Comparing tests for soil fertility. III. Evaluation of phosphate availability in Alfisols by Olsen, Mehlich 3, electro-ultrafiltration, and the paper-strip methodology procedures

The suitability of the 'iron-impregnated paper-strip' (Pi) methodology for the assessment of phosphate (P) availability in soils has been tested on the basis of a comparison with the classical Olsen method as well as with the new-generation electro-ultrafiltration (EUF) and Mehlich No. 3 (M3) extraction procedure. The EUF extractions were performed by a first 30 min run at low (L) energy conditions (20°C, max 15 mA, and 200V), followed by a 10 min run at high (H) energy conditions (80°C, max 150 mA, and 400V). The total EUF-P extracted (T-EUF-P), given by the sum L-EUF-P + H-EUF-P, was also considered. The investigation was carried out on twenty samples of representative Alfisols ('Term Rossa') from the pedoclimatic environments of the Mediterranean area. The soil samples were characterized by a large variability of their available-P values, whose ranges were, as mg · kg-1 soil, 0.1-34.0 for L-EUF-P, 0.2-39.3 for HEUF-P, 0.3-73.3 for T-EUF-P, 0.8-113.6 for Olsen-P, 1.0-122.4 for Pi-P, 1.1-224.6 for M3-P. The respective mean values were 4.6, 5.1, 9.6, 17.2, 20.1, and 29.7 rag P/kg soil, with ratios L-EUF-P:H-EUF-P:T-EUF-P:Olsen-P:Pi-P:M3 -P increasing as 0.27:0.29:0.56: 1.00:1.17:1.73. A good compliance among all methods was verified; the most significant correlations were determined for Pi-P versus M3-P (R2=0.993***) and for Pi-P versus Olsen-P (R2=0.988***), clearly indicating the liability of the Pi procedure for the assessment of the P fertility in Alfisols. A multiple linear regression analysis revealed that all the available P values, afar from the individual procedure, wore significantly (P<0.01) dependent on M3 exchangeable iron (Fe) (positively) and aluminum (Al) (negatively). This could lead one to argue that the prevailing available soil P source in the investigated Alfisol samples was that connected with the labile Fe-P pool. Withal, one could infer that the Al-M3 and Fe-M3 parameters should also be considered for the making of P fertilizer recommendations. From this standpoint, the M3 is a well-proved multielement extractant suitable for the simultaneous determination of available P and exchangeable Fe and Al as well.