The Collaborative International Dictionary of English v.0.48:

Topsoil \Top"soil`\, n.
   The upper layer of soil; surface soil.
   [1913 Webster]

WordNet (r) 3.0 (2006):

topsoil
    n 1: the layer of soil on the surface [syn: topsoil, surface
         soil]

Moby Thesaurus II by Grady Ward, 1.0:

64 Moby Thesaurus words for "topsoil":
   acres, alluvion, alluvium, arable land, band, bed, bedding, belt,
   clay, clod, couche, course, crust, deck, dirt, dry land, dust,
   earth, floor, freehold, gallery, glebe, grassland, ground, land,
   landholdings, layer, ledge, level, lithosphere, marginal land,
   marl, measures, mold, overlayer, overstory, real estate,
   real property, region, regolith, seam, shelf, sod, soil, stage,
   step, story, stratum, subaerial deposit, subsoil, substratum,
   superstratum, terra, terra firma, terrain, territory, the country,
   thickness, tier, underlayer, understory, understratum, woodland,
   zone