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TECHNICAL REPORTS

Ecological Risk Assessment

An Environmental Threshold for Degree of Phosphorus Saturation in Sandy Soils

Statistics Department, 522 McCarty Hall, University of Florida, Institute of Food and Agricultural Sciences, Gainesville, FL 32611-0510

^* Corresponding author (vdna{at}ifas.ufl.edu).

Received for publication March 14, 2003. There is critical need for a practical indicator to assess the potential for phosphorus (P) movement from a given site to surface waters, either via surface runoff or subsurface drainage. The degree of phosphorus saturation (DPS), which relates a measure of P already adsorbed by a soil to its P adsorption capacity, could be a good indicator of that soil's P release capability. Our primary objective was to find a suitable analytical protocol for determining DPS and to examine the possibility of defining a threshold DPS value for Florida's sandy soils. Four farmer-owned dairy sprayfields were selected within the Suwannee River basin and soil profiles were randomly obtained from each site, as well as from adjacent unimpacted sites. The soil samples were divided either by horizon or depth, and DPS was determined for each soil sample using ammonium-oxalate (DPS_Ox), Mehlich-1 (DPS_M1), and Mehlich-3 (DPS_M3) extracts. All methods of DPS calculations were linearly related to one another (r² > 0.94). Relationships between water-soluble P and DPS indicate that the respective change points are: DPS_Ox = 20%, DPS_M1 = 20%, and DPS_M3 = 16%. These relationships include samples from Ap, E, and Bt horizons, and various combinations thereof, suggesting that DPS values can be used as predictors of P loss from a soil irrespective of the depth of the soil within a profile. Taking into consideration the change points, confidence intervals, agronomic soil test values, and DPS values from other studies, we suggest replacing Mehlich-1 P values in the Florida P Index with the three DPS categories (DPS_M1 = <30, 30–60, and >60%) to assign different P loss ratings in the P Index.

Abbreviations: DPS, degree of phosphorus saturation • DPS_M1, degree of phosphorus saturation calculated as [M1-P/0.5(M1-Fe + M1-Al)] x 100 • DPS_M3, degree of phosphorus saturation calculated as [M3-P/0.5(M3-Fe + M3-Al)] x 100 • DPS_Ox, degree of phosphorus saturation calculated as [Ox-P/0.5(Ox-Fe + Ox-Al)] x 100 • M1-Al, M1-Fe, and M1-P, Mehlich 1–extractable aluminum, iron, and phosphorus, respectively • M3-Al, M3-Fe, and M3-P, Mehlich 3–extractable aluminum, iron, and phosphorus, respectively • Ox-Al, Ox-Fe, and Ox-P, oxalate-extractable aluminum, iron, and phosphorus, respectively • STP, soil test phosphorus • WSP, water-soluble phosphorus

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