Diagnosing molybdenum deficiency in cereals

Many sandplain soils in WA were molybdenum deficient in their natural state. Molybdenum deficiency is difficult to detect in the field and can decrease wheat yield by up to 30% before symptoms become obvious. Deficiency becomes more common as soils acidify. Molybdenum overuse can cause the disease molybdenosis in livestock grazing following pastures.

What to look for

Paddock

At low levels of nitrogen the crops are pale with some limpness.
Patches of good growth often found on higher pH areas such as ash heaps or burnt windrows.
As nitrogen levels increase, symptoms become more specific with all but the the oldest leaves pale green when nitrogen nutrition is adequate to high.

Plant

Middle leaves have a speckled flecking and mild yellow stripes.
Leaf tips are scorched at high nitrogen levels.
Severely deficient plants have white heads, shrivelled grain and delayed maturity.

Where does it occur?

Soil type

Soil ph

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.
Molybdenum is relatively immobile in soil and can become unavailable to crops in dry conditions.
The use of root-pruning herbicides, particularly groups A and B can induce molybdenum deficiency.

Management strategies

Spraying foliar

Soil application

Seed treatment

Lime application

Foliar spray (effective only in the current season), seed treatment or soil fertiliser.
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.
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.
Deep-placed molybdenum increases the chances of roots being able to obtain enough molybdenum when the topsoil is dry.
Liming productive acid soils to maintain topsoil pH above 5.5 is important for maintaining molybdenum availability.
Molybdenum fertiliser has a very low residual life in highly acidic wodjil sandplain soils. Molybdenum seed dressing or using seed from alkaline soil paddocks is a cost-effective way of avoiding deficiency on these soils.

How can it be monitored?

Tissue test

Use youngest fully-emerged leaf to test for molybdenum. Levels less than 0.075 milligrams per kilogram (mg/kg) indicate deficiency. Take paired good/poor plant samples when possible.
There is no reliable soil test for molybdenum.

Further information

Plant nutrition [1]