The Collaborative International Dictionary of English v.0.48:

Basic slag \Basic slag\
   A by-product from the manufacture of steel by the basic
   process, used as a fertilizer. It is rich in lime and
   contains 14 to 20 per cent of phosphoric acid. Called also
   Thomas slag, phosphatic slag, and odorless phosphate.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

basic slag
    n 1: slag produced in making steel; low in silica but having
         large amounts of calcium phosphate; useful as fertilizer