Molybdenum is essential to plant growth as a component of the enzymes nitrate reductase and nitrogenase. Legumes need more molybdenum than other crops, such as grass or corn, because the symbiotic bacteria living in the root nodules of legumes require molybdenum for the fixation of atmospheric nitrogen. If insufficient molybdenum is available nodulation will be retarded and the amount of nitrogen fixed by the plant will be limited.
If other factors are not limiting the amount of molybdenum will determine the amount of nitrogen fixed by the plant. Increasingly vigorous plant growth, higher protein contents and greater buildup of nitrogen in the plant and soil accompany nodulation and symbiotic microbial activity.
Source: Albrigo, L. G., Szafranck, R. C.and Childers, N. F., The Role of Molybdenum in Plants and Soils, Climax Molybdenum Co., Supplemental volume, 1966.