Yellow leaves: start by asking "old leaves or new leaves?"
The same yellow leaf can mean nitrogen, magnesium, iron, waterlogging or an alkaline soil — and the fixes point in opposite directions.
Two steps: (1) Is the yellowing on lower/older leaves or on new growth? (2) Are the veins still green, or is the whole leaf pale?
Step 1: where it starts
| Where | What it means | Most likely |
|---|
| Lower (older) leaves first | Nitrogen, magnesium and potassium can be moved inside the plant — it takes them from old leaves | N / Mg / K deficiency |
| New leaves and shoot tips first | Calcium, iron and boron do not move; only new growth is hit | Fe / Ca deficiency |
| Whole plant pale, thin leaves | Usually a root problem, not a missing nutrient | Waterlogging, heavy soil, over-fertilising |
Step 2: veins separate magnesium from iron
- Magnesium: starts on old leaves, veins stay green while the tissue between them yellows; leaves get stiff and brittle. Worse in alkaline soil.
- Iron: starts on new leaves — interveinal yellowing/whitening with green veins — then spreads to older leaves.
- Nitrogen: old leaves go uniformly pale, plant looks small and thin; no green veins left.
- Potassium: leaf tips and margins on old leaves yellow, brown and scorch first, like fire damage.
- Calcium: growing tips die back, margins yellow, leaves crinkle; roots suffer too.
What to do (match the cause)
- Nitrogen: 0.1%–0.2% urea as a foliar feed once every 2 weeks, 1–2 times.
- Iron: ferrous sulfate sprayed on leaves or watered in every 2 weeks, 2–3 times; if the soil is alkaline, change or acidify it.
- Magnesium: 1%–3% magnesium sulfate foliar spray, or water in 1% magnesium sulfate / magnesium nitrate.
- Waterlogging or heavy soil: fix drainage and aeration first — a damaged root cannot take up anything.
- Over-fertilising: flush the pot once, stop feeding for 1–2 weeks, then judge by new growth.
Three common mistakes
- Feeding nitrogen at every yellow leaf — in alkaline soil it makes iron chlorosis worse.
- Ignoring the roots: rot also makes the whole plant pale and limp; feeding it hurries the decline.
- Expecting a fix overnight: nutrients take 1–2 weeks to reach new leaves. Judge by new leaves, not whether the old ones turn green again.
← Growing guides
