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

Paddock

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.

Plant

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".

Where does it occur?

Soil type

Dry conditions

Spraying herbicide

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.

Management strategies

Spraying foliar

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?

Tissue test

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.

Further information

Plant nutrition [1]