Measurements of bi-directional ammonia exchange over a fertilized soybean canopy are presented for an 8-week period during the summer of 2002. The modified Bowen-ratio approach was used to determine fluxes from vertical NH3 and temperature gradients in combination with eddy covariance sensible heat fluxes. The measurement site is located on the Coastal Plain of North Carolina in an area with high NH3 emissions from animal production and fertilizer use. Ambient NH3 concentrations ranged from 0.01 to 43.9 micro-g m-3 with a median value of 8.6 micro-g m-3. A mean flux of -10.8 nano-g m-2 s-1 indicates that the canopy was a net sink for NH3, though emission fluxes occurred frequently during the late morning and early afternoon. Average canopy and stomatal compensation points of 7.1 and 12.6 micro-g NH3 m-3 are inferred from relationships between flux and ambient concentration. The mean deposition velocity during the experiment was 3.3 mm s-1. Deposition velocities were, on average, higher during the day (4.3 mm s-1) than at night (2.1 mm s-1) and nighttime values were largest when the canopy was wet (3.3 mm s-1). Measured deposition velocities indicate a large canopy resistance (median = 228 s m-1), which is likely the result of very dry conditions. The median daytime canopy resistance for NH3 (211 s m-1) is similar to the bulk canopy resistance for water vapor (184 s m-1). The net flux during the experiment corresponds to a dry deposition rate of 0.7 kg NH3-N ha-1 for the entire summer compared to wet deposition of 1.9 kg NH4+-N ha-1 at a nearby site during the same period. Dry deposition of NH3 accounted for approximately 0.3% of crop nitrogen requirements.