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

Root hair

Time after P application

0 50 100 150 2000

Soil test P (ppm)

No manure

40 lb P/acre/yr

90 lb P/acre/yr

110 lb P/acre/yr

Mehlich-3 soil P (ppm)

0 200 400 600

Surface runoff

Intended Outcomes

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

Anaerobic Layer Lagoon

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

New pearl millet hybrid may be a promising feed grain

• 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

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

STPOrg. P

Inorg. P

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

Yearly Runoff Estimation

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

Effect of Buffer Width

If STP < 450 BF = e(-0.042 Width)

If STP > 450 BF = 1

Overall Structure

Phosphorus Index

Sediment Loss (RUSLE) (tons/ac/yr)

Bioavailable P in sediment (mg/kg)

Sediment Loss (RUSLE) (tons/ac/yr)

Bioavailable P in sediment (mg/kg)

X Buffer Effect

Overall Structure

Phosphorus Index

STP Org. PInorg. P

DRPDRP

Soil Test P mg P/L

Organic P mg P/L

Inorganic P mg P/L+ + X Leachate

Volume

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

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

Depth to Water Table Factor

0.0 0.2 0.4 0.6 0.8 1.0

Depth to Water Table Factor

Overall Structure

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

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

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

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

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

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

Soil Phosphorus Issues Miguel Cabrera and David Radcliffe Crop and Soil Sciences University of...

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