Which nitrogen source results in the least amount of ammonia volatilization when not incorporated?

Ammonium nitrate results in the least amount of ammonia volatilization when not incorporated because its nitrogen is present in both ammonium and nitrate forms. The ammonium form can be subject to volatilization under alkaline conditions, but it is relatively stable compared to urea, which can readily convert to ammonia gas if left on the soil surface without incorporation. Nitrate, on the other hand, is less volatile as it is more soluble and readily available for plant uptake, which further inhibits the potential for ammonia loss.

In contrast, urea can lead to significant ammonia volatilization if it remains on the soil surface due to the conversion of urea to ammonia through urease activity in the presence of moisture. UAN 28%, which contains urea as part of its mixture, exhibits similar volatility issues. Anhydrous ammonia, when applied as a gas, is highly volatile and can escape into the atmosphere if not properly managed.

Overall, due to its composition and how plants utilize the different forms of nitrogen, ammonium nitrate minimizes the risk of ammonia loss when not incorporated into the soil.