A PHOSPHATE AVAILABILITY INDEX-(F) FOR OPTIMIZING A SOIL PHOSPHATE REQUIREMENT RATING

A phosphate (P) availability index ''F'' (F) is proposed as a paramete r to evaluate the fraction of P added to soil which remains actually a vailable after a given time. A quick test to measure the F is also dis cussed. Twenty soils from the typical Mediterranean pedoclimatic envir onments, with contrasting chemical and physical properties, were utili zed 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) re actants. The F were given by the values of the b slopes of the respect ive linear regressions: extracted-P = a+b added-P, obtained by the var ious soil+P equilibrium procedures and the two P extraction methods. T he F were compared among them as well as with the classical Sorption I ndex (SI) parameter. The F obtained after 16 and 48 hours or 60 days b y Olsen-P measurements (F-Olsen) ranged from 0.25 to 0.75, 0.19 to 0.5 6, and 0.17 to 0.56, with mean values of 0.55, 0.38, and 0.36, respect ively. 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, a nd 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 correla ted 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 stric tly correlated, and practically coincident, to those calculated after 60 days, but not with the respective SI. Differently, the F-M3 after 1 6 hours showed the best correlations with both the other F values afte r 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.