Magnesium shortage: old leaves yellow between still-green veins
Old leaves, not new ones. Don’t mix with iron.
- Old leaves yellow between green veins, from the bottom up.
- That is not iron.
- Iron lock yellows new leaves; magnesium yellows old leaves. Get the leaf position wrong and you feed the wrong bag.
- Nitrogen shortage yellows the whole leaf more evenly; magnesium yellows between green veins.
- Don’t treat old-leaf striping as iron; don’t pour more K; don’t dump magnesium sulphate without a test.
- Foliar magnesium sulphate (first aid): Commonly 0.5–1%. A small patch first. Soil calcium-magnesium phosphate or magnesium sulphate: follow a soil test. When potassium is high, drop the potassium rate before you add magnesium
- Name what you see before you feed.
- If leaves, roots and the surface disagree, stop.
What you see in the field
Old leaves yellow between green veins, from the bottom up. That is not iron.First the leaf position: new or old, even yellow or between the veins. If that does not match, do not fertilise yet. Pull one plant and read whether the roots are white, brown or foul.
Tap to enlarge. The caption says what to photograph. Missing field photos are marked as to-be-added.
Why it looks like this
The field looks like this because the root cannot take water or nutrients the usual way. Fix the soil condition first — drainage, salt, acidity, compaction or a salt of fertiliser — then a thin feed. Rates live in the range on this page; confirm with a local station and the label.
Easy to mix up
Looks like “Iron lock”
Iron lock yellows new leaves; magnesium yellows old leaves. Get the leaf position wrong and you feed the wrong bag.
Looks like “Nitrogen shortage”
Nitrogen shortage yellows the whole leaf more evenly; magnesium yellows between green veins.
Do this / don’t do that
- 1Compare the worst plants with a neighbour ridge. Photograph whole plant, close-up and soil surface before you add fertiliser.
- 2Match roots, leaves and the soil surface to this guide. If they do not line up, do not copy a bag count.
- 3Use the rate range on this page only as a range. Confirm with a local station and the product label.
Don’t do this first
- Don’t treat old-leaf striping as iron; don’t pour more K; don’t dump magnesium sulphate without a test.
Rate range
Foliar magnesium sulphate (first aid)
Commonly 0.5–1%. A small patch first. Soil calcium-magnesium phosphate or magnesium sulphate: follow a soil test
When potassium is high, drop the potassium rate before you add magnesium.
These are ranges. Confirm with your local extension station and the product label.
Whether something is crowding magnesium out
If you just dumped potassium, read them together.
When you see Old leaves yellow between green veins, test. Take soil from the bad patch and from a greener neighbour, separately.
Lesson video
2026-03-17 · Li Zeyu · For learning. Serious damage: local agronomist or a soil test.
For learning only. Fertiliser and pesticide decisions need a local agronomist and a soil test.
Not sure? Photograph the field to identify the problem