Phosphorus in
Agricultural Watersheds
Photo by NRCS
A Literature Review
by
G. N. Zaimes, R. C. Schultz
Department of Forestry, Iowa State University, Ames, Iowa
January 2002
Phosphorus in Agricultural Watersheds
A Literature Review
Table of Contents
List of Figures
i
List of Tables
iii
Executive Summary
1.
Context of Problem
1.1 Agriculture’s contribution of nonpoint source phosphorus to surface waters
1.2 Phosphorus importance for agriculture
1.3 Phosphorus negative impacts on surface waters
1.4 Total maximum daily loads program
2.
Nonpoint Source Phosphorus Transport To Surface Waters
2.1 Terrestrial phosphorus processes
2.1.1 Phosphorus forms in water
2.1.1.1 Importance
2.1.1.2 New systematic nomenclature
2.1.2 Transport Mechanisms
2.1.3 Pathways
2.1.3.1 Importance
2.1.3.2 Classification
2.1.3.3 Major pathways
2.1.4 Hydrology
2.1.5 Terrestrial sources of phosphorus for surface waters
2.1.5.1 Atmospheric Deposition
2.1.5.2 Soil sources
2.1.5.3 Agronomic sources
2.2 Aquatic Phosphorus Processes
2.2.1 Aquatic processes in lotic systems
2.2.2 Flow regimes
2.2.3 Critical phosphorus concentrations for surface waters
v
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10
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10
16
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18
27
30
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3.
Land-use Practices and Best Management Practices
3.1 Land-use practices
3.2 Pastures
3.3 Cultivated Fields
3.4 Reducing phosphorus inputs to surface waters
3.5 Riparian buffers
3.5.1 Filter Strips
3.5.2 Riparian forest buffer
3.5.3 Riparian management system
36
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41
45
46
49
49
50
4.
Other factors that influence phosphorus transport
4.1 Different Scale categories
4.2 Watershed scale studies
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53
5.
Phosphorus-Index System
5.1 Watershed-Modified Phosphorus-Index
5.2 Iowa Phosphorus-Index
55
55
57
6.
Conclusion
60
7.
References
62
APPENXIX I Case Studies: Phosphorus Losses Under Different Land-use Practices
78
Appendix II -Table 1. Phosphorus losses under different land-use management’s through different
pathways and various P forms
79
Appendix I - References
98
APPENDIX II Original Phosphorus-Index
101
Appendix II -Table 1. The site characteristics, their weight factors, and the five P loss levels of the
original P index system
102
Appendix II - Table 2. The site vulnerability chart for the original P index system that indicates the
potential of a site to deliver P to surface waters
103
Appendix II - References
104
Acknowledgement
This review was prepared with the support of the Iowa DNR through a grant from USEPA under the Federal
Nonpoint Source Management Program (Section 319 of the Clean Water Act).
i
List of figures
Figure 1. Phosphorus transport includes terrestrial and aquatic processes.
5
Figure 2. Potentially mobile agricultural P inputs and the hydrologic pathways that transport P to
reach surface waters.
6
Figure 3. A model describing how NPS P reaches surface waters from terrestrial sources.
Hydrology provides the energy for P transport and the soil, atmospheric
deposition, and agronomic practices, are the sources. Phosphorus transport is
initiated by three mechanisms and P can reach the surface waters through one or
more of these pathways.
7
Figure 4. Hydrological pathways for P transport in the (a) soil profile include matrix and
preferential flow (b) slope/field scales include overland flow, interflow and land
drainage (e.g. tiles).
12
Figure 5. Mechanisms and forms of P transported in overland flow.
13
Figure 6. Phosphorus cycle in the soil with inputs, outputs, and transformations that take place in
the soil.
20
Figure 7. Relationship between soil P test, crop yield, and environmental problems due to
excessive soil P.
23
Figure 8. Percentage of soil samples testing high P soil levels in 1989.
24
Figure 9. Phosphorus cycle in the aquatic systems. In this cycle P moves from the water in the
bed and from the bed to the water while also transforming into different forms.
Terrestrial and streambank sources provide the inputs.
31
Figure 10. The fourteen ecoregions in the United States. The ecoregions were differentiated
based on geology, land-use, ecosystem type, and nutrient conditions.
35
Figure 11. Phosphorus concentrations (µg L-1), losses (g ha-1 yr-1) and forms (TP (<0.45) and TP
(>0.45)) in surface runoff under different land-uses.
36
Figure 12. Nonpoint source P contributors in 1980. Cultivated, pasture and range land are the
major contributors.
37
Figure 13. Phosphorus losses and its forms RP (<0.45) RP (>0.45), and UP (unf.) in overland flow
as a function of different crops and different tillage practices on several Southern Plain
watersheds, averaged over five years.
43
Figure 14. Nontilled row-cropped fields have more overland flow and less total
evapotranspiration in larger flow compared to riparian buffers that reduce stormflow and
increase baseflow due to higher infiltration and evapotranspiration. Reducing overland
flow and streambank erosion will decrease P losses.
46
Figure 15. Phosphorus movement in riparian forest buffers. Sediment and TP (>0.45) are filtered
from overland flow and TP (<0.45) can be taken up by biota of the living filter.
47
ii
Figure 16. The riparian forest buffer. The first zone consists of unmanaged forest, which protects
the streambank and provides woody debris. In the second zone, the forest is managed to
maintain nutrient uptake through vigorous plant growth. The third zone has
grasses with controlled grazing allowed under certain conditions.
50
Figure 17. The multi-species riparian buffer model. The first zone, is located along consists of a
managed tress and shrubs. They provide bank stability, wildlife habitat, are a nutrient
and sediment sink, and modify the aquatic environment. The second zone, consist of
native grass and forbs that intercept overland flow, increase infiltration and intercept
NPS pollutants.
51
Figure 18. The Riparian Management System (RiMS) consists of five practices: i) multi-species
riparian buffer with woody plants and warm-season grasses that intercept NPS from
adjacent land practices, ii) streambank bioengineering that provide bank stability,
iii) constructed-restored wetlands that intercept and filter NPS from subsurface tiles,
iv) cool-season or warm-season grasses can replace MRB that may be used for rotational
grazing with stream fenced out and v) instream structures like boulder weirs.
52
iii
List of tables
Table 1. Percentage of NPS P inputs from agriculture to surface waters for different countries.
2
Table 2. Limiting nutrients for various surface water bodies.
4
Table 3. Classification of P forms in water with the new suggested classification and the
currently established terms.
9
Table 4. Definitions for hydrologic pathways of P in the unsaturated zone. Terms are sorted
alphabetically. When there is more than one term describing a pathway, the preferred
term is indicated in italics. This classification is based on spatial and temporal scales
and planes of water movement. Please note that this is a nominal classification.
11
Table 5. Phosphorus concentrations in stream and drainage water under various flow regimes.
Ranges are in the parenthesis when available.
17
Table 6. The common chemical forms of P in soil and their characteristics, or implication for
potentially mobile P.
19
Table 7. Soil P surpluses for different developed countries under different agronomic land-use
practices.
22
Table 8. Percentage of TP (unf.) of A, B and C horizons of several kinds of soil. Methods used
to measure TP (unf.) in the soil were not described.
24
Table 9. Recommended agronomic and environmental soil P test threshold values with the
appropriate P management recommendations.
26
Table 10. The amounts of feces and phosphorus produce by different animals.
27
Table 11. Phosphorus concentrations from headwaters to downstream reaches.
31
Table 12. Recommended critical or threshold P concentrations for surface waters that may cause
eutrophication.
34
Table 13. Changes in physical and chemical properties from the headwaters to the downstream
reaches and some biological implications.
34
Table 14. The EPA recommended critical TP (unf.) concentrations for each some of the nutrient
ecoregions for phosphorus. The ecoregions are shown in Figure 10.
34
Table 15. Phosphorus removal rates for different crops. Phosphorus removal (kg ha-1) is
estimated by the mean P concentration and the mean yield of the crop for the United
States. All values are expressed on a fresh weight basis.
42
Table 16. The effectiveness of different types of buffers with different widths in removing NPS
P from overland flow.
48
iv
Table 17. The transport and source characteristics with their respective weight factors, and the
five P loss-rating levels of the modified P index system. The transport include soil
erosion, soil runoff class and return period/distance. The source characteristic include
soil P test, P fertilizer rate, P fertilizer application method, organic P application rate,
and organic P application method.
56
Table 18. The site vulnerability chart of the watershed-modified P-index system that indicates the
potential of a site to deliver P to surface waters.
57
Table 19. The site vulnerability rating (P hazard class) for the Iowa P-index.
59
v
Executive Summary
Phosphorus is a major nonpoint source pollutant that causes eutrophication in
surface waters. In the past interest in phosphorus, as a nonpoint source pollutant was not
as great as for nitrogen because phosphorus is generally less mobile than nitrogen in the
agricultural landscape. Phosphorus is immobile because it is easily adsorbed to soil
particles. However, high soil and streambank erosion can lead to increased amounts of
phosphorus in surface waters. Additionally, dissolved phosphorus contributions are more
significant than previously thought. These facts along with the heightening concern of
the impacts of poultry and livestock manure on surface water quality has increased the
interest in phosphorus movement and management in the landscape.
To be able to assess the potential of nonpoint source phosphorus pollution and to
develop proper management strategies to reduce phosphorus losses to surface waters it is
essential to understand the major processes involved in phosphorus transport. Both
terrestrial and aquatic processes are important. Terrestrial processes are responsible for
phosphorus inputs from upland and riparian areas while aquatic processes include
streambank inputs and instream phosphorus inputs, outputs and transformations.
A model describing phosphorus transport from terrestrial sources would include:
the energy for transport, the specific sources, the form and amount of phosphorus that is
potentially mobile, the mechanisms of transport and finally the specific pathways that
phosphorus will follow.
Precipitation provides the major source of energy for transport through its effect
on soil erosion and is dependent on watershed morphology, hydrology and land cover and
management. The major sources of phosphorus include atmospheric deposition (wet and
dry fall), the soil (the phosphorus levels in the soil), and various agronomic inputs
(fertilizer, manure, animal litter, animal feces and plant residue).
Dissolved, particulate, organic and inorganic phosphorus are the major terms used
to describe various forms of phosphorus in the environment. These terms describe the
physical and chemical conditions of phosphorus and are ambiguous because each term
may represent various different forms depending on the methods used to extract and
analyze them. As a result a new systematic nomenclature has been suggested to describe
phosphorus forms that incorporates the methods used to differentiate them.
