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

Abstract

Ammonia (NH3) 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 NH3 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−1air flow, 44.6 mg N kg−1air‐dried soil, and 100 mL of 0.02 M phosphoric acid to recover volatilized NH3. Cumulative NH3 loss ranged from 33.9 to 37.2% of the applied N in all trials. Initial NH3 volatilization losses were delayed by calcium and potassium sulfate coatings alone. The inhibitor NBPT reduced cumulative NH3 losses to 17.9 to 24.7% of applied N and delayed NH3 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 NH3 volatilization from 35.6 and 35.1% to 25.4 and 24.1%, respectively. In both trials, no difference in cumulative NH3 loss was observed when NBPT rates exceeded 0.04%. The inhibitor NBPT had the greatest influence on NH3 volatilization losses in these studies, though the K2SO4 coated urea also reduced NH3 volatilization and supplies a small amount of S for crop growth.