Core Ideas
Volatile NH3 loss in the field was quantified via the modified passive flux method.
Urease inhibitor (NBPT) or S-coated urea reduced N loss by 39 to 53% versus urea.
Urea plus ammonium sulfate (Urea-AS) reduced N loss by 19 to 28% versus urea.
A companion laboratory trial found the same relative efficacy among fertilizers.

Substantial ammonia (NH3) loss can occur when urea is broadcast without incorporation. Choosing an alternate fertilizer (instead of urea) is one option when incorporation of urea by tillage or sprinkler irrigation is not feasible. This research evaluated alternative N fertilizer products (vs. urea) using the modified passive flux method. Fertilizers were broadcast (168 kg N ha–1) on an Irrigon fine sandy loam soil (mesic Xeric Haplocambid), and not incorporated. Light showers (7 mm in 14 d after application) triggered NH3 loss. After 34 d, ammonia-N loss was 14 to 17% of N applied as ammonium sulfate (AS) or ASN (fused granule of ammonium sulfate and ammonium nitrate), 22% as S-coated urea (SCU), 28% as urea + urease inhibitor NBPT [N-(n-butyl) thiophosphoric triamide], and 33 to 37% as blended or fused urea-AS fertilizers (79:21 urea/ammonium sulfate, w/w) vs. 46% as urea. A companion 44-d laboratory study (16°C) demonstrated a similar ranking of fertilizer efficacy. We conclude that fusing AS with urea in a granule (instead of blending) did not reduce NH3 loss. Partial substitution of AN for AS in a fused granule (ASN) did not provide additional benefit in reducing NH3 loss vs. AS alone. Treated urea products (SCU and urea + NBPT) had greatest efficacy in reducing NH3 loss during the first 10 d after application, with reduced efficacy thereafter, likely due to the degradation of the coating (SCU) or the urease inhibitor NBPT.