The Collaborative International Dictionary of English v.0.48:

Fallow \Fal"low\, v. t. [imp. & p. p. Fallowed; p. pr. & vb.
   n. Fallowing.] [From Fallow, n.]
   To plow, harrow, and break up, as land, without seeding, for
   the purpose of destroying weeds and insects, and rendering it
   mellow; as, it is profitable to fallow cold, strong, clayey
   land.
   [1913 Webster]