Terry L. RobertsPotash & Phosphate Institute

Potash & Phosphate Institute of Canada

Agricultural Phosphorus … Environmental Concerns

Phosphorus in Agriculture Symposium

Rosario, Santa Fe, ArgentinaMay 8-9, 2003

Phosphorus … essential to all life

Managed properly:

Increases soil productivity

Enhances environmental protection

Phosphorus … essential to all life

Managed improperly: Increases environmental risks …

eutrophication of water bodies

Phosphorus … essential to all life

P enrichment of lake water > 0.02 ppm accelerates eutrophication

Phosphorus … essential to all life

Critical concentration in soil solution for crop plants … 0.2-0.3 ppm P

Soil Phosphorus

Strongly adsorbed on surface of soil particles

Forms insoluble compounds with Al, Fe, and Ca

Immobile in soilSome leaching of

soluble P in low-fixing soil

T h e P h o s p h o r u s C y c le

A n im a lm a n u r e s

a n d b io s o lid s M in e r a lfe r t i liz e r s

C r o p h a r v e s t

R u n o f f a n de r o s io n

L e a c h in g( u s u a lly m in o r )

O r g a n ic p h o s p h o r u s• M ic r o b ia l• P la n t r e s id u e• H u m u s

P r im a r ym in e r a ls( a p a t ite )

P la n t r e s id u e s

P la n tu p ta k e

S o i l s o lu t io np h o s p h o r u s• H P O 4

-2

• H 2 P O 4-1

S e c o n d a r yc o m p o u n d s

(C a P , F e P , M n P , A lP )

M in e r a ls u r fa c e s

( c la y s , F e a n d A l o x id e s ,

c a r b o n a te s )

In p u t t o s o i lC o m p o n e n t L o s s f ro m s o i l

A tm o s p h e r icd e p o s i t io n

Soil Phosphorus Loss

Main pathway for loss to surface water is runoff

Runoff carries suspended soil particles, adsorbed P, and some dissolved P

Tillage … dramatic effect in reducing runoff losses

Oklahoma Watershed

Little environmental threat from P

when application rates of fertilizer and manure are based on soil test recommendations

rates do not greatly exceed crop removal

good agronomic practices are employed

Nutrient Management Planning (NMP)

NMP becoming increasingly popular (legislated) in North AmericaEnvironmental incentive programs in 2002

Farm Bill in U.S.CAFO (Concentrated Animal Feeding

Operation) Final Rule (15,500 units) – require NMPs to be implemented by end of 2006

NMPs employed where surplus P threatens water quality … flexible and site-specific

Nutrient Management Planning (NMP)

NMP … science-based, utilize on-farm nutrient sources and employ well-established management practices …

Soil and tissue testing

Manure analysis Proper nutrient

application methods and timing

Conservation tillage Vegetative buffers Riparian zones Other available

technologies

Soil Testing … important component of nutrient management planning

Agronomic value … well recognized and well defined, but environmental use needs refining

Interpreting soil tests for environmental purposes should follow the same process for agronomic interpretation …evaluation of extractantsanalytical methodologycalibration to reflect environmental

impact

Environmental Soil Tests

Calibration and interpretation to predict potential environmental impact … more complicated than predicting crop response

Environmental Soil TestsDetermining nutrient loads that prevent

water quality degradation depends on:proximity to sensitive water bodiesuse of the watersocioeconomic factors of rural land

useother site-specific factors

Environmental Soil Tests

All areas of the landscape do not contribute equally to nutrient loss

All nutrients do not behave similarly … N is easily leached or lost in runoff and may

contribute to low dissolved oxygen (hypoxia)K is environmentally benignP moves slowly, but can leach under the right

conditions if soil levels become excessive

Risk of P loss depends on the nutrient source and mechanism for transport

Threshold P Levels

Topography, soils and cropping systems are diverse … development of a single threshold level unreasonable or inapplicable

Environmental P Index …

Uses soil testing, nutrient application ratesIncorporates erosion, leaching, runoff

potential, and proximity to water bodies to assess areas of potential risk

Developed by USDA-ARS as a screening tool to rank the vulnerability of fields to P loss in runoff

Phosphorus in the Watershed

Sharpley, Gburek, USDA-ARS; Beegle, Penn State University

Soil Test P Distribution

Mehlich-3 P

mg/kg

<30

30-100

>100

Sharpley, Gburek, USDA-ARS; Beegle, Penn State University

Vulnerability to P LossP loss

vulnerabilityLow (clear)

MediumHigh

Sharpley, Gburek, USDA-ARS; Beegle, Penn State University

P index vs. P threshold level

P index … practical means to rate the potential for offsite loss through runoff in areas of excess P or others areas prone to P loss … favored by fertilizer industry

