SODIUM MOLYBDATE

Synonym: Molybdic acid sodium salt dihydrate

CAS Number: 10102-40-6 
Linear Formula: Na2MoO4 · 2H2O 
Molecular Weight: 241.95  
EC Number: 231-551-7  

Sodium Molybdenum Oxide Dihydrate, Disodium molybdate dihydrate, Molybdic acid sodium salt dihydrate, Sodium molybdate(VI) dihydrate, Sodium dioxido(dioxo)molybdenum hydrate (2:1:2)

sodium molybdate
Sodium molybdate dihydrate
disodium molibdate
Molybdate (MoO4(2-)), disodium, dihydrate, (T-4)
sodium molibdate 2h2o
Molybdate (MoO42-), sodium, hydrate (1:2:2), (T-4)-


Sodium molybdate, Na2MoO4, is useful as a source of molybdenum.
It is often found as the dihydrate, Na2MoO4·2H2O.

The molybdate(VI) anion is tetrahedral. Two sodium cations coordinate with every one anion.

Sodium molybdate was first synthesized by the method of hydration.
A more convenient synthesis is done by dissolving MoO3 in sodium hydroxide at 50–70 °C and crystallizing the filtered product.
The anhydrous salt is prepared by heating to 100 °C.

disodium dioxido-dioxomolybdenum
disodium molybdate
disodium;dioxido(dioxo)molybdenum
molybdic acid (H2MoO4) , disodium salt
molybdic acid disodium salt
molybdic acid, disodium salt
sodium molybdate(VII) dihydrate

Uses
The agriculture industry uses 1 million pounds per year as a fertilizer. 
In particular, its use has been suggested for treatment of whiptail in broccoli and cauliflower in molybdenum-deficient soils.
However, care must be taken because at a level of 0.3 ppm sodium molybdate can cause copper deficiencies in animals, particularly cattle.
Sodium Molybdate is a crystalline powder essential for the metabolism and development of plants and animals as a cofactor for enzymes. (NCI)

It is used in industry for corrosion inhibition, as it is a non-oxidizing anodic inhibitor.
The addition of sodium molybdate significantly reduces the nitrite requirement of fluids inhibited with nitrite-amine, and improves the corrosion protection of carboxylate salt fluids.

In industrial water treatment applications where galvanic corrosion is a potential due to bimetallic construction, the application of sodium molybdate is preferred over sodium nitrite. 
Sodium molybdate has the advantage in that the dosing of lower ppm's of molybdate allow for lower conductivity of the circulating water. 
Sodium molybdate at levels of 50-100 ppm offer the same levels of corrosion inhibition that sodium nitrite at levels of 800+ ppm. 
By utilizing lower concentrations of sodium molybdate, conductivity is kept at a minimum and thus galvanic corrosion potentials are decreased Molybdenum chemicals are a specialty raw material that is widely used in the agricultural, lubricant, pigment & dye, and water treatment industries. 
In water treatment, sodium molybdate (Na2MoO4) has proven to be a superior corrosion inhibitor in open recirculating cooling systems and closed recirculating cooling systems. 
It is often applied at lower levels and in combination with other inhibitors, such as inorganic and organic phosphates. 
In addition to being a very effective inhibitor in both soft and hard water, molybdates are thermally stable and are also excellent passivators in the presence of oxygen. 
Furthermore, molybdate treatments are safer for the environment, and may be most appropriate where phosphate and/or zinc discharge is limited.

In industrial water treatment applications, sodium molybdate offers the best corrosion protection in systems of bimetallic construction versus sodium nitrite. 
In these mixed-metallurgy cooling systems, there is a high potential for galvanic corrosion. 
Sodium molybdate provides an advantage over using sodium nitrite because it is dosed at lower ppm(s), and this allows for lower conductivity of the circulating water. 
Because of the lower conductivity levels in the cooling water, the potential for galvanic corrosion is greatly decreased. 
(Sodium Molybdate at levels of 50 – 100 ppm offers the same level of corrosion inhibition that sodium nitrite does at levels of 800+ ppm.)

Safety
Sodium molybdate is incompatible with alkali metals, most common metals and oxidizing agents. 
It will explode on contact with molten magnesium. It will violently react with interhalogens (e.g., bromine pentafluoride; chlorine trifluoride). 
Its reaction with hot sodium, potassium or lithium is incandescent


Sodium Molybdate Dihydrate is generally immediately available in most volumes. 
Hydrate or anhydrous forms may be purchased. High purity, submicron and nanopowder forms may be considered. 
ATAMAN sells to many standard grades when applicable, including Mil Spec (military grade); ACS, Reagent and Technical Grade; Food, Agricultural and Pharmaceutical Grade; Optical Grade, USP and EP/BP (European Pharmacopoeia/British Pharmacopoeia) and follows applicable ASTM testing standards. 
Typical and custom packaging is available. 
Additional technical, research and safety (MSDS) information is available as is a Reference Calculator for converting relevant units of measurement.

Sodium molybdate corrosion inhibitor for boiler feedwater systems:

Molybdate-containing corrosion inhibotors are becoming more attractive in an increasing number of commercial applications. 
Sodium molybdate inhibits corrosion of low carbon steel, copper and brass in recirculating cooling water systems while being environmentally safe. 
These compounds are also economical at the concentration required for effective water treatment. 
Molybdenum is tolerated in various concentrations by many life forms. 
Molybdenum compounds are characterized as nontoxic in US Public Health Bulletin 293, by the Federal Hazardous Substances Labeling Act, and by the Occupational Safety and Health Act. 
The nature of molybdate inhibition was discussed. Molybdate inhibits by adsorption. This necessitates the use of more molybdate to protect steel in typical waters than is required for protection in distilled water. 
Sodium molybdate inhibition has been investigated in waters of varying composition and pH. 
Several multi-component inhibitor systems which contained sodium molybdate were studied. 
Several advantages of molybdates beyond general ferrous metals corrosion inhibition can be shown which have important commercial implications. 
These include inhibitions of pit propagation, and of crevice and nonferrous metals corrosion. 
The effect of sodium molybdate on the corrosion inhibition of copper was studied in soft water. 
Potentiodynamic polarization experiments were performed with aluminum in deaerated high chloride and high chloride + sodium molybdate solutions. 
The ability of a single chemical to inhibit the corrosion of more than one metal has important advantages in water treatment.


Sodium molybdate is an important source of molybdenum and mostly occurs as sodium molybdate dehydrate. 
Its chemical formula is Na2MoO4. The molybdate anion is tetrahedral.Two sodium cations coordinate for each anion.

Sodium Molybdate (Sodium Molybdate Dihydrate) is widely used in manufacturing, including agricultural fertilizers, pigments, catalysts, fire retardants, corrosion inhibitors, as well as water treatment.

Sodium Molybdate is widely used in manufacturing, including agricultural fertilizers, pigments, catalysts, fire retardants, corrosion inhibitors, as well as water treatment.

It is an essential micronutrient for plants and animals. 
It is commonly used for hydroponics and leguminous plants, such as peas, beans, lentils, alfalfa, and peanuts. 
Sodium Molybdate improves the uptake of nitrogen and promotes efficient fixing of atmospheric nitrogen by bacteria. 

Sodium Molybdate is used in water treatment, including industrial water treatment due to its low toxicity. 
The advantage of Sodium Molybdate in water treatment is that it is effective in low dosages, which maintains low conductivity of water and prevents corrosion by reducing galvanic corrosion potentials.

It is also used for metal surface treatment, including galvanizing and polishing.
 
Sodium Molybdate dihydrate, also known as disodium molybdate is an odourless white, crystalline powder with the chemical formula Na2MoO4. 
Manufactured from pure molybdenum ore this product is of an extremely high quality. 

Sodium Molybdate  is widely used in the water treatment industry as a corrosion inhibitor in water treatment products. 
It is also used in agriculture as a micronutrient for plants and used in the manufacturing process of pigments, lubricants and an additive for metal finishing.

Sodium Molybdate as a corrosion inhibitor
Sodium Molybdate is an ideal environmentally responsible corrosion inhibitor for water and cooling systems. 
Capable of working across a variety of temperatures and pH levels, sodium molybdate experiences no loss of chemical properties or effectiveness in a variety of hot or cold environments. 
When used, it is capable of inhibiting the corrosion of ferrous, copper and aluminium metals in the cooling water of both open and closed cooling systems. 

Sodium Molybdate in Agriculture 
Sodium molybdate offers a useful source of molybdenum which is an excellent soil micronutrient and essential for healthy plant growth making it a popular choice of fertiliser within the agricultural industry. 
Suitable for foliar or fertigation applications, it is used in small amounts to supply molybdenum to crops and livestock. 
Sodium molybdate is also added to cattle feed when treating copper deficiencies.

PRODUCTION
Sodium molybdate was first produced by hydration process. 
It can be produced by dissolving molybdenum trioxide in sodium hydroxide within a temperature range of 50-70°C and then crystallizing the filtered product. 
The final anhydrous product is obtained by heating it to 100°C.


MoO3 + 2NaOH + H2O → Na2MoO4·2H2O


The above reaction illustrates the production reaction, where molybdenum trioxide reacts with sodium hydroxide to produce sodium molybdate along with water.


USES
WATER TREATMENT
Sodium molybdate is preferred over sodium nitrite in the industrial water treatment process, where there is a potential of galvanic corrosion owing to its bimetallic construction. 
It facilitates lower conductivity in the coursing water when the dosage is in lower ppm's of molybdate.

Sodium molybdate can offer consumption restraint, within the range of 50-100 ppm. Whereas, sodium nitrate would have to be at levels of 800 ppm to offer the same consumption restraint. 
The lower concentrations of sodium molybdate would maintain low conductivity and therefore the potentials of galvanic consumption are diminished.

 

AGRICULTURE
The agriculture industry uses nearly half a million kilograms of sodium molybdate as fertilizer annually. 
It is important for the conversion of nitrates present in leaves to proteins and amino acids. 
Molybdenum is essential for the optimum growth of leguminous plants.

 

This chemical is used to treat whiptail disorder in plants such as broccoli and cauliflower, which occurs due to the deficiency of molybdenum in the soil. 
Its dosage must be within limits, which otherwise would lead to copper deficiencies in animals.

 

CHEMICALS TRACER
Sodium molybdate finds applications in medicinal chemistry and biochemistry. 
It is used to track numerous natural organic chemicals, which are colorless post chromatographical analysis, where it stains blue. 
The blue color is known as molybdenum blue.
 

CORROSION INHIBITION
Sodium molybdate is a non-oxidizing anodic inhibitor and therefore used in corrosion inhibition. 
This chemical reduces the requirement of nitrate by fluids inhibited with nitrite amine and subsequently enhances the corrosion security of carboxylate salt fluids.

 

PRECAUTIONS 
Sodium molybdate is not compatible with oxidizing agents, alkali metals and many normal metals. 
This chemical could blast, when coming in contact with liquid magnesium. 
It violently reacts with interhalogens such as bromine pentafluoride and chlorine trifluoride. 
Sodium molybdate results in incandescent reactions when treated with hot lithium, sodium or potassium.


Sodium molybdate
Disodium molybdate

CAS Number    
7631-95-0 
10102-40-6 (dihydrate)


Chemical formula: Na2MoO4
Molar mass: 205.92 g/mol (anhydrous)
241.95 g/mol (dihydrate)
Appearance: White powder
Density    3.78 g/cm3, solid
Melting point: 687 °C (1,269 °F; 960 K)
Solubility in water: 84 g/100 ml (100 °C)
Refractive index (nD): 1.714


Sodium molybdate [Wiki]
12680-49-8 [RN]
231-551-7 [EINECS]
7631-95-0 [RN]
Dinatriumdioxido(dioxo)molybdaen [German] [ACD/IUPAC Name]
Dioxo(dioxydo)molybdène de disodium [French] [ACD/IUPAC Name]
Disodium dioxido(dioxo)molybdenum [ACD/IUPAC Name]
Molybdenum, diolatodioxo-, sodium salt (1:2) [ACD/Index Name]
sodium molybdate (anhydrous)
Sodium Molybdate, anhydrous
[7631-95-0]
10102-40-6 [RN]
106463-33-6 [RN]
14666-91-2 [RN]
231-107-2 [EINECS]
disodium diketo-dioxido-molybdenum
disodium dioxido-dioxomolybdenum
disodium dioxido-dioxo-molybdenum
Disodium molybdate
disodium tetraoxomolybdate
disodium;dioxido(dioxo)molybdenum
EINECS 231-551-7
Molybdate (MoO42-), disodium, (T-4)-
Molybdate (MoO42-), disodium, (β-4)-
Molybdate disodium
Molybdic acid (H2MoO4) , disodium salt
Molybdic acid (H2MoO4), disodium salt (8CI)
MOLYBDIC ACID DISODIUM SALT
Molybdic acid, disodium salt
Na2MoO4
Natriummolybdat [German]
Natriummolybdat
Natriummolybdat [German]
sodium dioxido(dioxo)molybdenum
sodium molybdate (anh.)
Sodium molybdate (Na2MoO4)
Sodium molybdate (VAN)
Sodium Molybdate 35% Solution
Sodium Molybdate ACS
Sodium Molybdate Anhydrous
Sodium Molybdate Crystals, Technical Grade
Sodium molybdate dihydrate
Sodium Molybdate Dihydrate (Technical Grade)
Sodium Molybdate Solution 35%
Sodium molybdate(VI)
Sodium Molybdate, ACS Grade
sodium orthomolybdate
sodiummolybdate


Sodium molybdate
Disodium molybdate
7631-95-0
Sodium molybdate(VI)
Molybdate disodium
Natriummolybdat
sodium molybdenum oxide
Molybdic acid, disodium salt
Natriummolybdat [German]
Sodium molybdate (VAN)
Sodium molybdate (Na2MoO4)
UNII-948QAQ08I1
CCRIS 5442
EINECS 231-551-7
NSC 77389
sodium molybdate (anhydrous)
Molybdic acid (H2MoO4) , disodium salt
Molybdate (MoO42-), disodium, (T-4)-
disodium;dioxido(dioxo)molybdenum
948QAQ08I1
13466-16-5
Molybdate (MoO42-), disodium, (beta-4)-
Sodium dimolybdate
Na2MoO4
Sodium Molybdate Anhydrous
MoNa2O4
Molybdic acid (H2MoO4), disodium salt
Molybdenum (as sodium)
Anhydric sodium molybdate
disodium tetraoxomolybdate
sodium molybdate (anh.)
Molybdic acid, sodium salt
EC 231-551-7
Sodium Molybdate, anhydrous
Sodium molybdate, >=98%
Ddisodium Molybdate Dihydrate
Sodium Molybdate, ACS Grade
Molybdate (MoO42-), sodium (1:2), (T-4)-
sodium dioxido(dioxo)molybdenum
Sodium Molybdate 35% Solution
Sodium Molybdate Solution 35%
Sodium molybdate, LR, >=99.5%
Sodium Molybdate Crystals, Technical Grade
Sodium Molybdate Dihydrate (Technical Grade)
Molybdic acid (H2MoO4), disodium salt (8CI)
Sodium molybdate, anhydrous, powder, -100 mesh particle size, 99.9% trace metals basis

Other Names
Molybdic acid (H2MoO4), disodium salt, dihydrate
Molybdate (MoO42-), disodium, dihydrate, (T-4)-
Sodium Molybdate Anhydrous


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Ammonium Heptamolybdate
Ammonium Octamolybdate

Corrosion Inhibitors are used in both open & closed recirculating cooling water treatment products. 
Benzotriazole (BZT) is a similar, but less common inhibitor. Sodium Molybdate is also used in cooling water treatment as a corrosion inhibitor. 
Molybdenum is used both as an inhibitor and as a trace chemical to monitor dosage measurements due to its yellow color.

