Diagnosing potassium deficiency in canola
Potassium (K) is a major nutrient that is increasingly required as soil reserves become depleted.
What to look for
- Smaller sometimes wilted plants with bulging scorched older leaves, and less symptoms on heavier soils, and old windrows.
Paddock
- Smaller sometimes wilted plants with bulging scorched older leaves, and less symptoms on heavier soils, and old windrows.
- Plants are smaller, tend to wilt and young plants appear prostrate.
- Dull pale yellow to yellow-brown colour begins at leaf edges and develops white dead spots and blotches that gradually join.
- Leaf interveinal tissue bulges above the veins and the midrib bends backwards.
- Tissue discolouration and death gradually spreads inwards towards the midrib, but leaf veins remain green.
- Fewer and thinner flowering stems that set smaller pods
Paddock
What else could it be
| Condition | Similarities | Differences |
|---|---|---|
| Diagnosing group C herbicide damage in canola | Older leaf necrosis from edges | Potassium deficient plants have puckered leaves; seedlings rarely show these symptoms. |
Where does it occur?
Soil type
- Sandy soils and deep grey sandy duplex soils tend to be more susceptible to K deficiency
- High rates of hay or grain removal can result in K deficiency.
Management strategies
- Top-dressed potassium fertilisers will generally correct the deficiency
- Foliar sprays generally cannot supply enough potassium to overcome a severe deficiency and can scorch crops.
How can it be monitored?
Tissue test
- Use whole-top plant test to diagnose suspected potassium deficiency.
- Critical potassium levels vary with plant age and size, but a rough guide 2.8 per cent (seedling) to 1.3 per cent (rosette) indicate deficiency.
- 0-10 cm potassium soil test values higher than 52 mg/kg are unlikely to respond to added potassium. This is reliable in deep sands but may not be reliable for duplex soils that have potassium reserves in the clay.
- As potassium is retained by organic matter and clay, sandy duplex soil levels usually fall below 10 cm, but may rise near the clay subsoil.
- For 0-10 cm, potassium soil tests should read between 40 and 50 mg/kg. Take another sample at 30 cm to determine whether the level is lower or higher. It is also valuable to do subsoil sulphur and pH test.
See also
Further information
Where to go for expert help
Page last updated: Wednesday, 13 May 2015 - 1:27pm