As far as we know at present, plants use molybdenum for one purpose only, but that purpose is of tremendous importance—producing enzymes to enable the plant to obtain and use nitrogen:
- Legumes use molybdenum to fix nitrogen from the air.
- Other plants use it to pull nitrogen up from the soil.
- All plants use molybdenum to make nitrogen into important compounds, including amino acids and chlorophyll.
Most soils naturally contain enough molybdenum to sustain plant life. However, seaweed extract is a good supplemental source of molybdenum.
Molybdenum can be leached from sandy soils that receive too much water. Likewise, peaty soils are commonly low in many nutrients, including molybdenum. However, most cases of molybdenum deficiency in plants are caused by low pH.
Symptoms of molybdenum deficiency include:
- Downward leaf cupping.
- Bleaching and death of margins and areas between veins on leaves.
- Few root nodules on legumes.
- Nitrogen deficiency in legumes.
- Reduced flowering.
- Reduced yield.
Should a molybdenum deficiency present itself in your garden, orchard, or field, the most important thing to do is raise the pH to a more neutral level. Consider amending sandy soils with organic matter to improve nutrient retention. Peaty soils will probably require supplementation with compost or seaweed extract to boost nutrient levels.
Molybdenum toxicity is nearly unheard of in plants. For one thing, plants appear to have an enormous tolerance for this mineral. For another thing, when molybdenum levels are unusually high, plants do not appear to show symptoms of toxicity, but rather symptoms of other nutrient deficiencies.
One thing to watch out for, however, is the use of foliar sprays containing molybdenum, as they can burn the leaves of your plants. Chelated molybdenum is the most hazardous form, but even a natural seaweed spray can cause damage if the concentration is too high.