The Collaborative International Dictionary of English v.0.48:

Fertilize \Fer"ti*lize\, v. t. [imp. & p. p. Fertilized; p.
   pr. & vb. n. Fertilizing.] [Cf. F. fertiliser.]
   1. To make fertile or enrich; to supply with nourishment for
      plants; to make fruitful or productive; as, to fertilize
      land, soil, ground, and meadows.
      [1913 Webster]

            And fertilize the field that each pretends to gain.
                                                  --Byron.
      [1913 Webster]

   2. To fecundate; as, to fertilize flower. --A. R. Wallace.
      [1913 Webster]

WordNet (r) 3.0 (2006):

fertilize
    v 1: provide with fertilizers or add nutrients to; "We should
         fertilize soil if we want to grow healthy plants" [syn:
         fertilize, fertilise, feed]
    2: make fertile or productive; "The course fertilized her
       imagination" [syn: fertilize, fecundate, fertilise]
    3: introduce semen into (a female) [syn: inseminate,
       fecundate, fertilize, fertilise]

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "fertilize":
   backset, compost, cross-fertilize, cross-pollen, cross-pollinate,
   cross-pollinize, cultivate, culture, cut, delve, dig, dress,
   enrich, fallow, fatten, fecundate, fecundify, feed, force,
   fructify, get with child, get with young, harrow, hoe, impregnate,
   inseminate, list, manure, mulch, nourish, plow, pollen, pollinate,
   pollinize, prolificate, prune, rake, spade, spermatize, thin,
   thin out, till, till the soil, weed, weed out, work