The SC soybean board funded this project in 2019 and 2020, which allowed us to produce a model for predicting leaf nitrogen levels from aerial images and assess nitrogen application as a crop rescue strategy. The conclusions from these initial experiments are that:

• There is a direct correlation between soybean leaf nitrogen levels and yield
• Aerial image analysis can be used to estimate leaf nitrogen levels within a field
• Yield of nitrogen deficient plants can be rescued by nitrogen application as late as R1

Together, these results suggest the economic feasibility of rescuing yield in nitrogen deficient soybean patches. However, additional real-world trials are needed to improve our ability to prescribe nitrogen application. This proposal seeks to apply our initial results to grower’s fields to more directly assess the levels of nitrogen application that produce the best possible revenue.

Experimental Design

We will identify 5 chlorotic patches of grower’s fields after the plants get large enough to visually identify the chlorosis (V5-V6). The research area will be marked out into randomized plots with 4 replicates of 4 treatments (Table I). The plots will each be 10 feet of two adjacent rows.

Initial Data collection
Aerial images and handheld NDVI of these plots will be taken to document the size and visual characteristics of the initial chlorosis. A composite soil sample from the area will be used to test for fertility and nematodes. Plant tissue will be collected from the most recently matured trifoliates from twelve plants in each replicate to determine the average initial nitrogen levels of the chlorotic patch. Plant height will be measured as the average plant height of six row feet of plants near the center of each plot.


Nitrogen application
At the V5-R1 stage, plants will be side dressed with urea about 6 inches from the plants in 4 treatments (Table I). It will be applied by hand and raked into the soil to prevent it from washing into other plots. Based on our preliminary experiments, we have concluded that higher nitrogen rates (i.e. 240 #/ac) are not feasible because of scorching and input costs. Thus, we plan to test low to moderate ranges of nitrogen application as shown below.

Table I - Treatments
Rescue #/acre N Input cost/acre ($0.35 per #)
0 $0
40 $14
80 $28
120 $42

Post nitrogen application data collection
Approximately one month after the nitrogen application, measurements will be taken to determine the effect of the rescue nitrogen application. Aerial images and handheld NDVI of these plots will allow us to determine canopy coverage and color. Plant tissue analysis of each plot will allow us to determine how much the nitrogen levels were improved by the treatment. We will then be able to assess if canopy color can be used as a proxy for nitrogen levels. Plant height and canopy coverage will used to determine the effect of the treatments on overall growth.

As the plants mature, we will hand harvest and thresh a sample from each plot to determine yield. This yield data will be used to calculate the revenue produced for each treatment.

• Determine the effectiveness of different rates of nitrogen application at rescuing nitrogen deficient soybeans
• Determine the profitability of nitrogen rescue treatment
In addition to these deliverables, the information gained from this study will provide the preliminary data needed for development of analytical tools and software that will allow growers, consultants, specialists, and breeders to use image data or leaf nitrogen to prescribe rescue treatments.

Updated April 15, 2021:

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Updated July 15, 2021:

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Updated October 13, 2021:

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Updated December 14, 2021:

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Updated January 17, 2022:

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Rescue application of nitrogen increases yield and profit in nitrogen deficient soybeans depending on current price conditions. We found that side dressing 40-50 pounds per acre of nitrogen before R1 stage is sufficient in many cases to raise leaf nitrogen levels to normal levels. Thus, identification of nitrogen deficient patches and a prescribed rescue application with variable rate technology has the potential to increase productivity and profitability.

This project will be successful if it can provide baseline measurements of how much nitrogen fertilizer is needed to rescue full yield potential and/or maximize profitability in nitrogen deficient soybeans. Together this will provide an avenue to produce effective resources for soybean management.