Long-term Optimum Nitrogen Rate for Corn Yield and Soil Organic Matter

Decisions on optimum nitrogen (N) rates are often made on the basis of single-year responses. Data are limited on the long-term impact on productivity and soil organic matter using fertilizer application rates higher or lower than these short-term optima. This controlled experiment was designed as a base for testing the application of dynamic soil-crop-atmosphere models as predictors of N rates for corn that optimize sustainability. The specific objectives include the assessment of both short- and long-term effects of N rate and application timing on productivity, environmental impact, profitability and cropping system sustainability, as well as the validation of crop models. Treatments include five application rates of N fertilizer ranging from 27 to 230 lb/A in a factorial arrangement with two times of application (pre-plant and sidedress) and differing levels of N applied the previous year.

Over the course of this trial so far, optimum N rates have ranged from 120 to over 230 lb/A, correlating directly to yields ranging from 115 to 220 bu/A. In seven of the eight years, optimum N application rates exceeded previous local recommendations. Based on results from this research, new Harmonized General (N Calculator) and Soil Nitrate (PSNT) Nitrogen Rate Recommendations for Ontario Corn were approved in December 2014 and have been implemented. The new recommendations allow the use of the pre-sidedress soil nitrate test, along with yield goal, to guide N recommendations. Yield results for the first six years up to 2014 were published in Better Crops 99(2):16-18.

Yield response to N in 2016 was influenced by drought in the early season. Results contrasted with those of 2015, in that yields and optimum N rates were higher with pre-plant than with sidedress timing. For pre-plant, an optimum N rate of 183 lb/A produced a yield of 187 bu/A. For sidedress, an optimum N rate of 133 lb/A produced a yield of 167 bu/A. Check plots continue to show little difference in yield, whether following corn fertilized with 130 lb N/A or 30 lb N/A. Partial N balance (N in grain divided by N applied) was 65% for pre-plant and 79% for side-dress and recovery efficiency (increase in N uptake divided by N applied) was 55% for pre-plant and 60% for sidedress.

Future work aims to integrate the information on soil N, soil organic matter, and yield resiliency to climate, using the DNDC model to assess sustainability impacts of N rate and timing decisions. This project also has received support from the Ontario Agri Business Association for sampling soil residual nitrate and soil organic carbon, and from Fertilizer Canada to measure nitrous oxide emissions. The study continues in 2017.