Dissolution, physical and incidental are mechanisms that are responsible for the
transport of phosphorus. Dissolution is the transport of dissolved phosphorus as in soil
leaching. Physical mechanisms refer to the transport of particulate phosphorus as in soil
erosion, while incidental mechanisms are controlled by unique conditions like the
instantaneous, short-term transport of surface applied phosphorus fertilizer after an
intense rainfall.
The major pathways that moving phosphorus follows include overland flow,
matrix flow, preferential flow, interflow and land drainage (ditches, tiles and moles).
A model for aquatic processes includes phosphorus inputs from streambank
erosion, phosphorus outputs from rivers and streams and phosphorus cycling within the
aqueous ecosystem. Phosphorus outputs from rivers and streams originate from
terrestrial inputs plus the addition of phosphorus from streambank erosion and
resuspension from the streambed. Instream cycling includes phosphorus transformations
between dissolved phosphorus (more readily bioavailable) and particulate phosphorus
vi
(less bioavailable) that regulate phosphorus bioavailability to organism in the water
column and the hyporheic zone. Additionally, it is very important to recognize the
impact of flow regimes on aquatic processes and the critical phosphorus concentrations
needed for eutrophication.
Land-use practices can have a major impact on phosphorus losses to surface
waters. The two major agricultural land-use practices that contribute phosphorus to
surface waters are cultivation and grazing. Both increase phosphorus as they become
more intensive. Grazing increases phosphorus losses because of the addition of
phosphorus from animal feces, and trampling that can increase overland flow, peak
discharges and streambank erosion. Cultivation in row-crop agriculture increases
phosphorus losses because much of the soil surface is bare and susceptible to direct
raindrop impact, is compacted from machinery and surface sealing and may have added
fertilizer and/or animal wastes. Best management practices can reduce phosphorus
losses, by reducing overland flow and streambank erosion and should be concentrated in
riparian areas that may be the major contributors of phosphorus to surface waters if they
are cultivated or heavily grazed. Riparian areas should be the focus for best management
practices because the sediment that is detached and moved in these areas has a greater
probability of reaching the stream than sediment that is detached in the uplands. Best
management practices in the riparian area include filter strips, riparian forest buffers, and
riparian management systems.
To further understand the processes and pathways of phosphorus movement and
the impacts of land-use on phosphorus losses, research is needed at different scales and in
different regions of the country. More watershed scale studies should be conducted.
However, their costs are very high and results from more field and plot scale studies
should be used to extrapolate to the watershed scale with the use of models.
The phosphorus index was developed by the USDA-NRCS to assess potential
losses at the filed scale. It has since been modified to be used at the watershed scale.
Additionally, most states, including Iowa, have modified the original index for use at the
field scale in their respective states.
Nonpoint source phosphorus pollution is not an easy problem to solve. It will
take a combination of research, the modification of existing and development of new best
management practices and rigorous application of those best management practices on
the landscape.
1
1. CONTEXT OF PROBLEM
1.1 Agriculture’s contribution of nonpoint source phosphorus to
surface waters
Nonpoint source (NPS) pollutants are the number one water quality problem in
the United States (USEPA, 1997a). The most common NPS pollutants are sediment and
nutrients (nitrogen (N) and phosphorus (P)). Agriculture is the nation’s leading NPS
contributor, and is responsible for degrading 60% of the impaired river kilometers and
50% of the lake hectares in the country (USEPA, 1997a). Cultivation, fertilizer and
pesticide application, irrigation, planting, harvesting, confined animal facilities, and
grazing are the major agricultural activities that cause NPS pollution (USEPA, 1997b).
Agricultural activities in the U.S. annually contribute1.9 billion metric tons of sediment
(USDA-NRCS, 1997a). In Iowa, agriculture contributes 28.3 metric tons of sediment ha1
yr-1 (sheet and rill erosion and ephemeral gullies) 9.8 kg N ha-1 yr-1 and 0.3-2.0 kg P ha-1
yr-1 (USDA-NRCS, 1997b).
This review will concentrate on P as a NPS pollutant from agricultural sources.
Past perceptions were that NPS P movement to surface waters was minimal, because P
was primarily held by soil particles. However, studies at both the plot and watershed
scale show evidence of significant P losses from agricultural fields (Sharpley et al., 2000;
Withers and Jarvis, 1998). These losses, typically are below 1 kg total P ha-1 yr-1, and
although negligible from an agronomic point of view, can have a significant
environmental impact (Heckrath et al., 1995). In Illinois, agricultural P contributions
more than doubled in areas with smaller urban population densities as P sewage effluent
contributions decreased, although riverine P loads were relatively similar to the P loads in
the higher urban population density areas (David and Gentry, 2000).
Powlson (1998) mentions three factors that have led to a greater realization of the
importance of P movement from agricultural fields. The first is that very low P
concentration can cause eutrophication. The second is that significant amounts of P
losses are through interflow and field drains that reach surface waters. The third is a
greater recognition of the importance of soil erosion and overland flow to P transport.
Even though P is strongly held by soil, surface erosion mobilizes P when soil particles are
detached (Addiscott et al., 2000).
More evidence of the importance of agricultural P inputs to surface waters is that
the removal of point sources does not significantly reduce P inputs to surface waters
(Sharpley et al., 2000). In lakes in Denmark and Ireland, reduction of point sources did
lower P concentrations but without any appreciable improvement in water quality (Foy et
al., 1995; Kronvang et al., 1993). This was attributed to P inputs primarily from
agricultural sources. The easier identification and recent control of point sources has led
to the belief that NPS of P in agricultural runoff now contributes a greater proportion to
surface water inputs than point sources (Sharpley et al., 1994) (Table 1).
The importance of agricultural P contributions to surface waters is recognized in
management plans for improving water quality. In Illinois, regional rivers contributed a
large fraction of the P loads to the Mississippi River. To reduce these P loads, a
reduction of P from agricultural sources has to be addressed (David and Gentry, 2000).
To improve drinking water quality, New York City decided to implement best
2
Table 1. Percentage of NPS P inputs from agriculture to surface waters for
different countries.
Country
Netherlands
Italy
Federal Republic of Germany
North Sea catchment basin
Sweden
Denmark
Norway
United Kingdom
U.S.A.
New Zealand
U.S.A., Illinois
Illinois River
All other rivers
Agriculture Share
%
24
33
38
25
16
70
27
35
84
90
53
30
67
Reference Year
1985 e
1986 e
1987 e
1987 e
1987 e
1988 f
1988 e
1995 d
1998 c
2000 b
2000 a
a
David and Gentry (2000), b Gilligham and Thorrold (2000), c Carpenter et al. (1998), d Heckrath et al.
(1995), e Isermann (1990), f Chiaudani and Premazzi (1988).
management practices (BMP’s) to reduce NPS P from agricultural sources rather than
build a new water sewage treatment facility (Sharpley et al., 2000).
Agricultural P inputs to surface waters have increased because of intensive
livestock grazing and because the combined fertilizer and manure inputs in excess of crop
requirements that have led to a build up of soil P levels (Sharpley et al., 1994). These
practices have altered the soil and landscape hydrology. The final amount of P entering a
stream depends on agricultural management practices, the soil type and its associated
chemical, physical and biological characteristics, the type of the runoff events and
concentrations of P in the runoff, and the nature of the receiving water (Abrams and
Jarrell, 1995). Because so many factors influence P inputs, this literature review will
concentrate on identifying agricultural P sources, pathways and processes that control P
transport, their influence on P availability and their potential impact on surface water
quality (Edwards and Withers, 1998; Johnes and Hodgkinson, 1998).
Although research on NPS P has been conducted for the last 25 years, more
studies are necessary to identify the environmental impacts from agricultural practices
(Johnson et al., 1997). Sharpley and Tunney (2000) suggest four main areas of research:
1) soil P testing for environmental risk assessment; 2) pathways of P transport within a
watershed: 3) BMP’s development and implementation; and 4) strategic initiatives to
manage P. They go on to say that future research should be interdisciplinary and involve
soil scientists, hydrologists, agronomists, limnologists, animal scientists, rural
economists, and social scientists. In addition, David and Gentry (2000) suggest more
research is needed to correlate P loads in streams to P loads from agricultural production.
3
1.2 Importance of phosphorus in agriculture
Phosphorus is an essential macronutrient that is required to meet global food
requirements and make crop and livestock production profitable (Hedley and Sharpley,
1998). The most important function of P in plants is the storage and transfer of energy,
and cell division (Norfleet, 1998; Troeh and Thompson, 1993). Plant cells need to have
adequate P before they divide. Additionally, P increases seed production, root growth,
grain, fiber and forage yield, enhances early plant maturity and stalk strength, and
promotes resistance to root rot disease and winter kill (Norfleet, 1998).
Phosphorus is the mineral with the most known biological functions in animals
(Beede and Davidson, 1999). Phosphorus is involved in most energy transactions, in the
acid-base buffer systems of the blood and other fluids, and in cell differentiation of
animals. Bones and teeth contain almost 80% of the P found in the bodies of animals.
Every cell in the body also contains P as phospholipids, phosphoproteins and nucleic
acids. Growing cattle contain 6-8 mg dl-1, while mature cattle contain 4-6 mg dl-1 in their
blood plasma. Whole blood contains 6-8 times more P than plasma.
1.3 Negative impacts of phosphorus on surface waters
Eutrophication has been recognized as the main cause of water quality
impairment (Environment Agency, 1998; European Environment Agency, 1998; USEPA,
1996). Phosphorus has been identified as the primary limiting nutrient causing
eutrophication of many surface waters (Table 2) (Daniel et al., 1998). Eutrophication
increases the growth of undesirable algae and aquatic plants that replace benthic
organisms and submerged macrophytes (Sharpley et al., 2000; Carpenter et al., 1998;
Norfleet, 1998; Kotak et al., 1994; Martin and Cooke, 1994). The death and
decomposition of algae cause oxygen shortages that restrict water use for fisheries,
recreation, navigation, industry and drinking. Coarse, rapid-growing fish replace highquality edible fish. Undesirable odors and surface scum are produced from the decaying
algae, with mosquito and other pest insect population increasing. Water transparency and
the aesthetic value of the surface water are decreased. Potentially toxic dissolved
compounds are produced that may harm livestock, wildlife, and the cost of purification
increases. Finally, cyanabacteria blooms can lead to drinking water unpalatability.
Land management practices can be used to effectively control the movement of P
into surface waters. Phosphorus transport is easier to control than nitrogen and carbon
(C), two other nutrients involved in creating water quality problems (Sharpley et al.,
2000). It is very difficult to control the exchange of N and C between the atmosphere and
water, the fixation of N by some blue-green algae, and the mobility of N in surface and
subsurface flow (Hession and Storm, 2000). Although P transport is easier to control
than N transport, very small P concentrations (as low as 10 µg L-1) cause eutrophic and
hypereutrophic conditions (Sharpley et al., 2000; Haygarth et al., 1998a; Powlson, 1998).