Sodium Nitrite is another common metal corrosion inhibitor for iron & steel and is typically used in closed-loop cooling water treatment products, such as chiller systems. 
(Other allied products which accompany these products are borates which act as buffers/stabilizers and have general cleaning attributes as well.)

Corrosion Inhibition:

CORROSION INHIBITION - 
In 1939, two patents first described the use of readily soluble sodium, potassium, and ammonium molybdates as corrosion inhibitors for motor vehicle engine coolants. 
These and other inorganic molybdates are now among the most popular corrosion inhibitors because of their favorable properties and behavior. 
Molybdate is an anodicinhibitor, i.e. it inhibits by increasing the polarization of the anode component of the metal corrosion cell. 
Basically, Molybdate forms a transparent passivating film that inhibits the corrosion of ferrous, aluminum, and cupreous metals over a wider pH range than any other inhibitor apart from chromate. 
It does this by precipitating escaping metal cations as molybdate species to block anodic sites and strengthen developing metal oxide films.
Furthermore, molybdate does not lose it’s chemical properties and effectiveness when it encounters ‘hot spots’ or increased temperatures; nor does molybdate breakdown in chlorinated systems or when chlorine is used as a biocide like other inhibitors (e.g. Belcor 575).

Molybdates are seldom used alone because the concentration level required for effective corrosion retardation make it commercially unattractive (e.g. 250 mg/L) – as with other anodically active inhibitors, molybdate efficacy is improved and its concentration requirement significantly reduced when it is combined with other synergistic chemicals. 
Among the best known of these synergists are amines (e.g. Cyclohexylamine, DEAE), phosphonates (e.g. HEDP, PBTC), azoles (e.g. tolyltriazole, benzotriazole), and soluble zinc salts (e.g. zinc sulfate).

The largest consumption of molybdates for corrosion inhibition is in the treatment of cooling water in open and closed cooling systems

Commercial water treatment programs commonly employ molybdate with as many as four or five other components to inhibit and/or control corrosion, algae & microorganism growth, pH regulation, and scale or solids dispersion. 
A typical formulation of a copper corrosion inhibition treatment that has retarded corrosion to a rate of 0.021 mils/yr. follows:

Sodium tolyltriazole 50% solution 1 mg/L
Sodium molybdate dihydrate 5 mg/L
HEDP 3 mg/L
Zinc sulfate 2 mg/L

Other products and processes which utilize molybdates include hydraulic & metalworking fluids, temporary rust-preventive coatings, pitting inhibitor for stainless steels in mineral acids, passivation treatments for galvanized zinc and tin plate, rinses for phosphate conversion coating, aluminum anodizing processes, hot forging lubricants, boiler waters, engine coolants, as well as many others


A shiny grey metal, which looks quite a lot like steel, molybdenum has been around in nature since time immemorial, however because of its limited occurrence, it has always been considered extremely valuable. 
Molybdenum has been used widely as an alloying element as well as a metallic coating. 
The oxides of moly find usage in several industries, including petrochemical, water and chemical treatment.

Moly compounds and their usages:

Molybdenum disulphide, or Molybdenite, is a naturally occurring ore and molybdates are chemical products that are made from this ore. 
The formulation is done by swapping out the sulphur atoms with oxygen ones and this leads to the formation of molybdic acid or pure molybdic oxide. 
Depending on what base is being used and under what conditions the basification is being done, multiple molybdates can be produced and the most common ones include pure molybdic oxide, sodium molybdate dihydrate and ammonium dimolybdate. 

Molybdic oxide finds several uses – as a component in certain types of steel, for the manufacture of other moly compounds and most importantly, as a chemical in water treatment facilities in the form of corrosion inhibitor formulations. 
It can help prevent corrosion on steel and aluminium and in places where the water needs to be heated, such as in boilers and water heating systems, sodium molybdate is most commonly used. Other uses include temporary coatings against rusting, pitting inhibitors for stainless steels, hydraulic and metalwork fluids, in rinses for phosphate conversion coating and aluminium anodizing processes.

Usage of molybdates as corrosion inhibitors:

The earliest patents related to the usage of molybdates as corrosion inhibitors, date all the way back to 1939, wherein they were considered as appropriate for coolants for motor engine vehicles. 
Recent studies have proven that molybdate is an anodic inhibitor – this means that it inhibits the anode component in the cells that causes corrosion, by increasing the polarization. 
In simple terms, the molybdate when used under the correct circumstances forms a protective layer, which inhibits the corrosion caused due to aluminium, ferrous and cuprous metals. 
More importantly, molybdates do not lose any of their chemical properties or efficacy, when they encounter elevated temperatures or breakdown in systems that have chlorine in them.

Perhaps the biggest advantage of using molybdates is that very low concentration levels are required to be considered effective corrosion inhibitors. 
The most common usage of molybdates for corrosion inhibition is in open and closed water cooling systems. 
In a typical commercial treatment program, there could be four to five other compounds being used with the molybdate, as it will be the combination that will ward off corrosion, regulate pH levels, control the growth of algae or other microorganisms.

Moving onto why molybdenum compounds are used in water treatment chemicals:

Most large scale commercial complexes, such as office buildings, hospitals and other such commercial institutions will utilise some version of a cooling tower. 
The heat that is generated through the HVAC systems in the building needs to be dispelled and this takes place through the cooling towers and the process of evaporation. 
When evaporation happens, there is a concentration of mineral salts and then solubility levels of these salts hit saturation point, they will start to form scales. 
The water in the cooling towers can be recycled several times, however there is always the need for certain chemicals, because with water, there is always the chance of corrosion and fungal or microbial build-up. 
Some of the most commonly used chemical products for corrosion inhibition include nitrite, borate and silicate and phosphoric acid for scale inhibition.

The use of sodium molybdate for closed water systems has been a long accepted practice and it is able to not only prevent corrosion, but also assist in the elongation of cooling tower lifecycles. 
Sodium molybdate inhibits the corrosion of low carbon steel, copper as well as brass in cooling water systems, which function on a recirculating basis and are environmentally safe. 
Previously, it was chromate that was used, however, when chromate was found to be toxic, it was banned, spurring the increased usage of molybdenum.  

Not only do these molybdate compounds have several commercial applications, they are also considered the most economical when taken in reference to water treatment; what makes it the most appropriate choice is the fact that molybdenum is safe and non-toxic for humans, in most forms. 
Studies and trials have shown that sodium molybdate’s efficacy might be altered depending on the water composition and other water treatment chemicals being used.


WHAT ARE THE USES OF MOLYBDIC OXIDE AND MOLYBDATES?

Molybdic oxide is used as a component of some types of steel, as a water treatment chemical and also as a reagent for the manufacture of other molybdenum compounds. 
The primary function of molybdic oxide as a component in steel and as a water treatment chemicals is the prevention of corrosion.


Molybdates are also very good corrosion inhibitors for steel and aluminium and these products are also used in water treatment applications. 
In particular, sodium molybdate is often used in boiler and heating system water treatment chemical formulations. 
Another use of sodium molybdates is a bioavailable source of molybdenum, particularly in the formulation of fertilisers for leguminous crops.


Ammonium molybdates are also sometimes used as corrosion inhibitors and sometimes used as a bioavailable source of molybdenum. 
These materials are also used in catalysis in industrial chemical manufacture.


Proprietary Molybdate Based Corrosion Inhibitors
Proprietary molybdate based corrosion inhibitors usually consist of an aqueous solution of sodium molybdate, a pH buffer, possibly a dispersing agent, and an azole. 
These inhibitors must not be added to a system that is used for direct/indirect heating/cooling of a potable water system.
Corrosion protection is provided by a protective “barrier” film that is formed by a chemical reaction between molybdate and iron. 
Therefore, initially the iron surface must be reasonably clean and free of corrosion products.
Molybdate based corrosion inhibitors are easy to test, and they provide excellent corrosion protection irrespective of the amount of air ingression into the system & are economical to use when the make-up rate is low.
However, if the make-up is excessive or continuous, the hardness that is introduced into the system will precipitate the molybdate, thus resulting in increased inhibitor demand and corrosion of the iron material in the system. 
Also, because the molybdenum concentration in the total waste water that is discharged to the sanitary sewer system must be less than the maximum limit of 5 mg/l Mo indicated in the

Water Treatment Program Environmental Guideline, molybdate based corrosion inhibitors have a high environmental impact.
Typically, a molybdenum concentration of 50-150 ppm Mo is maintained in the system, and the pH & TDS levels are maintained within their respective ranges of 9.0-10.5 & 2500 micromhos/cm maximum.

Sodium Molybdate is a crystalline powder essential for the metabolism and development of plants and animals as a cofactor for enzymes. 


Sodium molybdate (anhydrous) is an inorganic sodium salt having molybdate as the counterion. 

Sodium Molybdate as a Corrosion Inhibitor of Mild Steel in Natural Waters

Abstract
Sodium molybdate (Na2MoO4) is one of many proposed replacements for chromate-based inhibitors of steel corrosion. 
However, its ability to protect steel in natural waters, especially flowing natural waters, has not been examined in detail. 
Part 1 of this study detailed effects of flow rate on the corrosion rate of rotating cylinder electrodes (RCE) of ASTM A36 steel (UNS K02600) exposed to 125 ppm Na2MoO4 solutions under different flow conditions. 
Part 1 also discussed limitations of electrochemical impedance spectroscopy (EIS) as a method of examining electrochemical behavior in low-conductivity environments. 
Part 2 examined the behavior of A36 steel RCE exposed to various concentrations of Na2MoO4. 
For polished samples, a critical concentration of Na2MoO4 existed, beyond which the corrosion rate increased. 
However, no critical concentration existed for samples that possessed a corrosion product prior to the addition of inhibitors.
Oscillatory open-circuit potential behavior and EIS measurements suggested periodic active/ passive transitions on pre-corroded samples.

Sodium molybdate is a source of molybdenum oxide, and this chemical has a variety of useful industrial, commercial, and agricultural purposes. 

1. Agricultural Additive For Fertilizer
Sodium molybdate is widely used as an agricultural additive on farms. 
It’s an ideal choice for fertilizer applications. 
This is because the basic chemistry of molybdate compounds like sodium molybdate include molybdenum oxide at its highest oxidation state.
This means that the chemical is highly-soluble in water. 
This means that fertilizers using sodium molybdate easily combine and mix with water and soak into soil, delivering molybdenum oxide and other valuable micronutrients into the roots and minimizing runoff, which wastes chemical compounds and can have negative environmental consequences.
Sodium molybdate is particularly popular among farmers who primarily focus on legumes like lentils, beans, alfalfa, and peanuts. 
It helps with the uptake of nitrogen, ensuring efficient nitrogen-fixing for these plants, and allowing nitrogen to be synthesized into ammonia and essential amino acids.

2. Hydroponic Farming & Agriculture
Similarly to traditional soil-based fertilizer applications, sodium molybdate can be used in hydroponic farming, which uses inert substrates as the growing medium instead of soil. 
Mineral nutrient solutions are delivered directly to the plants using water, so highly-soluble nutrients and fertilizers – such as sodium molybdate – are very desirable for these purposes. 

3. Corrosion Inhibitor 
Sodium molybdate is commonly used as a metal corrosion inhibitor for iron and steel, and is commonly found in water treatment products like chiller systems, where bimetallic design and construction can raise the risk of metal corrosion. 
This additive is primarily used in closed-loop systems, and is regarded to be far superior to other corrosion inhibitors like sodium nitrate. At concentrations of just 50 to 100 ppm, sodium molybdate offers superior performance compared to 800+ ppm concentrations of sodium nitrate. 
4. Nutritional Supplement
Some people may choose to supplement their diets with sodium molybdate. 
These products can be found on their own, but molybdenum is typically found in multivitamins and complex vitamins. 
Typical doses for dietary supplements range from about 50 mcg to 500 mcg (micrograms) of sodium molybdate. 
Most people do not need an additional source of molybdenum, as this micronutrient is present in a wide variety of foods, such as legumes, yogurt, potatoes, whole-grain bread, beef liver, spinach, corn, cheese, tuna, and more.
However, in individuals who may have an improper diet or who wish to ensure they get adequate micronutrients, sodium molybdate is a good option. 
Cases of toxicity due to excessive intake of molybdenum are rare, and usually only occur due to exposure in the mining and metalworking industries, so supplementing with sodium molybdate is typically harmless. 

