Molybdenum

Molybdenum is a cofactor for nitrate reductase, the enzyme plants use to convert absorbed nitrate into forms usable for amino acid synthesis, and for nitrogenase, the enzyme that rhizobia bacteria use to fix atmospheric nitrogen in symbiosis with legume roots. Because of this, molybdenum status affects both direct nitrate use and, in legumes, biological nitrogen fixation.

Unlike most micronutrients, molybdenum becomes more available as soil pH rises. Deficiency is therefore concentrated on acidic, often high-rainfall or heavily leached soils, and has historically been corrected by liming to raise pH as much as by direct molybdenum application.

• Acidic soils are the main setting for molybdenum deficiency
• Liming to raise soil pH increases molybdenum availability
• High-rainfall, leached soils are more prone to deficiency than drier, less-leached soils

In legumes, molybdenum deficiency impairs nitrogen fixation, producing poor nodulation and nitrogen-deficiency-like symptoms even when soil nitrogen is adequate. In brassica crops, a classic symptom is "whiptail" — narrow, distorted, and often chlorotic leaves.

Molybdenum is rarely directly toxic to plants at levels found in agricultural soils. However, excess molybdenum accumulated in forage can cause molybdenosis in grazing ruminants such as cattle and sheep, a disorder that induces secondary copper deficiency in the animal — a notable example of a plant-nutrient issue with direct implications for livestock health.

Molybdenum and sulfur show a documented antagonism, with high sulfate availability reducing molybdenum uptake. Molybdenum is also functionally tied to nitrogen: in legumes it is essential for the nitrogen-fixation process itself, and in other crops it is required for the enzyme that processes absorbed nitrate.