Introduction

1. Efficacy of Commonly Used Extractants for Determining P Retention Capacity in Florida Soils D. Chakraborty*, V.D. Nair and W.G. Harris Soil and Water Science Department., 106 Newell Hall, Gainesville, FL 32611 Introduction Materials and Methods Conclusions • 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. • 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 + 0.175 M ammonium oxalate solution as extractant. • M1- Fe and Al determined after extraction with a double acid solution ( 0.05 M HCl + 0.0125 M H2SO4) at a 1:4 soil:solution ratio. • M3- Fe and Al were determined by extracting soil with 0.2 M CH3COOH + 0.25 M NH4NO3 + 0.015 M NH4F + 0.13 M HNO3 + 0.001 M EDTA at a 1:8 soil:solution ratio. • 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. Table 1: Mean value of extractable Fe and Al in Ox, M1 and M3 extractants Figure 1: Relationship between Mehlich 3Fe and OxalateFe for Bh and Bt horizons Figure 2: Mehlich 3Fe +Al versus OxalateFe +Al for Bh and Bt horizons † Values within parenthesis are standard deviations Ox: Oxalate; M3: Mehlich 3; M1: Mehlich 1; n: number of soil samples. Results and Discussion • 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 NH4F 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. 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. 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. Figure 3: Relationship between Mehlich 1Fe +Al and Mehlich 3Fe +Al for Bh and Bt horizons Objectives References • 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. • Logan, T. J. 2001. 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. 2004. A capacity factor as an alternative to soil test phosphorus in phosphorus risk assessment. New Zealand J. Agric. Res. 47:491-497. Acknowledgments This research was supported in part by a grant from the Florida Department of Agriculture & Customer Services (FDACS). I would like to thank MyrleneChrysostome, ManohardeepJosan, Solomon Haile and Dawn Lucas for their help at various stages of the work.