Molybdenum is a plant micronutrient. Molybdenum is only required in very small amounts but it is important for nitrogen metabolism; without molybdenum, plants may be able to take up nitrogen but if it's in the form of a nitrate (NO3‑) they can't process it and use it for it's intended purpose (to make amino acids and proteins for instance). Legumes may also have difficulty 'fixing' nitrogen (more accurately, the legumes have trouble using nitrogen and the nitrogen fixing bacteria they associate with have trouble converting atmospheric nitrogen to a form that the plant can use). Molybdenum also plays an essential role in the use of phosphorus within plants. Without molybdenum, plants may be able to take up inorganic phosphorus but they will struggle to convert that phosphorus into an organic form that they can use.
Factors that affect the availability of molybdenum include:
- Excess water ‑ high rainfall and frequent irrigation can wash molybdenum from the soil.
- Soil pH ‑ molybdenum is more easily uptaken by plants when the pH of the soil is high and less easily uptaken when the pH is low.
- Nitrogen ‑ the type of nitrogen in the soil and in any applied fertiliser can affect the amount of molybdenum that is required by plants. When more of the nitrogen is in the form of amonium (NH 4+), less molybdenum is required. When more more of the nitrogen is in the form of nitrate (NO3‑), more molybdenum is required.
Ideally, for healthy and productive soil the concentration of molybdenum in the soil should be at least 2 mg/kg.
Acidic soils and those comprised primarily of sandstone are more likely to be deficient in molybdenum.
Symptoms of Molybdenum Deficiency
Molybdenum deficiency stunts plant growth and plants may appear to have a nitrogen deficiency (because they're unable to use nitrogen properly). Leaves may also exhibit spots of chlorosis between the veins and along the edges. Eventually, the edges of leaves may turn brown and die though this only occurs in some plant species. Symptoms appear in older leaves first. The pollen of molybdenum deficient plants will usually be less viable than that of healthy plants so grain and fruit production is often reduced. Molybdenum deficiency is usually seen first in cauliflower as it has a high molybdenum requirement. Affected cauliflower plants will usually have cupped leaves.
Treating Molybdenum Deficiency
If your soil isn't neutral then adding lime may improve the uptake of molybdenum by your plants as molybdenum is most available when the pH of the soil is at least 7 though if the pH is at least 6, most plants should be able to uptake enough molybdenum. An increase in organic matter is also useful as it should help prevent molybdenum from being leached from the soil.
If soil tests reveal that the soil contains too little molybdenum, then you can use a molybdenum fertiliser to increase the level. Seaweed extracts may contain small amounts of molybdenum and are useful if you garden organically. Otherwise, sodium molybdate and ammonium molybdate are useful synthetic fertilisers that contain molybdenum. As sodium can be harmful to plants, I would use ammonium molybdate unless the soil contains large amounts of nitrogen.
Molybdenum toxicity is rare and it is of greater concern to the animals (generally ruminants) that eat plants containing excess molybdenum than it is to the plants themselves. Symptoms of toxicity are similar to deficiency and the more obvious symptoms of toxicity are actually symptoms of deficiency of other nutrients because high levels of molybdenum can reduce the absorption of other nutrients ‑ copper in particular. If molybdenum toxicity is ever suspected, an application of sulfur can reduce molybdenum uptake until the soil level drops and especially if the pH of the soil is greater than 7; care must be taken though to prevent other nutrient imbalances. In such circumstances, molybdenum containing fertilisers should not be used.