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It takes 4 to 5 pounds of nitrogen to produce a bushel of soybeans. For example, a 70 bushel-per-acre soybean crop requires about 300 pounds per acre of nitrogen for stover and grain production. Of this, approximately 75 percent is removed with the grain harvest.

Soybeans actually require more nitrogen than corn—a fact that is often overlooked or not addressed.  Much of soybeans’ nitrogen needs are met by soil bacterium Bradyrhizobium japonicum, often referred to as rhizobia, in a process known as nitrogen fixation through nodulation of soybean roots.  Although this symbiotic relationship provides a significant amount of nitrogen for soybean production, the full nitrogen requirements of soybeans often go unmet, potentially limiting yield.

Nodulation results from a chemical signaling process that soybean roots target towards rhizobia, indicating that the plant needs nitrogen. Approximately 10 to 14 days after rhizobia colonize soybean roots, nodules will become visible. When planting into warm soils (70° to 80° F), nodules appear as early as a week after emergence. The earlier in the plant’s life cycle this process begins, the greater the potential for adequate nodulation and effective nitrogen fixation.

Many factors can influence development of adequate and effective nodulation:

• The amount of available soil nitrate in the upper rooting zone (1 to 4 inches deep)
• Desirable soil moisture levels
• Adequate soil aeration
• Proper soil pH
• Absence of salinity
• Organic matter
• Soil temperature
• Rhizobia quality

In 2017, early planted soybeans experienced cool and wet conditions throughout Illinois during much of April and May. Once they are effectively established, soybean nodules prefer cool soil temperatures during late vegetative and early reproductive stages (Figure 1). However, very cool soil temperatures during and after emergence can delay or even inhibit nitrogen fixation.

Figure 1.  Correlation of average July soil temperature and soybean yield. Cooler soil temperatures during peak nitrogen fixation can be a contributing factor to higher yields.

Previous soybean nodulation research indicates that a root zone temperature (RZT) of at least 77° F is ideal for adequate and effective nodule development. RZT of 59° to 63° F can delay nodule development and initiation of N2 fixation by four to six weeks.  In addition, a RZT near 50° F can cause nodulation activities to cease. 

Figure 2 shows 2-inch daily soil temperatures at Champaign, Illinois, from the Illinois State Water Survey’s Water and Atmospheric Resources Monitoring (WARM) Program. This soil temperature data suggests that nodule development in early planted soybeans may have been delayed because of cool soil temperatures until mid May, when RZTs reached 75° to 80° F. 

Figure 2.  2-inch soil temperature data from Champaign, IL.

Soybean plants should normally show adequate and effective nodulation by the V2 growth stage, as seen in Figure 3.  If cool, saturated and anaerobic soil conditions delay or limit nodule development, soybean plants may show a yellow “transitioning” phase for a longer period early in the growing season until they can obtain nitrogen through fixation of atmospheric N2 via nodulation. This is most common during June.

Figure 3.  A well-nodulated V2 soybean plant from Beck’s Practical Farm Research site near Downs, Ill.

Here are soybean management strategies to assist with adequate and efficient nodulation:

• Plant early, but only into warm soils. This will result in nodules working longer to fix more nitrogen.
• Plant lower populations if planting early. This will produce more photosynthate to feed plants and nodules.
• Plant beans at a depth that will help buffer soil temperature and moisture changes. (1.5-inch depth will buffer soil temperature and moisture more than beans planted at 1 inch)
• Correct soil pH to 6.5 to 7.0 to help supply proper calcium levels, improve soil tilth and help maximize nodule efficiency. Buffer soil temperatures in the upper root zone by utilizing no-till and planting early.

Utilize high quality inoculants when planting soybeans to help maximize nodule formation.

If soybeans are poorly nodulated, a foliar-applied nutritional product containing molybdenum can assist in improving nodulation.

Chad Kalaher is a CCA soy envoy for East Central Illinois. He is also an agronomist for Beck's Hybrids. He has a bachelor's degree in agronomy from the University of Illinois and a master's degree in weed science from North Carolina State University.