When urea is broadcast and not worked into the soil there is a risk of some of the nitrogen being lost to the atmosphere as ammonia. In an effort to measure the effectiveness of urea compared to ammonium nitrate as sources of nitrogen for topdressing on established grass sod, an experiment comparing these two sources at rates to supply 0, 50, 100 and 150 pounds of nitrogen per acre was conducted in 1971.
Thomas, Grant and Miller, Harold F., "Comparison of Ammonium Nitrate and Urea as Nitrogen Source for Topdressing Established Grass Sods" (1972). Agronomy Notes. 123.