Principal Investigator: Fabian Fernandez
Organization: University of Minnesota
Award Amount: $100,000
Start Date: December 2021 | End Date: May 2023
Project Manager: Aicam Laacouri (Aicam.Laacouri@state.mn.us)
FINAL PUBLISHED REPORT is available in the Minnesota Water Research Digital Library
Preliminary Recommendations for Residual Soil Nitrogen Testing and N Rate Adjustment for Minnesota
It is common to apply nitrogen (N) fertilizer prior to or at corn planting in Minnesota. Residual soil nitrate testing can be used to quantify the amount of mineral nitrogen (N) present in the root zone and is an important N fertilizer management tool. It is among the practices recommended to be implemented under the Groundwater Protection Rule.
The current recommendations are based on research conducted nearly 40 years ago. They do not account for the generally wetter climate Minnesota is experiencing, advancements in fertilizer application technology, including variable rate nitrogen fertilizer application, or in-season assessments of nitrogen needs, including the use of in-season soil nitrate testing. They also do not align completely with the BMP regions used by the University of Minnesota for their other nitrogen fertilizer BMPs.
This project inventoried and assembled a dataset of existing corn grain yield responses to pre-and at-planting and in-season nitrogen fertilizer rates over contrasting spring weather conditions, soil textures, cropping rotations, and regions of Minnesota including data from University of Minnesota research projects, the MDA's Nutrient Management Initiative, and other relevant studies. Based on the assembled data, the project completed a synthesis study evaluating the uses and applications of pre-plant soil nitrate test (PPNT) and pre-sidedress nitrate test (PSNT) for nitrogen rate adjustment.
Conclusions
The study showed that PSNT from the 0-1 foot depth during the V4 to V6 corn development stages can be a useful tool to adjust N rates compared with the PPNT. The major limitation of the PPNT is related to wet spring conditions that can result in substantial N loss. The study showed a critical PSNT of 20 ppm is needed to achieve 97% of the potential corn grain yield in the Southern sub-regions of Minnesota, which is where most corn is grown in the state. The study also showed that 12.5 lbs. N/acre are needed to increase soil nitrate concentrations by 1 ppm in situations where the PSNT is below 20 ppm. Although comprehensive in terms of data size, this is an initial assessment and additional work is being done to refine the preliminary findings of this report. Additional field studies to validate these findings would be necessary to ensure consistent and reliable information for Minnesota corn growers.
