A nitrification inhibitor applied with ammonium sulphate to bare soil and to grass increased the persistence of ammonium-N in soil and decreased the amount of nitrate-N leached from bare soil. Ammonia was volatilized more rapidly from bare soils treated with ammonium sulphate plus the inhibitor than when ammonium sulphate alone was used; the inhibitor increased the amount of ammonia volatilized from grass covered soils eight times.