NH4OAc-EDTA-EXTRACTABLE PHOSPHORUS AS A SOIL TEST PROCEDURE

Several different extractants may be used to estimate plant-available soil P. Each laboratory utilizes the extractant that best predicts yields on its range of soils as determined by soil test correlations. The Agricultural Extension Service Soil Testing Laboratory of Texas A&M University is the only state-sponsored laboratory that routinely uses acidic ammonium acetateethylenediaminetetraacetic acid (NH4OAc-EDTA) to extract P from all soils. Little information is available comparing the predictive abilities of this solution with other commonly used P extractants. NH4OAc-EDTA was compared with Bray I, Bray V (1:50 soil:extractant), Olsen, and ammonium bicarbonate extractants for their capacities to predict wheat (Triticum aestivum, L. cv “Coker 916‘’) dry matter yields in the greenhouse, critical soil P levels, and plant total P uptake. The three clayey soils studied represented slightly acid (Burleson), highly alkaline (Latium), and strongly acid (Vamont) soil reactions. Phosphorus addition rates of 0, 8, 16, 32, and 64 ppm were utilized in this study. NH4OAc-EDTA-extractable P was most closely correlated with Olsen-, Bray I-, and Bray V-extracted P from the Burleson, Latium, and Vamont soils, respectively. Critical soil P levels for wheat yields varied less with NH4OAc-EDTA than with other extractants. Latium soil required the least added P to achieve the critical level, whereas Vamont soil required the most. Bray I and Bray V best predicted wheat dry matter yields on the soils studied. Bray I also best estimated wheat total P uptake. Although NH4OAc-EDTA did not produce the greatest correlation between dry matter yield and estimated available P on any soil, relationships were still highly significant (R2 = 0.92, 0.97, and 0.80 for Burleson, Latium, and Vamont soils, respectively). A potential advantage of NH4OAc-EDTA is that extractable cations can be determined from the same extract.