Molybdenum deficiency in plants results in stunted growth, interveinal, marginal chlorosis, pale green coloration, leaf rolling or cupping, deformation, and withering. Extreme defiance will result in marginal and in-between margin scorching, necrosis, or even leaf death.
Discover more about molybdenum deficiency in plants, including causes, symptoms, and diagnosis. We will also give you treatments or fixes and a lot more.
Overview
Molybdenum (Mo) has roles in nitrogen assimilation (conversion of nitrate to nitrate for making proteins and amino acids required for growth and development) and nitrogen fixation in legumes. Also, it participates in the synthesis of some plant hormones and the degradation of purine and sulfate.
Plants absorb molybdenum as molybdate (MoO42–), and plants need a very small amount, about 0.1-2.0 ppm, for optimum growth and development. Too much will cause toxicity, while too little will cause deficiency symptoms.
What causes molybdenum deficiency in plants
Most soils naturally have enough molybdenum, and a deficiency is rare unless the soils are acidic with a pH of less than 5.5. Also, it can occur in highly weathered, infertile, leached, or have some cationic metal oxides like iron, manganese, or aluminum. Also, clay minerals may result in a deficiency.
Here is more on these possible causes:
- Acidic soils: Acid soils will always be molybdenum deficient since molybdate will bind with any cationic minerals just as it happens with phosphate reducing its availability.
- Soils with Fe, Al, or Mn oxides and clay minerals: Amorphous iron, aluminum, or manganese oxides and clay minerals tend to adsorb molybdenum, reducing its availability in the soil. However, adsorption will only occur in acidic pH, with optimum values being 4-5.
- Highly weathered: Highly weathered soils will have released most of the Mo they initially naturally had, making them deficient.
- Podzols, podzolic soils, or iron pan soils: These are infertile, acidic soils that will tend to have low amounts of available molybdenum.
- Infertile soils: Most infertile soils low in phosphorus, sulfur, and other minerals in their natural state will also be low in molybdenum.
- Leaching: Sandy and well-drained soils may result in a deficiency. However, since it strongly sorbs to some metal oxides (Fe, Al, Mn), clay minerals, carbonates, organic matter, etc., molybdate is relatively resistant to leaching if you compare it to chloride or nitrate.
Besides these causes, feeding with nitrate fertilizer may induce deficiency sooner than ammonium fertilizer since plants need molybdenum to convert nitrate to ammonia.
What are the molybdenum deficiency symptoms?
Molybdenum deficiency symptoms will first show in the older and middle leaves and spread to younger, upper leaves. However, these signs may occur on the entire plant since it is mobile.
Also, the symptoms closely resemble nitrogen deficiency since plants lack enough nitrate reductase to convert nitrate to usable organic nitrogen. Also, legumes cannot fix nitrogen in root nodules.
Some of the common symptoms include the following:
- Slow or stunted growth and plants showing less vigor as they cannot make some amino acids required for growth and development.
- Leaves start turning pale green, including on the margins and in-between veins, beginning with the lower and middle ones.
- Lower/middle or older leave marginal, interveinal yellowing or chlorosis that will spread to upper leaves.
- Leaf rolling or cupping, crinkling, deformation, and withering with some vegetables, especially cauliflower, will have whiptail.
- Due to nitrate accumulation in leaves, there is marginal and interveinal scorching or necrosis as plants can no longer convert it to usable form due to less nitrate reductase. This marginal and interveinal browning usually occurs in advanced deficiency stages.
- You will notice fewer and smaller root nodules in legumes that will be greenish or colorless and not the usual pink color when healthy.
- Restricted flowering (formation and development) with some eventually withering and confined fruit setting
- Leaf death
- Reduced plant yield and poor health
Besides these generalized symptoms, some plants may have unique symptoms. See more on molybdenum symptoms in cabbage, citrus, cannabis, tomato, cauliflower, corn, poinsettias, cucurbits, and more.
What does Mo deficiency resemble?
Molybdenum deficiency resembles nitrogen deficiency since plants cannot utilize nitrate, and legumes cannot fix nitrogen in root nodules.
Also, marginal and interveinal necrosis or tissue death will resemble boron toxicity or salinity damage. However, molybdenum deficiency will cause necrosis to a much lesser extent.
Diagnosis
Symptoms may help you make an intelligent guess. Check for whiptails on cauliflowers around the area and patchy distribution of the various symptoms affected plant. Deficiency rarely affects the whole plant.
Next, since there are no reliable soil tests for molybdenum, conduct a soil pH test. It will help you make an intelligent guess since a molybdenum deficiency is rare in soils whose pH is 6.0 or higher.
For more conclusive results, submit the youngest but fully emerged leaf for molybdenum tests. Levels below 0.075mg per kg will indicate a deficiency. Also, a nitrate test will be handy since its accumulation will rule out nitrogen deficiency which presents similar symptoms.
Lastly, apply sodium or ammonium molybdate to leaves or the soil around the base to see if plants respond. If there was a deficiency, you will notice improved growth and leaves regaining proper coloration compared to untreated ones.
How to fix or treat molybdenum deficiency in plants
If diagnostics positively identifies molybdenum deficiency as the cause of the various symptoms, you can fix the deficiency by the following methods:
1. Liming to lower pH
You can fix molybdenum deficiency by amending soil pH below 6.0 since acidic soil reduces molybdate availability. In such cases, apply lime or dolomite at a rate of 2-8 tonnes per hectare depending on the initial soil pH and whether the soil is heavily textured or sandy.
However, liming will take a long to show results and is not the preferred method unless it has other benefits for your plants. Also, it is more expensive.
2. Use molybdenum fertilizers
The other faster and more effective way to deal with Mo deficiency is to use molybdenum fertilizers, i.e., for soil application, foliar, and seed.
- Soil fertilizers: You can opt for broadcasting, fertigation, banding, or spraying before planting, with usual application rates being 0.5-2lb per acre, depending on plant requirement and the extent of the deficiency. Use this method if soil pH isn’t the cause since acidic soils will still make the fertilizer unavailable.
- Foliar fertilizers: By applying 50-200g per hectare of foliar molybdate fertilizers, you can quickly correct any deficiency and, if used early, prevent it altogether. Select water-soluble ones like sodium or ammonium molybdate.
- Seed treatment: Treating seeds with 7-100g molybdate fertilizer per hectare will provide enough molybdenum plants need for growth. It requires less molybdate, and the results are excellent. However, please don’t overdo it, as it will affect germination rates.
3. Apply dead decomposing matter, including mature
Increasing organic matter, especially in infertile soils, may help raise the amount of Mo available for plants. Also, it will improve soil structure, aeration, and other nutrients.