Considering that eutrophication exhibits high spatial and temporal variability and that the
natural background levels can be a significant contributor of NPS P (28.4%) (Council of
Environmental Control, 1989), solving P pollution problems will not be simple (Edwards
and Withers, 1998). Natural background levels of P refer to the P that would be found in
4
Table 2. Limiting nutrients for various water bodies. a
System
Rivers and Streams
Point source dominated
Without P removal
With P removal
Nonpoint source dominated
Estuaries
Fresh water region
Point source dominated
Nonpoint source dominated
Brackish region
Saline region
Lakes
Large
Nonpoint source dominated
Small
Point source dominated
a
N/P ratio
Limiting nutrient
<<10
>>10
>>10
N
P
P
>>10
<<10
≈10
<<10
P
N
P or N
N
>>10
P
<<10
N
Sharpley et al. (1994).
a water body in an undisturbed system (absence of human-induced change) (Salloway,
2001; Taylor and Kilmer, 1980).
1.4 Total maximum daily loads program
The United States Environmental Protection Agency (USEPA) has developed the
Total Maximum Daily Loads (TMDL) program to regulate water pollution. This
program is based on section 303(d)(1) of the Clean Water Act (CWA) (USEPA, 2000a).
Under this program, States provide a list of all their impaired waters to the EPA, and
develop TMDL for sediment and nutrients (N and P) for their impaired waters from point
sources and NPS. The USEPA (1999) defines TMDL as “ a calculation of the maximum
amount of a pollutant that a water body can receive and still meet water quality standards,
and an allocation of that amount to the pollutant’s sources.” TMDL standards are based
on scientific criteria and the waterbody use (e.g. drinking, swimming, fishing). The
TMDL programs in the future will also require implementation programs based on
scientific data (USEPA, 2000b). The TMDL program will have a significant impact on P
management in agricultural practices.
5
2. NONPOINT SOURCE PHOSPHORUS TRANSPORT TO SURFACE
WATERS
To identify and quantify NPS P loads to water bodies it is necessary to understand the
processes and pathways that control P transport (Johnes and Hodgkinson, 1998). Phosphorus
transport can be divided into terrestrial and aquatic processes (Figure 1). Terrestrial processes
describe P inputs to surface waters from the upland and riparian NPS sources, such as natural,
agriculture and urban ecosystems, and point sources (Pierzynski et al., 2000). Aquatic processes
describe P inputs from streambank erosion, P outputs from rivers, streams and other watercourses, and the P cycling within the aqueous ecosystem that regulates P bioavailability to
aquatic organisms (Pierzynski et al., 2000). This review will focus on how agriculture P inputs
reach and move through lotic systems. However, it is very difficult to quantify, with any degree
of certainty, the terrestrial and aquatic processes (Edwards and Withers, 1998).
2.1 Terrestrial phosphorus processes
Terrestrial processes carry P above and below the soil surface before it reaches surface
waters (Figure 2). Potentially mobile P describes P in the terrestrial landscape that is in forms
that can be transported by water and is a nonquantitative concept (Haygarth and Jarvis, 1999).
Figure 1. Phosphorus transport includes terrestrial and aquatic processes (Pierzynski et
al., 2000).
6
The main sources of the potentially mobile P are fertilizers, manure, including that in pastures
and that associated with confinement facilities, animal litter, plant residue, soil P and
atmospheric deposition (Figure 2). The potentially mobile P that is actually transported by the
various hydrologic pathways to surface waters is called total transported P. Nonpoint source P
transported through these hydrologic pathways is accomplished by three mechanisms (incidental,
physical, dissolution) with hydrology the main driving force (Figure 3). To better understand
these mechanisms the forms of P in water will be described first.
2.1.1 Phosphorus forms in water
2.1.1.1 Importance
The distinction between P forms is necessary because of their differences in adsorptiondesorption reactions, transport, and potential bioavailability to aquatic organisms (Edwards and
Withers, 1998). For example, dissolved P primarily consists of orthophosphate that is
immediately available for algal uptake while particulate P is a long-term source of P for aquatic
biota (Sharpley et al., 1994). A better understanding of P forms and the mechanisms by which
they are exported to surface waters is required to minimize transport (Haygarth et al., 1998a).
Transport of P in the terrestrial environment can be via solution or with sediment
movement (Sharpley and Mendel, 1987). Dissolved (soluble) P in runoff originates from the
Riparian
zone
Agricultural
crop
Water
course
Atmospheric
deposition
Hydrological
pathways
Potentially mobile P
Fertilizer
Manure
Animal
litter
Soil
processes
Transported P
Plant
residue
Overland flow
Soil P
Interflow
Mineralogy
Preferential
flow
Matrix flow
Tile flow and
Groundwater flow
Figure 2. Potentially mobile agricultural P inputs and the hydrologic pathways that
transport P to reach surface waters.
7
THE PROBLEM
P transported to surface waters
PATHWAYS
Matrix flow
Overland flow
Preferential flow
Land drainage
Interflow
(ditches, canals, tiles, moles)
MECHANISMS
Incidental
Physical
Dissolution
P FORMS IN WATER
Particulate P
Potentially Mobile
Dissolved P
P SOURCES
Agronomic
(fertilizer, agricultural
wastes, plant residue)
External
Sources
Soil P
(mineralogy, internal cycling)
Internal sources: weathering
Atmospheric
deposition
ENERGY
Hydrology
Figure 3. A model describing how NPS P reaches surface waters from terrestrial sources
(modified from Haygarth and Sharpley, 2000). Hydrology provides the energy for P
transport and the soil, atmospheric deposition, and agronomic practices, are the sources.
Phosphorus transport is initiated by three mechanisms and P can reach the surface waters
through one or more of these pathways.
8
release of P from a thin zone of surface soil (1-2.5 cm) and/or vegetative material that interacts
with rainfall (Sharpley et al., 1994; Sharpley et al., 1992; Sharpley, 1985). Dissolved P transport
depends on desorption-dissolution reactions (Sharpley et al., 1992; Sharpley, 1985) and on the P
content in surface soil (Sharpley at al., 2000). Desorption, dissolution, and extraction of P from
the soil, crop residues, livestock manure or surface-applied fertilizer lead to dissolved P
movement in runoff. Particulate (suspended, or sediment-bound) P is associated with soil and
vegetative material eroded during runoff. In most cases, particulate P is typically the dominant
form of P lost (David and Gentry, 2000; Vaithiyanathan and Correll, 1992; Bottcher et al., 1981).
In 116 agricultural watersheds particulate P was on average 86% (ranged from 44-98%) of the
total P loads (kg yr-1) (Prairie and Kalff, 1986). However, to get the full impact of P losses both
dissolved P and particulate P must be measured (Stevens et al., 1999).
Algal available P is dissolved P and the portion of particulate P that is in equilibrium with
dissolved P (Sharpley et al., 1996). Sonzogni et al. (1993) defined algal available P as “the
amount of inorganic P a P-deficient algal population can utilize over a period of 24 h or longer.”
Algal available particulate P is important because the excess of this form will cause
eutrophication. Algal available particulate P is a function of soil loss, particle size enrichment,
and chemical properties of the eroded material that governs P adsorption and availability.
Reduction in total P loads may not lead to reduction of eutrophic conditions in many cases,
because it only reflects a decrease in particulate P (Sharpley et al., 1992). Instead, the
measurement of algal available P is essential to accurately estimate the impact of agricultural
management practices on surface waters (Sharpley et al., 1992).
2.1.1.2 New systematic nomenclature
The characterization of the various forms of P associated with water transport depends on
the filtration and chemical methods of analysis. Dissolved P and particulate P is differentiated
with filtration methods while the inorganic P and organic P is differentiated with chemical
methods. There are many different methods that are used to differentiate P fractions. Because
the terms in the above section describe the physical and chemical conditions of P and not the
methods that are used to differentiate the P fractions they are ambiguous. For example to
differentiate dissolved P and particulate P different size filter membranes might be used. In this
case the dissolved and particulate fractions would differ depending on the filter membrane size.
A new systematic nomenclature has been suggested based on the operational definitions
of the filtration and chemical methods (Table 3). For P forms described by the filtration
methods, Haygarth and Sharpley (2000) suggest that samples be defined by the filter size, with a
suffix of the filter pore size or the term (unf.) when they are not filtered (Table 3). Typically, a
0.45 µm membrane filter is used. According to Haygarth and Sharpley (2000), P that passes the
0.45 µm membrane filter is defined as dissolved or soluble P (TP (<0.45)). The P that does not
pass through the membrane is the particulate, sediment-bound or suspended P (TP (>0.45)).
From the chemical methods the Mo-blue reaction method (Murphy and Riley, 1962), is
most commonly used to estimate orthophosphate (an inorganic P form). In reality the
preparation of the sample causes this method to overestimate orthophosphate. Comparing
orthophosphate estimated by the Mo method would be different from estimates from other
methods. The new terms for P, suggested by Haygarth and Sharpley (2000), based on the Mo
reaction method are reactive (RP), unreactive (UP) and total P (TP) (Table 3). Specifically, RP
is reactive with Mo, while UP is unreactive with Mo, and TP is the sum of RP and UP.
9
Table 3. Classification of P forms in water with the new suggested classification and the
currently established terms. a
New classification
TP (unf.)
TP (<0.45)
TP (>0.45)
RP (unf.)
UP (unf.)
RP (<0.45)
UP (<0.45)
RP (>0.45)
UP (>0.45)
a
b
Established terms b
Total P (TP) -from a unfiltered sample Total dissolved P (TDP), soluble P
Particulate P (PP), sediment-bound P,
suspended P
Total reactive P (TRP),
Total organic P
Molybdate-reactive P (MRP), dissolvedreactive P (DRP), soluble reactive P (SRP),
dissolved molybdate-reactive P,
orthophosphate, inorganic P, phosphate
Dissolved organic P (DOP), soluble
organic P (SOP), dissolved nonreactive P
(DNRP)
Molybdate-reactive particulate P (MRPP),
particulate reactive P
Particulate organic P
Modified from Haygarth and Sharpley (2000).
May not necessarily be inclusive.
Finally, Haygarth and Sharpley (2000) recommend that the terms bioavailable P and
available P should not be used because of their inherent value judgment. Instead, P should be
described by the organism to which it is bioavailable. For example, for P in surface waters that
can potentially cause eutrophication the proper term is algal-available P (AAP).
