Can plant tissue testing be used to adjust N fertilizer rates for corn?
Carrie Laboski and John Peters, Extension Soil Scientists
There has been recent interest among producers and agronomists to use corn plant tissue testing to adjust sidedress N fertilizer rates. The challenge with this strategy is that plant tissue testing has not been calibrated to make recommendations for sidedress N application in Wisconsin.
Plant tissue testing was developed as a means to help diagnose problem areas in fields. The sufficiency ranges for each nutrient are guidelines to suggest whether or not the plant has already taken up adequate levels of a specific nutrient for a particular growth stage. Soil test levels, environmental conditions, and hybrid can affect tissue nutrient concentrations and make exact interpretation of tissue test results challenging.
A whole plant corn sample collected when the plants are 12 inches tall and analyzed for total N content will only give an indication of N availability to that point in the growing season. It is not predictive of how much N will continue to be available to the crop. For example, if a field has not received manure in several years, the previous crop was corn, and the only N applied was 20 lb N/a in starter fertilizer, it is possible for a tissue test of 12 inch-tall corn to come back in the sufficient range. However, should this be interpreted that no sidedress N is needed? The answer is no. Failure to apply N in this situation will likely result in reduced yields and profitability in most years.
The presidedress soil nitrate test (PSNT) is a useful tool for adjusting sidedress N application rates. The PSNT is a 12-inch soil sample that is taken when the corn is about 12 inches tall. The PSNT provides a measure of how much soil and manure/legume N has mineralized to that point in the growing season. The PSNT results can then be used to determine a N credit that can be subtracted from the targeted N application rate. The PSNT can be used to confirm forage legume and manure N credits. It is not recommended for use on sand and loamy sand soils or where more than 20 lb N/a in starter fertilizer was applied. In addition, the PSNT is not meant to verify the efficacy of preplant fertilizer N applications. For more information on the PSNT please see: UWEX publication A2809 (http://learningstore.uwex.edu/Nutrient-Application-Guidelines-for-Field-Vegetable-and-Fruit-Crops-in-Wisconsin-e-file-P1484.aspx).