Late Spring Nitrate Test - N Decision Response Tool

Iowa Nitrogen Initiative trials, 2023–2025  ·  1-foot sampling depth

This tool is designed to help users interpret soil nitrate test results, often referred to as the late spring nitrate test (LSNT). Enter your LSNT result in parts per million (ppm) NO₃-N from a 1-foot soil sample along with the maximum yield loss (bu/ac) you are willing to accept by forgoing further N application. Results should be interpreted in the context of your specific operation, using the full range of outcomes shown in the tool.

The tool uses data from 62 on-farm trials (136 unique measurements) conducted in partnership with the Iowa Nitrogen Initiative (2023–2025) to estimate the probability of a yield increase to additional nitrogen, and the additional sidedress N (lbs/ac) needed to reach the Economic Optimum N Rate (EONR). A maximum yield of 230 bu/ac is assumed based on the average yield in Iowa N Initiative trials, which ranged from 210–260 bu/ac (20–80th percentiles).

Each point represents one observation within an N rate trial, showing the percentage of maximum yield achieved at a given soil nitrate level. The green curve shows the estimated probability of a yield increase exceeding your selected threshold.

Each point represents the additional nitrogen (lbs/ac) needed to reach the Economic Optimum N Rate in that trial. The green line and shaded band show the local median and 25th–75th percentile range across trials.