the low end | 285 to 370 |
| Ca | 60 to 90 | 85 to 120, aim for the high end | 120 to 215 |
| Mg | 28 to 40 | 40 to 60, aim for the high end | 55 to 60 |
| S | 32 to 47 | 47 to 70 | 70 to 145 |
Two ratios matter more than any single number. In a standard Dutch tomato supply the K:Ca ratio is roughly 1.7:1 by weight (371 ppm K to 216 ppm Ca) and the Ca:Mg ratio is close to 3.7:1 (216 to 58). On fresh coir, push both towards calcium and magnesium for the first month. Keep potassium at the bottom of its range, calcium at the top, and raise magnesium to 50 to 60 ppm from the start. It costs very little. After the first month, move back to the standard ratios.
Coco-specific A/B feeds have this adjustment built in. With a general hydroponic feed, add calcium nitrate and magnesium sulphate (or a cal-mag product) in the early weeks instead of just turning up the EC.
Micronutrients
Coir doesn't change micronutrient needs, but root-zone pH does, so use a chelated iron that holds at your pH (DTPA in the normal coir range, EDDHA if it drifts above 6.5). A published coir tomato solution used 0.8 ppm iron, 1.5 ppm manganese, 0.4 ppm zinc and 0.05 ppm boron. Check water boron first: 0.5 ppm is safe, 1.0 ppm is the upper limit.
Water quality: bicarbonate and sodium
Your water is part of the recipe.
Bicarbonate (alkalinity)
Alkalinity, not water pH, is what pushes the root zone up over weeks. Water with an alkalinity of 120 ppm as calcium carbonate is likely to cause high substrate pH problems. Below 80 ppm is low, 100 to 200 ppm is moderate, and above 200 ppm is high. Plugs and small volumes of coir are the most sensitive, with problems appearing from 50 to 75 ppm. Up to 150 ppm is manageable in larger containers with an acidic fertiliser, 150 to 300 ppm is increasing concern, and above 300 ppm acid injection is the only practical fix.
How to adjust: neutralise alkalinity with acid in the stock or at the injector. Phosphoric acid suits moderate alkalinity but adds phosphorus you must subtract from the recipe. Nitric acid adds nitrogen in the same way, which is useful because you need it anyway. Sulphuric acid is the strongest option for very hard water. Citric acid is too weak above about 100 ppm bicarbonate. Don't take alkalinity to zero, or the solution loses all its buffering and pH will swing with every stock change.
Sodium and chloride
Coir already brings sodium with it, so you have less headroom than on rockwool. General greenhouse guidelines put irrigation water sodium at 50 ppm and chloride at 70 ppm, with an EC under 0.75 mS/cm for propagation and under 2.0 for general production. Above that, plan on reverse osmosis, rainwater blending or a higher leach fraction.
In the root zone, Dutch closed-system tomato growers were allowed to discharge drain water once sodium reached 8 mmol/L (about 184 ppm), and trials have since run tomato at 15 to 25 mmol/L without a measurable yield loss, as long as the other cations stayed above their minimums. The practical response when sodium creeps up is to cut potassium sulphate and chloride-bearing fertilisers, keep calcium and magnesium at full strength, and raise the drain fraction. Strawberries have far less tolerance than tomato.
Drain fraction and drain EC
Drain EC tells you whether the coir is holding or shedding salts.
| Setting | Drain fraction | Drain EC vs feed |
|---|---|---|
| Pots and small containers, daily feed | 10 to 20% of volume applied | Within 0.2 to 0.3 mS/cm of feed on fresh coir, up to about 1.0 above feed once established |
| Slabs and grow bags, fruiting crops | 20 to 50% in summer, around 20% in low light | 0.5 to 1.1 above feed, so feed 2.6 and root zone 3.7 is a normal pair for tomato |
| Any system, drain heading towards twice the feed EC | Raise volume, not strength | Flush with full-strength feed at low EC, not plain water |
A drain EC lower than the feed in the first two weeks is normal on fresh coir and means the substrate is still loading up with calcium and magnesium. Hold strength and let it climb. Drain EC rising faster than the feed means the plant is taking water faster than nutrients. Lower the feed a step and increase volume. When the gap gets large, flush with low-EC feed rather than water: plain water strips the calcium you've spent weeks putting on the exchange sites and the coir releases potassium again. Our guide on how to flush coco coir covers the procedure.
