TeX source:

\overset{-3\,\,+1\,\,}{NH_{4}^{+}}+\overset{+5\,-2\,\,\,}{PO_{4}^{3-}}\rightarrow \overset{-3\,+1\,\,\,}{NH_{3}}+\overset{+1\,+5\,-2\,\,\,}{HPO_{4}^{2-}}