(NH₄)₃PO₄ → 3NH₄⁺ + PO₄³⁻
the hydrolysis of the cation
NH₄⁺ + H₂O ⇄ NH₃ + H₃O⁺
the hydrolysis of the anion
PO₄³⁻ + H₂O ⇄ HPO₄²⁻ + OH⁻
HPO₄²⁻ + H₂O ⇄ H₂PO₄⁻ + OH⁻
H₂PO₄⁻ + H₂O ⇄ H₃PO₄ + OH⁻
a possible scheme of hydrolysis
(NH₄)₃PO₄ + 3H₂O ⇄ 3NH₃·H₂O + H₃PO₄