soil phosphorus issues miguel cabrera and david radcliffe crop and soil sciences university of...
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Soil Phosphorus Issues
Miguel Cabrera and David Radcliffe
Crop and Soil SciencesUniversity of Georgia
Intended Outcomes• Understand how P affects water
quality• Understand how P behaves in soil• Understand why manures present
a special problem with P• Understand how to use the
Phosphorus Index
How P Affects Water Quality• High levels of nutrients cause accelerated eutrophication
(algal blooms)• P controls eutrophication in fresh water• Lakes are more sensitive than streams• Eutrophication can increase:
• aquatic weeds• algal growth• low oxygen levels,• fish kills, • bad-tasting water (geosmin)
Natural Process Accelerated by land use
Accelerated Eutrophication• Lake in Canada• Divided by plastic curtain• For 8 years
– Nitrogen added each year to one side
– Nitrogen and phosphorus added to other side
• Every year there was an algal bloom in response to adding phosphorus
• www.umanitoba.ca/institutes/fisheries/eutro.html
How P Affects Water Quality• High levels of nutrients cause accelerated eutrophication• P controls eutrophication in fresh water• Lakes are more sensitive than streams• Eutrophication can increase:
• aquatic weeds• algal growth• low oxygen levels,• fish kills, • bad-tasting water (geosmin)
How P Affects Water Quality
Summer 2007
Georgia Lakes with P TMDLs
• Lake Lanier (2006)• Lake Allatoona (2006)• Carters Lake (2006)• Lake Walter F George (2006)
Intended Outcomes
• Understand how P affects water quality
• Understand how P behaves in soil• Understand why manures present a
special problem with P• Understand how to use the
Phosphorus Index
How P Behaves in Soil
Fe, Alhydrous
oxides
PP P PPP
P
P
PP P
PP
P
P
P
P
PP
PP P
P
P P
P
PP
P
P
PP
P
PP
P
P
P
P
Root hair
Time after P application
P
P
PP
P
P
P
P
P
P
How P Behaves in Soil
0 50 100 150 2000
10
20
30
40
Soil test P (ppm)
Soi
l dep
th (
inch
es)
No manure
40 lb P/acre/yr
90 lb P/acre/yr
110 lb P/acre/yr
How P Behaves in Soil
Mehlich-3 soil P (ppm)
0
0.5
1
1.5
2
0 200 400 600
Dis
solv
ed P
in s
urfa
ce r
unof
f (p
pm)
220
175
Surface runoff
Intended Outcomes
• Understand how P affects water quality
• Understand how P behaves in soil• Understand why manures present a
special problem with P• Understand how to use the
Phosphorus Index
Why Manures Present a Problem
• Soil P levels high enough to cause runoff P concentrations > 1 ppm unlikely with fertilizer
• Cost discourages over-application of P
• Over-application of P occurs with manures
• N-to-P ratio in manures is not the same as what crops need
Why Manures Present a Problem
• Most crops need about 8 lbs available N for every lb of P• N:P ratio of 8:1
• In typical broiler litter there are about 35 lb of available N and 30 lb of P per ton of litter• N:P = 1.2:1• For every 8 lb of N, litter supplies 6.7 lb of P
• 6.7 times as much P as crop needs
N:P Ratios in Manures
Type of Manure Available N:P Ratio
Over-application of P
Anaerobic Swine Lagoon
2.9 2.8 times crop needs
Anaerobic Dairy Lagoon
2.0 4.0 times crop needs
Anaerobic Layer Lagoon
4.5 1.7 times crop needs
Broiler Litter 1.2 6.7 times crop needs
Cycling of P: Before World War II
Grain P
Manure P
Grain P
Manure P
Cycling of P: After World War II
Manure PManure P
P RockP Rock
Grain P
Manure PManure P
Regional Imbalance of P
• Typical poultry ration contains 60% corn
• Less than 10% of the corn used in Georgia poultry feed is produced in the state
• Georgia imports 135 million bushels of corn per year
• Nearly 8,000 tons P imported each year
Long-term Solution: Close the Loop
• One way to restore the P cycle would be to feed more locally-grown grain
• Return manure to these fields
• Expand acreage for manure application
• Corn acreages could be expanded
• But corn has problems with drought-tolerance, aflatoxin, and pests
Long-term Solution: Close the Loop
