Concentrated superphosphate is made at the TVA Fertilizer Works at Wilson Dam by mixing 78% phosphoric acid from the electric furnace process with rock phosphate and curing the fresh superphosphate by storage in large piles for several weeks; the cured superphos-INDUSTRIAL AND ENGINEERING CHEMISTRY Vol. 37, Nx 9* Average weighted on basis of superphosphate produced each month. » Determined as perchloric acid-insoluble material.___ 6 In addition, there are a number of other minor constituents. A typical sample of plant acid contained 0.18% K,