Strong alkalis in laboratory bottles. From left to right the alkalis are: sodium hydroxide, ammonium hydroxide, and potassium hydroxide. All three have the standard 'corrosive' warning sign on them, but ammonium hydroxide has an additional 'dangerous for the environment' warning sign. This is because ammonium hydroxide is particularly toxic to aquatic organisms. Also, because of its high nitrogen content, it can upset the balance of nitrates in the soil.