Research HighlightsCan Nitrogen Rescue a Flooded Soybean Crop?
In this article, you’ll find details on:
- Soybean plants are sensitive to excess water
- Soybean can survive short-lived flooding events
- Mitigation practices for flooded soybeans are limited
- Applying nitrogen as a broadcast fertilizer may help soybean plants recover
By John Gaska, Spyros Mourtzinis, and Shawn Conley
Dept. of Plant and Agroecosystem Sciences
University of Wisconsin, Madison
The spring of 2024 brought many challenges to Wisconsin soybean growers, including heavy precipitation and late planting. Many soybean fields were unplantable until late May, and some of those that were planted were later underwater. Soybean seeds or plants submerged for several weeks are usually not salvageable. However, in fields where temporary flooding occurs and water recedes within a few days or over a week, soybeans can survive, continue to grow, and produce a crop. Often, these soybeans may be injured or produce lower yields. While yield may be limited, replanting may not be feasible if the field remains wet. Yield losses are usually not noted for soybeans submerged for less than 48 hours.
Several factors can dictate how well a particular soybean field will tolerate saturated soils and flooding. These include the growth stage, the rate of soil drying, soil texture, subsurface drainage, soil temperature, and the duration of the flooding. In addition to limiting oxygen to the roots, flooding can cause root diseases to flourish, leach nitrogen from the soil, and restrict the uptake of essential plant nutrients.
The Approach
Since flooding can remove nitrogen (N) from the soil through leaching, and oxygen is essential for general soybean growth and nitrogen fixation activities on the roots, the team hypothesized that additional N in the form of a broadcast fertilizer might benefit soybeans that have experienced flooding or saturated soils. These soybeans may lack the N needed for optimal growth from both the soil and the nitrogen fixation process.
A replicated trial was initiated in July 2024 to test the effects of broadcast nitrogen (N) fertilizer on soybeans stressed by flooding and saturated soils. The team selected a soybean field at the Agricultural Research Station in Arlington, WI, planted on May 6, 2024, in 15-inch rows, and had experienced significant flooding shortly after planting (Image 1). The field had a very gradual slope, resulting in areas where soybeans were underwater for extended periods and areas with only saturated soil conditions.
Main plots consisted of replicated strips of urea (46-0-0) fertilizer (30 lbs N/acre) broadcast on July 8, 2024, to V6/R1 growth stage soybeans using a drone equipped with a broadcast spreader (Images 2 and 3). The entire field was imaged several times during the season using a multispectral drone equipped with a RGB camera and four multispectral cameras (green, red, red edge, and near-infrared) to scan and analyze crop growth. This drone also carried a built-in sunlight sensor to capture solar irradiance. NDVI (normalized difference vegetation index) data were calculated from the field images.
The main plot of broadcast N was divided into 30 x 25 ft. subplots to encompass a spectrum of flooding intensities. Grain yield data were determined for each plot in October.
The Results
Yield data analysis showed a 17% increase for the N fertilization treatment over the non-treated control across all ranges of flooding (Fig. 2).
NDVI, a measure of plant health and biomass, was also higher in the N-treated plots compared to the non-treated control on three dates in July and August (Fig. 3). This can partially explain the yield difference due to N fertilization.
Managing flooded soybean fields is challenging due to limited options. If flooding occurs early in the season and the soil dries enough to support machinery, replanting can be an option. Aerating the soil with cultivation can also be beneficial. To reduce artificial stress on the crop, delay any foliar applications that may injure the plants.
Later season flooding is even more difficult to manage. Options include harvesting non-flooded portions of the field first and then waiting for wet spots to dry up. Additionally, there is a risk that grain quality in flooded soybeans may be compromised. Inspect grain from affected areas before mixing it with good grain.
Flooding can affect crop fields in various ways. Good management practices should be used on all soybeans to make the best decisions for each situation.
This study was for one year only. Funding for this project was provided by the Wisconsin Soybean Marketing Board.
Reprinted with permission.
References and Other Resources
Assessing Flood Damage to Soybean – Wisconsin Cool Bean article
Coulter, J., S. Naeve, D. Malvick, and F. Fernandez. 2018. Flooded soybean. Univ. of MN Extension.
https://extension.umn.edu/growing-soybean/flooded-soybean#sources-543410
Scott, H.D., DeAngulo, J., Daniels, M.B. & Wood, L.S. 1989. Flood duration effects on soybean growth and yield.
Agronomy Journal, 81,631-636.
https://doi.org/10.2134/agronj1989.00021962008100040016x
Sullivan, M., VanTooai, T., Fausey, N., Beuerlein, J., Parkinson, J., & Soboyejo, A. 2001. Evaluating on-farm flooding impacts on soybean. Crop Science, 41, 93-100.
https://doi.org/10.2135/cropsci2001.41193x
Published: Feb 3, 2025
