Nitrates
Nitrates were identified on Mars in 2015 by the Curiosity rover [1]. They contain Nitrogen, an important elements for life.
Up to 25% of the original atmospheric nitrogen of Mars might be stored on the Martian Cryolite (frozen surface), in molecules such as HNO3.[2] See also Atmospheric loss.
Nitrates could serve as fertilizer, or as a source of nitrogen for the settlement atmosphere.
Freeing the nitrogen might be part of an overall Mars terraforming plan.