Rusty brown spots starting on the older/middle leaves, some interveinal yellowing. Coco coir, RO water, feeding GH trio. Week 4 veg. pH going in is ~5.9.
Is this classic cal-mag or am I actually in pH lockout? Don't want to overcorrect.
Cal-mag deficiency or pH lockout? Week 4 veg, coco [pics]
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CalMagCarl Vegging
Asked 2h ago · Nutrients & Feeding
Best answer
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LivingSoilLuke Flowering
Nutrients & Feeding
In coco with RO water this is almost always a cal-mag deficiency — RO strips calcium and magnesium, and coco holds onto what calcium there is, so you have to supply it every feed. Rusty spots on older-to-mid leaves + interveinal yellowing is the textbook signature.
Fix: add cal-mag first to your RO, then your base nutes, then pH. In coco feed to ~10–20% runoff every watering and check runoff pH/EC. Your 5.9 going in is fine (coco target 5.8–6.2), so it's supply, not lockout. As a starting point, add roughly 150–200 ppm of cal-mag to your RO every feed, then base nutes, then pH — you should see clean new growth within a few days.
Marked helpful by CalMagCarl
241 replies
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DWC_DanaFloweringHelpful· 2h ago
Coco + RO = cal-mag every single feed, no exceptions. I add it to the water before anything else.
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TrichomeTomHarvest MasterHelpful· 2h ago
Confirm it's not lockout by checking runoff pH/EC, not just input. If runoff pH is drifting below 5.5 you've got a secondary problem.
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FirstGrowFionaSeedling· 2h ago
The 'cal-mag before base nutes' order matters — add it last and you can get precipitation/cloudiness in the res.
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CalMagCarlVegging· 2h ago
Runoff was 5.7 / EC a bit higher than input. So buildup but not crazy. Adding cal-mag to every feed now.
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AutoflowerAlSeedlingHelpful· 2h ago
Don't strip the affected leaves — they won't recover but the plant still uses them. New growth coming in clean is your sign it worked.
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LivingSoilLukeFlowering· 1d ago
@CalMagCarl with runoff EC climbing, plain-water-with-cal-mag every 3rd feed to rinse a little. Don't let coco accumulate salts.
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TentLifeTinaVegging· 1d ago
RO is great but in coco you're remineralizing from zero. Some folks run 50/50 RO/tap just to keep baseline cal/mag.
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DWC_DanaFlowering· 1d ago
If you switch to a base line with calcium built in (some coco-specific A/B), you can dial back separate cal-mag. GH trio needs the extra.
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TrichomeTomHarvest MasterHelpful· 23h ago
Quick myth-bust: cal-mag isn't a cure-all. If new growth keeps spotting after correcting, recheck pH and root health before adding more.
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FirstGrowFionaSeedling· 20h ago
Silica also helps calcium uptake/structure — add it first of all if you run it (before cal-mag), and give it a minute before the rest.
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CalMagCarlVegging· 18h ago
48 hours after adding cal-mag every feed: new growth at the top is clean and green. Old spots stayed (expected). Huge thanks all.
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AutoflowerAlSeedlingHelpful· 14h ago
Glad it's sorted. For coco, 'feed more often, slightly weaker' beats 'feed strong, less often' almost every time.
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TentLifeTinaVegging· 11h ago
Bookmarking the feed order: water → silica → cal-mag → base A → base B → pH. That sequence saved my res.
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LivingSoilLukeFlowering· 8h ago
Exactly that. And keep that runoff EC in check — coco rewards consistency more than any soil.
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