Cornstalk Nitrate Test Reveals Profitable Range for Nitrogen Fertilizer Addition

With the high cost of nitrogen (N) fertilizer this growing season, many farmers adjusted their fertility programs to increase efficiency. Now that the growing season is ending, some may be wondering whether their N management in corn was successful. The late-season cornstalk nitrate test (CSNT) can help diagnose this.

Conducting a CSNT allows producers to identify fields with low, optimal, or excessive N availability to the corn crop over the growing season. In Pennsylvania, results are interpreted as follows: less than 700 ppm nitrate-N is considered low, while greater than 2,000 ppm is seen as excessive.

However, there's uncertainty in interpreting results within the 250-700 ppm range, which may be optimal or marginal depending on the state. Researchers have analyzed CSNT results from various N rate trials across Pennsylvania between 2020 and 2025, finding that profitable yield response to N fertilizer addition corresponded to CSNT levels between 115 and 330 ppm nitrate-N.

A nuanced interpretation of CSNT results suggests that less than 250 ppm is a range where yield and economic losses are likely. The 250-700 ppm range encompasses optimal economic returns with some margin of safety, while the 700-2,000 ppm range provides a greater margin against N deficiency but increases the likelihood of economic losses from overfertilization.

To use CSNT results effectively, producers should keep strips of previous management practice in each field to compare and assess whether changes in N application rates provided a positive return on investment. They should also consider general weather patterns throughout the growing season and adjust management to improve fertilizer efficiency if necessary.