Coir-Slab Fertigation: When Drain Fraction Collapses
A 0.3 ha greenhouse in Jinju grows 1,200 cherry tomato plants in coir slabs fed by drip fertigation. The target is feed solution EC 2.4 mS/cm, pH 5.8, and 25-30 % drain. Over ten days the drain fraction falls to 8 %, slab EC rises to 3.9 mS/cm and slab pH to 6.5. Upper-truss leaf margins are scorched, trusses 3 and 4 show blossom-end rot on about 15 % of fruit, and root tips are no longer reaching the slab edges. The grower has responded by adding a calcium nitrate drench and raising the feed EC to 2.8 to push growth. Night temperatures have climbed and the crop is carrying a heavy fruit load. The injector was serviced two weeks ago and both A and B stock tanks are still more than half full. The grower asks whether to change the formula or the irrigation schedule first, and whether the calcium drench will correct the fruit symptoms.
1. Explain what the drain percentage, slab EC and slab pH readings indicate together, and trace the mechanism that links them to the marginal scorch and the blossom-end rot.
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The three readings describe one problem, not three.
Drain fraction is the leaching fraction: it is the only route by which salts leave the slab. At 8 % the slab is being leached far too little for the feed concentration, so salts accumulate. Slab EC of 3.9 against a feed of 2.4 is the accumulation signal - the root-zone solution is roughly 1.6 times the feed.
High root-zone EC raises osmotic potential, so water uptake falls and the plant cannot hold turgor in the fastest-transpiring tissue. That is the scorch on the upper trusses, which are the youngest and most demanding.
Blossom-end rot is a calcium delivery failure to distal fruit tissue. Calcium moves almost entirely in the xylem with the transpiration stream, and it is not redistributed from older tissue. The same osmotic and moisture swing that scorches the leaves reduces sap flow to trusses 3 and 4 exactly when those fruit are expanding fastest, so the distal tissue is starved and collapses.
Slab pH 6.5 is the secondary clue: above about 6.2 in coir, phosphate and micronutrients such as Fe, Mn and B become less available, independently worsening tip growth. All three readings point to water management, not to a formula error.
2. Set out the corrective actions for the next 72 hours in priority order, and state the measurements that would confirm each action worked.
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Restore water first; every other correction depends on it.
- Increase irrigation frequency and the number of shots per day in small steps until the drain fraction returns to 25-30 %. Do not raise feed EC. In warm nights with a heavy fruit load, start earlier and finish later; the classic error is a timer set during a cool week and never revisited.
- Measure drain EC and drain pH from the same slabs daily at the same time, and compare with the feed. Success looks like drain EC falling to within 0.5-0.8 mS/cm of the feed and drain pH tracking the feed.
- Check emitter discharge and distribution uniformity. At 8 % drain, one blocked emitter at the far end of a row mimics a whole-system failure. Catch 20 emitters for one minute and compare volumes.
- Weigh 3-5 slabs before the first and after the last irrigation. That separates too little water from channelling through the slab.
- If drain EC has not fallen within 48 hours, run a deliberate flush at feed EC 1.8-2.0 and normal pH, then resume the target formula.
Confirmation: new white root tips at slab edges within 7-10 days, no new blossom-end rot on trusses 5 and above, and stable drain EC. Fruit already affected will not recover and should be removed.
3. Evaluate the two grower responses - the calcium nitrate drench and raising the feed EC to 2.8 - and state what you would do instead, with reasons.
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Both responses push in the wrong direction.
Calcium drench: blossom-end rot here is not caused by a shortage of calcium in the slab but by calcium not reaching the fruit. The A tank already supplies Ca, and tissue analysis would almost certainly show adequate levels. Adding more Ca at root level raises EC further and increases salt load at the moment the plant is already osmotically stressed. It also invites cation competition: Ca2+ competes with Mg2+ and K+ at the root, so a heavy drench can induce Mg deficiency. There is no reason to expect it to fix the fruit symptoms.
Raising feed EC to 2.8: this compounds the fault. At 8 % drain, a 2.8 feed drives slab EC past 4.5 within days and deepens the osmotic stress that caused both symptoms.
What to do instead: hold the formula at the crop-stage target, repair the leaching fraction, and only then review the formula. If the water source carries high sodium or bicarbonate, adjust the formula for that specifically. Also check the injector's EC probe against a handheld meter - a drifted probe makes a correct feed read wrong, and vice versa.