Source : Webster's Revised Unabridged Dictionary (1913)

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}.

Source : WordNet®

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