Risk of P Movement from Soils of the Suwannee River Basin W.G. Harris 1, V.D. Nair 1, R.D. Rhue 1, D.A. Graetz 1, R.S. Mylavarapu 1 C.C. Truman 2 1 Soil and Water Science Department, University of Florida 2 USDA-ARS, Tifton, Georgia USDA-Initiative for Future Agriculture and Food Systems Willie Harris, Vimala Nair, and Dean Rhue

Topics Hydrology – Soils –P (& N) Transport Predicting Vertical P Movement in Soils

30 km Drainage: Surface vs. Subsurface Lower Suwannee Basin Important factor in potential P transport

Surface Drainage of “Flatwoods” (“Surface” entails some shallow subsurface flow in E horizon, which is minimally retentive of P) E Bh Spodosol Aquitard 2 m Runoff: High Erosion: Low Leaching: Low Denitrification: High P loss risk: High

Subsurface Drainage (Karst) (Flow in soil zone mainly vertical) E E Bt EntisolUltisol 10 m Runoff: Low Erosion: Low Leaching: Variable Denitrification: Low P loss risk: Variable

Sand grain coatings, their presence or absence, makes a big difference in P retention capacity

Toward a More Quantitative Site Specific Assessment of Leaching Potential Capacity Approach –What is the remaining capacity? Retardation Approach –How many “pore volumes” before “breakthrough” of elevated P concentrations at a given depth.

Calculated Remaining P Sorption Capacity vs. P Concentrations in Porewater Remaining “Safe P Storage Capacity” for sandy soils- SPSC (PSR 0 – Soil PSR)(Oxalate Fe + Al)

Minimally-impacted dairyHeavily-impacted dairy 2 m “Safe” P Storage Capacity (mg kg -1 )

Modeling P Retardation: RPA = “Relative P Adsorption Capacity”

Capacity Approach Retardation Approach Captures previous loading effect? Captures time factor? Works for naturally phosphatic soils? YesNot well Yes Indirectly No Leaching Assessment Approaches are Complementary

Summary of Findings, to Date 1.Predicted vs. observed P movement - reasonably close. 2.Typical “safe lifespan” of application site - a few years *. 3.Phosphate effects realized later than nitrate effects. (BUT …) 4.Phosphate effects realized longer than nitrate effects. * A loamy horizon (Bt) could extend this greatly, barring preferential flow.

Acknowledgements Technical staff: Bill Reve, Greg Means, Dawn Lucas, Keith Hollien Members of the Florida P-Index Committee. Graduate student: Myrlene Chrystostome, Omar Harvey, Daniel Herrera, Rav Ramnarine, Leighton Walker