Risk of P Movement from Soils of the Suwannee River Basin
W.G. Harris1, V.D. Nair1, R.D. Rhue1, D.A. Graetz1, R.S. Mylavarapu1
C.C. Truman2
1 Soil and Water Science Department, University of Florida2 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. SubsurfaceLower Suwannee Basin
Important factor inpotential 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: HighErosion: LowLeaching: LowDenitrification: High
P loss risk: High
Subsurface Drainage (Karst)(Flow in soil zone mainly vertical)
E E
Bt
Entisol Ultisol
10 m
Runoff: LowErosion: LowLeaching: VariableDenitrification: 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
y = -958.09x + 196.97
R2 = 0.8412
-4000
-3500
-3000
-2500
-2000
-1500
-1000
-500
0
500
1000
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
P concentration, mg L-1
Ca
pa
cit
y f
or
sa
tura
tio
n r
ati
o 0
.15
, k
g P
/ha
/cm
Remaining “Safe P Storage Capacity” for sandy soils- SPSC
(PSR0 – Soil PSR)(Oxalate Fe + Al)
0-20 -20 0 2020 40 60 -40-60
Minimally-impacted dairy Heavily-impacted dairy
2 m
“Safe” P Storage Capacity (mg kg-1)
Modeling P Retardation:RPA = “Relative P Adsorption Capacity”
CapacityApproach
RetardationApproach
Captures previous loading effect?
Captures time factor?
Works for naturally phosphatic soils?
Yes Not well
Yes
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
