Molybdenum (Mo) is one of seven recently identified trace elements that are essential for plant growth. It is the only transition element in group VI in the periodic table that is essential for normal growth, metabolism, and reproduction of higher plants. The biological importance of Mo in plants is due to its highly beneficial action in the fixation of nitrogen, from the air, by the nitrogen-fixing bacterium (Azotobacter chroococcum). After the establishment of its essentiality by scientists in Australia more than half a century ago, its deficiency has been reported in several countries in a variety of crops. The agricultural researchers in Australia were able to overcome the symptoms of Mo deficiency in tomatoes (Lycopersicon esculentum Mill.) by addition of minute quantities of Mo in the nutrient solution. Some of the crops considered most sensitive to Mo deficiency are clovers (Trifolium subterraneum L.), cauliflower (Brassica oleracea var. botrytis L.), broccoli (Brassica oleracea L. Botrytis Group), rape (Brassica napus L.), beet (Beta vulgaris L.), spinach (Spinacea oleracea L.), lettuce (Lactuca saliva L.), and alfalfa (Medicago sativa L.).

Among the micronutrients, Mo is an exception in that it is readily translocated, and its deficiency symptoms generally appear on the whole plant. The deficiency symptoms for other micronutrients appear on the young leaves at the top of the plant because of their inability to translocate within the plant. Molybdenum deficiency emerges as general yellowing and stunting of the plant, interveinal mottling, and cupping of the older leaves, followed by necrotic spots at leaf tips and margins.