Molybdenum (Mo) is important in ecosystems as a micronutrient for both plants and animals. It can also accumulate in the environment in toxic concentrations. Molybdenum is used widely in industrial societies and is an important fertilizer element in some agricultural systems. Soil Mo averages approximately 1.0–2.3 mg kg–1 as a crustal constituent, making it 53rd in abundance (Krauskopf, 1979), but it can accumulate as a result of biogeochemical cycling to 300mgkg–1 or more in shales rich in organic matter. However, the common range of Mo concentrations in U.S. soils is 0.8–3.3 mg kg–1 (dry weight) (Kubota, 1977). In soils, Mo can be found in four major fractions: (1) dissolved Mo in soil solution (watersoluble), (2) Mo occluded with oxides (e.g., Al, Fe, and Mn oxides), (3) Mo solid phases [e.g., molybdenite (MoS2), powellite (CaMoO4), ferrimolybdite (Fe2(MoO4)3), wulfenite (PbMoO4)], and (4) Mo associated with organic compounds.
Numerous processes take place in soil solution, including plant uptake, ion complexation, adsorption and desorption, and precipitation and dissolution (Figure 2.1). As shown in Figure 2.1, Mo solid phases dissolve upon contact with water and provide dissolved Mo in soil solution. The free molybdate ion reacts with metals to form complexes and ion pairs in soil solution. Plants absorb dissolved Mo, mainly as, from soil solution. Removal of by plants disrupts the electroneutrality of a soil solution.