Phosphorus is added to media as phosphate, most commonly in the forms of NaH2PO4, KH2PO4 or K2HPO4. All are weakly acidic (pKa > 6) soluble salts. Phosphate can precipitate at higher pH either as Apatite with Ca or through co-precipitation with Ca or Mg carbonates and hydroxides. For this reason, we add our phosphate stock aseptically to marine media after autoclaving.