What About Corn N Fertilizer Effect on Following Soybean Crop?
The corn-soybean rotation is a prominent system in U.S. production agriculture. In the quest for high corn yields, nitrogen [N] fertilizer is generally applied at a high rate prior to and/or during the corn growing season. However, in-season issues during the corn cycle of this rotation can result in some of this added N not being used–i.e. some of the N not used by the corn crop will remain in the soil. Then the question becomes “Will N carryover from an N-fertilized corn crop affect a following soybean crop?”.
Results from research conducted in Kansas are reported in an article titled “Footprints of corn nitrogen management on the following soybean crop” by Correndo et al. [Agron. J. 2022;114:1475-1488], and provide some answers to the above question. Pertinent points from that article follow.
• Much of the prior research regarding N in the corn-soybean rotation system has dealt with the amount of N from a preceding soybean crop that will be available to a following corn crop. Previous research has documented an antagonism between soil N supply and N derived from soybean nitrogen fixation [SNF]. Thus, a large N surplus from a previous corn crop may be detrimental to SNF.
• The objectives of the research described in the above-linked article were to evaluate the residual effect of N fertilization of a corn crop grown in rotation with soybean on 1) seed yield of a following soybean crop, 2) seed yield, seasonal N supply, and SNF for a case study with one sequence of the rotation, and 3) the apparent N budget, calculated as added fertilizer N minus amount of N in harvested corn grain, and its relationship to yields of following soybean crops.
• There were two Case Studies. Case Study I was a long-term irrigated experiment that was established in 1983 at Topeka, Kansas [39°09' N lat.] on a silt loam soil. N was applied pre-sowing to the corn crop at rates of 0, 40, 80, 160, and 240 lb./acre [1983-1994] and 0, 80, 120, 160, and 200 lb./acre [1997-2020]. Case Study II was a single-sequence of a corn-soybean rotation conducted with and without irrigation at Scandia, Kansas [39°49' N lat.] on a silt loam soil. N was applied to the corn crop at stage V5 at rates of 0, 54, 107, 161, and 214 lb./acre.
• In Case Study I [long-term experiment], average soybean yields ranged from 62.5 to 80.4 bu/acre [1984-1994] and from 41.7 to 65.6 bu/acre [1998-2020]. Most of the yield variability resulted from the different conditions among years. Across the years, there were no significant differences in soybean yield among any of the N treatments used in the studies.
• In Case Study II, average soybean yields ranged from 68.5 bu/acre [nonirrigated] to 75.9 bu/acre [irrigated], and there was no residual effect of N rate that was applied to the previous corn crop.
• The estimated apparent N budget that resulted from the previous corn crop ranged from -117 to 62.5 lb. N/acre in Case Study I and from -96.4 to 74.1 lb. N/acre in Case Study II. These variations in the apparent N budget following the corn crop had no significant effect on the following soybean crop in the rotation.
• Overall, the authors concluded that their results highlight the lack of effect of proper corn N management on a following soybean crop in the corn-rotation system.
• The authors propose, based on research results cited in the article, that a large N surplus following corn in the rotation may limit soybean yields if the initial N availability resulting from such a surplus reduces the quantity of N that would be derived from SNF more proportionally than the gain in soil N supply. They also noted that potential N losses from soil increase as soil N surpluses become large. Both of these occurrences resulting from N over-fertilization of the corn crop in the rotation are unacceptable.
• And finally, the authors concluded from their results that producers who apply an amount of N to corn that is near the economic optimum N fertilizer rate for the anticipated corn yield will not generate a significant N surplus that could compromise SNF levels of a following soybean crop. Thus, soybean yields are not likely to be affected by proper N management for the previous corn crop.
Takehome Message. The overriding point from these results is that producers should identify the economically optimum N rate that should be used for corn grown in a corn-soybean rotation in the Midsouth. This rate likely will vary among the various soil types [e.g. silt loam vs. clay] and between nonirrigated and irrigated environments in the region. This knowledge will involve producers knowing the anticipated corn yield on sites used for this rotation, and then applying the amount of N needed to achieve this corn yield level on those sites. This information should be available from extension and industry specialists within the states of the region.
Composed by Larry G. Heatherly, Nov. 2022, larryh91746@gmail.com