On average, a soybean crop needs 315 lb N per acre, about 60 percent of which (190 lb) goes to seed production and 40 percent (125 lb) goes to stover and roots. In most cases all of this need can be supplied from the environment and additional nitrogen isn’t necessary. (In sandy or low organic soils or for yields over 70 bu/ac additional nitrogen may be recommended.)
Soybeans fix nitrogen from the atmosphere when nitrogen-fixing rhizobia bacteria are present in the soil. Nitrogen fixation is a result of the symbiotic relationship between rhizobia and the soybean plant and is evident in nodules on soybean plant roots. Most studies show that between 50 percent and 60 percent of the nitrogen is from N fixation.
A well nodulated plant should have five to seven nodules on the primary root.
Checking for nodulation can help assess a field and make adjustments for the next crop; however, checking as early as two weeks after emergence allows time to apply nitrogen during the season if plants are not developing nodules. (The best time to apply nitrogen in soybean is between flowering and pod fill.)
Inoculating Soybean Seed
The least expensive way to supply nitrogen to soybean is by inoculating the seed at planting.
Research in Michigan on new ground where soybeans had not been grown showed as much as a 45 percent yield increase for soybeans that had been inoculated compared to those that hadn't. Michigan soils generally have higher organic matter and greater mineralization; Nebraska growers might see an even higher difference.
For fields where soybean has not been grown previously, inoculation is essential. For such fields, a soil-applied inoculant may provide greater yield potential than a seed-applied inoculant. For fields where soybean has been previously grown, either a soil-applied or a seed applied inoculant is good insurance for providing adequate nitrogen for the crop.