The Collaborative International Dictionary of English v.0.48:

B-horizon \B-horizon\ n.
   that layer of soil in a well-developed soil lying immediately
   below the A-horizon, and which contains deposits of organic
   matter leached from surface soils.

   Syn: B horizon.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

B-horizon
    n 1: immediately below the A-horizon; contains deposits of
         organic matter leached from surface soils [syn:
         B-horizon, B horizon]