A plant with yellowing leaves is the most-searched problem in indoor growing and the most misdiagnosed. The instinct is to reach for a bottle. Most of the time the bottle isn't the answer — and adding more feed to a plant that can't use what it already has makes things worse. This guide is the diagnosis first, then the fix.
Before you blame the nutrients
Three things cause more "deficiencies" than any actual shortage of nutrients, and each one makes the plant unable to take up food that's already in the pot:
- Wrong pH. Every nutrient is only available to roots within a pH band. Outside it, the nutrient is present but locked out. Soil wants 6.0–6.8; coco and hydro want 5.5–6.3. Above about 6.5 iron and manganese lock out (young leaves yellow); below about 5.8 calcium and magnesium do (older leaves yellow, new growth twists). Test the water going in and the runoff coming out.
- Overwatering. Roots sitting in a wet pot can't breathe and stop taking up nutrients. The plant droops, then yellows from the bottom up — which looks exactly like nitrogen deficiency. If the pot is heavy and the medium is wet an inch down, the fix is to stop watering, not to feed.
- Salt build-up. Weeks of feeding without runoff leaves salts accumulating in the medium. Runoff EC creeps far above what you're putting in, roots get burned, and the plant shows scorched tips and deficiency signs at once. Flush with plain pH'd water until runoff drops, then resume at a lower dose.
Check those three before reading on. If pH is right, the pot isn't waterlogged and runoff isn't sky-high, then it may genuinely be a deficiency — and the leaves will tell you which.
The one rule that identifies most deficiencies
Nutrients are either mobile or immobile inside the plant.
- Mobile nutrients (nitrogen, phosphorus, potassium, magnesium) can be moved from old leaves to new growth. So when they run short, the plant robs its oldest, lowest leaves first. Symptoms start at the bottom.
- Immobile nutrients (calcium, iron, sulfur, manganese, zinc, boron) can't be moved once placed. When they run short, new growth at the top suffers first while old leaves look fine.
So the first question is always: where on the plant did it start? Bottom means mobile; top means immobile. That alone halves the list.
The common ones, and what to do
| Nutrient | Where it shows | What it looks like | Usual cause | Fix |
|---|---|---|---|---|
| Nitrogen (N) | Old leaves, bottom up | Whole leaf goes uniformly pale then yellow; slow, spindly growth | Underfeeding, or a soil that's used up | Feed a veg-formula (higher N) at normal strength; in soil, top-dress with a nitrogen-rich amendment |
| Phosphorus (P) | Old leaves | Dark, dull, bluish-green leaves; purple or red stems and leaf undersides; stunted | Cold roots (below ~60 °F) or low pH locking it out, more often than a real shortage | Warm the root zone, correct pH, then a bloom feed if it persists |
| Potassium (K) | Old leaves | Yellow then brown scorching along the leaf edges, curling; weak stems | Underfeeding in flower; excess calcium or magnesium competing | Bloom feed (higher K); check you're not overdoing cal-mag |
| Magnesium (Mg) | Old leaves | Yellow between the veins, veins stay green (interveinal chlorosis), working upward | Very common in coco, with RO water, or when potassium is high | Epsom salt, 1 teaspoon per gallon, in the water or as a foliar spray; or a cal-mag supplement |
| Calcium (Ca) | New growth | Twisted, distorted or crinkled new leaves; brown dead spots on young leaves; blossom-end rot on tomatoes and peppers; tip burn on lettuce | Coco or RO water without cal-mag; inconsistent watering (calcium only moves with water); low pH | Cal-mag every feed in coco; keep watering consistent; correct pH |
| Iron (Fe) | New growth | Young leaves yellow to almost white between green veins; old leaves unaffected | pH too high (above 6.5) — nearly always | Lower the pH; chelated iron only if pH was already right |
| Sulfur (S) | New growth | Uniform pale yellow on young leaves, no vein pattern | Rare; RO water, very clean media | Epsom salt covers it (magnesium sulfate) |
| Manganese / Zinc | New growth | Interveinal yellowing on young leaves with small dead spots (Mn); tiny new leaves with short spacing between them (Zn) | pH too high | Lower the pH; a micronutrient supplement if it persists |
Two look-alikes worth separating: magnesium vs iron both give yellow-between-green-veins, but magnesium starts on old leaves and iron on new. And nitrogen vs overwatering both yellow from the bottom, but nitrogen deficiency leaves the plant perky and pale while overwatering makes it droop.
Nutrient burn: the deficiency that's actually an excess
Dark, almost glossy green leaves with crispy brown tips that then spread inward, and leaf edges curling down like a claw. That's too much feed, not too little — and it's the most common problem of all in the first month, because every label's recommended dose is generous. Flush with plain pH'd water to 20–30% runoff, wait a few days, then resume at half the dose you were using.
A method that works
- Look at where it started — bottom or top of the plant.
- Check pH of the water in and the runoff out. Fix it first; wait three or four days. A remarkable number of "deficiencies" resolve here.
- Lift the pot. Heavy and wet? That's your problem. Let it dry to half weight.
- Measure runoff EC. More than about 50% above what you're feeding means salts are building up: flush.
- Only now match the leaf to the table and treat the one nutrient it points to. One change at a time, then give it a week. New growth is the verdict — damaged leaves don't recover, so judge the leaves that form after the fix.
What to have on the shelf
- A pH pen and an EC/TDS meter — the two tools that turn guessing into knowing. Budget ones are fine; calibrate the pH pen when you buy it.
- pH up and down for adjusting water.
- Cal-mag, essential in coco or with RO water, useful everywhere.
- Epsom salt for magnesium — pennies, from any pharmacy.
- A basic veg and bloom feed, which covers everything else. Full details in How to use nutrients properly.
The one-line version
Old leaves first means nitrogen, potassium or magnesium; new leaves first means calcium or iron. But check pH, watering and salt build-up before any of that — they're the cause more often than an empty bottle, and feeding a locked-out plant only adds to the problem.
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