Monoammonium phosphate (MAP) is a widely used source of phosphorus (P) and nitrogen (N).* It’s made of two constituents common in the fertilizer industry and contains the most phosphorus of any common solid fertilizer. MAP has been an important granular fertilizer for many years. It’s water-soluble and dissolves rapidly in adequately moist soil. Upon dissolution, the two basic components of the fertilizer separate again to release ammonium (NH₄⁺) and phosphate (H₂PO₄⁻), both of which plants rely on for healthy, sustained growth.