Molybdenum

Select enzymes require molybdenum to perform redox reactions such as nitrogen metabolism, hormone production and sulfur metabolism. Too much nitrogen in fertiliser can cause negative environmental impacts and molybdenum increases the efficiency of the enzyme that absorbs nitrogen, meaning the total concentration of nitrogen in the soil decreases. The bioavailability of molybdenum is dependent on the pH of the soil; as the pH decreases, the solubility and therefore availability of the molybdenum also decreases. Some plants require molybdenum to fix atmospheric nitrogen. 

Because only a  very small amount of molybdenum is required by plants, there is a very narrow window between deficiency and toxicity and is normally a product of incompatible soil pH. A deficiency of molybdenum becomes apparent under 0.2 ppm in dry matter, and causes the leaves to have interveinal mottling, chlorosis (yellowing) of older leaves and necrotic spots at leaf tips. Too much molybdenum in an agricultural condition is rare, but can cause tomato and cauliflower leaves to turn purple and legume leaves to turn yellow. Common forms of adding molybdenum are sodium or ammonium molybdate applied in solution to either the leaves or the soil.  

Hooda, P. (2010). Trace elements in soils. Wiley.