2.1.2 Transport Mechanisms
The three main P transport mechanisms are dissolution, physical, and incidental
(Haygarth and Jarvis, 1999). Dissolution describes the transport of TP (<0.45) from the soil
particle or adsorption site to the soil solution. Dissolution is a micro-scale soil profile
mechanism that is determined by chemistry. Examples of dissolution are mineralization, enzyme
hydrolysis, adsorption-desorption, solubilization of P from saturated soils, and leaching
(Haygarth and Sharpley, 2000; Haygarth and Jarvis, 1999). Leaching is the elluviation of solutes
through soil. Although it is a process term, it commonly is used inappropriately to denote a
pathway. Adsorption and desorption continue to occur once the TP (<0.45) is in solution
(Sharpley et al., 1994). The magnitude and direction of P transformations depend on TP (<0.45),
TP (>0.45) and sediment concentration in the runoff.
Total P (>0.45) is primarily moved by physical mechanisms. As the intensity of the
physical mechanism increases, TP (>0.45) concentration in runoff increases (Sharpley et al.,
1992). In contrast to dissolution, this mechanism is a macro-scale process (Haygarth and Jarvis,
10
1999). Examples are soil erosion, and displacement and entrainment of colloids and submicronsized material (Haygarth and Sharpley, 2000).
Fertilizer and manure P inputs to the soil are retained by smaller particles, so the added P
is not redistributed uniformly through the whole soil profile (House et al., 1998). Soil P levels
are higher in the top 5 cm of the surface soil. Soil detachment and transport in surface runoff
preferentially erode finer particles. This results in eroded material with higher TP (>0.45)
content in the runoff compared to the soil in the source area (Sharpley et al., 2000). This
phenomenon is referred to as enrichment. The smaller, lighter particles are transported greater
distances and are more likely to enter surface waters (House et al., 1998). Because of the
selective erosion of finer material from the surface few centimeters, TP (>0.45) losses are much
larger than that predicted from whole soil profile analysis (House et al., 1998).
The incidental mechanism is conceptually different than the dissolution and physical
mechanisms, because it is controlled by instantaneous unique conditions (Haygarth and Jarvis,
1999). Examples are short-term transport of farm amendments like P fertilizer, manure or
animal feces. This short-term transport occurs after effective rainfall removes the amendment
shortly after it has been applied. Land-use management factors and hydrology have major
impacts on all three P transport mechanisms (Sharpley et al., 2000).
2.1.3 Pathways
2.1.3.1 Importance
To effectively mitigate P transport to surface waters the pathways of P movement must
be identified (Sharpley et al., 2000). The pathways that P follows depend on the form and
quantity of P that is transported to the surface waters (Sharpley and Syers, 1979a; Foy and
Withers, 1995). For example, RP (unf.) dominates in surface pathways because it reacts with the
upper surface horizons (Haygarth et al., 1998a). In contrast, more than 50% of the P in
subsurface (tile) drained flow is UP (unf.) because it moves more freely in the soil profile.
2.1.3.2 Classification
Classification of the hydrologic transport pathways is difficult because of the large
variability that exists in the spatial and temporal scale of water flow. The latest classification of
pathways, by Haygarth and Sharpley (2000), is organized around spatial and temporal
characteristics (Table 4). The spatial characteristics considered are the plane and scale of water
movement. The plane refers to the vertical and lateral direction of water flow. The spatial scale
is divided into: i) the soil profile (commonly in the vertical plane), ii) the slope/field (commonly
in the lateral plane), and iii) the watershed/catchment (the largest scale) (Figure 4). The temporal
scale can be minutes, hours, or days depending on local conditions.
2.1.3.3 Major pathways
The most important pathways for P transport from agricultural landscapes are (Figure 4):
i) matrix flow, ii) preferential flow, iii) overland flow iv) interflow, and v) land drainage. These
P pathways are interrelated during an event. For example, the pathway down a slope can be
11
Table 4. Definitions for hydrologic pathways of P in the unsaturated zone. Terms are
sorted alphabetically. When there is more than one term describing a pathway, the
preferred term is indicated in italics. This classification is based on spatial and temporal
scales and planes of water movement. Please note that this is a nominal classification. a
Term
By-pass flow
Interflow
Land drainage
Scale
Soil
Slope/field
Subcatchment
& Slope/field
Time
Min/h
Min/h
Min/h
Plane
Vertical
Lateral
Lateral
Leakage
Slope/field
Not applicable
Both
Macropore flow
Soil
Min/h
Vertical
Saturated (soil)
flow
Seepage
Soil
Days
Lateral
Slope/field
Not applicable
Lateral
Soil solution
Soil
Not applicable
Both
Subsurface flow
Surface runoff
Throughflow
Slope/field
Slope/field
Soil and
slope/field
Slope/field
Min/h
Min/h
Not applicable
Lateral
Lateral
Both
Min/h
Lateral
Matrix flow
Soil
Days
Vertical
Overland flow
Slope/field
Min/h
Lateral
Percolating
water
Pipe flow
Piston flow
Preferential flow
Soil
Not applicable
Both
Slope/field
Soil
Soil
Min/h
Not applicable
Min/h
Lateral
Vertical
Vertical
Return flow
Slope/field
Min/h
Lateral
Runoff
Slope/field
Min/h
Lateral
Vertical
saturated flow
Vertical
unsaturated flow
Soil
Days
Vertical
See preferential flow, but occurring laterally over
capped, compacted, or slowly permeable
horizons.
Implies a type of soil water movement-in this
case uniform vertical movement downward,
common in very porous media, such as sandy
textured soils.
Movement of water exclusively over the soil
surface during heavy rain.
General nonspecific term describing water
movement.
Lateral subsurface preferential flow.
See for matrix flow.
Implies a type of soil water movement-in the
case of vertical movement along larger subsoil
pathways, e.g., wormholes and fissures, often
occurring in unsaturated conditions.
Where a subsurface flow pathway emerges at the
soil surface.
General hydrological term describing the lateral
movement of water off land above- and belowground, causing a short-term increase in flow at
the catchment outlet. Can refer to qualified
pathway (e.g., surface runoff), but also has been
used to describe process and water media.
See matrix flow
Soil
Min/h
Vertical
See preferential flow
Unsaturated
flow
a
Haygarth and Sharpley (2000).
Definition
See preferential flow.
Lateral flows below the soil surface.
Water and solute (+entrained solids) export to
catchment resulting from land drainage practices:
anthropogenic.
General non specific term for describing water
and chemical movement.
See preferential flow; must be tightly defined
when used, otherwise best avoided.
See matrix flow, but the plane is lateral not
vertical.
General nonspecific term describing water
sampled from the soil environment by whatever
means-not a pathway.
Nonspecific term describing water movement,
implies emergence at or near the ground surface.
General lateral flow below the soil surface.
See overland flow.
See percolating water.
12
Matrix flow
Preferential flow
Impermeable substrate
Permeable substrate
Overland
Topsoil
OverlandInterflow
flow
Stream
River
Water table
Stream
River
Tiles (Land Drainage)
Groundwater flow
No Groundwater
Groundwater
Topsoil
Figure 4. Hydrologic pathways for P transport in the: (a) soil profile including matrix and
preferential flow (Haygarth and Jarvis, 1999) and (b) slope/field scales including overland
flow, interflow and land drainage (e.g. tiles).
either overland flow, or deep and shallow interflow, with potential interchanges between them as
the degree of soil saturation, slope and infiltration capacity changes (Johnes and Hodgkinson,
1998). Runoff, a very common term used as a hydrologic pathway should not be used to
describe a specific pathway (Haygarth and Sharpley, 2000). If used, it should be used in a very
general sense to describe all the pathways at the slope/field scale. The five pathways mentioned
occur in the unsaturated zone. Groundwater flow (saturated conditions) can also be a major
pathway by which water reaches streams. Not many studies have looked at the P contributions
13
of groundwater flow. The first reason is that groundwater P contributions are small compared to
P transported in the unsaturated zone (Peterjohn and Correll, 1984). However, Phillips et al.
(1982) found that RP (<0.45) concentrations in groundwater did not change with depth and over
a number of years but were 10-20 µg L-1, concentrations that are high enough to cause
eutrophication. The second reason is that groundwater nutrients are difficult to measure. The
nutrients are typically transported in anoxic and hypoxic environments and when the
groundwater reaches the surface waters it is oxygenated and the nutrients precipitates in the
surface sediments (Kalff, 2001).
i) Matrix flow: Matrix flow is the uniform downward movement of water through the
macro- and micro-pores of the soil. The scale is the soil profile, the plane is vertical, and time is
measured in days. Phosphorus is immobile and easily adsorbed in the soil. This is especially
true for the subsoil horizons that are P deficient (Sharpley et al., 2000). Typically, there is little
movement of P in the matrix flow pathway because the surface soil accumulates most of it
(Mozaffari and Sims, 1994; Oldham, 1998). Sandy, acid, organic soil with low P fixation and
holding capacities can have increased P in matrix flow (Sharpley et al., 2000).
ii) Preferential flow: Preferential flow is also the downward movement of water but in
larger subsoil pathways, such as fissures and cracks, burrows, and wormholes. The scale is the
soil profile, the plane is vertical, and time is measured in minutes or hours. Water moves faster
with less chance of adsorption and therefore greater transport of TP (<0.45) and TP (>0.45)
(Addiscott et al., 2000). Preferential flow can occur in heavy, clay-rich soils because of the
cracks that develop in response to drying and wetting, and where there is a heavy population of
earthworms and other burrowing organisms.
iii) Overland flow: Overland flow is the downslope movement of water over the soil
surface during heavy rainfall events (Figure 5). The scale is the slope or field, the plane is
lateral, and time is measured in minutes or hours. It has been traditionally considered as the
major transport pathway for P in agricultural landscapes (Sharpley et al., 1993; Oldham, 1998).
Erosion of
TP (>0.45)
Overland Flow
TP (<0.45)
TP (<0.45) and TP (>0.45)
Figure 5. Mechanisms and forms of P transported in overland flow (Daniel et al., 1994).
14
Overland flow can carry P in solution (TP (<0.45)) and the P in the eroded soil (TP
(>0.45) (Sharpley et al., 2000), and is generated by intense rainfall where infiltration capacities
cannot keep up with rainfall intensity (Johnes and Hodgkinson, 1998). Although it contributes
only periodically to stream flow, the quantities of P losses are considerable (Sharpley et al.,
1976). Overland flow is efficient, first, because the largest concentration of P is in the surface
layers of the soil, and second, because the greatest concentrated hydrologic energy is on the soil
surface (Haygarth et al., 1998a). Soil physical properties, vegetation cover and slope steepness
determine the efficiency of this pathway (Sharpley and Syers, 1976a). Phosphorus losses from
overland flow can vary threefold as site hydrology, relative drainage volumes, and soil P release
characteristics change (Sharpley and Tunney, 2000).
iv) Interflow: Interflow is the lateral movement of water in the soil. The scale is the
slope or the field, the plane is lateral, and time is measured in minutes or hours. Typically,
interflow P losses are much lower than overland flow. For three corn-cropped watersheds, the
average RP (<0.45) losses from overland flow were 0.105, 0.250 and 0.381 kg ha-1, respectively,
while for interflow the P loses for the same watersheds were 0.028, 0.030 and 0.031 kg ha-1 over
a ten year period (Alberts and Spomer, 1985).
