The Collaborative International Dictionary of English v.0.48:

Top-dress \Top"-dress`\, v. t. [imp. & p. p. Top-dressed; p.
   pr. & vb. n. Top-dressing.]
   To apply a surface dressing of manureto,as land.
   [1913 Webster]

WordNet (r) 3.0 (2006):

topdress
    v 1: scatter manure or fertilizer over (land)