Efficacy of Commonly Used Extractants for Determining P Retention Capacity in Florida Soils.

With excessive use of fertilizers, soil phosphorus (P) in agricultural lands has increased leading to eutrophication of surface water. Environmental risk of P loss from soils can be calculated from the P saturation ratio (PSR), the molar ratio of [P] to [Fe and Al]. In acidic soils, Fe and Al oxides play vital roles in P sorption and thus in P retention. Al and Fe content in soils can be determined in Oxalate (Ox), Mehlich-1 (M-1) and Mehlich-3 (M-3) extractions. Oxalate extraction is not widely used in Florida; Mehlich-1 and Mehlich-3 are more commonly used, particularly for determining soil test P. The main objective of this study was to evaluate the P retention capacity as inferred from Fe and Al concentrations in Ox, M-3 and M-1 extractions in Bh and Bt horizons resulting from P movement from the surface horizon through the sandy Florida soil profiles. Several Spodosol and Ultisol sites from Florida were sampled by horizon. The pH was determined using 1:2 soil:water ratio. Oxalate-extractable Fe and Al were determined using 0.1M oxalic acid and 0.175 M ammonium oxalate solution. Soils were analyzed for Fe and Al in Mehlich-1 and Mehlich-3 solutions at 1:4 and 1:8 soil:solution ratios, respectively. The soils were mostly acidic. Results indicated that extraction efficiencies were: Ox _Al+Fe≥ M-3 _Al+Fe> M-1 _Al+Fe. Compared to M-3, Ox is more efficient in dissolving organically bound Al in Bh horizons. On the other hand M-3 has greater affinity for Al in Bt horizons likely due to presence of NH₄F in M-3. However, M-3 has very poor Fe extraction efficiency compared with Ox for both Bh and Bt horizons, where the trend is Ox _Fe>>M-3 _Fe>M-1 _Fe. Soil compositional differences between Bh and Bt horizons result in different metal and P release characteristics.