Stevens et al. (1999) found that interflow could not be ignored in many cases. In their
research, overland flow was the major pathway for a soil with a shallow silty clay loam A
horizon and a medium-heavy clay loam B horizon. But for a soil with a sandy A horizon
and a heavy clay B horizon with low hydraulic conductivity, interflow was the major pathway.
Interflow provides the major proportion of flow in many streams and can be the major
contributor of P, although it typically has low P concentrations (Sharpley et al., 1976).
Preferential flow and P forms less susceptible to adsorption also may enhance P losses in
interflow (Heckrath et al., 1995). High soil P levels can also increase P loss from interflow
(Sharpley and Tunney, 2000). Typically, sandy Spodosols (Sharpley et al., 1994), Histosols
(Izuno et al., 1991; Duxbury and Peverly, 1978) and poorly drained soils in arable regions
(Johnes and Hodgkinson, 1998; Deal et al., 1986) can have significant P losses in interflow.
v) Land drainage: Land drainage includes a number of management practices that result
in increased water and solute export from watersheds. The scale is the subcatchment, the plane
is lateral, and time is measured in minutes or hours. These practices can be divided in: open
channels, tiles, and moles. Their purpose is to remove water from the upper part of the soil
improving trafficability and decreasing waterlogging that damages the crops. Artificial drainage
reduces the overland flow volume and the amounts of P forms and sediment overland flow
transports (Sharpley et al., 1976; Turtola and Paajanen, 1995; Nelson et al., 1996). Note that
wetland drainage should be avoided because wetlands are nutrient sinks (Nelson et al., 1996).
Recent research indicates that land drainage can contribute significantly to and cause
eutrophication, although its contributions are less than from overland flow (Sims et al., 1998).
Open channels: Open channels include open ditches and canals (Schwab et al., 1993). In
this review, only ditches will be discussed because most of them drain to surface waters. In
contrast, canals carry irrigation water. Open ditches are primarily used in flat land with
impermeable subsoil and shallow topsoil because subsurface drainage is not economical and
practical. There are three types of open ditches (Schwab et al., 1993): i) bedding-shallow
depressions that are plow deadfurrows, ii) field ditches-drains that are more widely spaced than
deadfurrows, and iii) parallel open ditch-drains that machinery cannot cross.
Campbell et al. (1985) found that furrows and overland flow had similar P concentrations
(0.30 and 0.29 mg/L, respectively). However, Izuno et al. (1991) found that P concentration
15
decreased as the water moved to larger ditches due to dilution, assimilation, and adsorption.
Less contact with the surface soil, compared to overland flow, also suggests that water in ditches
could decrease NPS P contributions. Although P concentration might decrease, TP (unf.) loads
may still be high enough to cause eutrophication. Sallade and Sims (1997a and b) found that in
ditches in the Delaware Inland Bay watershed, 42 % of the winter and 53 % of the spring
samples had TP (<0.45) concentrations that could cause eutrophication.
The quality of drainage waters in ditches is also influenced by the crops that are present,
management practices that are used and field conditions (Izuno et al., 1991). Crops with high P
uptake in soils and high P mineralization can have very low P losses. In contrast, Coale et al.
(1994a) found TP (<0.45) and TP (unf.) losses between sugarcane and fallow plots from drainage
events were not different. Faster field drainage rates lead to smaller P losses (Coale et al.,
1994b). Under slower drainage rates the soil becomes saturated faster and for a longer period
that leads to higher TP (<0.45) losses although TP (>0.45) losses do not decrease compared to
fast field drainage rates.
Tiles: Tiles can be separated according to their inlets to: i) surface inlets and ii) blind
inlet (French drain) (Schwab et al., 1993). The surface inlet tiles have a surface intake structure
that removes surface water from potholes, road ditches and other depressions. In contrast, the
blind inlet tiles have no surface intake and are constructed by backfilling the trench. Blind inlet
tiles should be more successful in reducing P concentrations because the water that reaches these
tiles travel through the soil and P can be adsorbed. For surface inlet tiles, P losses could be
higher, since the water originates as overland flow. Surface inlet tiles could be successful in
reducing P losses if the water spends less time on the surface and causes less surface erosion,
thereby minimizing TP (>0.45) losses.
Many studies do not identify the type of tiles under investigation, but the ones that do
have focused primarily on blind inlet tiles. In this review, both types of tiles will be treated
similarly. Sharpley and Syers (1979b) found that tiles contributed less P to streams than
overland flow, but more than interflow. Haygarth et al. (1998a) found that TP (unf.) losses were
reduced by 30 % in tile drained lands compared to overland flow, while Schwab et al. (1980)
found a reduction of 45 %, because only a small portion of P originated from the surface when
soils were drained. Tiles were also much more successful in reducing P compared to surface
drainage ditches (Campbell et al., 1985, Sims et al., 1998). Tiles primarily reduced TP (>0.45)
(Bottcher et al., 1981).
Other studies have found large amounts of sediment in tile effluent, as much as 2,800 kg
ha-1 (Logan and Schwab, 1976). Particulate matter carrying P can be transported from the
topsoil to the tiles within hours (Laubel et al., 1999). As the water flows vertically through the
soil (matrix flow) to the drains adsorption decreases P concentration (Haygarth et al., 1998b).
However, when preferential flow occurs, adsorption is more limited because of the reduced
contact time between percolating waters and the soil, and sediments will be more easily
transported (Sharpley and Syers, 1979a).
Powlson (1998) believes that more P moves by preferential flow than previously thought
and this is why tile waters can promote eutrophication. Sims et al. (1988) report that ridge tillage
had higher P losses in tiles than conventional tillage because of the enhanced formation of
macropores that increased preferential flow. Similarly, fertilized forages had greater TP (<0.45)
losses than continuous corn because of the better established macropore network.
Even if tile water has low P concentrations, it can be a major pathway, because of the
large areas that have been tiled, that contribute significant amounts of water to stream flow. Tile
16
drain effluent can also contain greater P concentrations during flooding events that create
anaerobic or reducing conditions that lead to increased P mobility (Sawhney, 1978).
Phosphorus losses are significantly influenced by agricultural management practices
(Sims et al., 1998), crop cover (Sawhney, 1978; Phillips et al., 1982) and surface soil P. Greater
soil cover over tile lines decreases P concentrations (Bottcher et al., 1982). Phosphorus
concentrations in tiles increased substantially after the soil reached 57 mg kg-1 Olsen P even
though the subsoils were P-deficient (Heckrath et al., 1995; Hesketh and Brookes, 2000). This
soil P value is called the change-point. In soils with P values above the change-point, tile
effluent has elevated P losses because of P transport in preferential flow. Additionally when the
soil P is above the change-point it is well above what is needed for plants optimum yield.
Moles: Moles are cylindrical artificial channels in the subsoil similar to tile but not lined
and not as deep (Schwab et al., 1993). They have been successfully used in England and New
Zealand, but are not commonly used in the United States. They are a temporary method of
drainage and deteriorate within the first few years. Sharpley and Syers (1976a) found that moles
decreased overland flow and P transport. Doubling the mole spacing increased P losses
(Addiscott et al., 2000). As the spacing and the travel distance of the water increased, water
reaching the moles had greater amounts of P-enriched sediment.
2.1.4 Hydrology
Hydrology is the main driving force and carrier for P transport (Sharpley and Tunney,
2000; Haygarth and Jarvis, 1999; Edwards and Withers, 1998). Gburek et al., (2000) mention
that to assess water quality problems the hydrologic pattern (wet vs. dry periods) should be
considered. Phosphorus concentrations in surface waters do not follow a consistent seasonal
pattern (Owens et al., 1989). Instead annual and seasonal P concentrations are higher with
higher precipitation (Correll et al. 1999; Shirmohammadi et al., 1997) or peak discharges (Kolpin
et al. 2000). Major rainfall events can account for up to 90% of annual P losses (Sharpley et al.,
2000; Edwards and Withers, 1998; Schuman et al. 1973) with the lowest P concentration in
surface water during baseflow (House et al., 1998) (Table 5). Rainfall intensity, duration, and
timing as well as snowmelt events have major influences on P transport and loss (Truman et al.,
1993; Culley and Bolton, 1983) (Table 5). High antecedent soil water content also increases P
losses because of higher runoff volumes (Edward and Withers, 1998; Coale et al., 1994a).
Hydrologic effects can be further separated into temporal and spatial effects. The
temporal effects have two major levels of P transport. Level 1 is associated with baseflow, and
level 2 is associated with stormflow (Haygarth and Jarvis, 1999). Typically, in the temperate
region 7.5 mm d-1 of rain may cause erosion (Evans, 1978), while 5 mm h-1 of rain or more is
considered high intensity that can cause severe erosion (Boardman and Robinson, 1985). Spatial
hydrologic effects can be divided into soil profile, slope/field and catchment effects. Soil profile
describes the vertical water transport pathways (matrix and preferential flow) over centimeters to
meters of depth. Slope/field describes lateral water pathways (overland flow, interflow, and land
drainage) over hectares to square kilometers. Finally, catchments (watersheds) include land-use
variables from square kilometers and upward.
17
Table 5. Phosphorus concentrations in stream and drainage water under various flow
regimes. Ranges are in the parenthesis when available.
Flow
regimes
Stormflow
TP (unf.)
1.5 a
TP (< 0.45)
Land-use
-1
mg L
1.0 a
0.15 b
0.49 (0.14-2.37) c
1.20 (0.16-4.30) c
1.43 (0.50-5.21) c
0.46 (0.07-3.30) d
0.13 (<0.01-0.36) d
Pasture with dairy operations, stream water.
Grassland, tile drains.
Pasture, stream water.
Riparian pine afforested 1, stream water.
Riparian pine afforested 2’ stream water.
Cropland (91%), stream water.
Snowmelt
0.27 (0.09-0.90) d
0.155 (0.02-0.45) d
Cropland (91%)
Baseflow
0.8 a
0.4 a
0.050 b
Pasture with dairy operations, stream water.
