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.
Polyphosphates consist of a series of orthophosphates that have been chemically joined together.
Upon contact with soils, polyphosphates revert back to orthophosphates. The P in these products is present in both the orthophosphate and polyphosphate form. The P present in these two fertilizers is present in the orthophosphate form. The cost of converting rock phosphate to the individual phosphate fertilizers varies with the process used. More importantly, the processes used have no effect on the availability of P to plants.