 To determine the efficiency of Ox, M3 and M1 extractants for Fe and Al extractions from Bh and Bt horizons, which in turn will help in predicting the P retention capacity (inferred from Fe and Al concentrations) accurately.  To determine whether soil compositional difference between Bh and Bt horizons alter the efficiency of (Fe+Al ) extraction by the extractants.  To determine the efficiency of Ox, M3 and M1 extractants for Fe and Al extractions from Bh and Bt horizons, which in turn will help in predicting the P retention capacity (inferred from Fe and Al concentrations) accurately.  To determine whether soil compositional difference between Bh and Bt horizons alter the efficiency of (Fe+Al ) extraction by the extractants. Introduction Objectives Materials and Methods Results and Discussion Efficacy of Commonly Used Extractants for Determining P Retention Capacity in Florida Soils D. Chakraborty *, V.D. Nair and W.G. Harris  Logan, T. J Soils and environmental quality. In: Handbook of Soil Science. M. E. Sumner (ed.). CRC Press, Boca Raton, FL, pp. G155–G169. Chapter G6.  Nair, V.D., and W.G. Harris A capacity factor as an alternative to soil test phosphorus in phosphorus risk assessment. New Zealand J. Agric. Res. 47:  Logan, T. J Soils and environmental quality. In: Handbook of Soil Science. M. E. Sumner (ed.). CRC Press, Boca Raton, FL, pp. G155–G169. Chapter G6.  Nair, V.D., and W.G. Harris A capacity factor as an alternative to soil test phosphorus in phosphorus risk assessment. New Zealand J. Agric. Res. 47: References  M1 does not thoroughly extract Fe and Al from Bh and Bt horizons.  M3 and Ox have a 1:1 relationship in Bt horizons in terms of Fe and Al extraction efficiency.  Although M3 has complexing agents (EDTA and F - ) with affinity for metals, it is inefficient in extracting organically-complexed metals in Bh horizons.  Ox is a more efficient metal extractant than M1 or M3 for organically-complexed metals.  Compositional difference between Bh and Bt horizons results in different metal (Fe and Al) release characteristics.  Therefore, for accurate determination of remaining (environmentally “safe”) P retention capacity as calculated using PSR threshold (Nair and Harris, 2004) it is preferable to use Ox for Bh and M3 for Bt horizons.  M1 does not thoroughly extract Fe and Al from Bh and Bt horizons.  M3 and Ox have a 1:1 relationship in Bt horizons in terms of Fe and Al extraction efficiency.  Although M3 has complexing agents (EDTA and F - ) with affinity for metals, it is inefficient in extracting organically-complexed metals in Bh horizons.  Ox is a more efficient metal extractant than M1 or M3 for organically-complexed metals.  Compositional difference between Bh and Bt horizons results in different metal (Fe and Al) release characteristics.  Therefore, for accurate determination of remaining (environmentally “safe”) P retention capacity as calculated using PSR threshold (Nair and Harris, 2004) it is preferable to use Ox for Bh and M3 for Bt horizons. Conclusions This research was supported in part by a grant from the Florida Department of Agriculture & Customer Services (FDACS). I would like to thank Myrlene Chrysostome, Manohardeep Josan, Solomon Haile and Dawn Lucas for their help at various stages of the work. Acknowledgments Hypothesis  Soil compositional difference plays a role in the nature of metal complexation in Bh and Bt horizons and thus affects the amount of Fe and Al extracted by any particular extractant.  Six Spodosol sites and four Ultisol sites located in South Florida were sampled by horizon.  pH of the soils determined using 1:2 soil:water ratio.  Ox- Al and Fe were determined using a 0.1 M oxalic acid M ammonium oxalate solution as extractant.  M1- Fe and Al determined after extraction with a double acid solution ( 0.05 M HCl M H 2 SO 4 ) at a 1:4 soil:solution ratio.  M3- Fe and Al were determined by extracting soil with 0.2 M CH 3 COOH M NH 4 NO M NH 4 F M HNO M EDTA at a 1:8 soil:solution ratio.  Six Spodosol sites and four Ultisol sites located in South Florida were sampled by horizon.  pH of the soils determined using 1:2 soil:water ratio.  Ox- Al and Fe were determined using a 0.1 M oxalic acid M ammonium oxalate solution as extractant.  M1- Fe and Al determined after extraction with a double acid solution ( 0.05 M HCl M H 2 SO 4 ) at a 1:4 soil:solution ratio.  