Molybdenum deficiency

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Many sandplain soils in Western Australia 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. Symptoms will vary according to the plant nitrogen (N) status as molybdenum is essential for N fixation by rhizobia and essential for N conversion in non legumes.

What to look for

Paddock

  • Symptoms are difficult to detect in the field, particularly early in the season.
  • Patches of good growth often found on higher pH areas such as ash heaps or burnt windrows.

Plant

  • Molybdenum symptoms can vary with plant nitrogen status. At low levels of nitrogen the crops are pale with some limpness.
  • At adequate to high nitrogen levels molybdenum symptoms become more specific with all but the oldest leaves turning pale green.
  • 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.

What else could it be?

Condition Similarities Differences
Potassium deficiency Pale plants with leaf tip death Potassium doesn't cause head sterility, leaf symptoms worse on older leaves with marked yellowing around hte necrotic tips.
Nitrogen deficiency Smaller, pale, poorly tillered plants Plants are more uniformly affected on each soil type, no white, 'rat tail' heads or delayed maturity. Molybdenum deficient plants are more likely to be wilted.
Copper deficiency White heads, dirty strw, shrivelled grain, late tillers, delayed maturity Leaf symptoms worse on youngest leaves, weak straw.
Frost White heads, dirty straw, shrivelled grain, late tillers, delayed, maturity. No vegetative symptoms before head emergence, symptoms worse on frost prone soils or locations, independant of soil pH.

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 worse 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 soil.
  • The use of root-pruning herbicides, particularly groups A and B can induce molybdenum deficiency.

How is it treated?

  • 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, infurrow application or seed treatment reduces the risk of plant roots not being able to obtain the nutrient in dry seasons.
  • Deep-placed molybdenenum 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 costeffective way of avoiding deficiency on these soils.
  • Note that high levels of Molybdenum in the soil can cause the disease molybdenosis in livestock. Therefore it is important to consider residual levels when determining application rates.

How can it be monitored?

  • Use youngest, fully-emerged leaf to test for molybdenum. Levels less than 0.03mg/kg indicate deficiency. Take paired good and poor plant samples where possible.
  • There is no reliable soil test for molybdenum.

 

Contacts: 

Ross Brennan | Principal Research Officer
Department of Agriculture and Food, Western Australia
444 Albany Highway | Orana, Albany, WA 6330
t: (08) 98928474 | e: ross.brennan@agric.wa.gov.au | w: agric.wa.gov.au

AGWEST Plant Laboratories
Department of Agriculture and Food, Western Australia
3 Baron-Hay Court, South Perth WA 6151
t: (08) 9368 3721 | e: agwestplantlabs@agric.wa.gov.au | w: agric.wa.gov.au

This page has been developed in association with the wheat diagnostic tool, mycrop. If you would like to diagnose other in-crop yield constraints please visit www.agric.wa.gov.au/mycrop.

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