Bradyrhizobium japonicum Inoculation and Seed Priming for Fluid‐Drilled Soybean

Microbial inoculation coupled with seed priming has potential application in fluid‐drilled cropping systems. The feasibility of applying Bradyrhizobium japonicum during seed priming and at planting using cellulose‐based gels was evaluated with soybean, Glycine max (L.) Merr. A greenhouse study evaluated whether exposure to Strain USDA 110SK or 123SR for 24 h during seed priming increased soybean nodulation by those strains compared to a subsequently applied gel‐carried strain. Well‐nodulated soybeans resulted when inoculation occurred only during seed priming. However, whenever gel inoculation was used, soybean nodules were almost exclusively occupied by the strain applied with the gel, regardless of whether the root radicle had or had not emerged prior to inoculation at seed priming. Differences in condition of seedbeds and soils at planting, and between fluid and conventional planting systems, influenced results from two field sites in eastern Nebraska. Strain 123SR occupied 5 to 10% of nodules at a Sharpsburg‐soil (fine, montmorillonitic, mesic Typic Argiudoll) site when carried in a cellulose‐based fluid gel at planting. Inoculation during solid‐matrix seed priming had little effect on nodule occupancy and Strain 123SR was not recovered when peat was used as a carrier. In contrast, 123SR was rarely recovered, regardless of treatment, from a Kennebec soil (finesilty, mixed, mesic Cumulic Hapludoll). Fewer nodules formed on Kennebec‐ than Sharpsburg‐soil soybeans (17 vs. 31 nodules/plant). Increased grain yields were observed at the Kennebec site (2.87 vs. 2.24 Mg/ha) when fluid‐drilled soybean seed was solid‐matrix primed. Seed priming apparently aided soybean establishment under dry soil conditions. Field delivery of viable bradyrhizobia in gel was shown to be feasible for fluid‐drilled soybean. Short‐term exposure to Bradyrhizobium inocula during seed priming did not increase soybean nodulation by that strain.