Soybeans need nitrogen — and lots of it — to reach their full yield potential. As much as 300 pounds of nitrogen can be required to produce a yield of 75 bushels per acre.

“Some of the nitrogen comes from oxidation of organic matter in the soil, but the balance is produced by rhizobia bacteria residing in nodules on the plant’s roots,” says Jim Beuerlein, technical consultant to Becker Underwood and retired Ohio State University extension agronomist. “That means soybean roots need to be properly inoculated with the right strain of rhizobia to effectively and efficiently provide the nitrogen needed for maximum yield.

"Applying a modern soybean seed treatment system containing a rhizobial component can ensure that rhizobia are present in adequate numbers for the plant.”

Whether a commercial inoculant was applied to the seed or not, conducting in-field evaluations of soybean root nodulation can provide growers insight on how to increase yield potential the next time soybeans are planted in that field.

Soybean nodulation checks require only a few basic tools — a spade, a bucket of water and a sharp box cutter or blade — and the knowledge of what to be looking for. Conduct enough checks to provide a good representation of what’s taking place in the entire field.

Beuerlein offers the following tips for effective soybean root nodulation evaluation.

Timing — Start checking plants 40 days following emergence and until pods begin to fill.

Process — Use a spade to dig up as much of a plant’s tap root and lateral roots as possible. Plants 6 to 8 inches tall will have most of their roots within the top 10 inches of the soil surface.

Do not pull the plant, as this can cause nodules to be separated from the roots.

Carefully remove soil from around the root system. Soak roots in a bucket of water and gently remove the remaining soil.

Evaluation — Start with an examination of the area around the tap root. Look at the overall mass of nodules, not just the quantity of nodules. There should be at least eight to 20 large healthy nodules per plant.

Slice open several nodules and check the color inside:

  • Pink to bright red — Actively fixing nitrogen
  • White — Indicates colonization by rhizobia, but they are not yet mature or actively fixing nitrogen. Check again in a few weeks.
  • Green, brown, tan, black or mushy — Not active, ineffective or may be parasitic.

Record the number of nodules per plant, their color and location within the field for review in a postharvest evaluation.

“If too few nodules are found and plants appear pale green in color, consider the use of a soybean seed treatment containing an inoculant component before planting soybeans again in that location,” Beuerlein says.

Stresses on the plant and the rhizobia during the growing season can adversely affect the nitrogen-fixation process. Beuerlein says using a seed treatment system containing an inoculant component at planting can help soybeans reach even higher yields when any of the following conditions occur:

  • Cool, wet soils (i.e. no-till, conservation tillage, etc.)
  • Saturated soil conditions for 3 or more days since last inoculant application
  • Early planting
  • Low pH (less than 5.8) or high pH (greater than 8.5) soils
  • Soils with a calcium base saturation of 10% or less
  • Prior use of inoculant material(s) more than a year old and having an inadequate live bacteria count
  • Dry soil that has been 80 F or warmer for 3 or more days
  • Sustained use of some soil-applied pesticides
  • Low organic matter (less than 1%)
  • Soybeans not grown in previous years (i.e. continuous corn, CRP acres)
  • Crusted soils that limit the exchange of atmospheric nitrogen by root nodules