Interactive Effects of Landscape Position and Time of Application on the Response of Spring Wheat to Fall-Banded Urea

The objective of this experiment was to quantify the effect of landscape position and time of application on the agronomic efficiency of fall-banded urea [CO(NH2)2] for spring wheat (Triticum aestivum L.) grown in the eastern prairie region of Canada. Landscape positions in this experiment were defined as high and low based on their relative elevations to one another within the field. Fertilizer treatments included urea banded at three different times in the fall (early, mid-, and late fall), in the spring at planting, plus a control with no fertilizer N added. In the low landscape positions, grain yield, total crop N uptake, grain yield increases (GYI), and crop nitrogen use efficiency (NUE) for fall-banded urea (all relative to spring-banded urea) increased linearly with delayed application dates and declining soil temperatures on date of application. However, only crop N uptake and NUE were related to cumulative soil heat units (SHU) from date of fertilizer application until freeze-up, and no measure of crop response to N was related to cumulative nitrification heat units (NHU). In the high landscape positions, the performance of fall-banded urea was not related to any measures of time and/or soil temperature. These results can be used to predict the increase in crop response to fall-banded N as a result of delaying application in low areas of the landscape. Our study also shows that date of application and soil temperature are robust and practical indicators for determining the appropriate time to fall-band urea fertilizer in these areas.