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AbstractManagement of phosphorus (P) fertilizer in Arizona cotton production systems has been studied for many years. A series of field experiments have been conducted across Arizona since 1989 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 L.) to the application of P fertilizers and have been conducted over a wide range of environmental conditions. A summary of these field trials is presented in this paper. All trials were structured in a similar fashion with a control being compared to various rates of applied P fertilizer ranging from 0 to over 120 lbs P₂O₅ per acre. All trials had a similar experimental design with large plots (minimum of 0.16 acres) and treatments arranged in a randomized complete block design with a minimum of three replications in all cases. All applications were made either pre-plant or shortly after stand establishment. A wide range of soil test P levels were evaluated at many locations across the State. Results indicate that the critical level for sodium bicarbonate extractable P is 5 parts per million (ppm) with a 90% probability of a positive lint yield response when soil test levels fall below the critical level. Effective (positive crop response) fertilization rates range from 60-90 lbs of P₂O₅ per acre.