Acid soil locks phosphorus: purple, dwarf seedlings — fix acid before you pour more P
Iron and aluminium grab phosphate in acid ground.
- Purple, short, few roots — not always ‘no phosphorus in the soil’.
- Acid iron and aluminium nail P down.
- Chill eases when it warms; acid-locked phosphorus stays purple after a warm spell.
- Nitrogen shortage is yellow; phosphorus shortage is often purple-dark.
- Don’t pour P as a meal without opening the lock; don’t heap lime and phosphate the same day.
- Phosphate fertiliser: Band it from a soil test. Not “short, so double”. On acid soil, put it closer to the root
- Name what you see before you feed.
- If leaves, roots and the surface disagree, stop.
What you see in the field
Purple, short, few roots — not always ‘no phosphorus in the soil’. Acid iron and aluminium nail P down.Walk the worst strip. Pull one plant, rinse the roots at the ditch without rubbing the tips, and read white tips versus brown bald ones. A rusty film on water is extra, not required. Grain land and tea do not share a pH target.
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 “Chill injury”
Chill eases when it warms; acid-locked phosphorus stays purple after a warm spell.
Looks like “Nitrogen-yellow seedlings”
Nitrogen shortage is yellow; phosphorus shortage is often purple-dark.
Do this / don’t do that
- 1Purple, short, few roots — not always ‘no phosphorus in the soil’. Acid iron and aluminium nail P down.
- 2Test pH and available P. A little lime if too acid. Band P by the root. Foliar potassium dihydrogen phosphate as a labelled rescue.
- 3Don’t pour P as a meal without opening the lock. Use the range on this page only as a range. Confirm with a local station and the product label.
Don’t do this first
- Don’t pour P as a meal without opening the lock; don’t heap lime and phosphate the same day.
Rate range
Phosphate fertiliser
Band it from a soil test. Not “short, so double”
On acid soil, put it closer to the root.
Agricultural lime
Follow pH
Unlock first, then add phosphorus.
These are ranges. Confirm with your local extension station and the product label.
When you see Purple, test. If the leaf and the soil disagree, test before you pour.
Lesson video
2026-03-24 · 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