Threshold or critical soil test P level … less favored … concern that restricting P application (starter or manure) on high P soils that pose no environmental threat could negatively impact crop yields

P response on high P soil

Probability of response declines as soil test P increases, but even high testing soil can respond to starter P applicationsoil and climatic stress early in

growing seasonother production factors are optimum

Starter P increased irrigated corn yield and lowered grain

moisture in high P soil in North Carolina

P2O5 rate Yield Grain

kg/ha t/ha moisture, %0 12.0 20.7

65 12.7 18.0Soil test P = highGriffith 1992

Starter P response varies with tillage system

Tillage Corn yield, kg/haSystem -P +P IncreaseMoldboard plow 8,906 9,220 314Chisel plow 8,028 8,718 690Ridge plant 8,342 8,781 439Disk 8,404 8,906 502No-till 5,582 6,272 690

Griffith 1992 Bray P-1 = 22 ppm

How much soil P is too much?

0

20

40

60

80

100

Low Medium High

Soil Test PSoil Test P

PotentialPotentialEnvironmentalEnvironmental

ProblemsProblems

Rel

ativ

e cr

op y

ield

, %

Rel

ativ

e cr

op y

ield

, %

((Sharpley Sharpley et al. 1993)et al. 1993)

No agronomic need for soil test levels > 100 kg P/ha for non-vegetable crops

Relative crop yields plateau at high soil test levels, but high concentrations of P in soil are not toxic to plants …

How much soil P is too much?

The challenge is to set threshold soil test P levels … minimize eutrophic runoff without restricting P application on soils and management systems that can safely accommodate higher levels

Agronomic and environmental threshold soil

test P levelsState AgronomicEnvironmentalSoil TestArkansas 50 150 Mehlich-3Delaware 25 50 Mehlich-3Idaho 12 50-100 OlsenOhio 40 150 Bray-1Oklahoma 30 130 Mehlich-3Michigan 40 75 Bray-1Texas 44 200 Texas A&M

Sharpley et al. 1999

Cadmium (Cd) and Phosphate

P fertilizers naturally contain varying amounts of Cd and crop plants take up varying amounts of Cd

Concern… toxic levels of Cd on human health and its persistence in the environment

There are no indications that P fertilizer, when applied at agronomic recommended rates poses any real threat to human health or the environment

Cadmium Sources

Background concentrations …air ... < 0.1 to 150 ppb fresh water ... 0.001 to 1 ppmearth’s crust ... 0.1 to 0.2 ppm

< 0.2 ppm in igneous rocks> 4 ppm in black shales

phosphate rock ... amounts vary

Typical Cd Content of Phosphate Rock, ppm

Location averagerange

FSU and South Africa <1 <2 Florida, USA 8 3-

20Morocco 18 8-

75North Carolina, USA 40 20-

50Togo 55 42-

80Western USA 90 40-

150

IFDC and TVA unpublished data

Soil Cd content

Natural background of Cd in agricultural soils: < 0.2 to 0.4 ppm)

Native Cd levels increase through: weathering of parent materials atmospheric deposition from forest firesvolcanic activityiron and steel productionApplication of sewage sludge, manureP fertilization

Plant Available Cd

Plant available Cd varies with crop, soil characteristics, management, time, and environment

Soil factors:Total soil Cd contentSoil pHIonic composition of the soil solutionSoil salinity … Cl- and SO4

= ionsCEC (clay, organic matter), carbonateszinc

Plant Available Cd

Other factors:Cultivation (zero-till vs conventional

till)High Cd accumulating crops: flax,

sunflowers, durum wheat• Wide variation in Cd uptake between and

within crop species and within cultivars

Crop rotation

Regulations: Cd in P Fertilizer

Because of potential health and environmental concerns several countries have proposed regulating Cd in P fertilizersEuropean Countries: limits … 21.5 mg Cd/kg P2O5

to 90 mg/kg P2O5

China: proposed a national standard of 8 mg CD/kg fertilizer

Canada: maximum acceptable cumulative soil addition of 4 kg Cd/ha over a 45-year period

U.S.: a risk-based concentration of 10 ppm Cd per 1% P2O5 in the fertilizer has been proposed

Cadmium and Phosphate

Risk assessment studies conducted by the U.S. EPA and others have concluded that Cd in fertilizers does not pose any harm to human health or the environment.

Summary

Environmental concerns with P:Eutrophication from excessive

enrichment of P in surface waterAdverse health and environmental

impact of Cd in P fertilizers

Concerns are real, but manageable and should not overshadow the critical role of P in crop production

Summary

Greater concern …Using insufficient P, or not appropriately balancing the use of other essential crop nutrients with PUnder-fertilization of P results in less

biomass and less residues to protect the soil against erosion and build below-ground organic matter

Inadequate P use, relative to N, results in reduce N use efficiency

Thank You