A phosphate availability index ("F") for optimizing a soil phosphate requirement rating

A phosphate (P) availability index “F”; (F) is proposed as a parameter to evaluate the fraction of P added to soil which remains actually available after a given time. A quick test to measure the F is also discussed. Twenty soils from the typical Mediterranean pedoclimatic environments, with contrasting chemical and physical properties, were utilized in this investigation. Three different soil+P incubation periods, i.e., 16 and 48 hours and 60 days were considered. After equilibrium, the available P was extracted by both Olsen (Olsen‐P) and M3 (M3‐P) reactants. The F were given by the values of the b slopes of the respective linear regressions: extracted‐P = a + b added‐P, obtained by the various soil+P equilibrium procedures and the two P extraction methods. The F were compared among them as well as with the classical Sorption Index (SI) parameter. The F obtained after 16 and 48 hours or 60 days by Olsen‐P measurements (F‐Olsen) ranged from 0.25 to 0.75, 0.19 to 0.56, and 0.17 to 0.56, with mean values of 0.55, 0.38, and 0.36, respectively. The corresponding F by M3‐P (F‐M3) showed ranges from 0.19 to 0.89, 0.11 to 0.85, and 0.07 to 0.78, with mean values of 0.54, 0.40, and 0.41. The SI ranged from 4.7 to 65.7, with a mean value of 21.7. We found that both the F and the SI parameters were consistently correlated to some soil properties effective in soil‐P adsorption phenomena, with special reference to the cation exchange capacity (CEC). However, the F‐Olsen and the F‐M3 indexes calculated after 48 hours were strictly correlated, and practically coincident, to those calculated after 60 days, but not with the respective SI. Differently, the F‐M3 after 16 hours showed the best correlations with both the other F values after 48 hours (r = 0.938) and 60 days (r = 0.955) and the SI (r = 0.861), thus providing a reliable index for the P availability evaluation.