Grassland, tile drains.
Pasture, stream water.
Riparian pine afforested 1, stream water.
Riparian pine afforested 2’ stream water.
Cropland (91%), stream water.
0.18 (0.07-0.60) c
0.20 (0.06-0.47) c
0.32 (0.14-1.16) c
0.071 (0.02-0.61) d
a
0.031 a (<0.01-0.23) d
b
c
Shirmohammadi et al. (1997); Stamm et al. (1997); Smith (1992); d Culley and Bolton (1983).
2.1.5 Terrestrial sources of phosphorus for surface waters
Major sources of P that get into surface water in agricultural watersheds are atmospheric
deposition, the soil, and agronomic inputs. Phosphorus in the soils originates from weathering of
parent materials, as well as from external inputs of fertilizers, manure, animal feces and animal
litter. While the agronomic sources of P provide inputs to the soil they may also serve as direct
sources to surface water in surface runoff events.
2.1.5.1 Atmospheric deposition
The importance of atmospheric deposition depends on the ration of the drainage area of
the watershed to that of the actual surface water. In most lotic systems the drainage area is much
greater than the water surface area. As a result, the low concentrations of P in atmospheric
deposition are not a very important source compared to agronomic inputs in lotic surface waters
(Johnes and Hodgkinson, 1998; Ryding et al., 1990).
Atmospheric deposition (wet and dry) is 0.22 kg TP (unf.) ha-1yr-1 in England, 0.11 kg TP
(unf.) ha-1yr-1 in Scotland (Haygarth et al., 1998b), while in Illinois wet atmospheric deposition is
much lower, 0.02 kg TP (unf.) ha-1yr-1 (David and Gentry, 2000). Jordan et al. (1995) found in
the Rhode watershed in Maryland even smaller amounts of atmospheric deposition (wet and dry)
(0.0002 kg TP (unf.) ha-1yr-1). For the whole Rhode watershed, the atmospheric deposition (wet
and dry) amounts were 0.94 kg TP (unf.) yr-1 while the rest of the load from the watershed was
1749 kg TP (unf.) yr-1 (Correll et al., 1992).
Gibson et al. (1995) summarized a number of studies and found that wet atmospheric
inputs typically range from 0.05-0.40 kg TP (unf.) ha-1yr-1 and are not a significant source.
However, others have found that rainfall can contribute up to 25 % of annual P inputs (Johnes et
al., 1996) to freshwater and be a significant source of P eutrophication (Schindler, 1977; Lee,
18
1973). Schwab et al. (1980) found in a crop field in Ohio that rain water averaged 4.6 kg TP
(unf.) ha-1yr-1 while surface drains and deep pipe drains averaged only 1.9 and 1.6 kg TP (unf.)
ha-1yr-1, respectively.
2.1.5.2 Soil Sources
Soil P sources have a major influence on dissolution and physical modes of transport.
They control the release of P from the solid phase to the solution (dissolution) phase, and the
amount of P in the particulates that are transported by water or wind (physical) (Haygarth and
Jarvis, 1999). Although total soil P concentrations in the soil, range from 0-0.4 % with an
average of 0.05 % (kg/ha in the furrow slice) (Ryan, 1983), soil may be the major source of P to
surface waters (House et al., 1998). To understand the reasons for this seeming discrepancy it is
necessary to describe the different forms of P in the soil, the P cycle in soil, current soil P levels,
the influences of different soil textures, soil microorganisms and the current soil P test methods
that are used to identify the P in the soil.
Soil P forms: Phosphorus in the soil can either be in soil solution or in the soil matrix.
Typically, soil P is described in terms of the following relationship (Larsen, 1967):
Soil solution P ↔ labile soil P ↔ nonlabile soil P
In general, out of 1000 kg ha-1 of P in the soil only 1 kg ha-1 is in solution (Troeh and
Thompson, 1993). Soil typically contains 100-3000 mg orthophosphate kg-1 (Frossard et al.,
2000) the form that plants primarily take up. The dominant orthophosphate form changes with
the pH (Pierzynski et al., 2000):
pH
orthophosphate form
2.2
H3PO4 ↔ H2PO4
7.2
-
12.3
↔ HPO4 ↔ PO4-3
-2
The labile and nonlabile soil P describes the condition of P in the soil matrix (Troeh and
Thompson, 1993). Labile soil P refers to the P in the soil that readily replaces P lost from the
soil solution (e.g. plant uptake) within a period of several days or a few weeks to maintain
equilibrium. In contrast, nonlabile is not readily available.
The main forms of P in the soil matrix are inorganic P and organic P (Table 6) (Bowman,
1989; Tate and Newman, 1982; Syers, 1974) and they can be both labile and nonlabile.
Intensively managed agricultural soil with long-term fertilizer and manure inputs has higher
proportions of inorganic P (Hawkins and Scholefield, 1984) ranging from 8 to 72% of the total P
(Haygarth and Jarvis, 1999). Inorganic P is easily released in solution and is primarily
considered labile. Major inputs of inorganic P in soils are weathering (mineral apatite
Ca5(PO4)3F) and fertilizers. Little is known about the role of organic P in P transport although its
importance is increasingly acknowledged (Haygarth et al., 1998a; Hannapel et al., 1963a).
Organic P is derived from plant residues and excreta from above- and below-ground organisms
(Haygarth and Jarvis, 1999). The proportion of P in organic forms can range from 30 to 65 % of
the total soil P (Frossard et al., 2000). The most common organic compounds are phytic acid,
nucleic acids and phospholipids that range from 35, 2 and 1% (Anderson, 1980) to 50, 2 and 5%
(Haygarth and Jarvis, 1999), respectively of the total organic P in the soil matrix.
19
Table 6. The common chemical forms of P in soil and their characteristics and potential
mobility. a
Form
INORGANIC in solution
Orthophosphate (H3PO4)
INORGANIC in the soil matrix
Apatites
Hydroxyapatite (Ca10(PO4)6OH2)
Fluorapatite (Ca10(PO4)6F2)
Sodium phosphates
.
Pyrophosphate (Na4P2O7 H2O)
Polyphosphate (Na3PO3)n
Other calcium phosphates
.
Monocalcium phosphate (Ca(H2PO4)2 H2O)
.
Dicalcium phosphate (Ca(HPO4) 2H2O)
Tricalcium phosphate (Ca3(PO4)2)
.
Octacalcium phosphate (Ca8H2(PO4)6 5H2O)
Aluminum phosphates
.
Variscite (AlPO4 2H2O)
.
Taranakite (H6K3Al5(PO4)8 18H2O)
Wavelite (Al8(OH3(PO4))2)
Iron phosphates
.
Vivianite (Fe3(PO4)2 8H2O)
.
Strengite (FePO4 H2O)
Surface-adsorbed P
ORGANIC in soil matrix
Phytic acid or Inositol hexaphosphates
(IHP:(C6H6O6)(PO3)6)
Phosphate diesters (nucleic acids, DNA, RNA,
phospholipids)
Glucose P (D-Glucose-6-phosphate; D-Glucose-1phosphate)
Phosphonates (R-PO3)
Polyphosphanates (ATP,AMP)
a
Characteristics or Implication
Readily mobile but easily
adsorbed/immobilized
Very low solubility. Tend to be
present more in nonacid soils than
in acid.
Soluble. No known information
on the implications
Tend to form when fertilizers are
added to nonacid soils.
Tend to form when fertilizers are
added to acid soils that have
aluminum.
Tend to form when fertilizers are
added to acid soils that have iron.
Adsorbed on calcium, iron and
aluminum compounds.
Not generally thought to be easily
mobile except in sandy soils:
maybe highly sorbed but have
been noted in lake sediments
Probably mobile but confirmation
is required
Leaches through sandy soils
No known information on the
implications
No known information on the
implications
Haygarth and Jarvis (1999); Troeh and Thompson (1993); Anderson (1980); McClellan and Gremillion (1980);
Sample et al. (1980).
20
The established terms for P forms in the soil, organic, inorganic, labile and nonlabile, are
ambiguous because they are physical and chemical definitions (Haygarth, 2001; Sharpley, 2001).
As with the P forms in water, Haygarth and Sharpley (2000) suggest that an operational
definition should be incorporated with each term describing soil P. The classification of P in
solution is similar to the classification of P in water (Table 3). For the classification of P forms
in the soil matrix, the operational definitions of the procedure used to measure P is incorporated
(Bray-P, Mehlich-P, Olsen-P). More work is needed for the classification of P in the soil.
Soil P-Cycle: The P cycle in the soil includes inputs and outputs of P, as well as internal
cycling of P (Figure 6). A point to note about the soil P-cycle is that there is no process to
remove P to the atmosphere like in the N-cycle (denitrification) (Cooper and Gilliam, 1987).
Removal of P can occur by erosion of enriched sediment, desorption by moving water (runoff,
leaching, tiles), or crop and animal removal. The major internal P inputs are weathering and the
major external P inputs to soil include fertilizers, agricultural wastes, plant residues, atmospheric
deposition, and municipal/industrial by products. This review deals with NPS pollutants and will
not discuss municipal/industrial by-product inputs. Finally, the internal cycling processes
include immobilization-mineralization, adsorption-desorption, and precipitation-dissolution.
Internal Soil P Inputs
Weathering: Phosphorus in the soil originates primarily from weathering of minerals and
other more stable geologic materials (Pierzynski et al., 2000) and typically is a minor input for
soil in agricultural landscapes (Johnes and Hodginkson, 1998). However, Abrams and Jarrell
(1995) suggest that native soil P is a potential source for NPS P pollution. In other studies,
Atmospheric
deposition
Atmospheric
deposition
Agricultural
wastes
Fertilizers
H2PO4-, HPO4-2
Municipal/Industrial
by products
Plant
residues
Weathering
Figure 6. Phosphorus cycle in the soil with inputs, outputs and transformations that take
place in the soil (modified from Pierzynski et al., 2000).
21
geology is the most important factor correlating P concentrations in the stream water compared
to land-use practices (Thomas et al., 1992; Thomas and Crutchfield, 1974).
External P inputs into the soil
Atmospheric deposition: Wet atmospheric inputs typically range from 0.05-0.40 kg TP
-1 -1
ha yr (Gibson et al., 1995) and are a minor input for soil (Haygarth et al., 1998b).
