by Gurbir Dhillon Ph.D.
Environmental factors can affect the way nitrogen fertilizers work in a crop particularly when surface applied.
Surface applied ammonium-based fertilizers such as urea or urea ammonium nitrate (UAN) can lose nitrogen through ammonia volatilization (evaporation).
Similarly, the breakdown of farm manures or soil organic residues may also lead to ammonia volatilization. Multiple factors may impact the rate of ammonia volatilization and reduce N use efficiency.
Surface applied fertilizers or manure can lose 25 - 75% more through ammonia volatilization compared to when they are soil incorporated. Sandy, alkaline, and calcareous soils favor high ammonia volatilization especially if they are exposed to rapid wetting and drying.
A study in Montana, US observed 30 - 44% loss in urea applied to moist soil surface followed by subsequent slow drying without precipitation. Precipitation following urea broadcast helped reduce losses by moving the fertilizer into deeper soil layers.
High temperatures, that often occur on soil surfaces, and windy conditions also favor ammonia evaporation. Studies in western Canada observed volatilization losses of 38-46% of urea at 25oC compared to only 7% loss at 15oC temperature.
Urea application can artificially increase soil pH around urea granules temporarily and result in substantial losses even from soils with low pH. Polymer-coated urea generally decreases volatilization losses by slowing the conversion of urea to ammonium combined with lesser increase in soil pH around fertilizer granules.