WHAT IS SODIUM MOLYBDATE?
There are two main forms of Sodium Molybdate. Sodium Molybdate, Dihydrate is a crystalline powder. 
It loses its water of crystallization at 100 degrees Celsius. 
It is known to be less toxic than the other corresponding compounds of group 6B elements in the periodic table. 
Sodium Molybdate, Dihydrate is used in the manufacturing of inorganic and organic pigments, as a corrosion inhibitor, as a bath additive for finishing metals finishing, as a reagent for alkaloids, and as an essential micronutrient for plants and animals.


Sodium Molybdate, Anhydrous is a small, lustrous, crystalline plate. 
It has the melting point of 687 degrees Celsius and a density of 3.28 (18C). 
It is soluble in water and also noncombustible. 
Sodium Molybdate can be used for reagent in analytical chemistry, paint pigment, production of molybdated toners and lakes, metal finishing, brightening agent for zinc plating, corrosion inhibitor, catalyst in dye and pigment production, additive for fertilizers and feeds, and micronutrient.

WHY THE AGRICULTURE INDUSTRY USES SODIUM MOLYBDATE
Sodium Molybdate uses cover a wide range of fields, including manufacturing, metalwork, printing, and more. 
But the impact it can have on plants and animals has brought it into the forefront of use for the agriculture industry, to the tune of more than 1 million pounds of sodium molybdate fertilizer used per year.


The basic chemistry of a molybdate, such as sodium molybdate, contains the element molybdenum in its highest oxidation state, which in turn helps contribute to a high solubility of the chemical in water, a benefit in fertilizer application. 
This characteristic, when combined with sodium molybdate’s use as a delivery vessel for essential micronutrients (such as molybdenum) in plants, serves as another key reason for the choice of sodium molybdate fertilizer over other types of fertilizers used in agriculture.


Another touchpoint for this usage ties back to the hydroponic nutrient practice that is growing in popularity. 
Hydroponics is an agricultural method in which plants are grown without soil. 
Instead, they receive their essential micronutrients through a water solvent, a practice that has shown growth rates almost 50 percent faster than traditional soil-grown plants, in addition to a higher yield from hydroponic plants.


Sodium molybdate has seen a particularly strong uptick in usage among farmers of leguminous plants, such as alfalfa, peas, beans, lentils and peanuts. 
Included in fertilizer, it provides these plants with enhanced uptake of the essential nitrogen element, while also allowing for efficient fixing of atmospheric nitrogen found in the atmosphere by bacteria in the legumes. 
These bacteria convert the nitrogen into ammonia to synthesize amino acids within the plant.


Overall, the use of sodium molybdate in the agricultural industry can be summarized in that it is one of the few chemicals that can provide essential micronutrients and help drive plant function in a form that is both efficient and effective. 
Efficiency is shown not only by the relatively small amounts needed to make an impact on the treated plants, but also in the ability to administer the chemical in easily-absorbed water-based formats.

Molybdenum is an important plant micronutrient. 
Plants pick up molybdenum (as molybdate) from the soil and only small amounts (0.1 to 1.0 ppm) are necessary to meet their dietary requirements.
It is essential for the production of two major enzymes in plants – nitrogenase and nitrate reductase – which enable nitrogen to be obtained, or ‘fixed’, from air or soil


Nitrogen is needed for compounds such as amino acids, proteins and chlorophyll. 

Plants suffer from poor growth without it, leaves may become pale and deformed, buds and flowers may not develop properly and fruit setting can be restricted. 

Acidic soils prevent the uptake of molybdate even if there are sufficient quantities in the soil. 
In these instances, lime can be added to the soil to reduce acidity, helping to increase the uptake of molybdate. 

Soils in some regions of the world are naturally low in molybdenum. 

This can also occur in peat soils and in highly weathered soils with low levels of nutrients. 

Since the importance of molybdenum in tomato crops was first recognized in 1939, deficiency symptoms have been identified in a number of crops. 

The element is critical for the nutrition of legumes, cereal, lettuce, tomatoes, cabbage, cauliflower and citrus fruit. 

An international study involving field trials in 15 countries found that molybdenum deficiency was often only revealed by yield effects and without obvious symptoms of stress to the plant, yet was the most widespread deficiency after zinc and boron.1 

In Australia, molybdenum deficiency has been identified as the second most common micronutrient deficiency affecting large areas of cropland with acid soils2 and can impair yield in cereal crops by as much as 30%.3 

In China, molybdenum deficiency affects nearly half of all agricultural soils and has been identified as an important factor limiting yields of winter wheat and soya beans.4 
Molybdenum deficiency can be misdiagnosed as nitrogen deficiency and lead to the ineffective overuse of nitrogen fertilizer, which wastes resources and risks oxygen depletion in rivers and oceans. 

Certain nitrogen fertilizers can also cause acidification of the soil, which further restricts the uptake of any available molybdate. 

As the global population grows, food security will b come more important than ever. 

Global food prices have doubled in the last decade5 and demand for food and feed crops is estimated to double in the next 50 years as the global population approaches nine billion.

In the context of these challenges, optimizing existing production by correcting micronutrient deficiencies (where they exist) becomes even more important. 

The solution Increased demand for food requires increased agricultural production, for both food and feed crops. 

Optimizing existing agricultural productivity by correcting micronutrient deficiencies can help to provide more food for a growing population while minimizing the amount of additional land turned over to food production. 

This helps to preserve biodiversity and maintain resistance to some of the impacts of climate change 

How molybdenum can help Improving the quality of soils by correcting deficiencies of micronutrients including molybdenum has been shown to be effective in improving crop yields. 

Studies in Australia have demonstrated increases in grain yield of up to 60% following the application of molybdate.

Fertilizers are an ideal method of delivering molybdenum and other nutrients. 

The agrochemical industry has developed optimized blends of nutrients and micronutrients tailored to different regions, soils and crops. 

Molybdenum is typically delivered in the form of ammonium heptamolybdate, ammonium dimolybdate or sodium molybdate. 

Alternatively, farmers can treat the crop seed or apply specially formulated foliar sprays to correct molybdenum deficiency. 

A study in Egypt8 demonstrated that adding 24 mg of molybdenum per mandarin tree, in the form of a foliar spray containing sodium molybdate, increased fruit yield by 37% (Figures 1 and 2). 

Another study in Sweden9 showed that applying just 0.25 liters per hectare of a molybdate-based foliar spray increased the yield of rapeseed plants from 1.76 to 1.89 tonnes per hectare, as shown in Figure 3. 

Better micronutrient management can prevent the inefficient overuse of nitrogen fertilisers and therefore help to minimize nitrate run-off, saving resources and reducing pollution. Molybdenum is essential to plant growth. 

Deficiencies are often caused by acidic soils which prevent uptake and can be corrected by liming. 

However, where there is not enough in the soil, applying fertilizer, seed or foliar treatments containing molybdenum can increase productivity significantly. Correcting molybdenum deficiency also ensures that the use of nitrogen fertilizer is more efficient, cost effective and less harmful to the environment. 

Optimizing output from existing production minimizes the amount of additional land turned over to food production as demand increases, thereby helping to preserve biodiversity.

You may not have heard of the trace mineral molybdenum, but it is essential to your health.

Though your body only needs tiny amounts, it’s a key component of many vital functions. 
Without it, deadly sulfites and toxins would build up in your body.

Molybdenum is widely available in the diet, but supplements are still popular. 
As with many supplements, high doses can be problematic.

This article covers everything you need to know about this little-known mineral.

What Is Molybdenum?
Molybdenum is an essential mineral in the body, just like iron and magnesium.

It is present in soil and transferred into your diet when you consume plants, as well as animals that feed on those plants.

There is very little data on the specific molybdenum content of certain foods, as it depends on the content of the soil.

Although amounts vary, the richest sources are usually beans, lentils, grains and organ meats, particularly liver and kidney. 
Poorer sources include other animal products, fruits and many vegetables (1).

Studies have shown that your body doesn’t absorb it well from certain foods, particularly soy products. 
However, this is not considered a problem since other foods are so rich in it (2Trusted Source).

Since your body only needs it in trace amounts and it’s abundant in many foods, molybdenum deficiency is rare. 
For this reason, people don’t usually need supplements, unless for some specific medical reasons.

SUMMARY:
Molybdenum is found in many foods, such as legumes, grains and organ meats. Your body only requires it in trace amounts, so deficiency is extremely rare.

It Acts as a Cofactor for Important Enzymes
Molybdenum is vital for many processes in your body.

Once you eat it, it is absorbed into your blood from your stomach and gut, then carried to your liver, kidneys and other areas.

Some of this mineral is stored in the liver and kidneys, but most of it is converted into a molybdenum cofactor. 
Any excess molybdenum is then passed in urine.

The molybdenum cofactor activates four essential enzymes, which are biological molecules that drive chemical reactions in the body. 
Below are the four enzymes:

Sulfite oxidase: Converts sulfite to sulfate, preventing the dangerous buildup of sulfites in the body.
Aldehyde oxidase: Breaks down aldehydes, which can be toxic to the body. 
Also, it helps the liver break down alcohol and some drugs, such as those used in cancer therapy.
Xanthine oxidase: Converts xanthine to uric acid. 
This reaction helps break down nucleotides, the building blocks of DNA, when they’re no longer needed. 
They can then be excreted in the urine (8Trusted Source).
Mitochondrial amidoxime reducing component (mARC): This enzyme’s function isn’t fully understood, but it’s thought to remove toxic byproducts of metabolism (9).
Molybdenum’s role in breaking down sulfites is especially important.

Sulfites are found naturally in foods and also sometimes added as a preservative. 
If they build up in the body, they can trigger an allergic reaction that can include diarrhea, skin problems or even breathing difficulties (10Trusted Source).

SUMMARY:
Molybdenum acts as a cofactor for four enzymes. These enzymes are involved in processing sulfites and breaking down waste products and toxins in the body.


Very Few People Are Deficient
Although supplements are widely available, molybdenum deficiency is very rare in healthy people.

The estimated average daily intake of molybdenum in the US is 76 micrograms per day for women and 109 micrograms per day for men.

This exceeds the Recommended Dietary Allowance (RDA) for adults, which is 45 micrograms per day.

Information on molybdenum intake in other countries varies, but it’s usually well above requirements.

There have been a few exceptional cases of molybdenum deficiency, which have been linked to adverse health conditions.

In one situation, a hospital patient was receiving artificial nutrition through a tube and not given any molybdenum. 
This resulted in severe symptoms, including fast heart rate and breathing, vomiting, disorientation and eventually coma (12Trusted Source).

Long-term molybdenum deficiency has been observed in some populations and linked to an increased risk of esophageal cancer.

In one small region of China, esophageal cancer is 100 times more common than in the US. 
It has been discovered that the soil in this area contains very low levels of molybdenum, resulting in a long-term low dietary intake (13Trusted Source).

Furthermore, in other areas that have a high risk of esophageal cancer, such as parts of South Africa, molybdenum levels in hair and nail samples have been found to be low (14Trusted Source, 15Trusted Source).

It is important to note that these are cases in individual populations, and deficiency is not an issue for most people.

SUMMARY:
In a few cases, low molybdenum content in the soil has been linked to esophageal cancer. 
However, since the average daily intake of molybdenum in the US exceeds the RDA, deficiency is extremely rare.


Molybdenum Cofactor Deficiency Causes Severe Symptoms That Appear in Infancy
Molybdenum cofactor deficiency is a very rare genetic condition in which babies are born without the ability to make molybdenum cofactor.

Therefore, they are unable to activate the four important enzymes mentioned above.

It’s caused by a recessive, hereditary gene mutation, so a child would have to inherit the affected gene from both parents to develop it.

Babies with this condition appear normal at birth, but become unwell within a week, experiencing seizures that don’t improve with treatment.

Toxic levels of sulfite accumulate in their blood, since they are unable to convert it to sulfate. This leads to brain abnormalities and severe developmental delays.

Sadly, babies who are affected do not survive past early childhood.

Fortunately, this condition is extremely rare. Prior to 2010, there were only about 100 reported cases globally (16Trusted Source, 17).

SUMMARY:
Molybdenum cofactor deficiency causes brain abnormalities, developmental delays and childhood death. 
Fortunately, it’s extremely rare.

Too Much Can Cause Serious Side Effects
As with most vitamins and minerals, there is no advantage to taking more than the recommended amount of molybdenum.

In fact, doing so can harm your health.

The tolerable upper intake level (UL) is the highest daily intake of a nutrient that is unlikely to cause harm for almost all people. 
It is not recommended to regularly exceed it.

The UL for molybdenum is 2,000 micrograms (mcg) per day (18Trusted Source).

Molybdenum toxicity is rare and studies in humans are limited. 
However, in animals, very high levels have been linked to reduced growth, kidney failure, infertility and diarrhea (19Trusted Source).

On rare occasions, molybdenum supplements have caused serious side effects in humans, even when the doses were well within the UL.

In one case, a man consumed 300–800 mcg per day over 18 days. He developed seizures, hallucinations and permanent brain damage (20Trusted Source).

High molybdenum intake has also been linked to a number of other conditions.

Gout-Like Symptoms
Too much molybdenum can cause a buildup of uric acid due to the action of the enzyme xanthine oxidase.

A group of Armenian people who each consumed 10,000–15,000 mcg a day, which is 5–7 times the UL, reported gout-like symptoms (19Trusted Source).

Gout occurs when there are high levels of uric acid in the blood, which causes tiny crystals to form around the joints, leading to pain and swelling.

Poor Bone Health
Studies have shown that a high intake of molybdenum could possibly cause decreased bone growth and bone mineral density (BMD).

Currently, there are no controlled studies in humans. However, an observational study of 1,496 people found interesting results.

It found that as molybdenum intake levels increased, lumbar spine BMD appeared to decrease in women over the age of 50 (21Trusted Source).

