No gardener wants to see it. All the hard work you’ve put into preparing the soil and planting only to realize your cauliflower, cabbage, or broccoli are stunted and dying. A visual inspection causes you to suspect a molybdenum deficiency, and testing confirms it. But what does that mean? And is there anything that can fix it?
To treat a molybdenum deficiency in a plant, correct the pH level of the soil by reducing the acidity and apply a molybdenum fertilizer. Molybdenum is an element that is necessary for plant growth. When a plant is deficient in molybdenum, its leaves turn yellow or wilt, and its growth is stunted.
Molybdenum deficiencies in crops tend to be a rare occurrence, but they are also frequently misdiagnosed as a nitrogen deficiency. Before treating your garden unnecessarily or incorrectly, it is best to test the soil and possibly the plant tissue. The results will help determine the best way for you to treat your plants.
Table of Contents
- 1 How to Treat Plants Deficient in Molybdenum
- 2 What is Molybdenum and Why Do Plants Need It?
- 3 Signs of Molybdenum Deficiency
- 4 Too Much Molybdenum?
How to Treat Plants Deficient in Molybdenum
Treating a molybdenum deficiency in plants may require a multi-pronged approach. For a mild deficiency, you may have success by raising the pH level of the soil. For more serious cases, a foliar fertilizer may be needed in addition to addressing the soil’s pH level.
Sometimes the concentration of molybdenum in the soil may be low even if the pH level is more neutral. In that case, a molybdenum fertilizer applied to the soil would be a better choice.
Raise the pH of the Soil
Because a molybdenum deficiency is often tied to the pH level of the soil, that is one of the first things to address. As the soil becomes more acidic, the molybdenum binds to minerals within it and is less available to the plant. Raising the pH level of the soil will help release the molybdenum and make it available for use.
Acidic soil can be made more neutral by adding lime or wood ashes. Liming is a common way of reducing the acidity of soil. As the soil becomes more neutral or slightly alkaline, any molybdenum in it will be released for uptake in the plant.
One downside to addressing a molybdenum deficiency through the pH level of the soil is that it is a slower correction method. It can take several months for lime or wood ashes to raise the pH of the soil. Ideally, soil testing is done prior to planting so that corrective actions can be taken before the growing season, thus preventing any potential nutritional deficits.
Add Organic Matter
Organic matter in soil is animal or plant material that is in various stages of decomposing. Adding organic matter is one of the best ways to improve the fertility of soil. Different types of organic matter that can be easily added to garden soil include compost, mulch, cover crops, and farmyard manure.
When soil is rich in organic matter, molybdenum and other micronutrients are more abundant in forms that are usable by plants. Compost can be made from grass clippings, leaves, vegetable scraps, paper, and other items.
Seaweed is another consideration for your garden. It is rich in minerals and nutrients. Adding seaweed to your compost and mulching it into your soil will not only increase the organic matter in your soil, but it will give a boost to the trace elements in the soil. It contains both the macro and micronutrients needed by plants to grow.
Apply a Molybdenum Fertilizer
If the pH level of the soil is already in the neutral range, or if a more immediate result is needed, you may consider using a molybdenum fertilizer. Fertilizer may be applied in different ways: to the soil, the foliage, or the seeds.
Molybdenum can be added to the soil to help with deficiencies. Many times, it is mixed with other materials for fertilizing. Common fertilizers include the macronutrients nitrogen, phosphorus, and potassium. To help with even distribution on the soil, the fertilizer might be mixed with water and sprayed before planting.
Liquid applications of soluble molybdenum can be applied directly to the leaves, which is known as foliar application. While this does not address any deficiencies that may be in the soil, it will help with plants that are known to be lacking essential elements. Foliar treatments can help correct nutritional deficiencies quickly.
The best time to apply molybdenum fertilizer this way is when plants are young. Doing so will help prevent deficiencies before they occur. Foliar applications can also be applied to older plants that are showing signs of developing a molybdenum deficiency. Plants tend to respond to this type of treatment rapidly and recover their ability to process nitrogen.
Seeds can be treated with a molybdenum fertilizer to prevent deficiencies. This is done in regions where the soil is known to be acidic or deficient in molybdenum. The fertilizer is applied to the seeds before planting.
