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Molybdenum Functions, Sources, and Toxicity in Plants

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Molybdenum is one of the essential micronutrients that plants need. Plants absorb it as molybdate (MoO42–) and bind it with the organic pterin complex before converting it into a biologically active form, i.e., the molybdenum cofactor.

The molybdenum cofactor then binds with various enzymes that need it, including those involved in nitrogen assimilation (conversion of nitrate to nitrite and fixation of nitrogen in legume’s nodules), purine and sulfide degradation, and biosynthesis of some plant hormones.

Plants that don’t get enough molybdenum will end up with deficiency symptoms. However, too much will result in toxicity, especially to grazing animals.

Discover more about molybdenum absorption and availability, functions or roles, and how much plants require) and some common sources. We will tell you whether it is mobile or immobile and much more.

See molybdenum deficiency symptoms and fixes if you want to know more about deficiency causes, symptoms, and treatment and molybdenum fertilizers or supplements if you want to know what to use to fix deficiencies. Also, there is something about toxicity.

Molybdenum in plants functions, requirement, sources and toxicity
Molybdenum deficiency in maize/corn. Photo credit: Alandmanson, Wikimedia, CC BY-SA 4.0


Molybdenum (Mo), atomic number 42, is one of the group 6 metallic elements. This transitional element is one of the essential micronutrients for humans, animals, microbes, and plants.

Arnon and Stout first showed Mo as essential in 1939 as a necessary trace or micronutrient using a tomato grown hydroponically. Among its many roles, it plays a key role in catalyzing various reactions, including nitrogen assimilation, and a deficiency will result in some symptoms.

Absorption and availability

Plants will only absorb dissolved molybdenum in molybdate (MoO42–), a negatively charged ion (anion). It is the second most important anionic (negatively charged) plant nutrient besides phosphate.

Molybdate availability in the soil increases with pH, unlike other micronutrients like zinc, copper, iron, and manganese, whose availability increases will reduce pH.

Some of the factors that affect molybdate availability include:

1. pH

Usually, molybdate, the soluble molybdenum, is highly available at pH ranges of 6.0 or higher, with solubility increasing by 100-fold via mainly reducing adsorption by metal oxides for each unit increase in pH.

2. Fe, Al, Mn oxides, clay minerals, and carbonates

Since molybdate adsorbs on aluminum and manganese metal oxides, carbonates, and clay minerals, their presence can affect its availability. However, adsorption to positively charged metal oxides (Fe, Mn, and Al) depends on pH, with the highest adsorption observed at pH 4-5.

3. Sulfate and phosphate

High sulfate (SO4-) reduces absorption by competing for uptake sites and vice versa. In contrast, phosphate or phosphorus increases absorption or availability by reducing or causing a release of molybdate adsorbed to soil particles.

Therefore, applying single superphosphate (SSP) will decrease the amount absorbed while triple superphosphate (TSP) increases.

4. Dissolved organic compounds (organic matter)

The presence of organic matter, especially fulvic and humic acid, binds with molybdate, reducing the amount available to adsorb onto metallic oxide. This will also have a net increase in the amount available in the solution.

5. K, Na, Ca, and Mg

Potassium, sodium, calcium, and magnesium will bind with soluble molybdate forming complexes that reduce the amount available for adsorption by metallic oxides. This increases the amount available in the solution.

6. Moisture content

Soil moisture influences the molybdate available, with swampy organic-rich soils and peat marshes accumulating more.

7. Soil drainage

Well-drained sandy soils may result in deficiency since they can leach significantly. However, how much leaches depends on pH, with acidic ones resulting in very little leached.

8. Nitrate nitrogen

The presence of nitrate nitrogen (NO3) increases molybdate uptake compared to ammonium.


Plants absorb molybdenum as molybdate (MoO42–), a negatively charged anion. Its availability depends on pH, soil texture, drainage, moisture content, metal oxides (Fe, Al, and Mn), and clay minerals. Also, nitrate nitrogen, sulfate, phosphate, potassium, calcium, magnesium, and sodium will affect availability.

What does molybdenum do for plants – Functions

Molybdenum has many important roles in plants, without which plants will have some deficiency symptoms, a reason why it is an essential plant nutrient.

However, the molybdate that plants absorb is not biologically active and will first bind with the organic pterin complex or backbone and then be converted to the molybdenum cofactor or Moco. Moco will bind with the various enzymes that require Mo, i.e., molybdoenzymes.

Therefore, the primary function of molybdenum is to form molybdoenzymes such as nitrate reductase and nitrogenase. Others important ones are xanthine dehydrogenase, aldehyde oxidase, and sulfite oxidase.

Nitrate reductase (NR) is the primary nitrogen-assimilating enzyme that converts nitrate to nitrite in all plants. Nitrate will then undergo reduction to ammonia used to make amino acids and proteins necessary for plant growth.

On the other hand, nitrogenase is present in legume nodules bacteroids. This molybdenum-based enzyme helps in nitrogen-fixing, i.e., conversion of atmospheric nitrogen to ammonia (N2 to NH3) compounds by rhizobia, a group of symbiotic bacteria.

These symbiotic bacteria need about ten times more molybdenum for nitrogen fixing than host legumes. Therefore, deficiency symptoms will first show in legumes before other plants on the same soil.

