superphosphate or superphosphate of lime, Ca(H₂PO₄)₂, is a compound produced by treating rock phosphate with sulfuric acid or phosphoric acid, or a mixture of the two. It is the principal carrier of phosphate, the form of phosphorus usable by plants, and is one of the world's most important fertilizers. Ordinary superphosphate contains about 20% available phosphate; double superphosphate (also called treble superphosphate) contains 40%-50% available phosphate.