Phosphorus in Agriculture Symposium
Rosario, Santa Fe, Argentina
May 8-9, 2003
Agricultural Phosphorus …
Environmental Concerns
Terry L. Roberts
Potash & Phosphate Institute
Potash & Phosphate Institute of Canada
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
on surface of soil
particles
 Forms insoluble
compounds with Al,
Fe, and Ca
 Immobile in soil
 Some leaching of
soluble P in lowfixing soil
Input to soil
Loss from soil
The Phosphorus Cycle
Crop
harvest
Animal
manures
and biosolids
Atmospheric
deposition
Mineral
fertilizers
Plant
residues
Organic phosphorus
•Microbial
•Plant residue
•Humus
Plant
uptake
Im
mo
bil
iza
Mi
ne
tio
ral
n
iza
tio
Soil solution
n
Leaching
(usually minor)
phosphorus
•HPO4-2
•H2PO4-1
W
ea
th
er
in
g
 Strongly adsorbed
Component
Primary
minerals
(apatite)
ion
orpt
Ads
n
rptio
Deso
Dissolution
Precipitation
Runoff and
erosion
Mineral
surfaces
(clays, Fe and
Al oxides,
carbonates)
Secondary
compounds
(CaP, FeP, MnP, AlP)
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 America
 Environmental 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 wellestablished management practices …
 Soil and tissue testing
 Conservation tillage
 Manure analysis
 Vegetative buffers
 Proper nutrient
 Riparian zones
application methods
and timing
 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 extractants
 analytical methodology
 calibration to reflect environmental
impact
Environmental Soil Tests

Calibration and interpretation to predict
potential environmental impact … more
complicated than predicting crop response
Environmental Soil Tests

Determining nutrient loads that prevent
water quality degradation depends on:
 proximity to sensitive water bodies
 use of the water
 socioeconomic factors of rural land use
 other 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 benign
 P 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
rates
 Incorporates 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 Loss
P loss
vulnerability
Low (clear)
Medium
High
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
application
soil and climatic stress early in growing
season
other production factors are optimum
Starter P increased irrigated corn
yield and lowered grain moisture in
high P soil in North Carolina
P2O5 rate
kg/ha
0
65
Griffith 1992
Yield
t/ha
12.0
12.7
Grain
moisture, %
20.7
18.0
Soil test P = high
Starter P response varies
with tillage system
Tillage
System
Moldboard plow
Chisel plow
Ridge plant
Disk
No-till
Griffith 1992
Corn
-P
8,906
8,028
8,342
8,404
5,582
yield, kg/ha
+P
Increase
9,220
314
8,718
690
8,781
439
8,906
502
6,272
690
Bray P-1 = 22 ppm
How much soil P is too much?
 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 levels
State
Agronomic Environmental Soil Test
Arkansas
50
150
Mehlich-3
Delaware
25
50
Mehlich-3
Idaho
12
50-100
Olsen
Ohio
40
150
Bray-1
Oklahoma
30
130
Mehlich-3
Michigan
40
75
Bray-1
Texas
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 ppm
 earth’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
average
FSU and South Africa
<1
Florida, USA
8
Morocco
18
North Carolina, USA
40
Togo
55
Western USA
90
range
<2
3-20
8-75
20-50
42-80
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 fires
 volcanic activity
 iron and steel production
 Application of sewage sludge, manure
 P fertilization
Plant Available Cd
 Plant available Cd varies with crop, soil
characteristics, management, time, and
environment
 Soil factors:
 Total soil Cd content
 Soil pH
 Ionic composition of the soil solution
 Soil salinity … Cl- and SO4= ions
 CEC (clay, organic matter), carbonates
 zinc
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 fertilizers
 European 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 water
 Adverse 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 P
 Under-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