Diagnosing nitrogen deficiency in canola
Nitrogen deficiency is the most common nutrient deficiency in canola especially during cold, wet conditions and in sandy soils in high rainfall areas. Hybrid varieties can display leaf purpling with adequate nutrient levels.
What to look for
- Smaller less branched plants with red to purple or yellow leaves.
- Symptoms are worse in wetter seasons, on lighter soil areas and sometimes on non- legume header rows.
- Mildly deficient plants are smaller with paler green and more erect leaves. Deficient seedlings have reddened cotyledons.
- Oldest leaves develop whitish-purple veins and mild purple pigmentation that starts at the end of the leaf and progresses to the base on both sides of the leaf.
- The whole leaf then turns yellow or pinkish-purple. Developing leaves are narrow and more erect.
- Established plants that become nitrogen deficient develop yellowing on leaf margins that spreads in toward the midrib between the veins. The midrib becomes discoloured then the leaf dies.
- From stem elongation, the main stem is thinner and branching is restricted. Flowering time and pod numbers are reduced.
What else could it be
|Diagnosing beet western yellow virus in canola||Purple-red colours spreading from end of oldest leaves||Affected plants are stunted rather than smaller and thinner.|
|Diagnosing damping off in canola||Reddened cotyledons and seedling older leaves||Damping off however causes stunted plants with pinched roots or hypocotlys. Often plant death occurs|
|Diagnosing sulphur deficiency in canola||Purple leaves||Sulphur deficiency affects younger leaves the most.|
|Diagnosing phosphorus deficiency in canola||Purplish older leaves||Phosphorus deficient plants have purpling on leaf margin, then the leaf turns bronze|
Where does it occur?
- Nitrogen deficiency can occur on most WA soils but is most common in the following situations:
- In cold, wet conditions that slow nitrogen mineralisation and uptake of nitrogen.
- Soils with very low organic matter.
- High rainfall on sandy soils can result in nitrogen leaching.
- Nitrogen fertiliser or foliar spray.
- There is a risk of volatilisation loss from urea or nitrate sources of nitrogen. Loss is greatest from dry alkaline soils with dewy conditions, but rarely exceeds 3 per cent per day.
- The yield potential for canola is established during stem elongation and the budding stage, so all nitrogen should be applied before this stage of growth (8 to 10 weeks).
- Unlike cereals, canola does not ‘hay off’ when too much nitrogen has been applied, but nitrogen reduces oil content, particularly with late application.
How can it be monitored?
- Use whole top-of-plant test to diagnose suspected deficiency. Critical nitrogen levels vary with plant age and size, but as a rough guide 2.7 per cent (seedling) to 3.2 per cent (rosette) indicate deficiency.
- Nitrogen soil testing by itself is of little value for most soils.
- Models that combine 0-10 cm mineral nitrogen and soil organic carbon, soil type, and legume history are valuable for nitrogen fertiliser calculation.
- Leaf colour symptoms are not a reliable guide for hybrid varieties
Where to go for expert help
+61 (0)8 9690 2174
Page last updated: Wednesday, 13 May 2015 - 1:29pm