Agricultural wastes: Animal manure, litter from animal confinements, and feces are the
major agricultural wastes. Animal manure is animal waste from confinement facilities that are
applied to cropfields. Cultivated soils that have repeated manure applications have consistently
higher soil P (Mehlich I) than the field borders that do not receive manure (Mozaffari and Sims,
1994). Animal litter includes bedding mixed with animal manure. Soils with broiler litter
applied for twenty years had 86 times more Mehlich-III P in the plow layer than unamended soil
(Oldham, 1998). Typically, manure or animal litter applications are designed to meet crop N
levels. Low N:P ratios in manure and animal litter lead to over application of P compared to the
needs of crops (Sharpley et al., 1996). Animal feces is animal waste that is deposited by animals
in pastures and rangelands, and can be a significant input on pastures, especially if they are
overgrazed. Haygarth et al., (1998b) found feed supplements were a large source of high P
levels in dairy manure with 70% of P intake excreted in animal feces (Tamminga, 1992).
Fertilizers: For decades P fertilization rates have exceeded the amount of P removed by
crops and that has resulted in increased soil P (Sharpley et al., 1994).
Plant residue: Plant material that is not harvested and is left in the fields can increase P
levels, especially if P fertilization is not incorporated in the soil profile (Sharpley et al., 1994).
On no-till fields, that had surface applied fertilizer P the soil total P levels increased by six times
within a couple years (Griffith et al., 1977).
Internal Cycling
Adsorption-desorption, and precipitation-dissolution are the major soil inorganic P
transformations (Pierzynski et al., 2000) (Figure 6). Adsorption refers to the chemical bonds that
form between orthophosphate anions (H2PO4-, HPO4-2) and soil colloids. Clays, oxides and
hydroxides of Fe, cations of Al and calcium carbonate, and organic matter are the main reactive
soil phases. Desorption, is the release of P from the solid phase into the soil solution. Plant
uptake, runoff, and leaching cause desorption. Precipitation is the formation of insoluble
compounds in soils and dissolution is the reverse process. Orthophosphate anions reacting with
Ca, Al, and Fe are the most common precipitated P products. Mineralization-immobilization is
the major soil organic-inorganic P transformation (Pierzynski et al., 2000) (Figure 6).
Mineralization is the decomposition of organic matter by microbes that results in the release of
inorganic P and is highly dependent on soil conditions. Immobilization, in contrast, is the
conversion of mineral (inorganic) P to biochemical compounds (organic P) by soil
microorganisms and is part of the active soil biomass fraction of the soil. Internal cycling is the
major replacement mechanism of P in the soil solution with labile soil P. The replacement can
either occur with desorption of adsorbed P ions on the soil or dissolution of P compounds.
Soil P levels: Similar soils may have different P losses to surface waters because of
different soil P levels (Pote et al., 1999). Soil can be a source of P for surface waters when P
levels are high (Abrams and Jarrell, 1995). Phosphorus can build up in soils because it is fixed
in soils and not easily leached (Haygarth et al., 1998a). As soil P levels get higher in the surface
22
soil, the risk of P moving to the surface water by soil erosion and overland flow becomes greater
(Mitchell, 1998). Important relationships between P losses to surface waters and soil P levels
have been observed in grasslands (Smith et al., 1995), arable lands (Heckrath et al., 1995), forest,
and cropped watersheds (Sharpley et al., 1994). Vaithiyanathan and Correll (1992) found that
soil P content and soil loss in runoff were highly correlated (r2 = 0.96) for both forested and
cropped watersheds in the Atlantic Coastal Plains.
Intensive agriculture has maintained a P surplus in the soil (Table 7) from P fertilizers
and animal waste inputs that exceed P outputs (Sharpley et al., 1994). Producers have applied P
at rates exceeding crop uptake (Pote et al., 1996). The high P levels in the top 5-10 cm with the
rapid decrease of P below this depth, indicates that P has accumulated in the soil from external
sources (Haygarth et al., 1998a; Cooper and Gilliam, 1987). Globally, from 1950 to 1995 ~600
x 106 Mg of fertilizer P were applied on the earth’s surface (primarily in croplands) while only
~250 x 106 Mg were removed from croplands through harvest (Carpenter et al., 1998).
Additionally, animal manure has added 50 x 106 Mg that leads to a net addition of P during this
period of 400 x 106 Mg (Carpenter et al., 1998). Once soil P levels exceed crop P requirements,
the potential of P losses with runoff and erosion is greater than any agronomic benefit (Figure 7).
In Ireland, the average soil P test has increased 10-fold in the last 45 years (Tunney et al., 1997).
Table 7. Soil P surpluses for different developed countries under different agronomic landuse practices.
Country
Management
Denmark b
cropfields
Federal Republic of Germany c
cropfields
cropfields
Germany Democratic Republic c
cropfields
Europe b
cropfields
Italy b
cropfields
Lombardia, Italy b
cropfields
Netherlands (two references) b
cropfields
United Kingdom b
cropfields
England U.K. b
grassland, dairy farm
Belgium b
grassland, dairy farm
Denmark b
grassland, dairy farm
Europe b
grassland, dairy farm
Brittany, France b
grassland, dairy farm
Italy b
grassland, dairy farm
Lombardia, Italy b
grassland, dairy farm
Netherlands b
grassland, dairy farm
United Kingdom b
grassland, dairy farm
England U.K. b
grassland, dairy farm
Devon, U.K. b
grassland, dairy farm
Northern Ireland, U.K. b
grassland, dairy farm
Pennsylvania, U.S.A. b
grassland, extensive sheep
Scotland, U.K. b
poultry-grain farm
Delaware, U.S.A. a
various
Inland Bay watershed, Delaware, U.S.A. a
various
Sussex County, Delaware, U.S.A. a
various
Delaware, U.S.A. a
a
Pierzynski et al. (2000); b Haygarth and Jarvis (1999); c Isermann (1990).
Surplus
kg P ha-1 yr-1
49
55
71
17
11
50
94
3
2
38
11
24
92
21
32
49
14
14
27
24
11
0.24
34
52
70
30
23
Soil P Test (mg kg-1)
Figure 7. Relationship between soil P test, crop yield, and environmental problems due to
excessive soil P (Pierzynski et al., 2000).
In Illinois, large inputs of fertilizer and manure occurred from 1965 to 1990 with a net
input of 230 kg P ha-1 yr-1 (David and Gentry, 2000). Since 1990, the P cycle has become
balanced because P fertilization decreased to the point where net P export matches inputs. But
most of the P surpluses from the last 25 years are still in Illinois soil because once high levels of
soil P have been attained, considerable time is required for significant depletion (Sharpley et al.,
1994).
Phosphorus accumulations (Figure 8) in the United States have reached a point were soils
are considered ‘over-fertilized’ because their P levels exceed crop needs (Sharpley and Smith,
1989a). Thomas and Crutchfield (1974) found no direct correlation between stream water P
concentrations and concentrations of fertilizers applications. Halving P fertilizer applications did
not significantly lessen P losses in mole drain flow (Addiscott et al., 2000). In both cases, the
fertilizers had no effect because the potentially mobile P levels in the soil were very high and
soils are the primary contributor to P losses in runoff (House et al., 1998; Pote et al., 1996).
Soil Type: Soil textural differences may account for differences in TP (unf.) in the soil
(Table 8) but also in different P losses to surface waters (Stevens et al., 1999; Heckrath et al.,
1995). Organic soils have greater P losses than mineral soils (Duxbury and Peverly, 1978).
Sandy A-horizon soils and clay-cultivated layers can have very high P losses (Catt et al., 1998).
These differences are due to different soil dispersion and soil chemical transformations. In many
cases soil with higher soil P levels are less threatening to surface water because of low
dispersivity and dissolution, and high adsorption. Low dispersivity and dissolution, and high
adsorption decrease the chances of soil P transport.
Soil dispersivity is promoted with high pH, high percentage of larger particles, low salt
concentrations and high ratio of monovalent to divalent and trivalent cations. The major affect of
high soil dispersivity is that it increases soil erosion and TP (>0.45). However, soil dispersion
24
Figure 8. Percentage of soil samples with high soil P levels in 1989 (Sharpley et al., 1996).
Table 8. Percentage of TP (unf.) in A, B and C horizons of several kinds of soil. Methods
used to measure TP (unf.) in the soil were not described. a
Percentage P (%) (kg ha-1 in furrow slice)
Soil Type
Soil Order
A horizon
B horizon
C horizon
Miami Silt loam
Alfisol
0.035
0.031
0.035
Barnes silt loam
Mollisol
0.100
0.065
0.065
Holdrege silty clay loam
Mollisol
0.087
0.092
0.096
Mackburg silty clay loam Mollisol
0.061
0.072
0.086
Marshall silt loam
Mollisol
0.052
0.044
0.070
Otley silty clay loam
Mollisol
0.055
0.055
0.070
Richfield clay loam
Mollisol
0.044
0.017
0.039
Sharpsburg silty clay loam Mollisol
0.059
0.073
0.081
Nipe clay
Oxisol
0.250
0.140
0.140
Becket fine sandy loam
Spodosol
0.057
0.035
0.031
Davidson clay loam
Ultisol
0.044
0.087
0.105
Maury silt loam
Ultisol
0.136
0.158
0.240
Sassafras sandy loam
Ultisol
0.048
0.035
0.031
Houston black clay
Vertisol
0.065
0.074
0.039
a
Troeh and Thompson (1993).
also influences losses from fertilizer and manure. Phosphorus losses from fertilizers are greater
in more dispersible soils (Addiscott et al., 2000; Catt et al., 1998). In dispersible soils, it is
recommended that the fertilizer or manure be applied when soil moisture content is below field
capacity (Catt et al., 1998).
The soil chemical transformations that are most affected by soil type are adsorptiondesorption. These transformations control P transport between the solid phase and soil solution
and subsequently the vulnerability of soil P to losses in water as TP (<0.45) (Frossard et al.,
2000). Soils with the highest P adsorption are usually, acidic, and high in clay or Fe/Al oxides,
particularly amorphous oxides (Pierzynski et al., 2000). Large areas with high clay content are
25
important P sinks (Cooper and Gilliam, 1987). High levels of reactive Fe and Al in the soil can
also reduce P transport (Abrams and Jarrell, 1995). In general, weathered soils that have more
clay and Al/Fe oxides adsorb greater amounts of P (Pierzynski et al., 2000). In soils with high
pH (calcareous) the existence of CaCo3 and Fe oxides increase P adsorption (Pierzynski et al.,
2000). Adsorption maxima give the long-term capacity of soil to retain P (Sharpley et al., 1994).
Sandy soils are more susceptible to P leaching when fertilizer is added because of low P
adsorption (Pierzynski et al., 2000). In soils with high organic matter (peats and heavily
manured soils), P mobility is enhanced because the colloidal surfaces responsible for P
adsorption are coated by soluble organic matter, and because organic P complexes leach faster
and to greater depths. Soils with high equilibrium P concentration at zero adsorption have a
greater tendency to desorb TP (<0.45) in runoff (Wolf et al., 1985).
