Risk of P Movement from Soils of the Suwannee River Basin

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