Introduction

Molybdenum (Mo) deficiencies have been reported from many countries around the world, mainly in acidic soils. Sandy soils are Mo-deficient more often than are loam or clay soils. Most Mo deficiencies are associated with legume crops, because Mo is an essential constituent of enzymes necessary for fixation of nitrogen (N) by bacteria growing symbiotically with legumes. Molybdenum is also required in other enzyme systems in all plants.

The availability of Mo in soil increases with increasing soil pH. Therefore, liming a soil to the recommended pH range may increase the plant availability of soil Mo sufficiently that Mo fertilization may not be required. This chapter discusses the sources of Mo and the methods of applying Mo fertilizers to those crops that require additional Mo to produce optimum crop yields.

Molybdenum Sources

There are fewer sources of Mo for fertilizers than there are for the other micronutrients (Table 11.1). Ammonium and sodium molybdates and molybdic acid are soluble compounds. These sources of Mo are sometimes applied with other fertilizers or are used as foliar sprays. Both MoO3 a nd Mo frits are insoluble in water, but are effective if applied as fine powders; MoO3 is applied as a seed coating in many cases.

Municipal sewage sludges and fly-ash materials, both waste products, contain Mo that is available to crops. Because these two materials are applied to soils at relatively high rates (5–401 tha–1), their Mo contents should be considered when determining their application rates.