A comparative study of urea hydrolysis rate and ammonia volatilization from urea and urea calcium nitrate

A comparing of urea hydrolysis and NH₃ volatilization from urea supergranules and urea calcium nitrate (UCN, a new fertilizer produced by Norsk Hydro A/S, Norway) was made on two different flooded soil types, a high-CEC clay loam (Ås) and an intermediate-CEC clay loam (Kinn).Nitrogen loss by ammonia volatilization was reduced from 17% by surface application of urea supergranules (USG) on flooded Ås soil to 3% and 6% by UCN briquettes at either the same urea or nitrogen concentration as USG. A significant reduction was even found with the surface application of prilled UCN, 12% and 18% N-loss for prilled UCN and urea, respectively. The floodwater pH and NH₄⁺ content was lower with UCN than urea, which reduced the potential for ammonia volatilization.NH₃-loss (5%) was significantly less when USG was surface applied on Kinn soil, while NH₃-loss from UCN briquettes was independent of soil type. The reduction in NH₃-loss from USG on Kinn soil was due to a decrease in the pH and NH₄⁺ content of the floodwater caused by a reduced rate of urea hydrolysis.The rate of urea hydrolysis was lower with UCN than USG in both soils, but the difference between UCN and USG was greater in the Ås soil than in the Kinn soil. Three days after deep placement (10 cm), 18% of UCN urea and 52% of USG urea were hydrolyzed in Ås soil, while only 12% UCN and 17% USG were hydrolyzed in the Kinn soil.The surface application of USG on flooded soil reduced the rate of urea hydrolysis as compared to deep placement. 30% and 17% of USG urea was hydrolyzed after four days on Ås and Kinn soil, respectively. During the first few days the rate of hydrolysis of UCN was more affected by the soil type than the application method. Four days after surface application 32% and 13% UCN urea was hydrolyzed on Ås and Kinn soil, respectively. The rate of urea hydrolysis exhibited a zero-order reaction when USG and UCN-briquettes were point placed in flooded soils.