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