By Charles Shapiro - Extension Soil Scientist—Crop Nutrition 

Agronomists and farmers love to put nitrogen on crops. I recall when I was a young boy, my job at home was to fertilize and cut the lawn. I quickly learned that if you did not pay attention to the distance between the passes of the spreader, there would be dark green strips with light green areas between. I used this knowledge to line out a baseball diamond in our backyard, once. My parents were not too happy.

This effect is most apparent on grasses, not legumes. We know that legumes, through symbiotic nitrogen fixation are capable of taking nitrogen from the air and supplying useable N to the plant. This process does not supply 100% of the nitrogen, so the rest needs to come from the soil or be supplied as fertilizer.

Two new publications, both with University of Nebraska authors and both published in the same journal, address the question of the nitrogen deficit between soil supply and nitrogen fixation. These are not the first articles from Nebraska to do this, and probably won’t be the last. At the end of this article I cite two earlier Nebraska articles on the subject. The two recent articles are:

1.Is soybean yield limited by nitrogen supply? Nicolas Cafaro La Menza, Juan Pablo Monzon, James E. Specht, and Patricio Grassini. 2017. Field Crop Research 213:204-212.
2.Soybean response to nitrogen application across the United States: A synthesis-analysis, available free online for the next 50 days. (A shorter version for the public is also available: Soybean Response to Nitrogen Application Across the US.) Mourtzinis, Spyridon, Gurpreet Kaur, John M. Orlowski, Charles A. Shapiro, Chad D. Lee, Charles Wortmann, David Holshouser, Emerson D. Nafziger, Hans Kandel, Jason Niekamp, Jeremy Ross, Josh Lofton, Joshua Vonk, Kraig L. Roozeboom, Kurt D. Thelan, Laura E. Lindsey, Michael Staton, Seth L. Naeve, Shaun N. Casteel, William J. Wiebold, and Shawn P. Conley. 2017. Field Crop Research.

Comparing Two Nitrogen Management Studies

The following summary is taken from the abstracts of each of the papers. (Metric units have been converted to English units.)

Table 1. Comparison of the findings from Cafaro La Menza and Mourtzinis publications.

Both of these papers have excellent literature reviews that go into more depth about nitrogen and soybeans. Anyone interested in the subject will find plenty to think about as well as citations to related research work for further study.

What We Learned

When we step back and look at the body of research new and old, we are left with several questions and a few answers. Most of the work shows that greater yield potential is needed before a nitrogen response is possible. This makes sense due to the increasing need for supplemental nitrogen that the plant cannot fix nor the soil supply. A problem remains, however, in that there is very high variability and very low predictability about when there will be a response. These two papers support the following:
1.Nitrogen may increase yields at higher yield potentials. The Cafaro La Menza paper reports an almost 4 bu increase per 15 bu yield potential increase. The Mourtzinis paper is consistent with this thought, but does not quantify the increase, and most of the increases they found were not significant.
2.Both papers report on experiments that used greater than economic nitrogen rates to get these increases.
3.The Mourtzinis paper does suggest that in non-irrigated production, greater than established population recommendations and N timings may produce a growing environment that will increase yield potential and hence, possible yield increases.

Neither paper makes the claim that nitrogen fertilizer by itself will increase, just that in high-yielding environments nitrogen is likely to have a marginal beneficial effect.

Impact of These Findings on Future Research, N Management

What remains to be discovered is why soybeans are so resistant to nitrogen response when the nitrogen balance inputs would lead one to suggest that supplemental nitrogen is needed. We know there is N substitution, that is, plants will give preference to mineral N at the expense of N fixation. Adding N fertilizer will depress N fixation to the extent that N uptake may not be too different. However, when we supply extremely high doses of N that are sufficient to supply all the N needed and then enough for many more bushels, yields are not increased enough to match the N supply. The hope is that future research will identify a timing/application/source/management system that overcomes this dilemma.

We also need to learn more about why results are so variable and if there are ways to increase N fixation in soybean.

In a larger context, even when we apply nitrogen to our main grass crop in Nebraska, corn, it’s not very efficient. Uptake of fertilizer nitrogen in corn rarely goes over 60% in the year of application although maybe reaching about 85% equivalent recovery in the long term. The data for soybeans shows fertilizer nitrogen efficiencies of much less than that. The nitrogen that does not go into the crop will remain in the soil, vulnerable to loss to the environment. This is costly for the purchaser and has consequences for the environment.

When considering nitrogen application on soybeans, first be sure the field is a high-yield producer. Applying nitrogen to soybeans is not going to take an unproductive field and make it more productive.

Resources

Citations of earlier University of Nebraska papers related to N and soybeans:

Utilization of N Fertilizer by Nodulating and Non-Nodulating Soybean Isolines. E. J. Deibert, M. Bijeriego, and R.A. Olson. 1979 Agronomy Journal, Vol. 71, pages 717-723.

Nitrogen fertilization of soybeans. R.C. Sorensen and E. J. Penas. 1978. Agronomy Journal. Vol. 70 (2), pages 213-216.

Growth and nitrogen fixation in high-yielding soybean: Impact of nitrogen fertilization. F. Salvagiotti, J.E. Specht, K.G. Cassman, D.T. Walters, A. Weiss, and A. Dobermann. 2009. Agronomy Journal, 101:958-970.

Irrigated soybean has a small response to nitrogen applied during early reproductive growth. C. S. Wortmann, C. A. Shapiro, R. Ferguson, and M. Mainz, 2012. Crop Management, doi:10.1094/CM-2012-0126-01-RS.

Originally posted by the University of Nebraska-Lincoln.