1. Soil Phosphorus Storage Capacity:
An Innovative Technology for Water Quality
Protection in Agroforestry Systems
Vimala Nair
Soil and Water Science Department, Univ. of Florida
Co-authors: PKR Nair, R Mosquera-Losada, G-A Michel,
D Howlett, and P Nkedi-Kizza
February 12, 2014
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2. Outline…
• Phosphorus Saturation Ratio (PSR)
• PSR and the Soil P Storage Capacity (SPSC)
– Determination of SPSC
• Application of the PSR/SPSC tool for water quality
protection in tree-based agricultural systems
– Silvopastures in Florida (Ultisols/Spodosols)
– Coffee Plantations in Uganda (Oxisols)
– Dehesa in Spain (Alfisols)
– Simulated Silvopastures in Spain (Inceptisols)
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3. Phosphorus Saturation Ratio (PSR)
25
PSR = Ex-P/ [ExFe + ExAl]
Ex = Oxalate, Mehlich 1 or
Mehlich 3
WSP, mg/kg
20
Threshold PSR
“change point”
15
10
5
0
0.1
0.2
0.3
PSR
0.4
0.5
The “change point” is determined statistically
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4. Threshold PSR
Water Soluble P (mg kg -1 )
25
Surface Horizon
20
• Ex-P/ [ExFe + ExAl]
(Ex = Extractable)
• Change point = 0.10
(Confidence interval = 0.05 – 0.10)
Subsurface Horizon
15
Threshold PSR
10
5
0
0
0.125
0.25
0.375
0.5
PSR
OX
Nair, V.D., K.M. Portier, D.A. Graetz, and M.L. Walker. 2004. J. Environ. Qual. 33:107-113
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5. The Approach – SPSC
SPSC = (0.1 – Soil PSR ) *
–
Oxalate Fe
56
Oxalate Al
* 31
27
(mg P kg-1)
Sink when soil PSR < 0.1 (positive SPSC)
Source when soil PSR > 0.1 (negative SPSC)
• SPSC can also be expressed in mmoles P kg-1, or kg P ha-1
• SPSC is additive; SPSC for horizons within a sandy soil can be
added providing a single value for a designated depth
• Used to predict the lifespan of a P application site
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6. SPSC and Water Soluble P (WSP)
600
SPSC (mg kg-1)
400
200
Positive SPSC
n = 604
Negative SPSC
Sink
0
-200 0
20
40
-400
-600
-800
-1000
Source
Water soluble P (mg kg-1)
60
y = -12.2x + 2.54
R² = 0.87
n = 147
• Location: Suwannee River
Basin, Florida, USA
• Soil: Entisols & Ultisols
• When SPSC is positive, soil is a
P sink
• When SPSC is negative, soil is
a P source
Chrysostome, M, V.D. Nair, W.G. Harris, and R.D. Rhue. 2007. Soil Sci. Soc. Am. J. 71:1564–1569.
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7. Soils under Coffee in Uganda
Water Soluble P, mg kg-1
20
15
Threshold PSR
10
5
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
-5
M3-PSR
SPSC (mg kg-1)
200
0
0
5
10
-200
-400
-600
7
Sink
15
20
• Location: Uganda
• Soils: Oxisols
• Coffea canephora and Coffea
arabica plantations in various
parts of Uganda
• Threshold PSR = 0.1
• When SPSC is positive, soil is a
P sink
• When SPSC is negative, soil is
a P source
Negative SPSC
Source
Positive SPSC
y = -26x - 14
R² = 0.76
Water Soluble P (mg kg-1)
Nkedi-Kizza, P. and V.D. Nair (unpublished data)

8. Silvopasture on Ultisols in Florida
Treeless
Pasture
• Location: Suwannee County,
Florida, USA
• Soil: Ultisols
• SPSC calculated to 1 m depth
Silvopasture
Treatment
SPSC
(kg P ha-1)
Silvopasture
342
Treeless pasture
-60*
Michel, G.-A., V.D. Nair, P.K.R. Nair. 2007. Plant Soil. 297:267-276.
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9. Simulated Silvopastures in Spain
• Location: Castro de Rey Farm,
Galicia, Spain.
• Soil: Inceptisols
• Mean SPSC in different soil
depths up to 100 cm between
pooled Betula alba and Pinus
radiata silvopasture treatments
versus pasture
Sand/Silt/Clay
Howlett, 2007
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10. Dehesa System, Extremadura, Spain
• Location: St. Esteban Farm,
Extremadura, Spain
• Soils: Alfisols
• SPSC in different soil depths
up to 100 cm at 2, 5, and 15
meters from individual Quercus
suber trees
Sand/Silt/Clay
Howlett, 2007
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11. Silvopasture on Spodosols in Florida
Soil P storage Capacity (SPSC), kg ha -1
-300 -200 -100
0
100 200 300 400
0
10
20
Pasture with trees
Depth, cm
30
40
Pasture with trees
Pasture without trees
50
60
Additional SPSC under
pasture with trees
70
80
90
Pasture without trees
100
Soil P storage capacity (SPSC) of soil profiles to a meter depth in a pasture with trees vs. a bahiagrass
pasture without trees.
Nair, V.D., P.K.R. Nair, R. S. Kalmbacher, and I.V. Ezenwa. 2007. Ecological Engineering 29:192-199.
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12. Summary & Conclusions
• A threshold PSR identifies the soil PSR at which the waterextractable P begins to increase
• SPSC is calculated based on a threshold PSR value
• When SPSC is positive (below the threshold PSR), soil is a P sink;
when SPSC is negative (above the threshold PSR), soil is a P source
• The PSR/SPSC approach can be used to predict P storage and loss
from agriculture and agroforestry systems
• In the presence of trees, the P storage within a soil profile is greater
than that without any trees under comparable ecological settings
“Trees for Life“:
Trees remove excess P from soils
Tree-based agricultural systems provide a greater environmental
service in regard to water quality protection compared to treeless ones
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13. Thank You!
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