Do High-Yielding Soybeans Need Supplemental Nitrogen?
Growing up on a university research farm, Mike Howell developed an interest in agriculture at a young age. While active in 4-H as a child, Howell learned to appreciate agriculture and the programs that would shape his career. Howell holds a Bachelor of Science degree in soil science and a Master of Science degree in entomology from Mississippi State University. He has more than 20 years of experience conducting applied research and delivering educational programs to help make producers more profitable. He takes pride in promoting agriculture in all levels of industry, especially with the younger generation.
Legumes, like soybeans, are capable of fixing nitrogen through a symbiotic bacterial relationship in nodules formed on the root structure. However, in high-yielding environments, supplemental nitrogen may be required to reduce this yield-limiting factor.
Soybean yields have consistently increased over the past 100 years (Figure 1), with record yields projected for 2018, and many growers approaching 100 bushels or more. Based on uptake values using our eKonomics Nutrient Removal Calculator, high-yielding soybean production of 65 bushels per acre would require 319 pounds of nitrogen. Increasing that yield to 100 bushels would require an additional 171 pounds of nitrogen. Therefore, research shows that soybeans can be responsive to supplemental nitrogen in those high-yielding environments (Salvagiotti et al., 2008).
Greater yields with current production increase nutrient demand. There has been much work completed in the past evaluating supplemental nitrogen for soybeans, but results have been inconsistent. Until recently, most of the research had been focused on pre-plant nitrogen applications, but now, in-crop nitrogen applications are being studied to determine if yields can be increased (specifically in high-yielding environments). High soil nitrate can inhibit nodule formation, so creating a nitrate-rich environment early during the growing season decreases the legume’s ability to fix its own nitrogen (Streeter, 1988), hence the desire to look at later season applications.
By supplying the crop with nitrogen later into the growing season, nitrogen will be available to the plant when it is most needed (during seed fill). As indicated in Figure 2, around 80% of the total nitrogen needs of the crop occur after bloom (R1). While maximum nitrogen fixation is reported to occur between R3 and R5, nitrogen fixation slows between R5 and R7, potentially leading to a nitrogen shortage during seed fill (Zapata, 1987). The hypothesis is that by supplying supplemental nitrogen to the plant that is available, or will become available, near the time of peak demand, increased yield potential of the soybean crop can be realized.
In 2013, a series of field trials were initiated in Mississippi to evaluate the addition of nitrogen at different timings and rates in soybeans. Trials were conducted in the Mississippi Delta on Deer Creek sandy loam and a Sharkey clay soil. Figure 3 shows the average yield response across both soil types over a three-year period. While several sources of nitrogen were evaluated in this study, ESN produced the largest yield increases and most consistent results. At both the V4 and R1 application timing, 80 pounds of nitrogen per acre produced the highest yield increase, 5 and 7 bushels per acre, respectively.
Evaluation of the literature published to date shows application of supplemental nitrogen does not always contribute to higher yields, especially in lower yielding environments. Additionally, there are likely other factors that impact the response of soybean to supplemental nitrogen, such as low-soil-pH-inhibiting root nodulation, high-yield conditions (pointed out here), and low residual nitrogen/low mineralization of nitrogen. Finally, nitrogen application rates should not be excessive if supplemental nitrogen is to be applied.
While the effects of supplemental nitrogen on soybeans are still being evaluated to better understand where yield gains can occur, this research conducted in Mississippi demonstrates that under high-yielding conditions, this practice can increase soybean yields and profitability.
- Bender, R.R., J.W. Haegele, and F.E. Below. 2015. Nutrient uptake, partitioning, and remobilization in modern soybean varieties. Agronomy Journal 107:563-573.
- IPNI. 2014. IPNI Estimates of Nutrient Uptake and Removal. Available at http://www.ipni.net/article/IPNI-3296. Verified March 9, 2018.
- Nordby, D. 2004. Pocket Guide to Crop Development. University of Illinois Extension C1389.
- USDA-National Agricultural Statistics Service. 2018.
- Salvagiotti, F., K.G. Cassman, J.E. Specht, D.T. Walters, A. Weiss, and A. Doberman. 2008. Nitrogen uptake, fixation, and response to fertilizer N in soybeans: A review. Field Crops Research 108:1-13.
- Streeter, J.G. 1988. Inhibition of legume nodule formation and N2 fixation by nitrate. CRC Critical Reviews in Plant Sciences 7:1-23.
- Zapata, F., S.K.A. Danso, G. Hardarson, and M. Fried. 1987. Time course of nitrogen fixation in field-grown soybean using nitrogen-15 methodology. Agronomy Journal 79:172-176.