Controlled studies in animals have supported these findings.

In one study, rats were fed high amounts of molybdenum. 
As their intake increased, their bone growth decreased (22Trusted Source).

In a similar study in ducks, high intakes of molybdenum were associated with damage to their foot bones (23Trusted Source).

Decreased Fertility
Research has also shown an association between high molybdenum intake and reproductive difficulties.

An observational study including 219 men recruited through fertility clinics showed a significant relationship between increased molybdenum in the blood and decreased sperm count and quality (24Trusted Source).

Another study also found that increased molybdenum in the blood was linked to decreased testosterone levels. 
When combined with low zinc levels, it was linked with a whopping 37% reduction in testosterone levels (25Trusted Source).

Controlled studies in animals have also supported this link.

In rats, high intakes have been linked to decreased fertility, growth failure of offspring and sperm abnormalities (26Trusted Source, 27Trusted Source, 28Trusted Source).

Although the studies raise many questions, more research is needed.

SUMMARY:
In rare cases, high intakes of molybdenum have been linked to seizures and brain damage. 
Initial studies have also suggested an association with gout, poor bone health and decreased fertility.


Preparation
may be obtained as a dihydrate by evaporating an aqueous solution of molybdenum trioxide and sodium hydroxide. 
Heating the dihydrate at 100°C converts it to the anhydrous salt.
Chemical Properties
solid

Uses
Reagent in analytical chemistry, paint pigment, production of molybdated toners and lakes, metal finishing, brightening agent for zinc plating, corro- sion inhibitor, catalyst in dye and pigment produc- tion, additive for fertilizers and feeds, micronutri- ent.
Definition
ChEBI: An inorganic sodium salt having molybdate as the counterion.
Hazard
Irritant.

Agricultural Uses
Sodium molybdate (NazMoO4-2H2O), which is an important molybdenum source, is applied along with other fertilizers or as a foliar spray (with 39% molybdenum). 
Sodium molybdate is the sodium salt of molybdic acid. Fusing molybdenum oxide with sodium carbonate or hydroxide makes sodium molybdate.
Molybdenum is an essential component of the enzyme nitrate reductase which catalyzes the conversion of nitrate (NO3-) to nitrite (NO2-). 
It is also a component of the nitrogenase enzyme involved in nitrogen fixation by root nodule bacteria of leguminous crops. 
Soaking seeds in sodium molybdate solution (made with slurry or dust) before sowing is an effective seed treatment. 
Sodium molybdate, the most commonly used fertilizer supplying molybdenum, is used as foliar spray, or in mixed fertilizers. 
It is also used in seed treatment.


Molybdenum Can Be Used as a Treatment for Some Diseases
In certain situations, molybdenum can help reduce the levels of copper in the body. 
This process is being investigated as a treatment for some chronic diseases.

Excess dietary molybdenum has been shown to result in copper deficiency in ruminant animals, such as cows and sheep.

Due to the specific anatomy of ruminants, molybdenum and sulfur combine in them to form compounds called thiomolybdates. 
These prevent the ruminants from absorbing copper.

This is not thought to be a nutritional concern for humans, since the human digestive system is different.

However, the same chemical reaction has been used to develop a compound called tetrathiomolybdate (TM).

TM has the ability to reduce copper levels and is being researched as a potential treatment for Wilson’s disease, cancer and multiple sclerosis (29Trusted Source, 30Trusted Source, 31Trusted Source, 32Trusted Source, 33Trusted Source, 34Trusted Source).

SUMMARY:
The product of a chemical reaction between molybdenum and sulfur has been shown to reduce copper levels, and is being researched as a treatment for chronic diseases like cancer and multiple sclerosis.

How Much Do You Need?
It is clear that both too much and too little molybdenum can be extremely problematic.

So how much do you actually need?

It is hard to measure molybdenum in the body, since blood and urine levels don’t necessarily reflect status.

For this reason, data from controlled studies has been used to estimate requirements.

Here are the RDAs for molybdenum for different populations (1):

Children
1–3 years: 17 mcg per day
4–8 years: 22 mcg per day
9–13 years: 34 mcg per day
14–18 years: 43 mcg per day
Adults
All adults over 19 years old: 45 mcg per day.

Pregnant or Breastfeeding Women
Pregnant or breastfeeding women of any age: 50 mcg per day.

SUMMARY:
Controlled studies have been used to estimate RDAs for molybdenum for adults and children, as well as women who are pregnant or breastfeeding.


The Bottom Line
Molybdenum is an essential mineral found in high concentrations in legumes, grains and organ meats.

It activates enzymes that help break down harmful sulfites and prevent toxins from building up in the body.

Situations in which people get too much or too little of the mineral are extremely rare, but both have been linked to serious adverse effects.

Since molybdenum is found in many common foods, the average daily intake exceeds requirements. 
For this reason, most people should avoid supplementing with it.

As long as you eat a healthy diet with a variety of whole foods, then molybdenum is not a nutrient to be concerned about.

The Role of Molybdenum in Agricultural Plant Production

• Background The importance of molybdenum for plant growth is disproportionate with respect to the absolute amounts required by most plants. 
Apart from Cu, Mo is the least abundant essential micronutrient found in most plant tissues and is often set as the base from which all other nutrients are compared and measured. 
Molybdenum is utilized by selected enzymes to carry out redox reactions. 
Enzymes that require molybdenum for activity include nitrate reductase, xanthine dehydrogenase, aldehyde oxidase and sulfite oxidase.
• Scope Loss of Mo-dependent enzyme activity (directly or indirectly through low internal molybdenum levels) impacts upon plant development, in particular, those processes involving nitrogen metabolism and the synthesis of the phytohormones abscisic acid and indole-3 butyric acid. 
Currently, there is little information on how plants access molybdate from the soil solution and redistribute it within the plant. 
In this review, the role of molybdenum in plants is discussed, focusing on its current constraints in some agricultural situations and where increased molybdenum nutrition may aid in agricultural plant development and yields.
• Conclusions Molybdenum deficiencies are considered rare in most agricultural cropping areas; however, the phenotype is often misdiagnosed and attributed to other downstream effects associated with its role in various enzymatic redox reactions. 
Molybdenum fertilization through foliar sprays can effectively supplement internal molybdenum deficiencies and rescue the activity of molybdoenzymes. 
The current understanding on how plants access molybdate from the soil solution or later redistribute it once in the plant is still unclear; however, plants have similar physiological molybdenum transport phenotypes to those found in prokaryotic systems. 
Thus, careful analysis of existing prokaryotic molybdate transport mechanisms, as well as a re-examination of know anion transport mechanisms present in plants, will help to resolve how this important trace element is accumulated.
Keywords: Molybdenum, molybdate transport, nitrate reductase, Moco, Vitis vinifera, Merlot, Millerandage, sulfate transport, nitrogen fixation, nitrogen metabolism, plant nutrition

INTRODUCTION
Molybdenum is a trace element found in the soil and is required for growth of most biological organisms including plants and animals. 
Molybdenum is a transition element, which can exist in several oxidation states ranging from zero to VI, where VI is the most common form found in most agricultural soils. 
Similar to most metals required for plant growth, molybdenum has been utilized by specific plant enzymes to participate in reduction and oxidative reactions. 
Molybdenum itself is not biologically active but is rather predominantly found to be an integral part of an organic pterin complex called the molybdenum co-factor (Moco). 
Moco binds to molybdenum-requiring enzymes (molybdoenzymes) found in most biological systems including plants, animals and prokaryotes (Williams and Frausto da Silva, 2002). 
The availability of molybdenum for plant growth is strongly dependent on the soil pH, concentration of adsorbing oxides (e.g. Fe oxides), extent of water drainage, and organic compounds found in the soil colloids. 
In alkaline soils, molybdenum becomes more soluble and is accessible to plants mainly in its anion form as  
In contrast, in acidic soils (pH <5·5) molybdenum availability decreases as anion adsorption to soil oxides increase (Reddy et al., 1997). 
When plants are grown under molybdenum deficiency, a number of varied phenotypes develop that hinder plant growth. 
Most of these phenotypes are associated with reduced activity of molybdoenzymes. 
These enzymes include the primary nitrogen assimilation enzymes such as nitrate reductase (NR), and the nitrogen-fixing enzyme nitrogenase found in bacteroids of legume nodules.
Other molybdoenzymes have also been identified in plants including xanthine dehydrogenase/oxidase involved in purine catabolism and ureide biosynthesis in legumes, aldehyde oxidase (AO) that is involved in ABA biosynthesis, and sulfite oxidase that can convert sulfite to sulfate, an important step in the catabolism of sulfur-containing amino acids (Mendel and Haensch, 2002; Williams and Frausto da Silva, 2002). 
There are recent review articles on molybdoenzymes in plants, animals and prokaryotes (Mendel and Haensch, 2002; Williams and Frausto da Silva, 2002; Sauer and Frebort, 2003) that cover the extensive literature on the regulation and formation of Moco and the activity of Moco with molybdenum-dependent apoenzymes. 
Instead of re-examining this important component of molybdenum nutrition, this review will instead re-examine the effects of molybdenum nutrition in agricultural plants and explore the poorly understood aspect of molybdenum transport into and within the plant. 
In prokaryotes and lower-order eukaryotes, the molybdate transport systems have been well defined and are characterized at both the physiological, biochemical and genetic levels (Grunden and Shanmugam, 1997; Self et al., 2001). 
Unfortunately, this wealth of sequence information has not translated into an improved understanding of how eukaryotic systems transport molybdenum. 
This is not surprising as the primary molybdate transport systems present in prokaryotes are members of the ATP-binding cassette (ABC) protein superfamily. 
Members of this superfamily extend into plants; however, the numbers are large, where in arabidopsis alone there is predicted to be at least 129 putative proteins in the genome (Sanchez-Fernandez et al., 2001). 
Secondly a large number of other putative transport proteins that may encode molybdate transport systems still remain uncharacterized in sequenced plant genomes (Schwacke et al., 2003). 
Nevertheless, the prokaryotic systems are good starting points to discuss the types of eukaryotic systems that may exist and direct future research into specifically identifying plant molybdenum transport systems.
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AVAILABILITY OF MOLYBDENUM IN AGRICULTURAL SOILS
Molybdenum is present in the lithosphere at average levels up to 2·3 mg kg−1 but can increase in concentration (300 mg kg−1) in shales that contain significant organic matter (Fortescue, 1992; Reddy et al., 1997). 
In agricultural soils, molybdenum is present as many different complexes depending on the chemical speciation of the soil zone. 
Mineral forms of molybdenum found in rocks include molybdenite (MoS2), wulfenite (PbMoO4) and ferrimolybdenite [Fe2(MoO4)]

The requirement of molybdenum for plant growth was first demonstrated by Arnon and Stout (1939) using hydroponically grown tomato. 
Plants grown in nutrient solution without molybdenum developed characteristic phenotypes including mottling lesions on the leaves, and altered leaf morphology where the lamellae became involuted, a phenotype commonly referred to as ‘whiptail’ (Arnon and Stout, 1939). 
The only trace element that could eliminate these phenotypes was found to be molybdenum. 
The first reported case of molybdenum deficiency in an agricultural context occurred in mixed pasture grasses in the Lofty ranges of South Australia (Anderson, 1942). 
Local pastoralists reported significant failures of well-irrigated pastures containing subterranean clover (Trifolium subterraneaum), perennial rye grass and Phalaris tuberosa. 
These pastures had been sown on sandy loam (ironstone) soils, which were low in nitrogen, slightly acidic (pH 5·5–6), rich in iron oxides and had received significant superphosphate treatments in previous years (Anderson, 1942, 1946). 
It was noted at the time that clover could grow in these soils after liming or when wood-ash was present (Anderson, 1942). 
It was later identified that molybdenum was the most abundant trace element present in the soluble and insoluble extractions of the wood-ash. 
Molybdate application at 2 lb per acre was capable of increasing lucerne yields approx. 3-fold over control plots (Anderson, 1942). 
Shortly thereafter, Davies (1945) and Mitchell (1945) demonstrated that the whiptail phenotype in cauliflower could be overcome with the addition of molybdenum to the soil. 
Walker (1948) observed that tomato grown in molybdenum-deficient serpentine soils could be rapidly rescued (return of green colour, loss of mottling) with application of sodium molybdate directly to the soil, or by leaf painting and leaf infiltration.

In contrast, molybdenum toxicity in plants under most agricultural conditions is rare. 
In tomato and cauliflower, plants grown on high concentrations of molybdenum will have leaves that accumulate anthocyanins and turn purple, whereas, in legumes, leaves have been shown to turn yellow (Bergmann, 1992; Gupta, 1997b). 
The greatest concern associated with high plant molybdenum levels is with crops used for grazing or silage production. 
Ruminant animals, which consume plant tissues high in molybdenum content, can suffer from molybdenosis, a disorder that induces copper deficiencies (Scott, 1972). 
Fortunately this disorder can be controlled by directly maintaining adequate Mo/Cu ratios in the rumen diet or by altering the availability of molybdenum to plants by changes in soil availability (pH adjustment).

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VISUAL SYMPTOMS OF MOLYBDENUM DEFICIENCY IN PLANTS
Molybdenum deficiencies have been documented in many plant species where phenotypes range in severity and appearance (Hewitt and Bolle-Jones, 1952a). 
In the Brassicaceae family, molybdenum deficiencies are strikingly pronounced and reproducible amongst many of its members. 
Visual effects in young plants include mottling, leaf cupping, grey tinting, and flaccid leaves which are often found on seedlings that remain dwarfed until dying (Hewitt and Bolle-Jones, 1952a). 
In older plants, where deficiencies have been rescued or when deficiency levels are modest, the symptoms appear in younger leaf tissues with the characteristic loss of proper lamina development (whip-tail), leathery leaves and meristem necrosis (Hewitt and Bolle-Jones, 1952b). 
Investigation into the ultrastructure of leaves exhibiting whip-tail indicated that chloroplasts near the lesions became bulbous and enlarged with spherical protrusions bounded by chloroplast and tonoplast membranes (Fido et al., 1977).

