PERFORMANCE INDEXES FOR TESTS OF SOIL NUTRIENT STATUS - EXTRACTABLE PHOSPHORUS

Soil phosphorus (P) extractants are often selected according to the co rrelation or regression between test values and crop performance (e.g. , P uptake and/or yield). Although this criterion is an essential dete rminant of extractant performance, it is often inadequate for evaluati ng whether extractants accurately discriminate between P-deficient and P-sufficient soils, or whether they produce reliable critical level e stimates or repeatable soil P measurements. Four supplementary indices were evaluated that may provide a more direct assessment of extractan t performance. The potential use and reliability of the indices were i nvestigated in an evaluation of four soil P extractants, Modified Truo g, Mehlich 3, Olsen, and ion-exchange resin, using data from a greenho use experiment. Coefficients of determination between relative dry mat ter yield and extractable P failed to identify differences among the e xtractants, ranging from 0.95 to 0.97. Coefficients of determination b etween extractable P and P added ranged from 0.96 to 0.97 except for o ne method at 0.83. The proposed indices, however, produced a ranking o f the extractants related to their performance. The Kappa efficiency ( K-EFF) index indicated that Mehlich 3 provided better detection of P-s ufficient and P-deficient soils than either Olsen or Truog (K-EFF valu es of 0.92, 0.83, and 0.68, respectively). These index values reflect that the extractants correctly detected P deficiency in 17 of 18, 17 o f 18, and 15 of 18 soils. The slight superiority of Mehlich 3 over Ols en was due to its correct detection of 9 of 9 P-sufficient soils while the Olsen and Truog extractants correctly detected 8 of 9 P-sufficien t soils. Further studies are needed, especially field studies, to dete rmine whether these indices accurately reflect the reliability of the extractants for use in diagnosis and recommendation. Because these ind ices directly assess success in identifying deficient and sufficient c onditions, their use in extractant evaluations should provide more spe cific, purposeful evaluations than methods based solely on correlation and regression.