Residual soil nitrogen levels may be higher than average heading into 2026 due to the relatively warm winter, meaning some fields may need less sidedress N fertilizer than in a typical year. The Late Spring Nitrate Test (LSNT) can be used to reduce uncertainty in N fertilizer needs, lowering the risk of both under and over application.
Using information from 62 Iowa N Initiative corn yield response to N trials (136 unique measurements), we developed an interactive decision support tool to help farmers and agronomists interpret LSNT results.
Link to LSNT decision support tool: https://facts.extension.iastate.edu/late-spring-nitrate-test
The tool requires two user inputs: the soil nitrate concentration in ppm N to 1 foot depth (the standard LSNT measurement) and an acceptable level yield loss if no further N fertilizer is applied (Fig. 1). The tool outputs: 1) the probability of a yield increase from additional nitrogen application (Fig. 2), and 2) the likely range of sidedress N needed to reach the Economic Optimum N Rate (EONR) when there is sufficient probability of a yield response (Fig. 3).
To use the tool, adjust the soil nitrate slider to your LSNT result and enter the yield loss you are willing to accept if no additional N is applied. The tool will automatically estimate the likelihood that yield will increase from additional N application and provide a probability-based range of N needed to reach the EONR.
For instance, with an LSNT of 7 ppm and an acceptable yield loss of 12 bu/ac, the tool suggests a 71% chance that yield will increase with sidedress application. The average amount of sidedress N needed to reach the EONR is 149 lbs N/ac, but this could range from 13–193 lbs N/ac. The range reflects real field-to-field variability in weather, soil type, and management — all of which affect in-season N mineralization and availability.
As the Iowa Nitrogen Initiative collects more data in 2026 and beyond, this range will continue to narrow. Furthermore, future versions of the tool may include inputs such as sampling depth, ammonium, sampling time, crop rotation, and soil organic matter levels on your farm.
We encourage you to try the tool with your own LSNT results and use it as one input for decision making alongside personal knowledge of your fields and operation.
Figure 1: Overview of the LSNT tool with inputs (soil nitrate and yield loss) and outputs (probability of yield increase and amount N-fertilizer required). The 25-75th percentile range indicates a rational range of N rates by removing the highest and lowest 25% of measurements.
Figure 2: The probability that yield will increase from a sidedress application (green curve), with lower probabilities suggesting lower likelihood of profitable response. This example uses 7 ppm LSNT and an acceptable yield loss of 12 bu/ac.
Figure 3: The range of possible sidedress application rates to reach the economic optimum nitrogen rate across the range of possible LSNT results (0-60 ppm).
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- Professor
- Integrated Cropping Systems
- Associate Professor
- Manure Extension Specialist
- Assistant Professor
- Extension Nitrogen Science and Soil Fertility Specialist
- Research Scientist II
- Associate Director Research Administration