While seed treatment is not really an option if you are just discovering a molybdenum deficiency in your current garden, it is something to consider when you plant next year.
See also: How to Lower Manganese Levels In Soil
What is Molybdenum and Why Do Plants Need It?
Molybdenum is a silver-gray metal found in the environment. It is commonly used in making metal alloys. It is also an important trace nutrient required for all life — both plants and animals. It is found in the soil and taken up by plants as they grow. The plants use molybdenum to help them process nutrients, especially nitrogen.
Animals need molybdenum to activate several different enzymes necessary for chemical reactions in their bodies. They acquire this essential element by consuming plants that contain molybdenum or other animals that have fed on the plants.
Other Nutrients Plants Need
In addition to air and water, plants require fourteen nutrients for healthy growth. They are found in the soil and taken up by the plant through its roots. Some of these are needed in larger quantities and are called macronutrients. Others are needed in small, or trace, amounts and are called micronutrients.
Molybdenum is one of the micronutrients that is essential for plant growth. While the amount a plant need is the lowest level of all the nutrients (along with nickel) when there is a deficiency plant growth is affected.
What Does Molybdenum Do for Plants?
Without molybdenum, plants cannot properly process nitrogen or phosphorus. Both are macronutrients that are critical for a plant’s ability to grow. Molybdenum enables a plant to make enzymes so they can use nitrogen either from the soil (as nitrate) or the air (called nitrogen fixation). It is also necessary to help plants convert phosphorus so they can use it.
Nitrogen is one of the most important elements for plants, and they need it in large amounts. Plants use nitrogen to create chlorophyll and for photosynthesis. Without nitrogen, plant growth slows down, and leaves turn yellow and fall off.
Molybdenum is essential for a plant to form an enzyme that enables it to take up nitrate from the soil. Without this enzyme, a plant cannot utilize the nitrogen available to it. Because of how closely molybdenum is linked to nitrogen use, the symptoms of a deficiency in molybdenum are similar to symptoms of a nitrogen deficiency.
Crops that are lacking in adequate levels of molybdenum will produce smaller yields. By correcting molybdenum deficiencies, you can greatly improve food production. Mild deficiencies, even before they are noticed because of other symptoms, can contribute to a decreased yield.
What Affects the Amount of Molybdenum in the Soil?
Molybdenum is a trace element that is found in most soils. Plants acquire it from the soil through their roots. However, the amount found is strongly dependent upon the pH level of the soil and other conditions.
There are several factors that can affect the amount of molybdenum in soil:
- Soil pH
- Organic matter
- Amount of water
- Type of nitrogen
When a soil’s pH level is low, it is said to be acidic. In acidic soils, molybdenum is less easily taken up by plants. Although it may be present in the soil, the molybdenum is unavailable to the plants because it binds to other minerals which cannot be taken up. A soil that is in the proper pH range allows the molybdenum to bind to the organics and other minerals in the soil.
The amount of molybdenum in the soil corresponds to the amount of organic matter in it. In fertile soils, there is a larger amount of organic matter. Molybdenum binds to this organic matter in a way that is usable by plants. Adding organic matter, such as farmyard manure or compost, can greatly improve a soil’s micronutrients and composition.
Amount of Water
The amount of drainage a soil has will affect the amount of molybdenum in it.
Sandy soils tend to be lower in available molybdenum because they are lower in organic materials that the molybdenum binds to. They also drain water rapidly and can leach larger amounts of molybdenum.
Wet soils, such as those found in peat marshes or swamps, have higher amounts of organic material. Because of the poor drainage, they tend to have higher concentrations of molybdenum which can be taken up by the plants that grow there. These plants may take up higher amounts of molybdenum, which is not harmful to the plant itself, but may be too high for the animals that eat them.
Type of Nitrogen
Nitrogen in the soil comes in different forms. The type of nitrogen affects the amount of molybdenum a plant needs. When the nitrogen is in the form of ammonium, the plant will need less molybdenum. More is required when the nitrogen is in the soil as nitrate.
Signs of Molybdenum Deficiency
All plants can be affected by a molybdenum deficiency. Some species show symptoms more than others. It can be difficult to tell if a plant is deficient in nitrogen, molybdenum, or some other essential nutrient. The best way to determine why a plant is struggling is through testing.