The other molybdenum cofactor and their uses or functions are as follows:

  • Xanthine dehydrogenase – Involved in purine catabolism or degradation to uric acid. Also, it has a role in the biosynthesis of ureide (in legumes).
  • Aldehyde oxidase (AO) – Biosynthesis of abscisic acid (ABA) and indole acetic acid plant hormones and catalyzing final carotenoid catabolism.
  • Sulfite oxidase: Helps convert (detoxify) sulfide to sulfate necessary in making sulfate-containing amino acids or formed during degradation of sulfur-containing amino acids.

From the above, we can conclude that molybdenum is essential in nitrogen assimilation in all plants and fixation in legumes. Also, it has a role in purine degradation, sulfide detoxification to sulfate, and in making plant hormones like abscisic acid and indole acetic acid.


Molybdenum is one of the least required micronutrients, together with nickel, with an amount as small as 50 grams sufficient for a hectare. Also, it is the least in plant tissue concentration together with copper.

For most plants, the molybdenum requirement is 0.1-2.0 ppm for optimal growth. However, some plants, especially legumes, cucurbits, and crucifers, require up to 5 ppm or more.

On the other hand, analysis of plant tissue or leaves has shown that the normal concentration is 0.01-1.5 ppm, with some plants having more, some up to 80 ppm, without showing any signs of toxicity.

Lastly, most agricultural soils have 0.6-3.5 ppm, averaging at 2 ppm, with available or soluble Mo standing at 0.2 ppm.

1. Plants that require high molybdenum

Responsive or plants that require high molybdenum include lettuce, cabbage, cauliflower, brussels sprout, alfalfa, peas, clover, soybeans, tobacco, sugar beets, spinach, broccoli, and tomatoes. Others are grapes, citrus, table beets, duckweed, poinsettia, primula, tobacco, and other legumes.

2. Plants that require moderate molybdenum

Grasses, corn, apples, barley, carrots, celery, cotton, grapes, potatoes, peaches, raspberries, rice, sorghum, and other small grains tolerate low molybdenum amounts.

What are the sources of molybdenum?

The primary source of molybdenum in the soil from weathering (solution and oxidation) of parent rocks, especially those with molybdenite (MoS2), ferrimolybdenite Fe2(MoO4), or wulfenite (PbMoO4) minerals.

These minerals are common in igneous and sedimentary rocks, with molybdenum making about 2.3 mg per kg of the lithosphere. However, concentrations go up to 300mg per kg in shale, which contains a considerable amount of organic matter. Also, soils derived from young granite have higher amounts.

Other source includes organic molybdenum from decaying organic matter, including farmyard manure, compost or mulches, and peat. Also, plants can get this micronutrient from molybdenum fertilizers or those that contain it.

Lastly, molybdenum in trace amounts also occurs in olivine. Also, some aluminum and iron oxides and hydroxides, olivine, and clay silicates have small quantities.  

Is Molybdenum mobile or immobile?

Molybdenum is mobile in plants, i.e., it can move via the xylem (transpiratory) and phloem transport system. Therefore, it can freely move from older to younger tissues.

However, older leaves will have a higher concentration since it readily binds with sugars, polyhydroxides, or sulfur-containing amino groups, which tend to be higher in older, lower leaves.

Deficiency symptoms and fixes

Molybdenum deficiency in plants is common in acidic, highly weathered, or infertile soils. Also, high in iron, manganese or aluminum oxides, clay minerals, and podzolic soils can be a reason.

Signs that your plant lacks Mo include stunted growth, yellowing of leaf margins or in-between leaves, and scorching or necrosis that will first affect leaves in the middle of the plant.

More signs are leaf cupping, rolling, crimping, deformation and withering. Also, restricted flowering, failure, or reduced fruit setting can happen.

However, the exact molybdenum deficiency signs may vary from one plant to another. We have common symptoms in citrus, cabbage, cannabis, corn, etc.

Managing molybdenum deficiency includes liming, using molybdenum fertilizers (soil, foliar, or seed treatment), and increasing organic matter.

Nitrogen fertilizer and molybdenum deficiencies

Since molybdenum affects nitrogen fixation by legumes, applying nitrogen fertilizer to legumes may help resolve some symptoms of nitrogen deficiency, like stunted growth and chlorosis that start with older leaves. However, this is not the recommended treatment.

However, for non-legumes, applying nitrogen fertilizer will not resolve but only worsen the various symptoms. 

Molybdenum toxicity in plants

Molybdenum toxicity in plants is rare, with plants capable of taking several times the optimum amounts without any toxicity symptoms. It is less likely to cause phytotoxicity than copper or zinc, with mounts as high as 10 ppm not toxic to most plants.

However, high amounts of molybdenum may interfere with iron metabolism, resulting in chlorosis and yellowing, including golden-yellow coloration in plants. Some symptoms of excessive molybdenum accumulation in leaves include purple coloration in tomato and cauliflower plants due to anthocyanins accumulation and yellowing leaves in legumes.

In maize, Mo phytotoxicity results in the accumulation of Mo in leaves and roots, reduced growth, and less uptake of other essential nutrients like nitrogen, magnesium, copper, phosphorus, and iron. 

Perhaps a more serious concern of molybdenum toxicity in plants is in pastures where tissue amounts of 5-10 ppm will cause molybdenosis in grazing animals. Molybdenosis occur due to copper deficiency as it binds with the excess molybdenum causing copper-deficiency symptoms. Therefore, always include copper when applying molybdenum to animal pastures.

 One study found arbuscular mycorrhizal (AM) fungi (AMF) To help manage Mo toxicity). It helps manage molybdenum phytotoxicity by reducing its movement to leaves or soil pH mediation. This use makes AMF vital in Mo-polluted sites but will still affect grazing animals.