Articles

Soybean Inoculation and Nitrogen Fixation

Key Takeaways

Soybeans have the greatest nitrogen (N) requirement per bushel among row crops
Biological N-fixation generally provides 40-70% of total soybean N requirement
Inoculum (Bradyrhizobium japonicum) may be necessary in fields not recently planted with soybean
Well-nodulated roots with active nodules are foundational for soybean growth and yield potential

Why Nitrogen Matters

Although the atmosphere has 78% N₂ gas, plants cannot use it directly. Instead, they must get N from the soil. Soybean is a legume that forms a symbiotic relationship with a specific bacteria called Bradyrhizobium japonicum to transform the atmospheric N₂ into plant-available N in the soil. These bacteria need to be introduced through inoculation because they are not naturally occurring in US soils. N-fixing bacteria colonize root hairs to form nodules. If nodulation and N-fixation fail, a 50-bu soybean crop would require 100-175 lbs N per acre in addition to the supply of N in the soil.

Fun Facts

40-70% | N-fixation generally provides 40-70% of total N for soybeans¹
3-4x | Soybean crops require three to four times more N per bushel than cereal crops such as corn, wheat, or rice.

Need to Know

N-fixing bacteria can be introduced through seed or in-furrow inoculation in the form of powders, liquids, or granular.

Bacteria are alive and should not be exposed to high temperatures or direct sunlight.

Even when soybean seeds are inoculated with the right bacteria and method of application, satisfactory nodulation is not guaranteed.

Development stages defined by Fehr and Caviness (1977)².

“V” stands for Vegetative stage. VE: emergence; VC: cotyledons fully expanded; V2: fully developed trifoliate leaf at node above the unifoliate leaf; V3: fully developed trifoliate leaf at node above the first trifoliate leaf; V4-V5: third trifoliate leaf fully expanded; V7-V8: sixth trifoliate leaf fully expanded; R3 (beginning of pod setting): a pod of 3/16 inch long at one of the four uppermost nodes on the main stem with a fully developed leaf; R5: (beginning of seed filling) a seed 1/8 inch diameter in a pod at one of the four uppermost nodes on the main stem with a fully developed leaf; R7 (physiological maturity): one “normal” seed-filled pod on a plant will have matured to a brown pod color; R8 full maturity: 95% of the plant’s pods will have matured to a final brown color.

Factors That May Compromise Nodulation and N-fixation:

Low or high temperatures
Low or high soil pH
High salinity
Dry or excessive water conditions
Nutrient deficiency
High soil N level

The formation of nodules can be observed as early as V1 to V2 growth stage a few weeks after the N-fixing bacteria have infected root hairs. Nodule development takes about 30 days to start fixing N₂ from the atmosphere. Fixation from the mature nodule will continue for the following 20 to 30 days. Active N-fixation is usually present by V3 to V4 and continues through reproductive stages, peaking between R5 to R6 (Below: Cross section of an active N-fixing nodule; pink interior). The number of nodules per root and the nodule size will increase as the plant develops. The N-fixing bacteria will infect new roots, develop new nodules, and fix more atmospheric N. In soybean rotational systems, an adequate level of N-fixing bacteria can survive in the soil for several years until the next soybean crop.

Determining Successful Nodulation and N-fixation

Scout for the presence of nodules after V3 to V4 stage.
Carefully dig up plants in several locations of the field to avoid removing nodules.
Wash the roots in a bucket of water.

Successful Nodulation

The plant has several active (pink) nodules and can fulfill soybean N requirements for an average yield (50 bushels per acre).

Unsuccessful Nodulation

Nodules are present, but they are green or white, which means they are not fixing N
No nodules are present and the plants show yellowing from N deficiency

What Now?

Additional soil inoculation will have no effect after planting if nodules are not already present
A rescue N application may be warranted (contact your local Extension office for recommended rates)
Inoculate with Bradyrhizobium japonicum the next time soybean is planted in the same field

Authors: Nicolas Cafaro, University of Nebraska; Shawn Casteel, Purdue University; Eros Francisco, Auburn University; David Holshouser, Virginia Tech University; Hans Kandel, North Dakota State university (Emeritus); David Moseley, Louisiana State University.

Read the fact sheet here.

References:

^¹Almeida, Luiz Felipe A., et al. (2023). Soybean yield response to nitrogen and sulfur fertilization in the United States: contribution of soil N and N fixation processes. European Journal of Agronomy, 145. https://doi.org/10.1016/j.eja.2023.126791

²Fehr, W. R., Caviness, C. E., & Vorst, J. J. (1977). Response of Indeterminate and Determinate Soybean Cultivars to Defoliation and Half‐plant Cut‐off 1. Crop Science, 17(6), 913-917. https://doi.org/10.2135/cropsci1977.0011183X001700060024x