An inhibitor is a compound added to a nitrogen-based fertilizer to reduce losses when the fertilizer has been applied to the crop. By extending the time the active nitrogen component of the fertilizer remains in the soil as either urea-N or ammonium-N, an inhibitor can improve nitrogen use efficiency (NUE) and reduce environmental emissions.

There are two main types of inhibitor that are added to nitrogen fertilizers:

† Urease inhibitors (UI), which inhibit the hydrolytic action of the urease enzyme on urea.

† Nitrification inhibitors (NI), which inhibit the biological oxidation of ammonium to nitrate.