nitrify

verb (used with object), ni·tri·fied, ni·tri·fy·ing.

1. to oxidize (ammonia, ammonium compounds, or atmospheric nitrogen) to nitrites, nitrates, or their respective acids, especially by bacterial action.
2. to impregnate with nitrogen or nitrogen compounds.
3. to treat or combine with nitrogen or its compounds.
4. (formerly) to convert into niter.

verb -fies, -fying or -fied (tr)

1. to treat or cause to react with nitrogen or a nitrogen compound
2. to treat (soil) with nitrates
3. (of nitrobacteria) to convert (ammonium compounds) into nitrates by oxidation