Effects of Cover Inoculation of Soybean on Nodulation, Nitrogen Fixation, and Yield1

Abstract In clay soils with no indigenous Bradyrhizobium japonicum, seed or soil implant inoculation of soybean [Glycine max (L.) Merr.] often results in nodules distributed mainly on the primary root and the initial portions of the secondary root, due to the poor mobility of B. japonicum in soil. To increase nodulation of soybean in silty clay loam soil (Vertic Xerothent), we used B. japonicum (USDA strain 122) cover‐inoculated with 100 m3 of irrigation water at the third node (V3) stage. Inoculation treatments included: (i) seed inoculation; (ii) seed and cover inoculation; and (iii) a control without inoculation. Two soybean cultivars were used. Seed plus cover inoculation doubled the biomass of the nodules compared to plants with seed inoculation only. Nodules produced after cover inoculation were very active in nitrogen fixation and showed increased nitrogenase activity during the reproductive stage of the plant. As a consequence, there was an increase in the N concentration in the stem, in seed protein (+6.7% in ‘Evans’, +16.0% in ‘Swift’), and in seed production per hectare (+16.1% in Evans, +10.5% in Swift).