Why Do Plants Need Molybdenum? A Micronutrient That’s Crucial To Plant Development

We all know that sunlight and water are the basis of plant growth. While they’re crucial for plant growth, there are also other things that the plants need. As a result, plant development is a little more complicated than what we learn in primary grade science classes. There are 17 essential nutrients that plants absorb from their surroundings. Each one has its importance for plant growth, but the amounts in which they’re needed vary. Molybdenum is one such essential nutrient that’s categorized under trace elements, or micronutrients, for plants.

Unlike major elements, like nitrogen, phosphorus, potassium, molybdenum, along with other micronutrients, are needed in tiny quantities. Though absorbed in trace amounts, molybdenum plays an imperative role in regulating many plant’s functions.

Why do plants need molybdenum, and how do you ensure its optimal supply to your plants? What are the deficiency symptoms, and how do you fix them? Read on, and you’ll learn all about the mineral and its role in plant growth.

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What Is Molybdenum?

Molybdenum (Mo) is a chemical element present in Group 6 of the periodic table. You may not have heard of it before, but it’s an essential trace mineral necessary for our health as well as that of plants. It’s present in foods, including milk, cheese, grains, and leafy vegetables.

Though it’s only needed in small amounts by our body, it contributes to many functions inside our body. Plants also depend on molybdenum for various functions. It’s among the seven essential plant nutrients that are described as micronutrients. The other six micronutrients include boron, zinc, manganese, copper, iron, and chlorine.

What Is The Function Of Molybdenum In Plant Growth?

So far, you know that molybdenum is a chemical element that plants and animals contain. But what does it do? Why do plants need molybdenum? There are two primary roles of the element in plant functions. Both of these contribute to healthy plant development. Plants need molybdenum for:

  • Creating an enzyme that helps the plants use nitrates present in the soil. The soil nitrates are first converted to nitrite and then to ammonia, which is used for synthesizing amino acids inside the plant.
  • Creating an enzyme in legume crops helps the symbiotic nitrogen-fixing bacteria fix the nitrogen present in the atmosphere.
  • Nitrogen (N) is one of the three primary macronutrients for plants alongside phosphorus (P) and potassium (K). Molybdenum helps the plants use nitrogen in forming compounds like amino acids and proteins. The same compounds further contribute to creating chlorophyll, which is essential to the survival of plants.
Without the proper supply of molybdenum, plants won't form the complex nitrogen compounds that keep the plants healthy and alive. So, as expected, you'll see that lack of molybdenum in the soil will result in quite similar symptoms to those of nitrogen deficiency.

Other than helping the plants use their nitrogen supply, molybdenum also contributes to more functions. For example, it converts inorganic phosphorus to organic phosphorus within the plant, contributing to plant growth.

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What Are The Ideal Concentrations Of Molybdenum In Soil?

Plants get their molybdenum supply from the soil. Though plants can generally tolerate high concentrations of molybdenum in the ground, their development will suffer if the soil lacks or is deficient in molybdenum. So what are the proper concentrations of molybdenum that we should be aiming for? How much molybdenum do plants need?

Molybdenum concentrations between 0.1 to 1.0 ppm in the soil are generally enough for most crops. In plant tissues, ideal molybdenum concentrations are between 0.3 to 1.5 ppm. However, in case of an imbalance of the nutrients, whether it’s a deficiency or an excess (which is rarely the case), the gardeners must respond promptly to correct the concentrations before taking a toll on the crop’s yield or quality.

Molybdenum Deficiency In Plants

You’ve already learned how molybdenum is essential to plants. If the soil lacks the nutrient or the plants aren’t absorbing enough to cover their requirements, it will show up in the plant’s performance.

What Are The Reasons For Molybdenum Deficiency?

Molybdenum is amply available to plants in alkaline soils. However, the element tends to be unavailable to plants in acidic soil even if it’s sufficiently present in the ground. As a result, most cases of molybdenum deficiency come from soils with pH levels less than 5.5. Sometimes, deficiency symptoms even result at pH levels less than 6.0.

