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 short and long-term effects of N rate and application timing on productivity, environmental impact, profitability, and cropping system sustainability and the validation of crop models, such as Maize-N.

The drought of 2012 reduced yields in this trial by about 35% in comparison to the previous three years. Optimum rates of N application, and N use efficiencies also declined. This was the first year in which the optimum rates observed did not exceed recommended rates. Timing of application did not affect optimum rates or yields attained. Unexpectedly, short-term rates did not produce differences from long-term rates, even though this was the fourth year on the long-term check plots receiving only 30 lb/A of starter N each year. Agronomic efficiency in 2012 was 16 as compared to 23 to 31 lbs of grain yield increase per lb of N applied in the previous three years. Recovery efficiency in 2012 was 41% as compared to 53 to 61% in the previous three years. Late September soil nitrate concentrations were higher where application rates exceeded optimum rates. 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. This additional support enables a more complete assessment of sustainability. In 2013, following five years of treatments, measurements of changes in soil organic matter are planned. ON-29