Deficiency symptoms can also be masked by the indirect effect of molybdenum on nitrogen assimilatory enzymes (i.e. NR). 
Many horticultural, cereal and legume crops growing at deficient molybdenum levels in the presence of nitrate fertilizers will develop pale green leaves and, at times, necrotic regions at leaf margins with accompanied decreases in overall plant growth (Hewitt and Bolle-Jones, 1952a; Agarwala et al., 1978; Chatterjee et al., 1985; Chatterjee and Nautiyal, 2001). 
Molybdenum-deficient oat and wheat develop necrotic regions on leaf blades, and seeds are poorly developed and shrivelled (Anderson, 1956; Chatterjee and Nautiyal, 2001). 
In maize, molybdenum deficiency shortens internodes, decreases leaf areas and causes the development of chlorotic leaves (Agarwala et al., 1978). 
In reproductive tissues in maize, molybdenum deficiency can alter the phenotypes in developing flowers, including delayed emergence of tassels, small anthers, poorly developed stamens, and reduced pollen grain development (Agarwala et al., 1979). 
Pollen that is released from the anthers has been shown to be shrivelled and have poor germination rates (Agarwala et al., 1978, 1979). 
In grapevines, molybdenum deficiency has recently been suggested as the primary cause of a bunch development disorder called Millerandage or ‘hen and chicken’ (Williams et al., 2004). 
Millerandage (Fig. 1) is characterized by grapevine bunches that develop unevenly, where fully matured berries are present in a bunch alongside a large number of fertilized underdeveloped berries as well as unfertilized swollen green ovaries (Mullins et al., 2000). 
Millerandage has been reported primarily in Vitis vinifera ‘Merlot’ but unpublished anecdotal reports have suggested the problem also occurs in Cabernet Sauvignon and Chardonnay cultivars (P. Dry, The University of Adelaide, Adelaide Australia, pers. comm.). 
In Merlot vines displaying Millerandage, other characteristic molybdenum-deficiency responses also appear including shortened zigzag-shaped internodes, pale-green leaves, increased cupped and flaccid leaves, and marginal leaf necrosis (K. Gridley, University of Adelaide, unpubl. res.).

BIOCHEMICAL RESPONSE IN PLANTS TO MOLYBDENUM DEFICIENCIES
Molybdenum deficiency affects plant metabolism at many different levels. 
The responses are strongly linked to the requirement of molybdenum for the various types of molybdoenzymes present in plants. 
Plant molybdoenzymes can be broken down to those involved in nitrogen reduction and assimilation [i.e. nitrate reduction (nitrate reductase; NR), nitrogen fixation (nitrogenase), purine catabolism (xanthine dehydrogenase/oxidase; XDH), abscisic acid (ABA) and indole-3 acetic acid (IAA) synthesis (aldehyde oxidase; AO)] and sulfur metabolism (sulfite oxidase; SO). 
The molybdoenzymes can be classified even further based on their interactions with Moco. 
NR and SO contain a dioxo-Mo co-factor, which activates the protein when it is inserted into the protein complex (Mendel and Haensch, 2002). 
XDH and AO have a monoxo-Mo co-factor which requires Moco insertion and then subsequent sulfuration of the Mo centre to activate the Moco/protein complex (Mendel and Haensch, 2002). 
Since molybdenum is involved in a number of different enzymatic processes, a defined plant response to molybdenum deficiency can be complex and thus difficult to assign causally to specific enzyme systems. 
This is particularly evident in molybdoenzymes involved in nitrogen metabolism where overall reductions in plant growth and health can alter plant development, susceptibility to pest damage, and fruit or grain development (Graham and Stangoulis, 2005).

Molybdate transport into plants
Since there is no known molecular mechanism controlling molybdate transport in plants, and higher organisms for that matter, we are left to speculate on the types of systems based on the information we have from prokaryote and whole-plant molybdenum nutrition studies. 
Unfortunately, linking prokaryotic molybdate transport systems to the processes, which occur in eukaryotes, is not direct as there is limited sequence homology to modABC, modE and ModF in either arabidopsis or rice genomes or any other large plant expressed sequence tagged collections or partially sequenced genomes. 
However, there are similarities in physiological responses to molybdenum between prokaryotic and eukaryotic systems, namely the close interaction with sulfate transport. 
Sulfate is a similar-sized anion to molybdate, and evidence from prokaryotic studies suggests that sulfate transport systems and selenate-sensitive anion channels are capable of molybdate transport (Self et al., 2001). 
Stout and Meagher (1948) first demonstrated that, in tomato, molybdate (99Mo) uptake in simple single salt buffer was significantly enhanced in the presence of phosphate and inhibited with sulfate. 
In a more representative nutrient solution where both phosphate and sulfate were present, sulfate was still found to be an effective competitor to molybdate uptake (Stout et al., 1951). 
In contrast, 99Mo uptake into tomato increased when phosphorus was withheld from the nutrient solution which could be quickly reversed with phosphorus re-supply (Heuwinkel et al., 1992). 
From this study, it would appear molybdate is bound and transported across the plasma membrane using a phosphorus transport system. 
However, firstly, the competition studies demonstrated that when phosphorus levels were adequate, low concentrations of molybdate failed to effectively compete with phosphorus and, secondly, accumulated molybdate did not quickly move from roots to shoots and was instead readily available for exchange with non-labelled molybdate (Heuwinkel et al., 1992). 
These data suggest the phosphorus transport system may effectively bind and accumulate molybdate but would appear to have limited impact on molybdate transport under good growing conditions where the soil has adequate amounts of available phosphorus. 
It is also interesting to note that sulfate accumulation was significantly repressed during the phosphorus starvation period (Heuwinkel et al., 1992), a result which strengthens the case for the involvement of sulfate transport systems in molybdate transport. 
Since the initial observation by Stout and Meagher (1948), sulfate has since been shown to be an effective regulator of molybdenum uptake in many plants under a wide range of growing conditions (see review by Macleod et al., 1997). 
The similar size of the two anions and the relative concentrations in the soil solution most likely contribute to the competition observed with sulfate. 
However, the effect of sulfate on molybdate uptake is not solely at the root/soil interface. 
Soybean plants showed decreased molybdenum levels in aerial parts of the plant as the sulfate supply increased (Sing and Kumar, 1979) even if molybdenum was applied as a foliar spray (Kannan and Ramani, 1978).

The influence of other ions on molybdate uptake is poorly understood. 
In excised rice roots, the uptake of molybdate (0·01 mm) was significantly enhanced in the presence of 0·1 mM FeSO4 but not in FeEDDHA (Patel et al., 1988). 
Interestingly, in free-living cowpea Rhizobium grown in iron-deplete conditions, the addition of high concentrations of molybdenum (1 mm) results in a release of a siderophore which appears to bind molybdenum and influences its uptake into the cell (Kannan and Ramani, 1978). 
Molybdate is highly mobile once in the plant where foliar absorption and translocation occur quickly. 
Williams (2004) showed that foliar-applied molybdate was rapidly distributed throughout the plant, including translocation towards the stem and roots within 24 h.
 Work completed by Ngaire Brady and colleagues (unpubl. res.) showed that foliar application of molybdate onto V. vinifera ‘Merlot’ restored NR activity in non-treated leaves elsewhere in the plant canopy (Fig. 3). Indeed, Brodrick and Giller (1991a), have shown good plant growth responses from foliar molybdenum application in the field. 
The mobility of molybdenum in plant tissues does appear to be genetically controlled. 
Brodrick and Giller (1991a) observed different molybdate partitioning patterns between two Phaseolus vulgaris cultivars. 
One variety had a distinct advantage in distributing molybdate to developing seeds, nodules, roots and pod walls (Smith et al., 1995).


PUTATIVE PLANT MOLYBDATE TRANSPORTERS
The close interaction between molybdate and sulfate transport in many biological systems suggests a similar transport system is likely be involved in the movement of molybdenum into and within plants. 
The first plant sulfate transporters (SHST1, SHST2, SHST3) were identified from sulfur-starved roots of the tropical forage legume Stylosanthes hamata (Smith et al., 1995). The SHST(1–3) clones were identified by their ability to functionally complement a yeast sulfate transport mutant YSD1 (Takahashi et al., 1996, 1999, 2000; F. W. Smith et al., 1997; Bolchi et al., 1999; Vidmar et al., 1999; Hawkesford, 2003). Since then a number of sulfate transport systems has been genetically identified and characterized in plants including genes from arabidopsis, barley, maize, potato, soybean and wheat (Hawkesford, 2003). In arabidopsis, there are 12 identified sulfate transporters with significant sequence homology and two more which are more distantly related (Hawkesford, 2003). This rich gene collection in many plant species has enabled distinct groups to be identified based on their sequences, cellular localization and response to sulfate (Takahashi et al., 1999). 
Group I sulfate transporters are high-affinity systems (KM 1·5–10 μm) primarily expressed in roots, and increase or decrease in expression in response to sulfur starvation or supply, respectively. 
Group II sulfate transporters are considered low affinity systems (0·1–1·2 mm) based on their functional properties when expressed in yeast cells. 
Group II transporters also respond to sulfur starvation through increased expression levels. 
Group III transporters are mainly expressed in leaf tissues and account for five of the 14 sulfate-like transporters identified in arabidopsis. 
For the remaining two groups there is less information on their functionality in plants. 
Initial reports indicated a member of group IV (AtSultr4;1) may be targeted to chloroplasts (Shibagaki et al., 2002), while group V members are distantly related to members of group I–IV and no functional experimentation has been completed on them. 
The role of the sulfate transporter family in plants is slowly becoming clearer. 

Recently, the arabidopsis AtSultr1;2, which is a member of the group I sulfate transporters, was shown to be involved in sulfate uptake in planta where a T-DNA lesion in the AtSultr1;2 locus allowed plants to grow on toxic concentrations of selenate and reduced its ability to accumulate sulphate into root tissues. 
There is an obvious requirement for more research into identifying the in planta function of the remaining sulfate transporters in plants before any of them can be nominated as putative molybdate permeases. 
However, one avenue of research that could be explored further is the role of these transport proteins when expressed in heterologous expression systems such as yeast cells. 
Although significant headway has been made in identifying genes encoding sulfate transport proteins very little information exists on the functional properties of most of these transporters in relation to anion selectivity, pH regulation and kinetic activities. 
Early studies in yeast demonstrated selenate and chromate as effective inhibitors o sulfate uptake (Breton and Surdin-Kerjan, 1977). 
Thus, selenate has been an effective screening tool to identify mutants that have disruptions in sulfate transport (Smith et al., 1995; Cherest et al., 1997). 
Using a selenate-resistant mutant YSD1, the selectivity of this mutant for sulfate transport and other anions such as molybdate is being explored. 
By removing molybdate from the media by activated charcoal scrubbing it has been possible to demonstrate that molybdate uptake at low external concentrations is also impaired in the yeast mutant (K. Gridley, unpubl. res.). 
This low molybdate media screen has been incorporated into ongoing experiments where selected plant sulfate transporters are being expressed in yeast and ranked on their ability to rescue growth on reduced molybdenum concentrations.

CONCLUDING REMARKS
Molybdenum nutrition is an essential component to healthy plant growth. 
Molybdate which is the predominant form available to plants is required at very low levels where it is known to participate in various redox reactions in plants as part of the pterin complex Moco. 
Moco is particularly involved in enzymes, which participate directly or indirectly with nitrogen metabolism. 
However, Moco is also uniquely involved in ABA synthesis where it has a significant effect on ABA levels in plant cells and consequently a role in water relations and transpiration rates through stomatal control and in stress related responses. 
There is significant scope in exploring practices, which optimize molybdenum fertilization in crops where nitrate is the predominant available N source or in nitrogen fixing legumes. 
There is also a large gap in the understanding of how molybdate enters plant cells and is redistributed between tissues of the plant. 
For instance the mechanism controlling molybdenum transport to nitrogen fixing bacteroids may be a unique control mechanism by which the plant can regulate the symbiosis indirectly through molybdenum availability to support nitrogenase activity. 
From our recent work with the grapevine cv. Merlot, we are starting to appreciate the influence of molybdenum on plant development and better understand mechanisms, which may be responsible for molybdenum uptake from the soil. 
It is ironic that it took a new industry to be expanded in South Australia where molybdenum first made its mark as an essential plant element to again reinforce the importance of molybdenum in plant development. 
Much more research is required to ascertain the simple processes involved in how plants gain access to molybdenum and how the element may be used in the future to expand growing areas where soil molybdate profiles limit plant growth.

MOLYBDENUM 
1. INTRODUCTION Of all the essential micronutrients or trace elements, molybdenum (Mo) is required in the smallest amount by plants. 
In Australia, molybdenum deficiency in pastures was first detected by CSIRO in South Australia in 1942. 
Since then, molybdenum fortified superphosphate has been applied to millions of hectares of legume pastures. 

2. MOLYBDENUM IN THE SOIL 
The parent rocks from which soils are formed are variable in their molybdenum content.
 Consequently, soils are equally variable in their molybdenum status, even granitic soils in the same district. 
Molybdenum is the least abundant of the trace elements in soils and very little is present in forms that are available to plants. 
It is fortunate that plants need such minute amounts of molybdenum. 
The availability of molybdenum is influenced by soil pH. 
Acid soils, i.e. pHw less than 6.0, and the presence of iron and aluminium oxides greatly reduce the availability of molybdenum. 
Most soil molybdenum is in mineral forms, but a small portion is held in organic matter. 
Molybdate is quite strongly sorbed or attached to clay particles or organic matter in soils, and is therefore not readily leached.
Of the anions (negatively charged ions) which are of importance as plant nutrients, molybdate is second behind phosphate in this respect, and much more strongly sorbed (resistant to leaching) than nitrate or chloride. 

