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 the fraction of phosphorus (P) added to soil remaining available after a given time. However, the classical F determination methods are laborious and time-consuming; so we devised more quick and simple procedure. This paper deals 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 Alfisol were examined: they were characterized by from low to high P Sorption Index (SI) values (4.6 to 35.4, mean 22.0) and from low to sufficient available Olsen-P contents (2.0 to 33.0 ppm, mean 11.3). The samples were supplied with 0 or 50 mg P per kg soil (P0 or P50, respectively), and then incubated for 2 or 24 hours (short term soil+P incubation periods), or 90 days (long-term period). At the balance, the available P was extracted by the compared methods and the respective F indexes were calculated in relation to the different amounts of available P extracted from P50 and P0 samples. For both P0 and P50 samples, and after the different incubation periods, all the compared 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 experiments, the extractable P values increased after P addition, but decreased the longer the incubation time: for P50 samples they were 36.9, 29.6, and 22.0 mg P kg-1 soil on the average after 2 h, 24 h, or 90 d, respectively, while 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 availability occurred within 2 hours from P addition. For all the methods, the F indexes 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 the F index determined after the shortest 2h- equilibration time is a quicker and cheaper soil test to evaluate changes in soil P availability.