A series of field experiments have been conducted across Arizona beginning in 1988 with the most recent trial conducted in 2004. These trials represent over 15 site-years of research investigating the response of upland cotton (Gossypium hirsutum) to the application of phosphorus (P) fertilizers. A wide range of soil P test levels were evaluated across the state during this period of time. Results indicate that the critical level for sodium bicarbonate extractable P is 5 parts per million (ppm). Effective application rates range from 60-90 lbs of P2O5 per acre. The most recent P fertility research has indicated a positive response to utilizing GPS/GIS technologies to make prescription applications of P fertilizers.