In Vitro Evaluation of Coatings to Control Ammonia Volatilization from Surface‐Applied Urea

Ammonia (NH 3 ) volatilization from surface‐applied N fertilizers containing urea can be substantial if environmental conditions are favorable. Physically coating urea with sulfate salts and urease inhibitor may reduce NH 3 volatilization and supply plant available S. The objectives of this study were to quantify in vitro N loss from surface‐applied urea; and measure the rate and total N volatilization loss from urea coated with calcium sulfate, potassium sulfate, alone and in combination with the urease inhibitor, N‐(n‐butyl) thiophosphoric triamide (NBPT). Six trials, lasting 14 d, were conducted using a laboratory system at 26°C, 1.00 L min −1 air flow, 44.6 mg N kg −1 air‐dried soil, and 100 mL of 0.02 M phosphoric acid to recover volatilized NH 3 . Cumulative NH 3 loss ranged from 33.9 to 37.2% of the applied N in all trials. Initial NH 3 volatilization losses were delayed by calcium and potassium sulfate coatings alone. The inhibitor NBPT reduced cumulative NH 3 losses to 17.9 to 24.7% of applied N and delayed NH 3 volatilization for 96 h after N application when applied at the 0.08% w/w application rate in trials I to IV. In trials V and VI, applying NBPT at 0.02% w/w reduced cumulative NH 3 volatilization from 35.6 and 35.1% to 25.4 and 24.1%, respectively. In both trials, no difference in cumulative NH 3 loss was observed when NBPT rates exceeded 0.04%. The inhibitor NBPT had the greatest influence on NH 3 volatilization losses in these studies, though the K 2 SO 4 coated urea also reduced NH 3 volatilization and supplies a small amount of S for crop growth.