the ion NH4 derived from ammonia; behaves in many respects like an alkali metal ion
"The fertilizer's high ammonium content acts much like a dissolved salt, allowing it to travel freely through the soil solution before plants can absorb it."
The univalent NH₄⁺ cation, derived by the protonation of ammonia
"In the laboratory, we synthesized ammonium salts by adding a strong acid to an excess of ammonia gas."