In the past, ground rock phosphate itself has been used as a source of P for acid soils. However, due to low availability of P in this native material, high transportation costs, and small crop responses, very little rock phosphate is currently used in agriculture. Figure 1.The process used in the manufacture of various phosphate fertilizers. The manufacture of most commercial phosphate fertilizers begins with the production of phosphoric acid. A generalized diagram showing the various steps used in the manufacture of various phosphate fertilizers is provided in Figure 1.
Phosphoric acid is produced by either a dry or wet process.
In the dry process, rock phosphate is treated in an electric furnace. Fertilizers that use white phosphoric acid as the P source are generally more expensive because of the costly treatment process. The impurities which give the acid its color have not been a problem in the production of dry fertilizers. The phosphoric acid produced by either the wet or the dry process is frequently heated, driving off water and producing a superphosphoric acid. The P in this acid is present as both orthophosphate and polyphosphate.