Molibdenum (Mo) is a component of the nitrogen biological fixation enzyme of bacteria and of the enzymatic complex of nitrate assimilation by the plants.
Its sufficiency for these processes in the plant depends on the availability in the seed or in the soil. The recommendation of Mo for soybean, in the Rio Grande do Sul State, was indicated for only one soil, a sandy Red Yellow Podzolic (São Pedro soil). With the objective of evaluating soybean response to Mo fertilization in the main soybean producting regions of the State of Rio Grande do Sul, 45 experiments were carried out during the season 1994/1995 and nine experiments in the 1995/1996 season. Two treatments were conducted under a completely randomized design, with three replications in the first year and four in the second year. The treatments were 0 and 40 g/ha of Mo, as ammonium molybdate. From the 49 remaining experiments, 22 showed grain yield higher than 9.5% when Mo was applied, but only in eight trials the yield increase was statistically significant. The results indicated that eventually Mo deficiency can occur in clayed soils, especially in soil with water-pH less than 5.5.