The leaching and chemical transformations of surface-applied urea under flood irrigation

The depth of leaching and the rate of chemical transformation of surface-applied granular urea was studied in the field and in intact soil cores in the laboratory following the flood irrigation of a young barley crop. In the field, preferential flow of urea occurred to 300–400 mm depth with the application of 50 mm of irrigation, but the urea concentration remained greatest in the surface (0–50 mm) layer. In contrast, leaching of urea was dominated by miscible displacement under 100 mm of irrigation in the field. Little urea remained in the surface layer after irrigation and maximum urea concentration occurred at 200–400 mm depth. There was no apparent significant redistribution of urea or its hydrolysis products through the profile after the initial sampling. Urea leaching patterns in intact soil cores were different to those in the field and were largely unaffected by irrigation volume. Preferential water flow occurred through the soil cores, although most of the applied urea was by-passed and remained in the surface layer. Transformation rates of urea were similar in the field and in soil cores. Urea was transformed to ammonium within 48 h, with almost complete conversion of ammonium to nitrate within 192 h of application.