Movement of urea in soils: Downward movement of urea in soils and possibility of loss by leaching dependent on rate of conversion to ammonium form
Fertilizer urea is considered—usually—to resemble nitrates in its mobility in soils because it is not strongly retained by soil colloids and appears to move freely in the soil solution. However, when added to a soil, urea is converted to ammonium carbonate, and when nitrogen is in the ammonium form it is leached only slightly. The downward movement of urea in soil and the possibility of leaching loss are dependent on the rate of hydrolysis—the conversion of urea to the ammonium form. The hydrolysis is accomplished through the activity of enzymes, present in all agricultural soils, although the rate of conversion varies somewhat among different soils. The accompanying table shows that in four soils the equivalent of 200 pounds per acre urea nitrogen was hydrolyzed to ammonia in two days, and in two of the soils there was virtually no urea remaining after one day. Thus there would be movement of urea nitrogen for only a day or two—or in some soils, only a few hours—because it would be converted to the ammonium form.