Phosphorous, periodic table P, is a light metal. It's most common isotope is P31 ,with 16 neutrons.
Phosphorous is one of the "CHNOPS" elements (Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, & Sulfur). All life is 99% composed of these elements. Of all the CHNOPS elements, phosphorus is the rarest one. Fortunately, Mars seems to be fairly rich in phosphates.
Phosphorous is available on Mars as phosphates. The Martian mantle is richer in potassium and phosphorus than Earth's, so the resource should be widely available, although concentrated ores need to be identified by exploration. The Columbia hills in Gusev crater have been shown by the Spirit rover to contain Phosphorous.