3. MOLYBDENUM IN THE PLANT 3.1 
Uptake and Functions Plant uptake of molybdenum is as the molybdate (MoO4 2-) ion. 
Uptake may be depressed by the presence of sulfate (SO4 2-) ions, which are much the same size as molybdate ions and have the same charge. 
Molybdenum is moderately mobile in plants and can move quite freely from older to younger tissue as required. 
Many large seeded annual plants (especially legumes) contain sufficient molybdenum to last the crop, as long as the seed came from plants that were adequately supplied with molybdenum.


Molybdenum is important in nitrogen metabolism, and the synthesis of protein. 
Two important processes in which it is involved are: 
• The reduction of nitrate (NO3 - ) to nitrite (NO2 - ), the first step in the synthesis of amino acids and protein. 
• In root nodules in legumes, Rhizobium bacteria require molybdenum to fix atmospheric or molecular nitrogen (N2). 
Symbiotic bacteria require about ten times more molybdenum for nitrogen fixation than does the host plant (for protein synthesis). 
Hence, molybdenum deficiency commonly occurs in legumes before it does in other plants when grown in the same soil. 
In non-legume plants, cruciferous crops (particularly cabbage and cauliflower) and cucurbits have a high molybdenum demand. 
Grasses are relatively tolerant of low molybdenum, and deficiency in cereals only occurs in extreme conditions. 

3.2 Molybdenum Deficiency in Plants Molybdenum deficiency is important and widespread on acid (low pH) soils. 
It occurs in pastures and crops in sandy soils in the south-west of Western Australia, parts of South Australia, Victoria and Tasmania, the coast and tablelands of New South Wales and coastal areas in Queensland. 
It commonly occurs in plants growing on sands and on podsolic soils derived from sedimentary rocks, in legume-based pastures, and in a number of vegetable crops. 
Molybdenum responses have also been reported in cereals in the Riverina, South West Slopes and Tableland areas of New South Wales, and in Western Australia. 
Deficiency symptoms vary between legume and non-legume plants :- In legumes, a lack of molybdenum prevents proper nodulation and fixation of molecular nitrogen (N2), by symbiotic Rhizobium bacteria. 
Symptoms of nitrogen deficiency are displayed by the plant (e.g. sub-clover). 
These symptoms can be relieved by applying nitrogen fertiliser (although this would not normally be the recommended treatment). 
Growth is stunted and nodulation is poor. 
The root nodules are green or colourless, not the typical healthy pink colour. 
In non-legume plants, symptoms specific to molybdenum deficiency occur, although plants suffer essentially from a shortage of protein, due to the failure to convert nitrate (NO3 - ) to amino acids. 
Nitrates can accumulate in the plant. 

Specific symptoms include:-
• Marginal chlorosis and eventual scorching of leaves of broad-leaved plants;
• “Whip tail” of cauliflower; 
• Spotting of citrus leaves; 
• In wheat, leaves are a pale colour and the plant has reduced foliage with short internodes. 
Young plants may even show white, necrotic areas extending back along the leaves from the tips, reduced tillering and ultimately death. 
In southern Australia, molybdenum deficiency may contribute to haying-off in cereals where nitrogen is applied. 
Patches of withered plants, which are unable to cope with high soil nitrate are symptomatic of molybdenum deficiency on red-brown earths in New South Wales, Victoria and South Australia.

In contrast to legumes, the symptoms of molybdenum deficiency in non-legume crops cannot be corrected by applying nitrogen fertiliser, but only by adding molybdenum. 
In fact, the addition of extra nitrogen may make the symptoms worse. 

3.3 Molybdenum Toxicity in Plants 
Excessive molybdenum levels in plants, implying high levels of available soil molybdenum, are typical of peats (highly organic soils), but plant performance remains unaffected by levels that pose animal nutrition problems. Most plants have such a high tolerance of excess molybdenum that there are few symptoms of toxicity. 4. CRITICAL VALUES The amount of plant-available molybdenum in soils, and that taken up by plants is small, and much less than for the other nutrients. Consequently, critical levels for soil and plant tissue analysis are lower than for other nutrients. This may require the use of more sophisticated laboratory methods or equipment capable of lower levels of detection. Incitec Pivot Limited does not analyse soils for plant-available molybdenum, and little use is made of such tests elsewhere in the world, as they lack reliability. 
Molybdenum is more likely to be required on soils which are acid (pHw less than 6.0) and high in iron or aluminium. 
Sandy soils, and those which are inherently infertile (low in phosphorus) in their natural state are typically low in molybdenum. 
Plant tissue analysis is a much better guide, but because of the added cost of analysis, molybdenum is not routinely analysed in samples submitted to the Incitec Pivot Laboratory. 
Molybdenum is tested on request only, as an optional extra. The molybdenum content of plant material is usually low and typically less than 1 mg/kg Mo in the dry matter. 
It is variable, and in pasture can range from 0.01 to several hundred mg/kg Mo. 
A molybdenum content of less than 0.1 mg/kg Mo in dried plant tissue (usually leaves) indicates molybdenum is deficient. 
Molybdenum toxicity in plants is rare. Compared to other micronutrients, molybdenum can be taken up in concentrations many times that regarded as necessary for optimal plant growth without toxic effects. 
Livestock grazing pastures high in molybdenum may be affected when the pasture itself it not. 
Where molybdenum values are about 5 mg/kg Mo or higher on a dry weight basis in pasture and forage, copper deficiency may be induced in grazing animals. 
Copper supplementation of livestock may be necessary. 
Copper deficiency can occur where molybdenum concentrations in pasture are less then 5 mg/kg Mo if dietary sulfur intake is adequate to high. 
This is attributable to the formation of insoluble copper sulfide in the gut. 
In the absence of a molybdenum test, plant tissue nitrate and total nitrogen (N) figures can be used to indicate if a plant may be suffering from molybdenum deficiency. 
High nitrate figures, coupled with low total N figures, indicate that the plant is taking up adequate nitrate, but not converting it to protein. 
While there may be other explanations, e.g. sulfur deficiency, molybdenum deficiency is often the cause.


MOLYBDENUM FERTILISERS Molybdenum is required in minute amounts, typically around 50 g/ha Mo in legume pastures, which remains effective for several years. 
Consequently, it needs to be applied as a dust or powder, if all plants in the field are to have access to molybdenum. 
Accurate and uniform coverage at such low rates, however, is not possible, so molybdenum needs to be applied with a carrier, e.g. another fertiliser, water or seed. 
The three most commonly used molybdenum compounds are: 
• Molybdenum trioxide (MoO3) 66% Mo 
• Ammonium molybdate (NH4)6Mo7O24.4H2O 54% Mo 
• Sodium molybdate Na2MoO4.2H2O 39% Mo Molybdenum trioxide is insoluble. 

Commercial formulations may contain less than 66% Mo. Ammonium molybdate and sodium molybdate are soluble in water. 
Sodium molybdate is the more soluble of the two. 

They provide much more uniform coverage than can be achieved by adding molybdenum to fertilisers that are applied dry to the soil. 
Foliar sprays (which would need to be applied on a regular basis at very low rates) are not practical in pasture. 
Molybdenum is best applied to the soil in pasture.


Molybdenum Fortified Fertilisers 
The use of molybdenum fortified SuPerfect (and SuPerfect Potash blends) offers a convenient way of applying phosphorus, sulfur, molybdenum (and potassium) simultaneously in the one operation to legume pasture. 
Molybdenum does not need to be applied every time that SuPerfect is, i.e. on an annual basis. 
It is typically applied once every 3 – 4 years. 
Molybdenum Solutions Ammonium and sodium molybdate can be used in the preparation of fertiliser solutions, which can be sprayed onto the soil or foliage. 
These provide uniform coverage. Molybdenum can be sprayed onto the soil during seedbed preparation for a new pasture, or during the fallow period in crops. 
Repat applications are not required for several years. 
Soil sprays are less suited to established pasture, as the spray is intercepted by the foliage. 
In turn, the molybdenum can be ingested by grazing animals, and returned/deposited unevenly to the field in dung and urine, or removed if cut for hay. 
Molybdenum ingested by livestock while grazing may also indce nutritional disorders. 
If molybdenum is to be applied through a boom in established pasture, do so when the pasture is short, e.g. after grazing or cutting for hay, so that the maximum amount of the spray reaches the soil.
Grazing should be delayed for at least one month and until such time that significant regrowth has occurred. 
Lower molybdenum application rates are required for foliar sprays than for soil application. 
Foliar sprays only need to be applied to crops that are susceptible to molybdenum deficiency. 
They are not required by all crops in the rotation. 
Seed Dressings Molybdenum trioxide, which is insoluble, should be used as the molybdenum source in seed coatings. 
The soluble molybdenum compounds, ammonium molybdate and sodium molybdate are not recommended as they are likely to harm the Rhizobium bacteria in the inoculum. 
For temperate legumes, the molybdenum trioxide should be thoroughly mixed with the quantity of lime to be used for coating the seed. 
This pre-mix is then applied to the seed after it has been treated with inoculant and adhesive.
Lime coatings of most tropical legumes is unnecessary, and may in fact be harmful. 
Tropical strains of Rhizobium are adapted to acid soil conditions. 
Here, coating materials such as ground rock phosphate or bauxite may be used. 
While normally coated onto legume seed (as part of the inoculation process) molybdenum trioxide can be coated onto non-legume seeds if necessary. 
Achieving a uniform mix and distribution of the molybdenum trioxide can present problems when coating seeds. 
Use an appropriate sticker (methyl cellulose) and seek advice on the pelleting process.

SOIL APPLICATION RATES 7.1 Pasture Recommended rates for soil application of molybdenum (Mo) in pasture vary, and are typically in the range of 50 to 100 g/ha Mo every 3 to 4 years. 
The application rate may need to be reduced where copper deficiency occurs in livestock. In Victoria, molybdenum is recommended at 50 to 60 g/ha Mo on pasture once every 8 to 10 years. 
More frequent applications are required (once every 5 to 6 years) in high rainfall areas (above 1 000 mm per annum) and on high phosphorus fixing soils, e.g. red clay loams. 
On occasions, deficiency has been recorded 2 to 3 years after the previous application. 
In white or sub clover pastures in New South Wales, SuPerfect Mo 0.025 % is commonly applied at 125 kg/ha every 3 to 4 years (in place of an annual application of Super). 
This supplies around 30 g/ha Mo. While this may be adequate in some soils and pastures, it is thought that higher rates or more frequent applications of molybdenum may be required in some circumstances. 
On tropical pastures, the general rate for molybdenum is 100 g/ha Mo every 3 to 4 years. 
This rate is increased to 200 g/ha Mo every 3 to 4 years on glycine, and on basaltic soils on the wet tropical coast of North Queensland, the frequency of application is increased to every second year. 
In drier areas, and on Siratro and the Stylosanthes species, the recommended rate of molybdenum is 50 g/ha Mo every 3 to 4 years. 
The application of molybdenum at high rates and/or on a too frequent basis can result in elevated concentrations of molybdenum in pasture, which in turn can be detrimental to livestock by inducing copper deficiency. 
While such risks are slight, care must be exercised in the application of molybdenum, particularly on light-textured sandy soils where copper is most likely to be deficient. 
Where copper deficiency has been diagnosed in livestock, or soil copper levels are marginal, it may be necessary to reduce molybdenum application rates. 
The most common way in which molybdenum is applied to legume based pasture is as molybdenum fortified superphosphate, which is used in place of ordi ary superphosphate, usually once every three to four years. 
Common addition rates of molybdenum to superphosphate are 0.015%, 0.025% and 0.05% Mo. 
The following table shows the amount of molybdenum applied at these concentrations at various product application rates.


Legume Grain Crops Where grain and oilseed crops are grown in rotation with legume based pastures, molybdenum is normally applied at the start of the pasture phase, as pasture legumes have a higher requirement for molybdenum and are more responsive than non-leguminous grain crops. 
Where pastures do not feature in the crop rotation and legume grain crops are to be grown, a practical way to apply molybdenum is to spray sodium molybdate onto the soil, e.g. in combination with a pre-emergence herbicide, provided no chemical compatibility problems exist. 
A typical application rate for sodium molybdate is 150 g/ha, supplying 55 to 60 g/ha Mo. 
This can provide protection against molybdenum deficiency for a number of seasons.

8. FOLIAR SPRAYS 

8.1 Horticulture Where molybdenum deficiency occurs in vegetable crops such as cauliflower and cucurbits, it is recommended that molybdenum be foliar applied. 
This can be easily done with early season crop protection sprays, and provides much more uniform coverage than can be achieved by incorporating molybdenum additives into basal planting fertilisers. 
Molybdenum is mobile in plants and is readily relocated from old to young plant parts during the growing season, so one or two early season sprays is usually all that is required. 
Two sprays are recommended in crops such as cauliflower, which is very susceptible to molybdenum deficiency. 
Sodium molybdate is more commonly used in spray solutions than ammonium molybdate. 
Typical spray concentrations for sodium molybdate are 0.04 % w/v (40 g/100L) in seedbeds, i.e. before transplanting, and 0.05 – 0.1 % w/v (50 – 100 g/100 L) in the field early in the life of the crop. 
Check compatibility before mixing with crop protectants. 
Avoid late season sprays, i.e. approaching harvest. 
These are usually ineffective, and may result in elevated concentrations of molybdenum in farm produce. 
Avoid over-application.

Compatibility in Solution Sodium and ammonium molybdate are compatible with most other fertilisers and trace elements. 
Do not mix with calcium fertilisers, e.g. calcium nitrate or calcium chloride, as insoluble calcium molybdate will be precipitated. 
When preparing fertiliser solutions, fill the tank with water to near capacity, leaving space for the added fertiliser, which should then be added slowly while agitating. 
Do not pre-mix. Fertiliser solutions should be prepared just prior to use, and not allowed to stand for an extended period, to minimise sediment formation and settling in tanks. 

