The importance of molybdenum (Mo) in plant nutrition is well established (Fageria, 1992; Marschner, 1995; Mengel et al., 2001). Molybdenum is required by higher plants in very small amounts (0.1 mg kg-1) (Graham and Stangoulis, 2007). However, it has crucial roles in plants via molybdoenzymes (Yu et al., 1999). The essentiality of Mo for plants was first established in 1938 by D. I. Arnon and P. R. Stout, who used tomato as a test plant in nutrient solution (Marschner, 1995; Fageria et al., 1997). Molybdenum is the least abundant of all the micronutrients in the earth’s crust, and the common range in soil is 0.2 to 5 mg kg-1, with an average value of about 2 mg kg-1 (Lindsay, 1979). Molybdenum deficiency is common in acid soils because Mo is adsorbed on Fe hydrous oxides and hydroxides as MoO42-. In several acid soils of Brazil, response of common bean to Mo application was observed only after the soil pH was raised above 5.5 (Franco and Day, 1980). This could be due to limitation of nodule or plant function or to inability of some bean cultivars to absorb Mo or move it to nodules under very acid soil conditions (Franco and Munns, 1981).