Changes of soil phosphorus availability in Italian alfisols as estimated by short-term soil plus phosphorus equilibration procedures using Olsen, Mehlich 3, and paper-strip methods

The soil availability index, "F", showed to be a suitable tool to assess th e fraction of phosphorus (P) added to soil remaining available after a give n time. However, the classical F determination methods are laborious and ti me-consuming; so we devised more quick and simple procedure. This paper dea ls with the reliability of Olsen, Mehlich 3 (M3), and the innovative "iron- impregnated paper-strip" (Pi) methods to evaluate the F index by short-term , double-point soil+P equilibration procedures. Representative Italian Alfi sol were examined: they were characterized by from low to high P Sorption I ndex (SI) values (4.6 to 35.4, mean 22.0) and from low to sufficient availa ble Olsen-P contents (2.0 to 33.0 ppm, mean 11.3). The samples were supplie d with 0 or 50 mg P per kg soil (P0 or P50, respectively), and then incubat ed for 2 or 24 hours (short term soil+P incubation periods), or 90 days (lo ngterm period). At the balance, the available P was extracted by the compar ed methods and the respective F indexes were calculated in relation to the different amounts of available P extracted from P50 and P0 samples. For bot h P0 and P50 samples, and after the different incubation periods, all the c ompared methods provided consistent and homogeneous extractable-P values (p <0.001). The following global conversion equations were calculated: M3-P =- 1.264*1.021 Olsen-P; Pi-P =-1.921*0.916 Olsen-P. For each set of experiment s, the extractable P values increased alter P addition, but decreased the l onger the incubation time: for P50 samples they were 36.9, 29.6, and 22.0 m g P kg(-1) soil on the average after 2 h, 24 h, or 90 d, respectively, whil e for P0 samples they did not vary significantly over time (10.7, 10.0, 9.7 mg P kg(-1) after 2 h, 24 h, or 90 d). The largest reduction in P availabi lity occurred within 2 hours from P addition. For all the methods, the F in dexes were inversely depending on the SI and both the F-2h and F-24h values were directly correlated to the respective F-90d. Our findings show that t he F index determined after the shortest 2h-equilibration time is a quicker and cheaper soil test to evaluate changes in soil P availability.