Other than this, sandy soils, peat soils, and highly weathered soils may also lack molybdenum, together with other necessary nutrients. According to research, high concentrations of sulfates can also reduce plants' uptake of molybdenum. Certain crops are more susceptible to molybdenum deficiencies. The symptoms usually show up in legumes, cruciferous vegetables, soybeans, tomatoes, grapes, duckweed, citrus, and poinsettias.
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What Happens When Soil Has Too Little Molybdenum?

Unlike other micronutrients, molybdenum is mobile, so the deficiency symptoms will initially appear in older leaves and spread towards the new growth. As a result, the leaves deform and turn pale and withered. As a result, the plants do not grow their best, and in later stages, flower formation and fruit development are restricted.

In cauliflowers, the midrib of the leaves continues to grow, but the leaf width is significantly reduced, resulting in narrow, distorted leaves. We call this condition ‘whiptail.’

How To Diagnose Molybdenum Deficiency In Plants?

The symptoms of molybdenum deficiency are very characteristic in cauliflowers. But, in many crops, they are not easily identifiable. It’s easy to confuse them with deficiencies of other elements, especially nitrogen.

One way to confirm molybdenum deficiency is to apply a solution of ammonium molybdate to the leaves or the soil around one of the affected plants. If growth improves and the leaves respond with a healthy green color compared to those of the untreated plants, it's confirmed that plants are suffering from molybdenum deficiency.

How Is Molybdenum Deficiency Treated In Plants?

There are several possible solutions to the symptoms of molybdenum deficiency, depending on the problem’s reasons.

1. Increase Soil pH

If soil acidity is causing molybdenum deficiency in plants, liming the soil can help improve the symptoms. With the increase in soil’s pH levels, if molybdenum is already present in the ground, it will become readily available to the plants.

The Optimal Soil pH For Most Plants Is Between 5.5 To 7.5
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2. Foliar Application

A quick fix is a foliar application of molybdenum fertilizer to the plants. Since plants only need it in small amounts, it will revitalize the plants quickly. But, if the soil lacks the element, a foliar spray will not fix the shortage.

3. Soil Amendments

If the soil pH is ideal, amending the soil with a molybdenum fertilizer supplement will improve the symptoms. It will also nourish the ground for your future crops. Single element applications include sodium molybdate and ammonium molybdate. A complete micronutrient fertilizer is enough to balance the trace elements in the soil. It will correctly address the deficiency without causing an imbalance in any of the other nutrients.

Plants that are growing in low molybdenum soils will also enjoy fertilizers containing NH4. NO3 fertilizers, on the other hand, won’t help in the condition.

Note: Always read the instructions before applying chemical fertilizers! An excess of chemicals can be as detrimental as their shortage, possibly even more. According to the concentrations mentioned in the package, use enough to compensate for the lack of nutrients.
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Is Molybdenum Toxicity Possible?

Molybdenum toxicity is very rare, but it may show up in certain crops. In most cases, plant tissues can have over 1000 ppm or more molybdenum and still not show any symptoms.

Although molybdenum toxicity isn’t usually problematic to plants, ruminant animals fed on crops with molybdenum levels higher than five ppm can get molybdenosis. Molybdenosis is a disease in which high levels of molybdenum compete with copper availability in the animals and induce copper deficiency symptoms.

What Are The Symptoms?

In rare cases, plants may show golden yellow leaves with an excess of molybdenum in the plant tissues.

So, What Are The Solutions?

Applying sulfur to the soil reduces the uptake of molybdenum by the plants and relieves the toxicity symptoms.

Final Words

Now you know the answer to – why do plants need molybdenum? Plants need their tiny doses of the trace element to perform well, and so do we.

While plants depend on the soil to provide them the proper share of molybdenum, we rely on the plants for our molybdenum supply. Therefore, ensure your soil is rich in all the beneficial nutrients to maintain a healthy, balanced diet for all.

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