Plants take up molybdenum (Mo) as the MoO42- anion. It’s required for the synthesis and activity of the enzyme nitrate reductase and. vital for the process of symbiotic nitrogen (N) fixation by Rhizobia bacteria in root nodules. It’s also needed to convert inorganic phosphorus (P) to organic forms in the plant.
Molybdenum deficiencies show up as general yellowing or stunting of the plant, and more specifically in the marginal scorching and cupping or rolling of leaves. An Mo deficiency can also cause N-deficiency symptoms in legume crops such as soybeans and alfalfa, because soil bacteria growing symbiotically in legume root nodules must have Mo to help fix N from the air.
Molybdenum deficiencies occur mainly in acidic, sandy soils in humid regions. Sandy soils, in particular, more typically lack Mo than finer-textured soils. Molybdenum becomes more available as soil pH goes up, the opposite of other micronutrients. Since Mo becomes more available with increasing pH, liming will correct a deficiency if soil contains enough of the nutrient. However, seed treatment is the most common way of correcting Mo deficiency because only very small amounts of the nutrient are required.
Heavy P applications increase Mo uptake by plants, while heavy sulfur (S) applications decrease Mo uptake. Applying heavy amounts of S-containing fertilizer on soils with a borderline Mo level may induce Mo deficiency.
Excessive Mo is toxic, especially to grazing animals. Cattle eating forage with excessive Mo content may develop severe diarrhea.
Symptoms of deficiency can vary across crop species, but similarities exist for how nutrient insufficiency impacts plant tissue color and appearance. Nutrient deficiencies are commonly associated with the physical location on the plant
(i.e., whether the symptoms are primarily observed on older versus newly formed plant tissue), but these symptoms can spread as the severity of the deficiency progresses.