As soybean yields increase, the probability of a profitable yield response to nitrogen fertilizer is also increasing. Research funded by ISA to the coinvestigators found that biological nitrogen fixation accounts for only 50% of soybean nitrogen demand.

Soybeans must capture the remaining 50% of nitrogen demand from the soil. This very important result demonstrates that high soil nitrogen availability is critical to achieve high-yielding soybeans. This research was presented at the ICM and ISA Research Conferences. On average, across 16 site-years represented in our ISA-funded study, the average soybean nitrogen uptake from the soil was 111 pounds nitrogen per acre.
Moreover, the average nitrogen balance (that is, grain nitrogen harvest minus biological nitrogen fixation) was -30 pounds nitrogen per acre. Our findings were later replicated by University of Nebraska-Lincoln and Kansas State University researchers.

Continued research in this area addressing opportunities to increase soil nitrogen availability for soybean is critical because the potential for profitable nitrogen fertilization of soybeans is increasing for at least three reasons:
1) Wetter weather is expected to decrease nitrogen fixation. The fixation process is extremely sensitive to water availability; too little and too much soil moisture reduces fixation. Over the past decade, Iowa soils have become measurably wetter.
2) Total nitrogen demand by soybean increases with increasing yield. Yet the proportion of total nitrogen derived from fixation declines with increasing yield. Hence, as soybean yields increase, soybeans will become even more reliant on soil nitrogen: One bushel of soybeans requires 4.2 pounds of nitrogen. At 55 bushels per acre, soybeans require 231 pounds of nitrogen with 49% coming from fixation (114 lbs) and 51% coming from the soil (117 lbs). At 65 bushels per acre, soybeans require 271 pounds nitrogen with 45% coming from fixation (123 lbs) and 52% coming from the soil (148 lbs).
3) The effects of increasing corn yields and winter cover crop production prior to soybeans have uncertain effects on soybean nitrogen fertility. There may be less soil nitrogen for soybeans for at least the following reasons: i) Higher corn yields increase soil nitrogen uptake and leave less residual inorganic nitrogen for the following soybean crop; ii) Greater corn residue inputs and cover crop residue inputs reduce soil inorganic nitrogen pool size owing to the high carbon-to-nitrogen ration of corn residue, which promotes microbial immobilization of ammonium and nitrate; iii) Greater corn residue inputs keep the soil cooler and wetter, which slows microbial nitrogen mineralization (that is ammonium and nitrate production) from soil organic matter and manure.

Alternatively, the high carbon-to-nitrogen ratio of corn residue and the nitrogen demand of cover crops may increase inorganic nitrogen retention in the soil and allow for greater nitrogen mineralization throughout soybean growth and production.

Together, these results indicate that corn residue production, cover crop residue production, and soybean yield potential should be positively associated with the probability of profitable soybean yield response to nitrogen fertilizer: high corn residue and cover crop production, coupled with high soybean yield potential, are likely to generate conditions when nitrogen fertilization is profitable.

1. Understand how nitrogen fertilizer management in corn affects soybean yield.

In addition, this main objective contributes to the objectives of the Iowa N Initiative:
2. Expand a nitrogen rate trial database that enables researchers to understand and interpret how various cropping systems, genetics, soil types, and weather-years affect optimum nitrogen fertilizer rates to corn. The database will ensure farmer anonymity and translate data into a public-facing tool (Objective 3) that allows all Iowans to observe rational nitrogen rates. The database will be 100% transparent so that ISA researchers can use it to advance science for the benefit of soybean farmers.
3. Update and enrich the ISU nitrogen fertilizer recommendation tool for corn (i.e., the MRTN) with a dynamic benchmarking tool that allows users to move beyond the average by identifying the genetic, environment, management, and weather factors behind each trial. This proposal to Iowa Soybean Association will contribute to our understanding about how soybean residue production and soybean management affects the optimum nitrogen rate to corn (Years 2-3).
4. Develop a forecasting tool that predicts cropping systems nitrogen dynamics across soil types and weather years, allowing farmers to respond to current conditions (Years 3-4).
5. Develop a hindcasting tool that allows farmers to compare what they did to what they could have done.
6. Conduct fundamental research on cropping systems nitrogen dynamics to make continuous improvements in our ability to accurately forecast and manage nitrogen dynamics (all years).

We will communicate the results of this work at the ISA and ICM research conferences and we will publish two extension publications. One will report the measured effects of previous corn yield, nitrogen fertilizer input, and residual inorganic nitrogen levels on soybean yield and yield potential. We will publish a second extension publication that addresses how these effects interact with cover crop production to affect soybean yield potential and soybean yield gap. This analysis will allow us to calculate probabilities of soybean response to nitrogen fertilizer and identify potential corn residue management strategies that can boost subsequent soybean yield. Finally, these analyses will add great value to the Iowa Nitrogen Initiative by explicitly integrating the effect of corn nitrogen fertility management on soybean management and production.

Updated September 5, 2025:
In this reporting period, we continued two major activities: 1) analysis of soybean yields following Iowa Nitrogen Initiative trials, and 2) identifying crop year 2024 Iowa Nitrogen Initiative trial participants who will be rotating to soybeans this crop year (2025) and have planted cover crops following crop year 2024 corn harvest. At present, we have not identified an effect of residual soil nitrate levels or corn yield (as a proxy for corn residue production) on the following soybean yield. Our database has excellent ranges of residual soil nitrate levels. However, one limitation of our analysis has become apparent: the Iowa Nitrogen Initiative trials, despite a wide range of nitrogen fertilizer inputs, have a relatively limited range of crop residue levels. Hence, we have been able to reject the hypothesis that residual soil nitrate levels, ranging from less than 10 to greater than 100 lbs N per acre, have a consistent, measurable effect on soybean yield; however, we cannot interpret any potential effect or lack of effect of corn residue production on soybean yield.

As part of our work, we have also analyzed the effect of cover crop following soybean on pre-plant soil nitrogen levels in corn and the economic optimum nitrogen fertilizer rate to corn. Soil nitrate plus ammonium levels following soybean with a rye cover crop compared to following soybean without a rye cover crop were significantly lower following soybean with the rye cover crop post-cover crop termination but prior to corn planting but significantly higher following soybean with the rye cover crop at the V6 corn growth stage. This analysis indicates that soil N retained by the rye cover crop is released during corn growth but perhaps later than ideal because despite the significantly higher N levels at V6, the economic optimum N fertilizer rate to corn was higher following soybeans with a rye cover crop vs. following soybeans without a rye cover crop. These results point to the importance of early-season N availability following soybean with rye cover crop. However, there may also be pre-existing differences in fields that are and are not cover-cropped. We are currently exploring these possibilities during the current reporting period.

Figure 1. Soil nitrate levels following a soybean crop with grass cover crops (mostly rye), legume cover crops, or no cover crops at spring pre-plant and post-plant (V6-V9) stage.
Figure 2: The effect of cover crop planting on the economic optimum N rate (EONR) to corn following soybean.

View uploaded report

This work will advance our understanding about how the management of corn impacts soybean production. Corn yield has large effects on crop residue and residual soil nitrogen going into the following soybean year. These factors interact with cover crops to affect soybean yield and environmental performance. In particular, large amounts of corn residue and cover crops continue to create management challenges. By understanding how these factors affect soybean production, we will help farmers better manage for soybean yield and environmental performance.