COMPARING TESTS FOR SOIL FERTILITY .3. EVALUATION OF PHOSPHATE AVAILABILITY IN ALFISOLS BY OLSEN, MEHLICH-3, ELECTROULTRAFILTRATION, AND THE PAPER-STRIP METHODOLOGY PROCEDURES

The suitability of the ''iron-impregnated paper-strip'' (Pi) methodolo gy 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 Meh lich No. 3 (M3) extraction procedure. The EUF extractions were perform ed by a first 30 min run at low (L) energy conditions (20 degrees C, m ax 15 mA, and 200V), folIowed by a 10 min run at high (H) energy condi tions (80 degrees 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 Alfi sols (''Terra Rossa'') from the pedoclimatic environments of the Medit erranean area. The soil samples were characterized by a large variabil ity of their availabIe-P values, whose ranges were, as mg . kg(-1) soi l, 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 respe ctive mean values were 4.6, 5.1, 9.6, 17.2, 20.1, and 29.7 mg P/kg soi l, 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 w as verified; the most significant correlations were determined for Pi- P versus M3-P (R-2=0.993**) and for Pi-P versus Olsen-P (R-2=0.988*** ), clearly indicating the liability of the Pi procedure for the assess ment of the P fertility in Alfisols. A multiple linear regression anal ysis revealed that all the available P values, afar from the individua l procedure, were 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 inves tigated Alfisol samples was that connected with the labile Fe-P pool. Withal, one could infer that the Al-M3 and Fe-M3 parameters should als o be considered for the making of P fertilizer recommendations. P;rom this standpoint, the M3 is a well-proved multielement extractant suita ble for the simultaneous determination of available P and exchangeable Fe and Al as well.