Soil Test Calibration Evaluations for Phosphorus on

Numerous field experiments were conducted at a wide range of sites in Arizona from 1988 through 1999 involving phosphate (P) fertilization of cotton (Gossypium spp.). A total of 21 site-years were used to study the effects of P on both Upland (G. hirsutum L.) and Pima (G. barbadense L.) varieties. The purpose of these experiments was to evaluate University of Arizona (UA) soil fertility guidelines with respect to soil test results (NaHCO3 extractable P) and to possibly fine-tune or calibrate these guidelines in relation to soil test P, applied P, and yield for common Arizona soils used in cotton production. Results from these experiments, based on soil test information, plant measurements, and lint yield showed no significant difference (P 0.05) due to treatments for all the studies with the exception of one P study conducted in Graham County in 1998 and another P study conducted in Pinal County in 1999. The 1998 Graham County site had a preseason soil test value of 7.6 ppm NaHCO3 extractable P. The 1999 Pinal County site had a preseason soil test value of 3.0 ppm NaHCO3 extractable P. Analysis of yield results vs. soil P show that soil test P levels greater than 5 ppm are consistently sufficient for both Upland and Pima cotton. Yield results vs. applied P (lbs. P2O5/acre) for both Upland and Pima did not indicate a positive response over the rates of fertilization tested (20-160 lbs. P2O5/acre). Based on the results from these studies, the current UA soil fertility guidelines for P fertilization of cotton appear to be valid. Furthermore, the data indicates that the UA soil fertility guidelines may be further refined to provide the following categories: < 5 ppm = high probability of response to an added P fertilization; 6-10 ppm = medium probability of response to an added P fertilization; and> 10 ppm = low probability of response to an added P fertilization