9. WITHHOLDING PERIOD BEFORE GRAZING Excess molybdenum in young regrowth, or that ingested as fertiliser dust with pasture, can induce copper deficiency in livestock. 
This is most likely to occur on sandy soils low in copper. 
Plant levels of molybdenum can be high for up to four weeks after application. 
It is advisable to spell treated paddocks during this period. 
If rain is not received or irrigation is not applied within a month, grazing may need to be deferred for longer. 

10. MOLYBDENUM IN ANIMALS 
10.1 Interactions While molybdenum is important in animal nutrition (in various enzymes), it is its relationship with other elements such as copper, sulfur and iron that are of more importance. 
Copper and molybdenum are mutually antagonistic, i.e. one restricts the uptake of the other by plant roots. 
Molybdenum itself is unlikely to be toxic to livestock, but an oversupply of molybdenum can induce copper deficiency in animals, particularly on light textured soils. 
Sulfur, consumed in protein or as sulfate, can also induce copper deficiency, due to the formation of insoluble copper sulfide in the gut. 
This can occur even where there is no shortage of copper in the pasture. Sulfur applied in pasture topdressing programs may contribute to copper deficiency in livestock.
 On the other hand, if the diet is low in sulfur and molybdenum, copper may accumulate in the liver and other tissues, resulting in copper toxicity.


Induced Copper Deficiency in Animals 
The application of molybdenum at high rates and/or on a too frequent basis can result in elevated concentrations of molybdenum in pasture, which in turn can be detrimental to livestock by inducing copper deficiency. 
While such risks are slight, care must be exercised in the application of molybdenum, particularly on light-textured sandy soils where copper is likely to be deficient. 
Where copper deficiency has been diagnosed in livestock, or soil copper levels are marginal, it may be necessary to adopt or consider the following management practices: 
(i) Apply molybdenum at lower rates on a more frequent basis. 
Molybdenum rates (per application and cumulatively) may in fact need to be reduced, particularly on sandy soils. 
Soil fixation of molybdenum (and other nutrients) is generally lower on light-textured soils than on heavier-textured (clay) soils. 
The molybdenum rate should perhaps be halved, with no more than 30 g/ha Mo being applied in a single application.
(ii) Do not treat the whole property with molybenum at the one time. 
Treat part of it each year and rotate stock between paddocks. 
(iii) Spell freshly fertilised paddocks for a month. 
If no rain is received or irrigation is applied within this time, delay grazing until after either event occurs. 
This helps prevent ingestion of fertiliser dust containing molybdenum which lodges on pasture leaves during application, or of young regrowth which may contain elevated levels of molybdenum. 
(iv) If copper deficiency is limiting pasture growth, it too should be included in the fertiliser program. 
(v) Direct copper supplementation of animals may also be required. 

10.3 Induced Copper Toxicity (Toxaemic Jaundice) in Animals 
The application of molybdenum can help reduce the incidence of toxaemic jaundice in areas where soil copper status is high and where establishment of legume pastures provides a diet higher in copper than do native pastures.
 Use of molybdenum-fortified superphosphate every 3 to 4 years in top-dressing programs helps alleviate the problem. 
Seek veterinary advice to confirm the diagnosis, and consult local advisers on rates and frequency of application.

WARNING The information contained in this publication is for use as a guide only. 
The use of fertilisers is not the only factor involved in producing a top yielding pasture or crop. 
Local soil, climatic and other conditions should also be taken into account, as these could affect pasture or crop responses to applied fertiliser. 
Before using fertiliser seek appropriate agronomic advice. Fertiliser may burn and/or damage plant roots or foliage. 
Foliar burn to the leaves, fruit or other plant parts is most likely to occur when different products are mixed and sprayed together, the water is of poor quality, or the spray is applied under hot dry conditions, eg. in the heat of the day. 

olybdenum Fertilizer
Molybdenum knowledge introduced
(1) the main types of molybdenum and nature

Commonly used in the production of molybdenum are ammonium molybdate, sodium molybdate, molybdenum trioxide, molybdenum slag, glass, fertilizers containing molybdenum.

(2) molybdenum fertilizer application

Molybdenum fertilizer on crop species responses: lack of molybdenum is leguminous crops, alfalfa most prominent, in addition to rape, cauliflower, corn, sorghum, millet, cotton, sugar beet molybdenum also have a good response.

Molybdenum fertilizer and soil conditions: effect of molybdenum fertilizer application, soil molybdenum content, morphology and distribution areas related to Southern Institute of Soil Science, Chinese Academy of Liu Zheng, etc. 
The molybdenum content of soil and fertilizer is divided into three areas, namely molybdenum fat significant area, molybdenum and molybdenum active area may be effective area. 
Soils in the northern zone of molybdenum fertilizer required significant molybdenum fertilizer crops as soybeans, peanuts, soil in the south zone of molybdenum fertilizer required significant molybdenum fertilizer leguminous green manure crops, peanuts, soybeans, citrus. 
Active area in molybdenum molybdenum fertilizer needed for the legume green manure crops, peanuts, soybeans, etc., and may be effective in areas molybdenum molybdenum fertilizer situation requires further experimental study.

Molybdenum fertilizer application techniques: molybdenum mostly used types of fertilizer (seed dressing, seed soaking) and foliar application. 
Seed dressing, per kilogram of seeds with ammonium 2g-6g, first dissolved in hot water, cold water and then diluted to 2% -3% of the solution, spraying the seeds with a spray, side spray edge mix, after marinated dried seeds can be planted. Soaking, the available concentration of 0.05% -0.1% ammonium molybdate solution to soak seeds for 12 hours. 
Yemianpenfei generally used for larger leaf crops in seedling and bud stage with 0.01% -0.1% ammonium molybdate solution, spray 1-2 times per 667m2 each spray 50.

Molybdenum fertilizer on winter wheat yield and its use of technology

In recent years, through the wheat molybdenum trials come to its molybdenum fertilizer and soil available molybdenum content was significantly negatively correlated Mo fertilizer can significantly improve cold hardiness of wheat, reducing the wheat frost damage. 
Now Mo deficiency symptoms of wheat, and the physiological role of molybdenum and molybdenum Mo fertilizer fertilizer use methods described below:


First, the lack of effective soil on soil classification molybdenum molybdenum content: less than 0.10 mg / kg for the low, 0.10 to 0.15 mg / kg is low, 0.16 to 0.20 mg / kg for medium, greater than 0.21 mg / kg for the rich. Soil molybdenum 0.15 mg / kg of wheat molybdenum soil threshold.


Second, the lack of symptoms wheat wheat molybdenum molybdenum (partial nitrogen and particularly low), the four-leaf stage in wheat seedling disease begins, initially in the upper part of the old leaves leaves produce white spots along the veins, gradually into a linear, sheet, until dry; but leaves lower is better. 
Only when the performance of a serious shortage of molybdenum leaf chlorosis, tip and leaf margin gray, flowering delayed maturation, grain shrinkage, husk growth is not normal.


Third, the physiological role of Mo fertilizer


(1) molybdenum application can promote wheat nitrogen metabolism. 
The main body of molybdenum in crop physiological functions that affect nitrogen metabolism, nitrate uptake by plants, it must be in the role of nitrate reductase, converted into ammonium nitrogen, to participate in plant protein synthesis, molybdenum nitrate reductase an indispensable component. 
Therefore, molybdenum wheat leaf nitrate will be the large accumulation to protein synthesis difficult. 
It has been determined: Wheat molybdenum molybdenum fertilizer soaking and spraying fertilizer, wheat plants within the nitrogen content increased 4.10% and 1.21%, while the protein content of wheat increased in vivo, amino acids (except proline) also increased significantly.


(2) Effects of molybdenum fertilizer phosphorus in the body can promote the metabolism of wheat. 
Molybdate affect pyrophosphate salts and orthophosphate salts chemical hydrolysis of esters, the body also affect crop organic phosphorus and inorganic phosphorus ratio. It has been determined: molybdenum wheat leaf blades high inorganic phosphorus than normal 4 to 6 times, again for 2 to 4 days after molybdenum, molybdenum raw organic phosphorus content in wheat leaves began to recover about 20 days, the body of organic phosphorus content of wheat compared to the control increased 15.20%.


(3) Mo can improve cold hardiness of wheat. 
Mo wheat at low stress, photochemical reactions and increased photosynthetic capacity, nitrogen metabolism, phosphorus metabolism strengthened, wheat leaf soluble sugar increased, thereby increasing the low spring wheat resistance to cold damage capability. 
According to the field survey, Mo wheat freeze injury mortality rate decreased from 10% to 18%.


(4) Mo can increase wheat chlorophyll content. 
Mo wheat after each period can significantly increase chlorophyll content, especially chlorophyll content increased. 
Therefore, delay leaf senescence, extending blade functional period, improved strength and photosynthesis of wheat photosynthetic capacity and time, increasing the grain weight, improve wheat production.


(5) Mo favor wheat dry matter accumulation and running. 
Mo later, with the increase in wheat plants photosynthetic area, promoting dry matter accumulation and running. 
According to the determination of molybdenum after the wheat plant dry weight ratio was increased (mature determination) 10.21%, the economic coefficient of 0.3761 up to 0.4177 in control.


Fourth, the yield molybdenum


According to Mo test: in the effective low molybdenum content in the soil, with a concentration of 0.05 to 0.10% of ammonium molybdate solution soaking wheat, wheat 502.7 kilograms per mu, representing increase of 16.47% in control 431.6 kg, with an average increase of 1.20 per spike , grain weight increased 1.01 g; in the effective low molybdenum content in the soil, with a concentration of 0.05 to 0.10% molybdenum.

Soybean hi molybdenum

 Nongyan said: "Soy molybdenum, beans hoard thickening." 
Molybdenum is indispensable for the growth and development of soybean trace elements. 
According to analysis, the content of molybdenum soybean plants is within the 1.9-91ppm (dry weight), more concentrated in the nodules, followed by seeds, legumes than molybdenum content of only 0.01-0.7ppm. Molybdenum can promote the body of soybean strains on phosphorus absorption, increased leaf chlorophyll, improve pod number, grain number and grain weight, and can improve the protein content of grain, and promote precocious. If the soil available molybdenum content of less than 0.01ppm, soybean vacancy will show symptoms of molybdenum, namely planting small, few and small nodules, lack of green between the veins, leaf distortion, reduced nitrogen fixation, nitrogen fixation is reduced. 
Practice has proved that the production of soybeans increased molybdenum molybdenum fertilizer, generally yield more than 10% or higher.
In soybean growth stages molybdenum application to foliar spray as well. 
The specific method is: in the soybean flowering acre with 25-50 grams of ammonium, watered 50-75 kg, made ​​of liquid fertilizer spraying, spray once every 7-10 days, even spray 2-3 times. If mixed with molybdenum and phosphate fertilizer spraying, the better. Preferably with ammonium phosphate, ammonium fertilizer per acre sprayed liquid plus 25-30 grams of ammonium phosphate, mixed melt, spray can, as with superphosphate per acre with 1 kg, 5 kg watered, soak for a day and night, whichever filtered supernatant was added ammonium fertilizer per acre spraying of liquid, mix spraying.

Sodium Molybdate is a free flowing soluble crystalline fertiliser and is used to supply the trace element molybdenum to crops and livestock in various situations. 
Sodium Molybdate is only required in very small quantities to satisfy annual plant requirements. 
Sodium Molybdate is suitable for foliar or fertigation application on a wide range of horticultural and broad acre crops and pastures. 

SODIUM MOLYBDATE BENEFITS 
• Supplies the essential trace element molybdenum to crops and livestock 
• Foliar applied to crops and pastures grown on acid soils where plant availability is low 
• Essential for conversion of nitrates in leaves to amino acids and proteins 
• Suitable for foliar or fertigation 
• Ideal for brassica, beans, peas, grapes, cucurbits, canola, clover and other crops and pastures susceptible to molybdenum deficiency.

Application
Sodium Molybdate can be used as a foliar or fertigation application in a regular nutrition program for
applicable crops and pastures. Multiple applications may be required if leaf analyses reveal ongoing
deficiency. 


Effect of sodium molybdate on the corrosion behavior of cold rolled steel in peracetic acid solution
Abstract
The effect of sodium molybdate (Na2MoO4) on the corrosion of cold rolled steel (CRS) in peracetic acid (PAA) solution was investigated by gravimetric measurements, Tafel polarization curves, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). 
All the data indicate that Na2MoO4 acts as a very good inhibitor in PAA solution. 
The inhibition efficiency increases with increasing concentration of Na2MoO4 and immersion time. 
The inhibition efficiencies, calculated from gravimetric measurements, Tafel polarization curves and electrochemical impedance spectroscopy, are in reasonably good agreement and are very similar in the three cases. Furthermore, polarization data show that Na2MoO4 behaves as an anodic passive type inhibitor. Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM) were used to characterize the corrosion surface. 
A probable mechanism is presented to explain the experimental results.

SODIUM MOLYBDATE
MOLYBDENUM (SODIUM MOLYBDATE)
Soluble form of molybdenum. Increases growth in pastures, particularly legumes. 
Excess levels of molybdenum can cause copper deficiency as higher than optimum levels of molybdenum cause an increase in copper excretion in ruminants. 
High levels of molybdenum have also been shown to cause higher levels of infertility in ruminants. 
This problem only effects ruminants it does not effect other monogastric animals such as horses.

