Superphosphate \Su`per*phos"phate\, n. (Chem.) An acid phosphate.

Superphosphate of lime (Com. Chem.), a fertilizer obtained by trating bone dust, bone black, or phosphorite with sulphuric acid, whereby the insoluble neutral calcium phosphate, Ca3(PO4)2, is changed to the primary or acid calcium phosphate Ca(H2PO4)2, which is soluble and therefore available for the soil.

Source: Webster's Revised Unabridged Dictionary (1913)