The Collaborative International Dictionary of English v.0.48:

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.
          [1913 Webster]

   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.
      [1913 Webster]

   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 a bacteria,
         such as members of the genus Azotobacter, formerly
         called nitrification ferments. In former times the
         process was extensively made use of in the production
         of saltpeter.
         [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

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)