Source : Webster's Revised Unabridged Dictionary (1913)

Nitrification \Ni`tri*fi*ca"tion\, n. [Cf. F. nitrification. see
   {Nitrify}.]
   1. (Chem.)
      (a) The act, process, or result of combining with nitrogen
          or some of its compounds.
      (b) The act or process of oxidizing nitrogen or its
          compounds so as to form nitrous or nitric acid.

   2. A process of oxidation, in which nitrogenous vegetable and
      animal matter in the presence of air, moisture, and some
      basic substances, as lime or alkali carbonate, is
      converted into nitrates.

   Note: The process is going on at all times in porous soils
         and in water contaminated with nitrogenous matter, and
         is supposed to be due to the presence of an organized
         ferment or ferments, called nitrification ferments. In
         former times the process was extensively made use of in
         the production of saltpeter.

Source : WordNet®

nitrification
     n 1: the chemical process in which a nitro group is added to an
          organic compound (or substituted for another group in an
          organic compound)
     2: the oxidation of ammonium compounds in dead organic material
        into nitrates and nitrites by soil bacteria (making
        nitrogen available to plants)