The Collaborative International Dictionary of English v.0.48:

Nitrify \Ni"tri*fy\, v. t. [imp. & p. p. Nitrified; p. pr. &
   vb. n. Nitrifying.] [Niter + -fy: cf. F. nitrifer. See
   Niter.] (Chem.)
   To combine or impregnate with nitrogen; to convert, by
   oxidation, into nitrous or nitric acid; to subject to, or
   produce by, nitrification.
   [1913 Webster]

WordNet (r) 3.0 (2006):

nitrify
    v 1: convert into nitric acid, nitrous acid, or nitrate,
         especially with the action of nitrobacteria
    2: treat with nitrogen or a nitrogen compound [syn: nitrify,
       nitrogenize, nitrogenise]
    3: treat (soil) with nitrates [ant: denitrify]