Deficiency symptoms on coir and the fix
Most "deficiencies" on coir in the first month are imbalances caused by the substrate, not a weak feed.
| Symptom | Where | Likely cause on coir | Fix |
|---|---|---|---|
| Interveinal yellowing starting at the margins of older leaves, later light brown dots | Lower leaves first | Magnesium, depressed by excess potassium from fresh coir or by high feed K | Raise Mg to 50 to 60 ppm, cut K to the low end, check drain K if you can |
| Dark sunken patch at the blossom end of fruit (blossom-end rot) | Fruit, often trusses 1 to 3 | Calcium not reaching fruit. On coir this is often uneven watering or high EC, not low Ca in the feed | Keep Ca at 120 ppm or more, even moisture, smaller more frequent feeds, lower EC in hot weather |
| Tip burn and distorted new leaves | Growing points | Calcium, same mechanism as above | As above, improve air movement so the plant transpires |
| Older leaves yellow then scorch from the edge, blotchy ripening with green shoulders | Lower leaves and fruit | Potassium, usually later in the crop once the coir has stopped releasing K and fruit load is heavy | Raise K towards 350 to 370 ppm in production, check EC isn't the real issue since high salinity mimics it |
| Yellowing between veins on young leaves, veins stay green | New growth | Iron lockout from high root-zone pH, often from high-alkalinity water | Acidify irrigation water, switch to EDDHA chelate, check drain pH |
| Small pale leaves, thin upright plant | Older leaves first, then whole plant | Nitrogen, either too little in the feed or a drain fraction so high that nitrate leaches | Raise N and reduce leach fraction |
The order of checks when something looks wrong on coir: drain EC and pH first, then water alkalinity and sodium, then the Ca:Mg:K balance of your feed. Only after that look at a single element. The guide to growing tomatoes in coco coir has stage-by-stage notes for that crop.
Putting it together
For the first month on fresh washed coir, run calcium and magnesium at the top of their ranges, potassium at the bottom, and a slightly higher drain fraction. Pre-wet the coir with that solution. Measure drain EC and pH twice a week. When drain EC settles close to feed EC, the exchange sites are full and you can return to standard ratios.
If you'd like washed, unbuffered coco peat in blocks, grow bags or open-top slabs for a feed programme like this, contact us with your crop and volumes.
FAQ
Do I need a coco-specific nutrient, or can I use my normal hydroponic feed?
You can use a normal feed if you add calcium and magnesium in the first few weeks and ease back on potassium. Coco-specific A/B feeds simply have that adjustment built in. After the coir's exchange sites are loaded with calcium, the two approaches end up in the same place.
How much cal-mag should I add to coco coir?
Think in ppm rather than millilitres. Aim for calcium of 120 ppm or more and magnesium around 50 to 60 ppm in the feed for the first month, including whatever your water already contains. Many cal-mag products also carry nitrogen, so count that towards your total N.
Should I feed coco coir with every watering?
Yes. Coir holds very little nutrient reserve of its own and plain water pulls calcium off the exchange sites and releases potassium. Feed with every irrigation, and use a low-EC feed rather than water when you need to flush.
What drain EC should I see on coco coir?
Close to the feed EC on fresh coir, then 0.5 to 1.0 mS/cm above the feed once the crop is established and fruiting. If drain EC heads towards twice the feed, increase irrigation volume and lower the feed strength a step.
Why are my lower leaves yellowing between the veins in coco?
That pattern on older leaves is usually magnesium. On fresh coir it happens because the substrate releases potassium, and excess potassium blocks magnesium uptake. Raise magnesium to 50 to 60 ppm, trim potassium and check that root-zone pH is still in range.
Does blossom-end rot in coir mean I need more calcium in the feed?
Not usually. Trials on coir show blossom-end rot incidence similar to rockwool and strongly tied to feed EC and water stress, not to the calcium concentration. Keep calcium at 120 ppm or more, but fix irrigation frequency, EC and humidity first.
How does hard water affect feeding coir?
Hard water usually means high alkalinity, which raises root-zone pH over time and locks out iron and manganese. Above about 120 ppm alkalinity as calcium carbonate, acidify the water with nitric, phosphoric or sulphuric acid and subtract the nitrogen or phosphorus the acid supplies from your recipe.
Can I reduce the feed strength late in the crop on coir?
You can lower EC in the final weeks for taste and shelf life, but keep the ratios intact. Dropping only potassium late in a fruiting crop brings on blotchy ripening, and dropping calcium brings tip burn and blossom-end rot in the last trusses.