Consider the Cornstalk Nitrate Test to Assess Your Nitrogen Management

Nitrogen is a challenging nutrient to manage because of its many forms, transformations, and loss pathways. Furthermore, despite a number of tools designed to improve N fertilizer recommendations in corn, such as the recently recalibrated pre-sidedress soil nitrate test or a new experimental tool to credit N supply from soil organic matter and cover crops, significant uncertainty remains in determining optimal N application rates. This uncertainty often leads growers to apply extra N fertilizer as an insurance against yield losses due to insufficient N. The late-season cornstalk nitrate test (CSNT) is a useful diagnostic tool to use at the end of the growing season to determine whether excess N was applied to a corn crop, which can inform management decisions in future years.

Conducting a CSNT allows producers to identify fields that were low, optimal, or excessive in the availability of N to the corn crop over the growing season. Historically, in Pennsylvania the CSNT has been interpreted as low when nitrate concentrations are less than 700 ppm nitrate-N, optimal when results are 700-2,000 ppm nitrate-N, or excessive when results are greater than 2,000 ppm nitrate-N. The CSNT is especially accurate at identifying fields where N was excessive. If your field falls into this category, with greater than 2,000 ppm nitrate-N, there is a strong likelihood that N applications could have been reduced without any yield losses, which should improve the economic and environmental performance of your system. This information could give you increased confidence in making incremental reductions in N applications in the next growing season.

Currently, there is less certainty in interpreting results of the CSNT with regard to the optimal range. The original research conducted to calibrate the CSNT in Pennsylvania, which was published in 2001, found that corn yields did not start to decline due to lack of N until stalk nitrate concentrations fell below 250 ppm nitrate-N. However, as a conservative safety factor to protect against potential yield losses due to N limitation, the optimum range of CSNT results was raised to a lower limit of 700 ppm nitrate-N in Penn State Extension literature. Researchers in other states also found CSNT results around 250 ppm nitrate-N best separated sites that were N deficient and N sufficient, but a variety of interpretations in the extension literature exist across states. For instance, some states consider the optimal range to be 250–2,000 ppm nitrate-N, some states consider 250–700 ppm nitrate-N to be in the "marginal" category, while others are consistent with Pennsylvania's current optimum range of 700–2,000 ppm nitrate-N. Given this variation, it may be confusing to interpret CSNT results that are between 250 and 700 ppm nitrate-N. Depending on whose interpretation you use, results in this range could be considered low, marginal, or optimal.

Recently, we have begun to analyze results of the CSNT in various N rate trials conducted across Pennsylvania between 2020 and 2023 to see if updates to the interpretive categories are necessary. Preliminary analysis of this data indicates that the most profitable yield response to N fertilizer addition corresponded to CSNT levels between 115 and 315 ppm nitrate-N. Interestingly, the 250 ppm nitrate-N threshold that many researchers have identified as separating N deficient and N sufficient sites is right in the middle of this range. In our recent dataset, observations with CSNT results below 115 ppm nitrate-N had an average net loss of $178/acre relative to management at the agronomic optimum N rate (AONR, the N rate where corn yields reached the maximum plateau), which is a result of yield losses from N limitation. On the other hand, observations with CSNT results greater than 315 ppm nitrate-N had an average net loss of $46/acre relative to the AONR due to the cost of additional N fertilizer without a corresponding yield increase. These results demonstrate the economic importance of maintaining sufficient N supply to prevent yield losses, while also highlighting that there can be meaningful opportunities to increase profits by reducing excess N applications where appropriate. Collecting CSNT samples on your corn this fall could help you understand where and how to refine your N management.

That said, N dynamics are notoriously variable across years. So, when choosing to change N application rates in future years based on CSNT results, it is worthwhile to keep some strips of your previous management practice in each field to compare and assess whether changes in N application rates provided a positive return on investment. It is also important to consider general weather patterns throughout the growing season, and if there were conditions that may have promoted N losses such as volatilization, leaching, or denitrification. Considering this information along with the CSNT results allows for best management practices to be implemented in future growing seasons. If your CSNT result is low, the correct management response may not necessarily be to apply higher rates of N fertilizer. Rather you may need to adjust management to improve the fertilizer efficiency, such as applying the fertilizer closer to the time of crop uptake, incorporating manure and urea-containing fertilizers, or using nitrogen stabilizers such as urease inhibitors, nitrification inhibitors or slow-release polymer-coated fertilizer. Adjusting management to improve efficiency helps the same amount of N fertilizer usage go further to improve crop yields and the overall economics of the crop production system.

The earliest that a producer can conduct a CSNT is at one-quarter milk line, which can be determined when the ear of corn is broken in two (Figure 1). The milk line is the border between the milk and starch layers that can be seen when looking at the broken half containing the ear tip. One-quarter milk line means that the outer one-quarter of the kernel has converted to starch. The latest that a producer can conduct the CSNT is three weeks after black layer (physiological maturity). A minimum of ten representative plant samples need to be taken within the cornfield or one stalk per acre of field size if the field is greater than 10 acres. Choose healthy disease-free samples to include in your CSNT. An eight-inch stalk sample needs to be taken beginning six inches above the soil surface. A set of pruning shears works well to assist you in cutting your samples. (Figure 2).

An alternative to cutting the stalks into two-inch segments is to quarter each stalk lengthwise with a knife and retain one quarter strip from each stalk to submit for analysis (Figure 3). Quartering stalk segments allows the sample to dry faster and is easier for the lab to process. Place samples in a paper bag to be mailed to the Penn State Agricultural Analytical Services Laboratory. If samples are being held over the weekend, place them in a paper bag and put in a refrigerator. Do not freeze the samples. The Agricultural Analytical Services Laboratory website has instructions on how to collect and submit a CSNT sample.

Figure 3. Cutting stalk segments into quarters and submitting one quarter from each stalk sampled reduces the volume of material for the lab to handle, speeding up the sample preparation process.