Plants Most Affected by Molybdenum Deficiency
The role of molybdenum was first recognized in tomatoes in 1939. Since then, the symptoms of molybdenum deficiencies have been found in many plant species. Plants in the Brassicaceae family, including broccoli, cabbage, cauliflower, turnips, Brussels sprouts, and many others, are particularly susceptible to molybdenum deficiencies.
While all plants need molybdenum to thrive, many plants have especially been noted as showing symptoms when there is a deficiency:
- Brussels sprouts
If you notice any of the symptoms described below in these plants, don’t rule out a molybdenum deficiency.
Symptoms of Molybdenum Deficiency
Short of testing the soil or plant tissue, how can you determine if your crops have a molybdenum deficiency? Careful observation can reveal several clues as to what is causing problems with your plants. Things to note are where on the plant these signs occur or at which point in the life cycle.
Plants suffering from a molybdenum deficiency have several characteristics that are easily seen:
- Chlorosis, or yellowing, of the leaves
- Whip-tail characteristics of leaves (thin and misshapen)
- Leaf cupping
- Wilted or shriveled leaves
- Stunted seedlings
- Poorly developed flowers and reduced pollen
In young plants, the leaves may be mottled, limp, or cupped. Seedlings do not thrive and die. In older or more established plants, existing leaves may yellow, and the edges turn brown. Symptoms may show up on new leaf growth as whiptail, or they may have a leathery appearance.
In your garden, you may first notice a molybdenum deficiency because of your cauliflower plants. They will often have cupped leaves or malformed curds. Molybdenum deficient plants also have less pollen than well-nourished plants and produce less fruit or grain as a result.
Similarity to a Nitrogen Deficiency
Since a lack of molybdenum will interfere with a plant’s ability to process nitrogen properly, the symptoms resemble a nitrogen deficiency. There are some subtle differences between the two which can help you determine the best way to treat your plants.
Some differences are that a molybdenum deficiency first shows in older leaves, and they become cupped and thickened with yellow patches closer to the leaf veins. A nitrogen deficiency shows as a general lightening of the entire plant and a yellowing of older leaves that progresses to the younger leaves.
Testing for a Deficiency
Testing your soil or the plant tissue can aid greatly in trying to figure out what is happening in your garden. Unfortunately, there is no specific soil test for a molybdenum deficiency. Molybdenum levels will show in plant tissue tests. However, because of the very small amounts of needed molybdenum, often these tests are expensive, and reliability is low.
Testing the Soil’s pH Level
One test that can help with treating a molybdenum deficiency is testing a soil’s pH level. Determining whether your soil is acidic, alkaline, or neutral can go a long way in knowing the best course of action.
On the pH scale, levels less than 7 are called acidic, and greater than 7 are basic, or alkaline. For plants, soil with a pH level between 6.5 and 7.5 is considered neutral. This is an ideal range for most plants to grow and take up the nutrients. While many micronutrients are less available as the soil becomes more alkaline, the opposite is true for molybdenum which becomes less available as the soil becomes more acidic.
Sending out a soil sample can determine where on the pH scale your soil falls. If your soil is too acidic, there are steps you can take to raise the pH level and bring it within the neutral range. Keep reading for ways to treat the pH level of the soil and a molybdenum deficiency.
Too Much Molybdenum?
Is there too much of a good thing? Do you have to watch out for molybdenum toxicity?
Plants can tolerate high levels of molybdenum and not show any symptoms. Molybdenum toxicity is a rare occurrence and may show as a golden yellow color of the leaves. Higher than needed amounts of molybdenum in a plant is unlikely to cause problems with the plant’s growth.
One thing to be concerned about is animals such as cows, sheep, and goats that eat grasses with higher-than-normal levels of molybdenum. The high levels of molybdenum react in their bodies, and they can develop a disease that causes a copper deficiency.
Molybdenum is an important micronutrient for plant growth. Without it, a plant will be unable to process nitrogen properly and will fail to thrive. Even without exhibiting outward symptoms of a deficiency, a plant may have levels that are too low and cause a reduced production of food.
Molybdenum deficiencies in plants are often found in areas with acidic soil, and treatments include raising the soil’s pH level and using a molybdenum fertilizer.