'Basic slag' definitions:

Definition of 'basic slag'

(from WordNet)
noun
Slag produced in making steel; low in silica but having large amounts of calcium phosphate; useful as fertilizer

Definition of 'Basic slag'

From: GCIDE
  • 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.]