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

Decisions on optimum 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 of 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 six of the seven 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 are en route to implementation. 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 was unusually linear in 2015, with optimum N rates in excess of 230 lb/A, the highest rate applied. Application of N at sidedress produced yields 10% higher than at pre-plant. Long-term rates produced yields slightly (4%) higher than short-term rates; check plots continue to show little difference in yield, whether following corn fertilized with N at 130 lb/A or at 30 lb/A. Partial N balance (N in grain divided by N applied) averaged 56%, and recovery efficiency (increase in N uptake divided by N applied) averaged 52% in 2015.

Further studies are continuing, to assess the impacts on soil mineral and total nitrogen, and soil organic matter. Remote sensing and crop models are to be applied to discern when in the season growth responses occur. This project also receives support from the Ontario Agri Business Association, for sampling soil residual nitrate and soil organic carbon, and from Fertilizer Canada, for measuring nitrous oxide emissions. The study continues in 2016.