A method for imparting flame retardancy to a substrate material. The method comprises adding to a substrate material a flame retardant composition. The flame retardant composition comprises at least one flame retardant salt, a nitrogen-containing compound, and optionally water. The at least one flame retardant salt comprises an ammonium salt of phosphoric acid. The ammonium salt of phosphoric acid comprises water soluble ammonium polyphosphate (APP). The water soluble ammonium polyphosphate has a total nitrogen as N from about 5 to about 15 weight percent, and a total phosphorus as P2O5 from about 30 to about 40 weight percent, based on the total weight of the ammonium polyphosphate (APP).