The Collaborative International Dictionary of English v.0.48:

Fertilizer \Fer"ti*lizer\, n.
   1. One who fertilizes; the agent that carries the fertilizing
      principle, as a moth to an orchid. --A. R. Wallace.
      [1913 Webster]

   2. That which renders fertile; a general name for commercial
      substances which make plants grow better, as manure,
      guano, phosphate of lime, ammonium nitrate, etc.
      [1913 Webster]

WordNet (r) 3.0 (2006):

fertilizer
    n 1: any substance such as manure or a mixture of nitrates used
         to make soil more fertile [syn: fertilizer, fertiliser,
         plant food]