New pearl millet hybrid may be a promising feed grain
Long-term Solution: Close the Loop
• New pearl millet hybrid is called “Tifgrain 102”
• Can be used as a substitute for corn grain in poultry, cattle, swine, and dairy rations
• Check with county agent about local markets
• http://pubs.caes.uga.edu/caespubs/pubcd/B1216.htm
Short-term Solution: BMP’s
• Alter feed ration to reduce P in manure• Add alum to waste water• Don’t apply manure when runoff is likely• Incorporate or inject manures• Avoid artificially drained fields• Grass filters and stream-side buffers• Use practices that reduce runoff and
erosion
Objectives
• Understand how P affects water quality
• Understand how P behaves in soil• Understand why manures present a
special problem with P• Understand how to use the
Phosphorus Index
Possible Phosphorus Pathways
Soluble P
Particulate P
Soluble P
Overall Structure
Soluble Phosphorus in Surface Runoff
Particulate Phosphorus in Surface Runoff
Soluble Phosphorus in Leachate
+
+
Phosphorus Index
Soluble Phosphorus in Surface Runoff
STPOrg. P
Inorg. P
DRP
Soluble Phosphorus in Surface Runoff
Soil Test P mg P/L
Organic P mg P/L
Inorganic P mg P/L+ + X Runoff (L)
Manure Water Soluble Factor
Poultry litter (with alum) 0.15
Poultry litter (no alum) 0.30
Layer manure 0.40
Dairy manure 0.65
Swine manure 0.40
Layer slurry 0.40
Dairy slurry 0.65
Swine slurry 0.25
Curve Number
55 60 65 70 75 80 85
Ye
arly
Ru
no
ff (m
m)
0
50
100
150
200
250
300
Yearly Runoff Estimation
Soluble Phosphorus in Surface Runoff
Soil Test P mg P/L
Organic P mg P/L
Inorganic P mg P/L+ + X Runoff
X Buffer Effect
Buffer Width (ft)
0 20 40 60 80 100
Buf
fer
Fa
ctor
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Effect of Buffer Width
If STP < 450 BF = e(-0.042 Width)
If STP > 450 BF = 1
Overall Structure
Soluble Phosphorus in Surface Runoff
Particulate Phosphorus in Surface Runoff
Soluble Phosphorus in Leachate
+
+
Phosphorus Index
BAP
Particulate Phosphorus in Surface Runoff
Sediment Loss (RUSLE) (tons/ac/yr)
Bioavailable P in sediment (mg/kg)
Particulate Phosphorus in Surface Runoff
x
Sediment Loss (RUSLE) (tons/ac/yr)
Bioavailable P in sediment (mg/kg)
X Buffer Effect
Particulate Phosphorus in Surface Runoff
x
Overall Structure
Soluble Phosphorus in Surface Runoff
Particulate Phosphorus in Surface Runoff
Soluble Phosphorus in Leachate
+
+
Phosphorus Index
STP Org. PInorg. P
DRP
Soluble Phosphorus in Leachate
DRPDRP
Soil Test P mg P/L
Organic P mg P/L
Inorganic P mg P/L+ + X Leachate
Volume
Soluble Phosphorus in Leachate
Estimation of Leachate Volume
Leachate Vol (mm) = (P - 0.4s)2/(P + 0.6s) x 25.4
P = Annual precipitation = 50 inchess = (1000/CN) – 10CN = curve number for percolation
Williams and Kissel (1991)
Hydrologic Group A B C D
CN for Percolation 28 21 17 15
Ye
arl
y L
ea
cha
te (
mm
)
200
300
400
500
600
700
A DB C
Hydrologic Soil Group
Volume of Yearly Leachate
1250 mm rain
Soil Test P mg P/L
Organic P mg P/L
Inorganic P mg P/L+ + X Leachate
Volume
Soluble Phosphorus in Leachate
Depth to Water Table Factor
Depth to Water Table Factor
0.0 0.2 0.4 0.6 0.8 1.0
Dep
th to
Wat
er T
able
(ft)
0
2
4
6
8
Depth to Water Table Factor
Overall Structure
Soluble Phosphorus in Surface Runoff
Particulate Phosphorus in Surface Runoff
Soluble Phosphorus in Leachate
+
+
Phosphorus Index
How to Use the P-Index
• Use P Index (Excel spreadsheet)
• P index =
Risk of soluble P in runoff
+ Risk of particulate P in runoff
+ Risk of soluble P in leachate
Phosphorus Index Inputs
• Phosphorus Sources
• Phosphorus Transport
• Best Mgmt. Practices (BMP’s)
P Index Input: P Sources
• Soil P (soil test P)
• Fertilizer P
• Manure P
P Index Inputs: Soil Test P
• Lab analysis: Mehlich 1
– UGA Soil Test Lab uses this method
• Other labs may use Mehlich 3
• The two methods give different numbers and there is no good way to convert Mehlich 3 to Mehlich 1
• Be sure your lab is using Mehlich 1 !