Molybdenum importance for appropriate plant functioning and growth is inconsistent by the most of the plants in respect to the total quantity that is obligatory for them. 
Molybdenum is a micronutrient that is directly involved in the metabolic functions of nitrogen in the plant. 
The transition metal molybdenum, in molybdate form, is essential for plants as a number of enzymes use it to catalyze most important reactions in the nitrogen acclimatization, the synthesis of the phytohormone, degradation of the purine and the detoxification of the sulfite. 
There are more than known 50 different enzymes that need Mo, whether direct or indirect impacts on plant growth and development, primarily phytohormones and the N-metabolism involving processes. 
On the other hand, in the synthesis of ABA uniquely Moco is involved, there on the level of ABA Moco effect is highly vital and ultimately by the response in the stress and the stomatal control, it has a very important role in the rate of transpiration and water relations. 
The practices that are involved in the fertilization of Mo optimization in crops, has a very important scope in discovering and improving these practices where the legumes are fixing the N or No3- is primarily source of available N. 
The deficiency of Mo and to enhance the molybdoenzymes activity, it may be very effective and vital important to use the spray of Mo as foliar application through the soil. 
The most recent understanding that from the soil how the plant gets access Mo or how they redistribute it is not still clear. 
However in the system f prokaryotes, it has been found that in plants it has likewise physiological Mo transport phenotypes. 
So, the mechanism of transport of Mo in the prokaryotes is needed as well as the reconsideration of anion transport mechanism that is in plants, will provide a help to solve that how this is accumulated. 
In this review, the discussion covers about the vital importance of Mo to enhance the productivity for optimizing the yield concentrating on metabolism, uptake, transport, storage, Mo cofactors, application, focusing on some other recent constrains in the recent situation of agriculture, where the yield and development in agriculture may be aided by increasing the Mo nutrition.


Sodium Molybdate
Disodium Molybdate

Sodium Molybdate dihydrate, also known as disodium molybdate is an odourless white, crystalline powder with the chemical formula Na2MoO4. 
Manufactured from pure molybdenum ore this product is of an extremely high quality.

Sodium Molybdate uses
Sodium Molybdate  is widely used in the water treatment industry as a corrosion inhibitor in water treatment products. 
It is also used in agriculture as a micronutrient for plants and used in the manufacturing process of pigments, lubricants and an additive for metal finishing.

Sodium Molybdate as a corrosion inhibitor
Sodium Molybdate is an ideal environmentally responsible corrosion inhibitor for water and cooling systems. 
Capable of working across a variety of temperatures and pH levels, sodium molybdate experiences no loss of chemical properties or effectiveness in a variety of hot or cold environments. 
When used, it is capable of inhibiting the corrosion of ferrous, copper and aluminium metals in the cooling water of both open and closed cooling systems. 

Sodium Molybdate in Agriculture 
Sodium molybdate offers a useful source of molybdenum which is an excellent soil micronutrient and essential for healthy plant growth making it a popular choice of fertiliser within the agricultural industry. 
Suitable for foliar or fertigation applications, it is used in small amounts to supply molybdenum to crops and livestock. 
Sodium molybdate is also added to cattle feed when treating copper deficiencies.


Molybdenum (Mo) is a trace element required in very small amounts for the growth of both plants and animals. 
Crop deficiencies of Mo are fairly uncommon, but when diagnosed, various soil and foliar fertilizers can be used to correct this condition. 
Deficiencies have been reported from acid, sandy soils and can be worse under minimum disturbance cropping systems.

Molybdenum in Plants
All plants require very small amounts of Mo for normal growth and development and Mo and nickel (Ni) are required in the lowest concentrations of all the essential nutrients. 
Within the plant, Mo is primarily used in the production of “molybdoenzymes” that regulate various plant functions. 
The most well known of these enzymes regulate nitrogen (N) nutrition. 
In non-legumes, Mo-enzymes regulate the conversion of nitrate into proteins (nitrate reductase). 
In legume crops, another Mo-enzyme (nitrogenase) is needed by the root nodule bacteria for N fixation. 
The Mo requirement of legumes is greater than that of grasses and other crops.

Molybdenum toxicity in plants is rare under most agricultural conditions. 
However, sheep and cattle feeding on plants with a high Mo concentration may suffer from molybdenosis. 
This condition is a result of high Mo concentrations suppressing the availability of dietary copper (Cu) in these animals. 
For this reason, regular applications of Mo are not recommended for pastures.

Legumes have a higher demand for Mo than cereals, and canola is similar to legumes. 
Vegetable brassicas have a relative high demand (see Table 2). 
The demand for Mo in canola is 5 to 6 times higher than for cereals.

Molybdenum in Soils
Plant-available Mo is in the anion form of MoO42-; or molybdate. 
It is released from solid minerals through normal weathering processes and then undergoes various reactions in the soil. 
Once it is dissolved, MoO42- anions are subject to adsorption processes on clays, metal oxides of iron (Fe), aluminum (Al), and manganese (Mn) as well as organic compounds, and carbonates.

The solubility of MoO42- is greatly influenced by soil pH, similar to the chemically analogous nutrient phosphate (PO43-). 
Molybdenum is the only micronutrient that has increased plant availability as the soil pH rises. 
Molybdate solubility increases approximately 100 times for every unit increase in soil pH.

Therefore, the use of lime to increase the pH of acid soils is an important management tool to improve Mo availability. 
Where soils have a pHCa of 5.0 or greater, it is uncommon to encounter Mo deficiencies.

Addition of sulfate (SO42-) fertilizer tends to decrease MoO42- uptake, as they both compete for root uptake sites. 
For example, one study showed that the plant Mo concentration of peanuts was decreased by more than 70% following fertilization with SO42- containing single superphosphate (SSP), but Mo concentrations increased by 20% following fertilization with triple superphosphate (TSP) fertilizer that contains no sulfate (Rabefka 1993). 
The addition of phosphate often results in the release of Mo that is adsorbed on soil solids, leading to greater Mo uptake and accumulation in plants.

Diagnosing a molybdenum deficiency
Because it is required in such small quantities, the diagnosis using soil or plant testing is analytically challenging and tests are more expensive than other soil or plant tests. 
Soil tests can be either CaCl2 or ammonium oxylate but neither test is reported as reliable in predicting Mo deficiencies over a wide range of soils (Brennan and Bruce, 1999).

Soil pH is an important determinant of the amount of available Mo, and risk of deficiencies increases where pHCa is less than about 4.5, and as soils acidify under agriculture, the amount of plant available Mo declines (Brennan and Bolland 2011). However, soil acidification can also increase the risk of potassium deficiency and aluminium toxicity, so the diagnosis can be difficult (Brennan et al. 2004).

Plant tissue tests can be used but, like soil tests, the reliability is relatively low. 
For example, Shovelton (1982) showed that sub-clover on Mo responsive sites generally had Mo concentrations of 0.1 to 0.2 mg/kg, but responses to added Mo were seen even when concentrations were 0.4 to 0.5 mg/kg. Despite this variability, critical youngest mature leaf Mo concentrations in mid-tillering wheat and pre-flowering canola have been reported as 0.05-0.09 mg/kg and less than 0.3-0.6 mg/kg respectively. 
Tissue tests are expensive as specialized equipment is required.

Molybdenum Deficiency Symptoms
Molybdenum is mobile within plants and deficiency symptoms can appear on the entire plant.
· Non-legumes: Since adequate Mo is essential for proper N metabolism, deficiencies commonly appear as stunted plants and failure of leaves to develop a dark green color. 
In more severe deficiencies, the leaves may develop a pale green or yellow area around the edges and between the veins. 
Advanced symptoms of insufficient Mo may appear as burning (necrosis) around the leaf edges and between the veins, because the plant cannot assimilate the nitrate and convert it to protein. The most severely affected plants can show empty heads (like Cu deficiency or frost) and delayed maturity. 
A well-known Mo deficiency symptom has been described for cauliflower, which develops a “whiptail” when the leaf tissue fails to develop surrounding the mid-leaf vein.
· Legumes: These plants have an additional requirement for Mo, since it is required for N fixation by the root nodule bacteria, in addition to the internal utilization of nitrate. The symptoms of insufficient Mo include a general stunting and yellowing, typically seen as a result of insufficient N supply. Nodules are green and small.

Fertilizing with Molybdenum
In many soils, application of a liming material to increase pH will release Mo from insoluble forms. 
For example, a study showed that addition of lime alone resulted in the same soybean yield as when Mo fertilizer was added to unlimed soil. 
However, the chemical release of soluble Mo following lime application may take weeks or months to occur.

If lime is not required for crop growth or when the Mo concentration of the soil is low, it may be useful to fertilize with additional Mo in the following ways:
· Soil: Molybdenum fertilizers can be banded or broadcast on the soil. 
It is commonly added in small amounts, ranging from 250 to 900 g/ha. 
It is often mixed with other fertilizer materials to help with uniform application or it may be dissolved in water and sprayed on the soil before planting. 
Molybdenum trioxide (MoO3) is only suitable for soil application due to its low solubility. 
The use of MnO3 at 150 g/ha has been shown to have a residual activity of up to 5 years (Kerridge & White 1977).
· Foliar: Soluble Mo sources, such as sodium or ammonium molybdate, are used for foliar application to plants. 
Foliar application of dilute solutions of Mo is generally most effective when applied at earlier stages of plant development. 
Foliar applications are beneficial for immediate correction of Mo deficiency symptoms, compared with soil applications, which have a longer residual benefit.
· Seed: Treatment of seed with small amounts of Mo fertilizer is common in regions where deficiency occurs. 
This technique ensures that each seed is uniformly provided a small, but adequate amount of Mo for healthy growth. 
Rhizobia inoculants for legume crops are sometimes amended with small amounts of Mo to promote vigorous N fixation. 
Excessively high application rates can lower seed germination or cause Mo accumulation to concentrations that may be harmful for grazing animals. 
Selecting seed from a crop with a good Mo history or from an alkaline soil can reduce the risk of deficiency (Brennan and Bolland 2007).

The selection of a specific Mo fertilizer depends largely on how the material will be applied. 
Some common fertilizer products containing Mo are given in Table 1.

Table 1. Some common fertilizer products containing molybdenum.
Name    Chemical Formula    Mo Content    Solubility
Sodium molybdate    Na2MoO4.2H20    39%    653 g/l
Ammonium molybdate    (NH4)2Mo7024.4H2O    54%    400 g/l
Molybdate trioxide    MoO3    66%    3 g/l

Crop Response to Molybdenum
The benefit of supplying adequate Mo most commonly relates to boosting the ability of plants to utilize N. Plant Mo deficiencies may not always require supplemental fertilization, especially in acid soils where application of lime will increase Mo availability to plants. 
Similarly, addition of P fertilizer releases Mo into solution after it exchanges with MoO4 2- on soil adsorption sites.

There is no evidence that high N supply can induce a Mo deficiency, or that crops – especially under high yielding situations become deficient in Mo as more N is supplied. 
In neutral to alkaline soils, there is adequate Mo to meet the demands of crops and pastures.

Table 2 shows some examples of Mo responses, all which come from situations where soil pH is very low, either from Western Australia or on the slopes in New South Wales. 
For example, Brennan and Bolland (2011) measured yield responses in canola to applied Mo in Western Australia on sites where pHCa <4.8. 
Where Mo is lacking, supplemental fertilization has resulted in large increases in plant growth and yield.


The importance of molybdenum for plant growth is disproportionate with respect to the absolute amounts required by most plants. 
Apart from Cu, Mo is the least abundant essential micronutrient found in most plant tissues and is often set as the base from which all other nutrients are compared and measured. Molybdenum is utilized by selected enzymes to carry out redox reactions. 
Enzymes that require molybdenum for activity include nitrate reductase, xanthine dehydrogenase, aldehyde oxidase and sulfite oxidase.
 Loss of Mo-dependent enzyme activity (directly or indirectly through low internal molybdenum levels) impacts upon plant development, in particular, those processes involving nitrogen metabolism and the synthesis of the phytohormones abscisic acid and indole-3 butyric acid. 
Currently, there is little information on how plants access molybdate from the soil solution and redistribute it within the plant. 
In this review, the role of molybdenum in plants is discussed, focusing on its current constraints in some agricultural situations and where increased molybdenum nutrition may aid in agricultural plant development and yields. 
Molybdenum deficiencies are considered rare in most agricultural cropping areas; however, the phenotype is often misdiagnosed and attributed to other downstream effects associated with its role in various enzymatic redox reactions. 
Molybdenum fertilization through foliar sprays can effectively supplement internal molybdenum deficiencies and rescue the activity of molybdoenzymes. 
The current understanding on how plants access molybdate from the soil solution or later redistribute it once in the plant is still unclear; however, plants have similar physiological molybdenum transport phenotypes to those found in prokaryotic systems. 
Thus, careful analysis of existing prokaryotic molybdate transport mechanisms, as well as a re-examination of know anion transport mechanisms present in plants, will help to resolve how this important trace element is accumulated.

Background The importance of molybdenum for plant growth is disproportionate with respect to the absolute amounts required by most plants. 
Apart from Cu, Mo is the least abundant essential micronutrient found in most plant tissues and is often set as the base from which all other nutrients are compared and measured. Molybdenum is utilized by selected enzymes to carry out redox reactions. 
Enzymes that require molybdenum for activity include nitrate reductase, xanthine dehydrogenase, aldehyde oxidase and sulfite oxidase.

• Scope Loss of Mo-dependent enzyme activity (directly or indirectly through low internal molybdenum levels) impacts upon plant development, in particular, those processes involving nitrogen metabolism and the synthesis of the phytohormones abscisic acid and indole-3 butyric acid. 
Currently, there is little information on how plants access molybdate from the soil solution and redistribute it within the plant. 
In this review, the role of molybdenum in plants is discussed, focusing on its current constraints in some agricultural situations and where increased molybdenum nutrition may aid in agricultural plant development and yields.

• Conclusions Molybdenum deficiencies are considered rare in most agricultural cropping areas; however, the phenotype is often misdiagnosed and attributed to other downstream effects associated with its role in various enzymatic redox reactions. 
Molybdenum fertilization through foliar sprays can effectively supplement internal molybdenum deficiencies and rescue the activity of molybdoenzymes. 
The current understanding on how plants access molybdate from the soil solution or later redistribute it once in the plant is still unclear; however, plants have similar physiological molybdenum transport phenotypes to those found in prokaryotic systems. 
Thus, careful analysis of existing prokaryotic molybdate transport mechanisms, as well as a re-examination of know anion transport mechanisms present in plants, will help to resolve how this important trace element is accumulated.

Molybdenum, molybdate transport, nitrate reductase, Moco, Vitis vinifera, Merlot, Millerandage, sulfate transport, nitrogen fixation, nitrogen metabolism, plant nutrition

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