In Vitro Evaluation of Coatings to Control Ammonia Volatilization From Surfaced Applied Urea.

Loss of nitrogen in the form of volatilized ammonia from surface applied nitrogen sources containing ammonium or urea can be substantial if environmental conditions are favorable. Coatings utilizing urease inhibitors are currently available for granular urea to control ammonia volatilization. The objective of this study was to quantify the control of ammonia volatilization using the urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT) coupled with physical coatings. Methods to quantify ammonia volatilization under field conditions have had little success due to altering the air movement and soil environment.  Evaluations were conducted using a laboratory system with the following characteristics: (1) constant air flow rate(1.00 L min^-1 over the soil and surface applied fertilizers; (2) constant temperature 26^°C; (3) humidity maintained at RH~100%,  and  (4) volatilized ammonia captured  in acid traps containing 100 ml of 0.02 M phosphoric acid.  A nitrogen rate of 100 kg N ha^-1 was used for all treatments in all trials except for the control soil which did not receive nitrogen, and volatilization losses were measured for 14 days. The soil used in the studies was a Fine-loamy, mixed, active, mesic Ultic Hapludalf. Treatments for the first two trials (I and II) of the study were non-coated urea, NBPT-coated urea using binder A, NBPT-coated urea using binder B, calcium sulfate (CaSO₄) coated urea with NBPT, potassium sulfate (K₂SO₄) coated urea with NBPT, and soil with no fertilizer N added.  In trial I, 37.2% of the nitrogen applied as non-coated urea was lost as ammonia compared to 25.1%, 25.0%, 23.7%, and 22.4% for NBPT-coated with binder B,  NBPT-coated using binder A, CaSO₄-coated, and K₂SO₄-coated respectively.  In trial II, non-coated urea lost 34.1% of the applied N through volatilization while the coated materials lost 18.1%, 19.9%, 19.7%, and 21.0% for NBPT-coated with binder B,  NBPT-coated using binder A, CaSO₄-coated, and K₂SO₄-coated respectively.  Volatilization reductions were measured for all coatings with NBPT, a known urease inhibitor, as well as for the physical coatings.  A delay in volatilization of approximately 48 hours was also observed in these trials.