M3- Fe and Al were determined by extracting soil with 0.2 M CH 3 COOH M NH 4 NO M NH 4 F M HNO M EDTA at a 1:8 soil:solution ratio. Table 1: Mean value of extractable Fe and Al in Ox, M1 and M3 extractants † Values within parenthesis are standard deviations Ox: Oxalate; M3: Mehlich 3; M1: Mehlich 1; n: number of soil samples.  Al plays a more vital role than Fe in P retention (Table 1).  M3 has very poor Fe extraction efficiency compared to Ox for both Bh and Bt horizons. (Fig 1).  Ox is more efficient in dissolving organically bound Al+Fe in Bh horizons compared to M3 (Fig 2).  M3 has high affinity for Al in Bt horizons due to the presence of NH 4 F in the extractant (Fig 2).  M1 has least extraction efficiency of Al+Fe in Bh and Bt horizons compared to M3 and Ox (Fig 3 and 4).  Al and Fe are generally associated with organic matter in the form of organo-metal complexes in Bh horizons whereas in Bt, Al and Fe mainly exists in inorganic forms.  Different nature of association of metals in Bh and Bt horizons alters the extractability of the commonly used extractants as observed in Fig 4.  Al plays a more vital role than Fe in P retention (Table 1).  M3 has very poor Fe extraction efficiency compared to Ox for both Bh and Bt horizons. (Fig 1).  Ox is more efficient in dissolving organically bound Al+Fe in Bh horizons compared to M3 (Fig 2).  M3 has high affinity for Al in Bt horizons due to the presence of NH 4 F in the extractant (Fig 2).  M1 has least extraction efficiency of Al+Fe in Bh and Bt horizons compared to M3 and Ox (Fig 3 and 4).  Al and Fe are generally associated with organic matter in the form of organo-metal complexes in Bh horizons whereas in Bt, Al and Fe mainly exists in inorganic forms.  Different nature of association of metals in Bh and Bt horizons alters the extractability of the commonly used extractants as observed in Fig 4. Figure 3: Relationship between Mehlich 1 Fe +Al and Mehlich 3 Fe +Al for Bh and Bt horizons Figure 1: Relationship between Mehlich 3 Fe and Oxalate Fe for Bh and Bt horizons Figure 2: Mehlich 3 Fe +Al versus Oxalate Fe +Al for Bh and Bt horizons Figure 4: Comparison among mean extractable Fe and Al by Mehlich 3(M3), Oxalate(Ox) and Mehlich 1(M1) extractions for Bt and Bh horizons. Soil and Water Science Department., 106 Newell Hall, Gainesville, FL  Excess Phosphorus (P) application in soils can lead to eutrophication of surface water (Logan, 2001).  P in sandy soils can be retained in Bh and Bt horizons.  In acidic soils Fe and Al oxides play a vital role in P sorption and P retention; Fe+Al is used as a surrogate for P retention.  Environmental risk of P loss from soils abruptly increases above a threshold molar ratio of P/(Al+Fe), called “P Saturation Ratio” (PSR) (Nair et al., 2004).  The amount of P that can be added prior to reaching this ratio can be calculated as the remaining “safe” capacity (Nair and Harris, 2004).  Al and Fe content in soil can be determined in Oxalate (Ox), Mehlich 1 (M1) or Mehlich 3 (M3) extractions.  Ox extraction is not widely used in Florida due to difficulties in the extraction procedure.  M1 and M3 are more common soil tests that are used.  It is vital to extract all metals associated with P retention in determining retention capacity.  Excess Phosphorus (P) application in soils can lead to eutrophication of surface water (Logan, 2001).  P in sandy soils can be retained in Bh and Bt horizons.  In acidic soils Fe and Al oxides play a vital role in P sorption and P retention; Fe+Al is used as a surrogate for P retention.  Environmental risk of P loss from soils abruptly increases above a threshold molar ratio of P/(Al+Fe), called “P Saturation Ratio” (PSR) (Nair et al., 2004).  The amount of P that can be added prior to reaching this ratio can be calculated as the remaining “safe” capacity (Nair and Harris, 2004).  Al and Fe content in soil can be determined in Oxalate (Ox), Mehlich 1 (M1) or Mehlich 3 (M3) extractions.  Ox extraction is not widely used in Florida due to difficulties in the extraction procedure.  M1 and M3 are more common soil tests that are used.  It is vital to extract all metals associated with P retention in determining retention capacity. P sourcenpHOx-AlOx-FeM3-AlM3-FeM1-AlM1-Fe (moles) Bh1905.8(1.3) †63(44)3(4)40(18)1(2)21(14)0.3(0.4) Bt1304.8(0.7)24(14)5(3)31(13)0.5(0.3)6(3)0.11(0.1)