Soybean producers know that soybean does not require nitrogen [N] fertilizer because, as a legume, the soybean plant “fixes” atmospheric N. This N fixation occurs in nodules as a result of the colonization of soybean roots by Bradyrhizobium japonicum bacteria that form a symbiotic relationship with the soybean plant. In this relationship, the soybean plant supplies energy to the bacteria, and the bacteria provide useable N to the soybean plant. The level of nodulation and the resulting nodule activity is positively correlated with the amount of N that is available to the soybean plant to support its growth, development, and seed yield.
According to myriad sources, counting nodules is the earliest and best way to determine the N health of the soybean plant. To assess nodulation and nodule activity, check soybean roots at about the V2-V3 developmental stages when adequate nodulation should be 5 to 7 nodules on the taproot. By about V3-V4, there should be 8-10 healthy nodules per plant, and 12 or more total nodules per inch of taproot at R1. The number and size of nodules should increase to about seedfill or R5. Nodules that are actively fixing N are pink or red inside; those that are mushy and brown are not fixing N.
An article titled “Counting the nodules that count, relationships between seed nitrogen and root nodules” by Zubrod was recently published in Natural Sciences Education 2022;51:e20088. Major points from that article follow.
• The study was conducted to explore nodulation in early soybean growth stages, and discern the relationships nodulation may have with N content in mature soybean seeds.
• The nodule data were collected from six genetically and phenotypically diverse soybean lines grown at two different sites in Iowa in 2018.
• There was a strong positive correlation between the number of nodules on the taproot at growth stages V1, V3, and V5 and the amount of elemental N in mature seeds.
• There was a strong positive correlation between V5 nodule volume on the taproot and the amount of elemental N in mature seeds.
• These results suggest that 1) a majority of N in mature soybean seeds comes from taproot nodules, and 2) the larger the volume of taproot nodules, the higher the percentage of N in mature soybean seeds.
• The author concluded that 1) root nodulation in the early stages of soybean development has a positive effect on elemental N in mature soybean seeds, and 2) nodules on the taproot have a more significant effect on the amount of N in mature seeds than do nodules located on secondary soybean roots.
The growth, development, and activity of the rhizobia bacteria in the nodules can be affected by soil factors such as moisture level, pH, temperature, oxygen content, and N content/N fertilizer application. Thus, favorable early-season growing conditions are very important for optimum nodule activity that is subsequently very important to the season-long health and vitality of the developing soybean plant and to the N content of mature soybean seeds.
As an aside to the nodule/nodulation information presented above, click here to access a White Paper on this website that discusses N fertilization for soybeans.
Composed by Larry G. Heatherly, May 2023, firstname.lastname@example.org