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 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. Grain N concentrations were higher with sidedress than with preplant fertilizer applications (0.62 versus 0.59 lb/bu). The same grain N concentration were observed comparing corn following corn that had been fertilized at N rates of 195 and 27 lb/A, respectively. These values compare to grain crude proteins levels of 8.3 and 7.8%. The 2013 season also saw the highest levels of N use efficiency in the five-year period. Agronomic efficiency in 2013 was 34 lb of grain yield increase per lb of N applied, as compared to 16 to 31 in the previous four years. Recovery efficiency in 2013 was 64% as compared to 41 to 61%. There were no significant effects of N rate, previous crop N rate, or application timing on the level of residual mineral N in the soil just after corn maturity in 2013 (average level was 40 lb/A).

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. The study will continue in 2014.