Seed Inoculation

Pawel Wiatrak, Jim Camberato, and Jason Norsworthy

The soybean plant is capable of supplying much of the nitrogen (N) for growth through N fixation. Living symbiotically (both organisms receive benefits) in the root nodules of the soybean plant, N-fixing bacteria, Bradyrhizobium japonicum, remove nitrogen from the air and provide it to the plant. If nitrate nitrogen is present in the soil, either from manure, sludge, or fertilizer, the bacteria do not fix nitrogen. The plant then preferentially uses the soil nitrogen until it is depleted, at which point the bacteria will again function to provide nitrogen to the plant.

Efficient strains of N-fixing bacteria are common in most South Carolina fields with a recent history of soybean production. Also, strains indigenous to our soils are extremely competitive with introduced strains when forming root nodules. For these reasons, it is very rare that soybeans will respond to seed-applied inoculants containing Bradyrhizobium japonicum.

It is important for farmers to maintain an optimum soil pH to ensure effective N-fixing activity. Low soil pH can sometimes cause a deficiency of molybdenum, a minor element critical for the N-fixing process. Molybdenum deficiency can be corrected by liming the soil to correct soil pH, or by adding a seed treatment of sodium or ammonium molybdate at 2 to 4 oz/Bu.

To determine if applying an inoculant will be justified, consider the following "rule of thumb." If the field in question has a recent (within 3 years) history of soybean production, an inoculant is not recommended (unless soybeans are planted on sandy soils).

The following precautions are suggested when using an inoculant:

  • If molybdenum and/or a fungicide are used, add these products to the inoculated seed in the planter-box immediately before planting.
  • If an inoculant is needed, purchase fresh material (bacteria only) and keep in a cool place until used. Thoroughly mix with moistened seed.