Transformation of Fall‐Banded Urea: Application Date, Landscape Position, and Fertilizer Additive Effects

A 2‐yr study was initiated in the fall of 2000 to generate fundamental information on the effects of application date, landscape position, and a combined urease and nitrification inhibitor (NBPT [N‐(n‐butyl) thiophosphoric triamide] and DCD [dicyandiamide], respectively) on the rate of transformation of fall‐banded urea fertilizer into NH 4 + and eventually NO 3 − under conditions typical for Manitoba, Canada. Landscape position did not have a large effect on the conversion of banded urea to NO 3 − under the moisture conditions present during the autumn period at the sites. Delaying application of fall‐banded urea fertilizer into the late fall and the presence of NBPT and DCD slowed nitrification and increased the recovery of fertilizer N as NH 4 + (%RFN as NH 4 + ) in the soil before it froze. Date of application, the soil temperature on the date of application, and the accumulation of SHU (soil heat units) and NHU (nitrification heat units) were all linearly related to the %RFN as NH 4 + Accumulated SHU and NHU were most strongly correlated with the %RFN as NH 4 + at the end of the fall, with and without inhibitors. The interactions between time, temperature, and nitrification demonstrate that producers in the Northern Great Plains region must consider the overall length of time that fall‐applied N fertilizer will be exposed to the soil before the soil freezes, even after the soil temperature has reached a given level and even if the fertilizer is banded.