Hybrid Tolerance and Rescue Nitrogen Applications in Saturated Soils

• Rescue nitrogen (N) applications can be economical in saturated soils.
• Nitrogen management is more important than the level of soil saturation.
• Hybrids tend to vary in their tolerance to waterlogged soils.

Saturated Soils

Excessive rainfall and soil types that are poorly drained can cause saturated or “waterlogged” soil within fields. At times of heavy precipitation, there can be ponding water in certain areas of a field. Prolonged wet soils will negatively affect crop growth and yield. Saturated soils reduce oxygen availability to the roots and increase risk of nitrogen loss through leaching and denitrification. The level of standing water, crop growth stage, air temperature and days of soil saturation all play a role in the degree of impact on yield.

Artificial Soil Saturation Trial

Golden Harvest Agronomy in Action Research conducted a trial at Slater, IA, in 2022 using surface drip irrigation to create artificial saturated soil conditions. There were three questions addressed in this study:

1. What impact does saturated soils have on crop growth and yield?
2. If early-season ponding water creates N deficiency, can plants be “rescued” with a sidedress application of N?
3. Do hybrids differ in how they tolerate low soil oxygen levels and N loss from saturated soils?

Treatments included two water regimes, two nitrogen programs and 10 Golden Harvest^® corn hybrids. The water regimes were either blocks watered repeatedly with surface drip irrigation to create artificial saturated soil or blocks that were rainfed. Irrigation began at the V5 growth stage with a total of 24 inches of water being applied over eight consecutive days to keep the soil saturated within designated blocks (Figure 1). When the ground was dry enough to walk on after irrigation, half of both water regime blocks received 50 lbs of N/acre sidedressed as 32% UAN dribbled on the soil surface along both sides of the crop row using a hand applicator. Irrigation treatment schedule and quantities are outlined in Graph 1.

Slater, Iowa, Weather Patterns

The base rate 160 lbs of N/acre was applied as broadcast urea on April 16. After the base rate of N was applied, Slater received a total of 4.3 inches of precipitation over several weeks delaying planting until May 16. After planting, there was an additional 5.4 inches of rainfall before any blocks of corn were irrigated. The spring rainfall events likely created wet anaerobic conditions conducive to N loss through leaching across the entire trial. Excessive irrigation began on June 15 and 24 inches of water was applied over the next seven days, creating saturated soils and N deficiency symptoms (Figure 2).

Following the last day of irrigation, a sidedress of 50 lbs of N/acre was applied along the crop row dribbled on the soil surface on June 23. During this application, the soil profile in the excessively irrigated blocks was full of water compared to the rainfed blocks that were dry. The rainfed blocks had only received 0.4 inches of rain over the previous 10 days with an average daily high temperature of 93^°F. After the sidedress application, there was no significant rainfall event of 0.5 inches or more until 0.7 inches of precipitation on July 6.

Yield Results

Nitrogen was a limiting factor across the entire trial indicated by the 13 and 58 bu/acre yield response to the sidedress N application in rainfed and excessive irrigation regimes, respectively (Graph 2). On average across all hybrids, excessively irrigating to create saturated soils reduced yields by 12 bu/acre in plots that only received the base rate of N (Graph 2). Nitrogen leaching and reduced oxygen availability in the soil stunted crop growth and ultimately yield. Surprisingly, in plots that received the base rate of N and the sidedress application, there was a 33 bu/acre yield advantage in the excess irrigation blocks compared to rainfed. Nitrogen was limiting in both the rainfed and excessive irrigation blocks because of the heavy spring rain events. It is speculated that the sidedress application was more effective on the excessive irrigation blocks because the soil was saturated and N was able to move through the soil profile to the rootzone. The rainfed blocks were dry during sidedress with little precipitation after the application, therefore the plants were not able to utilize as much of the sidedressed N. In addition, the lack of rainfall and high temperatures following the dribbled sidedress created conditions conducive to N loss through volatilization. These initial findings indicate that under waterlogged or ponding conditions, a rescue application of N can enhance yields.

Like many other management and environmental factors, hybrids varied in their tolerance to an extended period of saturated soils. Five of the 10 hybrids tended to be negatively affected by the lack of oxygen and nitrogen in the soil while the other five hybrids were more tolerant to saturated soils (Table 1). However, many of these responses were not statistically significant due to the large amount of variability across water regimes. Natural variation in drainage across the excessive irrigation blocks created variability in hybrid responses.

There can be many genotypic and phenotypic hybrid characteristics influencing why some hybrids tolerate waterlogged soils better than others. Plants at Slater were deficient in N regardless of water regime. On average, across both water regimes and all hybrids, there was a 36 bu/acre yield increase to the 50 lbs of N/acre sidedress application. Hybrids also varied in the level of response to N sidedress applications (Table 2).>

On average, the five hybrids that tended to be more negatively impacted by excessive water had a 62 bu/acre response to the rescue N application. In comparison, the five hybrids that tended to be more tolerant to the excessive water had a 54 bu/acre response to the sidedress N. Hybrids that showed less tolerance to stress, especially N stress, resulted in a larger yield increase to the additional N.

Summary

Initial findings from this study suggest yield decreases from waterlogged soils can be mitigated with rescue N applications. The nitrogen program had a larger effect on yield than soil saturation level, illustrating the importance of N management. In situations where areas of a field experience ponding, it is economical to sidedress/topdress additional N in those areas when the soil is fit. In this study, the sidedress application was made while the soil was still saturated which may not be applicable with heavy machinery. In addition, the sidedress application placed N directly near the rootzone compared to topdressing urea which is commonly used for rescue applications due to less corn being run over. Crop response to rescue N applications will vary based on application timing and method.

Hybrids tended to respond differently to being in waterlogged soils. Similar results were found in another study illustrating genetic differences in tolerance to saturated soils.¹ That study also concluded that waterlogged soils have a greater negative impact during early vegetative stages (V2-V7) compared to later vegetative and reproductive stages.¹

These results are from only one location which had several environmental interactions such as nitrogen loss prior to planting followed by excessively hot and dry conditions during June-August. Environmental conditions caused unique nitrogen loss and plant utilization scenarios for both base N and sidedress N applications. Natural variation in drainage across the excessive irrigation blocks caused high variability in hybrid responses. A previous study evaluating the effect of tile drainage on corn yield showed yields to be much more variable between drain tiles compared to over drain tiles, demonstrating the importance of tiling and adequate drainage.

Additional research is needed to confirm the consistency in response for water regimes, nitrogen management and repeatable differences amongst hybrids.

References

¹ Zaidi, P. H., Rafique, S., Rai, P. K., Singh, N. N., & Srinivasan, G. (2004). Tolerance to excess moisture in maize (Zea mays L.): susceptible crop stages and identification of tolerant genotypes. Field Crops Research, 90(2-3), 189-202.

² Nelson, K. A., Shannon, D. K., Thompson, A. L., Anderson, S. H. (2001). Site-specific evaluation of a claypan soil with subsurface drainage-impact on corn grain yield. University of Missouri Agricultural Experiment Station Publication.

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