A number of factors can affect the availability of molybdenum (Mo) to crops. The most important ones include the nature of the parent rock, soil pH, the organic matter in the soil, drainage, interactions with other nutrients, and plant species, plant part, and stage of plant growth at sampling. This chapter attempts to review the information available on the soil and plant factors that affect Mo uptake by plants in different parts of the world.
Molybdenum Occurrences in Parent Rocks
The concentrations and forms of Mo in rocks and soils tend to vary according to the particular origins and conditions of formation. Molybdenum is a versatile element insofar as valence is concerned, and it can precipitate under either oxidizing (Mo6+ predominant) or reducing (Mo4+) conditions (Manheim and Landergren, 1978). Consequently, there may be local enrichments or depletions, and recent work has largely been concerned with elucidating the sequences of occurrence, mobilization, and deposition in particular situations.
Occurrences in Igneous and Metamorphic Rocks
Igneous rocks make up some 95% of the crust of the earth (Mitchell, 1964), and Mo occurs in both acid and basic igneous rocks. Manheim and Landergren (1978) suggested an overall Mo content of nearly 2.0 ppm for granitic rocks and somewhat lower values for basalts.
Although the occurrences of Mo in metamorphic rocks have not been widely studied, metamorphism would be expected to alter the form and site of occurrence rather than the amount of Mo present.