Nitrogen fixation is critical for high soybean yields. For nitrogen fixation to occur, the nitrogen-fixing bacteria known as Bradyrhizobia japonicum must be established in the soil through seed inoculation. Soybean can obtain up to 50 to 75% of its nitrogen requirements from the air when nitrogen-fixing bacteria have established functioning nodules on the roots.

The soybean-bacteria symbiosis is mutually beneficial

The soybean plant gets nitrogen from the bacteria, while providing the bacteria with carbohydrates. When soybean seed germinates, the bacteria invade the root hairs of the seedling and begin to multiply. Nodules, which house the bacteria, form on the roots. Under field conditions, the first nodules form within a week after seedling emergence and become visible as they increase in size. Active fixation begins in the V2 to V3 stage, after which the number of nodules and the amount of nitrogen fixed continue to increase. The soybean demand for nitrogen is highest from the R5 to R8 stages.

Do you need to inoculate your fields?

The decision on whether or not to inoculate depends on whether the field has a recent history of healthy-looking soybean. Most soils in Iowa have a good population of B. japonicum if soybean has been grown in recent years. However, if a field is new to soybean, or has been out of soybean for more than three to five years, it is good insurance to inoculate.

Current recommendations for Iowa are to inoculate the seed if:
1) the field has never been planted to soybean
2) soybean have not been grown in the field in the past three to five years
3) the soil pH is below 6.0
4) the soil has a high sand content
5) the field has been flooded for more than a week, creating anaerobic conditions

Inoculation products

Bradyrhizobia japonicum can be added as a liquid, a granular peat inoculant, or as a peat-based powder. The different forms can be seed-applied or used in-furrow. Inoculant is relatively inexpensive and several new products have entered the market, creating a renewed interest in seed inoculation even on fields that have a history of soybean production.