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AbstractThis project is a continuation of an ongoing phosphorus (P) fertility evaluation that began in 1998, at Safford, AZ on a grower cooperator field. The field study conducted during the 2003 season consisted of five treatments of varying rates of P fertilizers from 0 through 165 lbs. of P₂O₅ per acre. Treatments were arranged in a randomized complete block design with three replications. Plot sizes were 12, 36 in. rows wide and extended the full length of the irrigation run of 1250 feet. Three of the five treatments (treatments 3, 4, and 5) consisted of varying rates of P fertilizer applied as a liquid blend (11-30-0-5S) proprietary to United Agri Products (UAP) injected in the soil at approximately the 4 leaf stage prior to the first post plant irrigation. An additional treatment (treatment 2) of calcium thiosulfate (CATS) proprietary to Tessenderlo-Kerley was also applied. In an effort to ensure uniform application of fertilizer nitrogen (N) across all treatments, a solution of urea-ammonium-nitrate (UAN) was applied to bring uniform the level of all treatments with respect to N fertilizer. Lint yield results indicate a strong positive response to applied P, consistent with the previous fours years of P fertility work done in this area. The highest applied P treatments resulted in the highest lint yields with an increase of approximately 155 lbs. lint/acre above the control, an increase of about 12%. Fiber quality data did not indicate any significant differences among treatments for any of the fiber quality parameters measured. Compiling all yield data from the prior six years and plotting percent relative yield as a function of applied P fertilizer demonstrates a positive response to P fertilization. These results indicate the need for local growers to consider P fertilization as an important component of their fertility program. This is particularly true for cotton fields that have had very little or no rotation with small grain crops that are commonly fertilized with a P based fertilizer such as 16- 20-0.