P Index Inputs: Manure P• Soil test lab analysis of dry and liquid manure will give
P2O5 in lbs– Dry manure: lb per ton– Liquid manure: lb per acre-in or lbs per 1,000 gal
• If you don’t use lab analysis, P2O5 content must be calculated– Example 1: Apply 5 ton of poultry manure/acre– Assume 1.5% P content for manure– 1.5% x 2,000 lb = 30 lb P per ton– 30 lb/ton x 5 ton/A x 2.29 = 344 lb P2O5 per acre– 2.29 conversion factor for converting P to P205
P Index Inputs Summary
• Manure/Fertilizer information– Source, application method, and rate– From manure test reports: P2O5 content
• Soil information– From soil test reports: STP– From county soil survey map: soil series
(hydrologic group) and depth to water table– From NRCS office: curve number and erosion rate– Curve number can also be obtained from TR-55
using soil series and management
P Index CategoriesP index range
Category Interpretation
0 to <40 Low N-based NMP is usually satisfactory.
40 to <75 Medium Use conservation practices and P applications that maintain a P Index < 75.
75 to <100 High Add conservation practices or reduce P applications to achieve a P Index < 75.
100 or greater
Very
High
Add conservation practices or reduce P applications to achieve a P Index < 100 in the short term. Develop a management plan with the goal of achieving a P Index < 75 within 5 years.
P INDEX VALUE
0 20 40 60 80 100 120 140
AN
NU
AL B
AP
LO
SS
(kg
P/h
a)
0
2
4
6
8
10
12
Fort Valley 1995
W.Unit 1997
Eatonton 1995
Eatonton 1996
Broiler Litter Applications
LOW MEDIUM HIGH V. HIGH
Example # 1 2 3 4 5
Manure Broiler Broiler
Crop Pasture Pasture
STP (lb/acre) 450 450
P2O5 (lb/acre) 344 344
Manure method Dec-Feb Dec-Feb
Soil series Cecil Cecil
Curve # 70 75
Erosion (T/acre) 0.1 0.1
Water table (ft) 15 15
BMPs none none
P index 100 148
Alternatives Time of application or rate
Example # 1 2 3 4 5
Manure Broiler Broiler Broiler
Crop Pasture Pasture Corn
STP (lb/acre) 450 450 20
P2O5 (lb/acre) 344 344 275
Manure method Dec-Feb Dec-Feb <30 days
Soil series Cecil Cecil Cecil
Curve # 70 75 80
Erosion (T/acre) 0.1 0.1 2.0
Water table (ft) 15 15 3
BMPs none none none
P index 100 148 63
Alternatives Time of application or rate
OK
Example # 1 2 3 4 5
Manure Broiler Broiler Broiler Broiler
Crop Pasture Pasture Corn Corn
STP (lb/acre) 450 450 20 400
P2O5 (lb/acre) 344 344 275 275
Manure method Dec-Feb Dec-Feb <30 days <30 days
Soil series Cecil Cecil Cecil Cecil
Curve # 70 75 80 80
Erosion (T/acre) 0.1 0.1 2.0 2.0
Water table (ft) 15 15 3 3
BMPs none none none none
P index 100 148 63 93
Alternatives Time of application or rate
OK Buffer, time of application, or rate
Example # 1 2 3 4 5
Manure Broiler Broiler Broiler Broiler Dairy
Crop Pasture Pasture Corn Corn Pasture
STP (lb/acre) 450 450 20 400 400
P2O5 (lb/acre) 344 344 275 275 227
Manure method Dec-Feb Dec-Feb <30 days <30 days Sprinkler
Soil series Cecil Cecil Cecil Cecil Cecil
Curve # 70 75 80 80 80
Erosion (T/acre) 0.1 0.1 2.0 2.0 2.0
Water table (ft) 15 15 3 3 15
BMPs none none none none None
P index 100 148 63 93 84
Alternatives Time of application or rate
OK Buffer, time of application, or rate
Summary• Excess P causes fresh water eutrophication• Build up of soil test P leads to P in runoff• Manures based on N supply too much P
• P imported in corn and soybean for rations• Growing pearl millet may help close P loop
• P Index is used to assess risk of P loss• Input: soil test P, fertilizer P, manure P • Input: soil series, curve number, erosion loss• BMP’s can be used to reduce P Index value
Reference Web Sites
• AWARE – www.engr.uga.edu/service/extension/agp2/
aware/newtools.php– P index calculator – Excel spreadsheet– P index documentation – pdf file
• NRCS– http://www.wsi.nrcs.usda.gov/products/W2
Q/H&H/Tools_Models/WinTR55.html– TR 55 for determining curve #’s