Phosphoric acid (0-54-0) is produced by treating finely ground rock phosphate with sulfuric acid or by smelting (burning) rock phosphate with coke and silica in an electric furnace and reacting with water. The sulfuric acid product is called wetprocess or green acid, and is most commonly used in the fertilizer industry. The smelted phosphorus product is called furnace acid, and is not commonly used for fertilizer because of cost, except for some specialty fertilizers.
Agricultural grade phosphoric acid is seldom used for direct application because of its very corrosive properties. Instead, it is used to make other forms of phosphorus fertilizers.
Superphosphoric acid (0-79-0) is made by dehydrating water from phosphoric acid. This product is a mixture of polyphosphate and orthophosphate. It is less corrosive than orthophosphate and can be stored in sub-freezing temperatures. This acid is used to produce ammonium polyphosphate fertilizers.
Ordinary superphosphate (0-20-0 + 12S) is produced by reacting sulfuric acid with finely ground rock phosphate. The highly insoluble phosphorus is converted to monocalcium phosphate monohydrate. This phosphate is about 85-90 percent water soluble. The product also contains about 12 percent sulfur as gypsum. It is an excellent source of phosphorus and sulfur. Previously, this fertilizer was very popular for Nebraska's sandy soils, especially where alfalfa is grown; but its use has declined as higher analysis materials have become available at lower cost.
Concentrated superphosphate (0-46-0) is manufactured by reacting phosphoric acid with finely ground rock phosphate. The granular product is predominately monocalcium phosphate, which is 85-95 percent water soluble.
Concentrated superphosphate has replaced ordinary superphosphate as a major phosphorus fertilizer. It can be used for direct soil application and is often used in bulk blend products.