Molybdenum deficiency in canola has not been diagnosed in the field in WA, and there are few symptoms described. Those described below are only a guide and should be verified by plant nutrient test.
Diagnosing molybdenum deficiency in canola
Many sandplain soils and gravelly sands in WA were molybdenum deficient in their natural state. Canola is susceptible and foliar spray responses have occured in trials, but deficiency is difficult to detect in the field. Molybdenum overuse can cause the disease molybdenosis in livestock, particularly after lime application.
What to look for
- Pale plants that are more common on acidic sandy soils.
- Pale and healthy plants can be intermixed.
- Worse symptoms are expected on acidic soils, and better plants in limed areas and windrows.
- Poor response to nitrogen fertiliser.
- Stunted pale plants.
- Tissue between veins of older leaves becomes paler green with scattered white dead spots that expand as deficiency worsens.
- Leaf margins are frequently brown and "scorched".
What else could it be
|Diagnosing zinc deficiency in canola||Smaller plants with pale leaves. Interveinal chlorosis||Zinc deficient plants are often stunted with shortened petioles, and do not have scorched edges.|
|Diagnosing copper deficiency in canola||Smaller plants with pale leaves. Interveinal chlorosis and scattered dead leaf spots.||Copper deficient plants have numerous yellow specks develops between veins of older leaves that progresses until the whole plant appears pale green.|
Where does it occur?
- Acidic yellow sandy wheatbelt sands required molybdenum when initially cleared, but deficiency is becoming more widespread as soils acidify
- Molybdenum deficiency is exacerbated in acidic soils and soils high in iron and aluminium oxides. It is particularly acute on acid wodjil soils that are too acidic for economic canola production.
- Molybdenum is relatively immobile in soil and can become unavailable to crops in dry soil.
- The use of root-pruning herbicides can induce molybdenum deficiency.
- Foliar spray (effective only in the current season), seed treatment or soil fertiliser
- In long term no-till paddocks frequent small applications of molybdenum via drilled, in- furrow application or seed treatment reduces the risk of plant roots not being able to obtain the nutrient in dry seasons
- As molybdenum is immobile in the soil topdressing is ineffective, only being available to the plant when the topsoil is wet
- Mixing molybdenum throughout the topsoil improves availability due to more uniform nutrient distribution.
- Deep-placed molybdenenum increases the chances of roots being able to obtain enough molybdenum when the topsoil is dry
- Liming acidic soils to maintain topsoil pH above 5.5 is important for maintaining molybdenum availability.
How can it be monitored?
- Use youngest fully-emerged leaf to test for molybdenum. Levels less than 0.07 mg/kg indicate deficiency. Take paired good/poor plant samples when possible.
- There is no reliable soil test for molybdenum.
- Soil pH less than 4.7 indicates likely molybdenum response. Maintaining a surface pH above 5.5 is more effective than adding extra molybdenum.