Ammonium dihydrogen phosphate (ADP), or monoammonium phosphate, NH₄H₂PO₄, is formed when a solution of phosphoric acid is added to ammonia until the solution is distinctly acidic. It crystallizes in tetragonal prisms. Monoammonium phosphate is often used in the blending of dry agricultural fertilizers. It supplies soil with the elements nitrogen and phosphorus in a form which is usable by plants. The compound is also a component of the ABC powder in some dry chemical fire extinguishers. This substance is also supplied in an emerald green or aquamarine crystal growing box kit for children. Solid monoammonium phosphate show a dissociation pressure of ammonia of 0. 05 mmHg at 125°C based on the decomposition reaction as follows: >2PO₄(s) D NH₃(g) + H₃PO₄(l). ADP is a widely used crystal in the field of optics due to its birefringence properties. As a result of its tetragonal crystal structure, this material has negative uniaxial optical symmetry with typical refractive indices n_o=1. 522 an n_e=1. 478 at optical wavelengths. ADP crystals are piezoelectric which is a property required in some active sonar transducers (the alternative being transducers that use magnetostriction). In the 1950s ADP crystals largely replaced the Quartz and Rochelle Salt crystals in transducers because they are easier to work than Quartz and, unlike Rochelle Salt, are not deliquescent