Ammonium is a nitrogen compound which is usually colourless and readily soluble in water. In solid form, ammonium is arranged in crystal structures. On heating, many ammonium compounds disintegrate into gaseous products (sometimes explosively).
Ammonium compounds are most often produced by direct reaction between dissolved or gaseous ammonia and an acid. They are of great technical and economic significance as they are used in commercial fertilisers, explosives and baking powder and also as fire-preventing additives in textiles.
Precipitation today contains far more ammonium than it did before the Second World War. Deposition of ammonium arises as a consequence of ammonia having evaporated from manure.
As the ammonium ion is a weak acid, nitrogen fertilisation with ammonium compounds has an acidifying effect.
Source: Swedish Environmental Protection Agency and Nationalencyclopedin 2010