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, such as Maize-N. Treatments include five application rates of N fertilizer ranging from 27 to 230 lb/A in a factorial arrangement with two times of application (preplant and sidedress) and differing levels of N applied the previous year.

The 2013 growing season saw the highest rainfall, the highest yields, and the highest optimum N rates of the five years of this field experiment. Over the course of this trial so far, optimum N rates have ranged from 120 to 215 lb/A, correlating directly to yields ranging from 115 to 220 bu/A. In four of the five years, optimum N application rates have exceeded local recommendations. Timing of application did not affect grain yields or optimum N rates. Neither were they affected by a wide range of N rates applied to the previous year's corn crop.

The 2014 growing season produced yields and N responses almost as large as those observed in 2013, despite the cooler temperatures. Data from the first six years are being summarized in a Better Crops with Plant Food article to be published in 2015.

In addition, several other projects have been initiated, with support from the governments of Canada and Ontario, and from the 4R Research Fund, to evaluate modeling approaches to account for weather impacts on corn N responses. The models to be assessed include SPACSYS and NLOS. This project also receives support from the Ontario Agri Business Association, for sampling soil residual nitrate and soil organic carbon, and from the Canadian Fertilizer Institute, for measuring nitrous oxide emissions. These data are currently being analyzed and summarized. The study continues in 2015.