Soil organisms: Soil organisms can influence P transport because they play a major role
in P immobilization-mineralization processes that control the transformations of P between
inorganic and organic forms (Frossard et al., 2000). Organic P forms are mineralized through
ingestion/excretion by soil organisms while inorganic forms are immobilized by retention within
their biomass (Patra et al., 1990; Brookes et al., 1982). Plant roots and soil microbes can cause
hydrolysis of organic P. Hannapel et al. (1963b) found that the addition of a microbial energy
source increased mobility of P by 38 times. Treatments with formaldehyde suppressed microbe
activity and reduced P mobility. When inorganic P was added in the residue, P transport was not
significantly increased. This indicated that the mobilization of P by the microbial population
was the most important factor in P transport (Hannapel et al., 1963a).
Earthworms increase soil P levels because their casts contain finer soil particles that can
release 4 times more inorganic P into solution than surface soil (Sharpley and Syers, 1976b).
However, absence of earthworms would reduce infiltration and the litter incorporation into the
soil thereby increasing overland flow and P losses (Radke and Berry, 1993; Sharpley et al.,
1979).
Some microorganisms can solubilize certain P forms and cause leaching (Illmer et al.,
1995), while others can excrete P (Haygarth and Jarvis, 1999). Passively, soil organisms can
also transport P as it is attached or contained within them (Haygarth and Jarvis, 1999). Finally,
when these organisms die, wetting/drying and/or freezing/thawing cause cell lysis of the dead
organisms that releases P that is available to the soil solution (Haygarth and Jarvis, 1999).
Soil P test: It is very difficult to rectify P losses in surface waters when soil P is high.
This is because reduction of surplus P in soil needs long-term planning (Sharpley et al., 2000).
Before meaningful planning can be conducted soil P analysis must be conducted. Most of the P
accumulation in the soil is in the upper few centimeters that are the most critical for P runoff to
surface waters (Ahuja, 1986).
The soil P tests (Bray-I, Olsen, and Mehlich-I and -III) are simple and inexpensive tools
that can assure optimum crop production and optimum nutrient availability for crops (Maguire et
al., 1998). From an environmental prospective, however, the agronomic soil P tests are not very
useful. These tests are based on crop nutrient requirements not on runoff water quality. In many
cases, they might underestimate potentially mobile P that could be transferred to surface waters
(Haygarth and Jarvis, 1999). For example, Olsen P represents only 1-5% of the total soil P.
However, the remaining 95-99% of the soil P might also be potentially mobile. The sampling,
analytical, interpretive, and educational role of soil P testing should be re-evaluated to meet
26
environmental goals (Sims, 1993). Alternative soil testing approaches should be considered for
soil P release to runoff. These approaches include the estimation of algal available P (AAP), or
water extractable P and the iron-oxide impregnated paper method (Pote et al., 1996; Sharpley,
1993). Another approach gaining acceptance is the P adsorption saturation of the top 5 cm of
soil (more effective than top 15 cm) (Sharpley et al., 1996).
Threshold soil P test values have been recommended (Table 9). However, different soils
can have different susceptibilities to P losses irrespective of soil test P (Sharpley et al., 2000).
Phosphorus management recommendations for soils with similar P, but contrasting land-use
management and topography, could lead to extreme differences in P losses. Pote et al. (1996)
Table 9. Recommended agronomic and environmental soil P test threshold values with the
appropriate P management recommendations. a
State
Arkansas
Agronomic b Environmental
Threshold values
-1
mg kg
50
150
Soil P Test
method
Management recommendations for water
quality protection
Mehlich-III
At or above 150 mg kg-1 soil P:
Apply no more P, provide buffers next to
streams, overseed pastures with legumes
to aid P removal, and provide constant
soil cover to minimize erosion.
Above 50 mg kg-1:
Apply no more P unless soil P is
significantly reduced.
Sandy soil-above 50 mg kg-1:
Silt loam soils-above 100 mg kg-1:
Apply no more P unless soil P is
significantly reduced.
Above 159 kg mg-1:
Reduce erosion and reduce or eliminate
P additions
30 to 130 mg kg-1 soil P:
Half P rate on slopes > 8%
130-200 mg kg-1 soil P:
Half P application rates and reduce
surface runoff and erosion
Above 200 mg kg-1 soil P:
P rate not to exceed crop removal
Below 75 mg kg-1 soil P:
P application not to exceed crop removal
Above 75 mg kg-1 soil P:
Apply no P from any source
Above 200 mg kg-1 soil P:
P addition not to exceed crop removal
Below 75 mg kg-1 soil P:
Rotate to P demanding crops and reduce
P additions
Above 75 mg kg-1 soil P:
Discontinue P applications
Delaware
25
50
Idaho
12
50 and 100
Olsen
Ohio
40
150
Bray-I
Oklahoma
30
130
Mehlich-III
Michigan
40
75
Bray-I
Texas
44
200
Wisconsin
20
75
Texas
A&M
Bray-I
a
Mehlich-I
Sharpley and Tunney (2000).
Agronomic threshold concentrations are average values for nonvegetable crops; actual values vary with soil and
crop type. In addition, vegetables have higher agronomic P requirements.
b
27
found that although soil P test values for three sites ranged only from 285-295 mg kg-1 (using
Mehlich-III), P losses in overland flow were 0.05, 0.16, 0.35 kg/ha during a 30-min rainfall
simulation due to runoff volume variations (Sharpley and Tunney, 2000). Pionke et al. (1997)
reported that most annual P export comes from a small area of the landscape. Therefore,
threshold values have little meaning if only the soil P test is used without defining the site’s
potential for overland flow and erosion. The P-index (discussed in more detail in a following
section) provides a more reliable tool to estimate the risk of P losses than soil P tests.
2.1.5.3 Agronomic Sources
Under the right conditions, agronomic P sources of animal feces, fertilizers, manure,
animal litter, and plant residue can make their way to surface waters very quickly making them
the most influential for the incidental transport mechanism.
Animal feces: Phosphorus from animals is predominantly excreted in feces (Betteridge
et al., 1986). Grazing animals can directly deposit feces in the surface waters (Shirmohammadi
et al., 1997). Feces can also move to surface waters by runoff. The best ways to reduce these
sources is to fence or restrict animal entrance into the stream, and decrease grazing densities in
riparian areas (Sharpley et al., 1994). Finally, different animals produce different amounts of
feces and have different amounts of P in their feces (Table 10).
Table 10. The amounts of feces and phosphorus produce by different animals. a
Animal
Dairy Cow
Beef Cattle
Swine
Nursery Pig
Growing Pig
Finishing Pig
Finishing Pig
Boar
Sheep
Poultry-Layers
Poultry-Broilers
Horse
a
Size
Total Manure Production
kg
68
113
227
454
635
227
340
454
567
16
29
68
91
159
45
2
1
454
Midwest Plan Service (1985).
kg d-1
5.4
9.1
18.6
37.2
52.2
13.6
20.4
27.2
34.0
Phosphorus
oxide (P2O5)
kg d-1
0.0104
0.0204
0.0372
0.0753
0.1502
0.0576
0.0866
0.1134
0.1442
Phosphorus
oxide (P2O5)
kg yr-1
4128
6804
13608
27670
52618
20412
30845
41278
51710
1.0
1.9
4.4
5.9
5.0
1.8
0.10
0.06
20.4
0.0054
0.0223
0.0500
0.0680
0.2676
0.0068
0.00011
0.00006
0.0476
1950
3720
8618
11340
9979
2495
422
195
17690
28
Fertilizers-Manure-Animal Litter: Fertilizers, manure, and animal litter applied to
cropfields can be a significant source of P for surface waters. They can be directly deposited in
stream water with improper application, but usually end up in surface water bodies with runoff.
In the United States 4100 million metric tons of phosphate fertilizer and 1.2 million metric tons
of organic phosphorus were applied in 1992 (USDA-NRCS, 1997c).
When fertilizers are applied to cropfields stream P levels are often elevated (Oldham,
1998; Sharpley and Rekolainen, 1997). Single additions of P fertilizers increase concentrations
and amounts of P in overland flow (Sharpley and Syers, 1979c). Fertilization even increases P
levels in tile drainage (Bolton et al., 1970). Sharpley and Menzel (1987) reported that P losses to
surface waters increased as the P fertilizer application rate increased across a range of different
land-uses and pathways. Typically, less then 5% of the P fertilizer applied is lost (Carpenter et
al., 1998).
Similar soils may have different P losses resulting from manure additions (Logan and
Schwab, 1976). One of the major problems with manure is that intensive animal production is
concentrated in small areas of the country and manure production often exceeds crop needs in
these areas. In the United States 84% of the cattle are produced by 4 % of the feedlots (NRC,
1993). In several counties in North Carolina, manure production exceeds crop P needs by 500%
(Baker and Zublena, 1995). With cost restrictions of moving excess manure (Sharpley et al.,
1994) this can lead to excessive P loading in these areas. Many have suggested that composting
and compacting manure can reduce transportation costs. Phosphorus losses from manure
applications can be up to 20%, if rainfall immediately follows applications (Carpenter et al.,
1998). Similarly, Sauer et al., (1999) found that 22% of P in poultry litter was transported
because P in the litter is highly soluble.
A major portion of P in manure samples is soluble in weak extractants, such as water or
bicarbonate (Dou et al., 2000). This indicates a high potential of P losses in runoff if the manure
is applied on the surface. Many studies have shown that P losses could be reduced by using
fertilizer instead of manure, although results have been inconsistent (Mikkelsen and Gilliam,
1995). Additionally, manure has a different pH, microbial population, soil atmosphere and water
holding capacity than soil, and manure application can change soil physical and chemical
properties. However, manure can provide plant available P and other nutrients for several years
after application compared to fertilizers, in addition to benefiting soil structure (Sharpley et al.,
2000).
Reducing P inputs from fertilizers-manure-animal litter: The rates of P applied should
not exceed that necessary to meet plant requirements (Sharpley et al., 2000). In soils with high P
levels, the manure applied should have low P that can be controlled by the addition of dietary
supplements, while fertilizer should have no P (Sharpley et al., 1994). In contrast, fertilization or
manure application on areas low in nutrients can increase vegetation cover that will reduce
runoff, erosion and P losses (Sharpley et al., 1994).
Phosphorus dietary inputs typically exceed P needs of ruminant and non-ruminant
animals and therefore increase P in their excreta. The best methods of decreasing P levels in
manure is to increase P assimilation efficiency in the feeds by livestock or by reducing P mineral
supplementation (Valk et al., 2000; Poulsen, 2000).
Using forms of fertilizer that are less water-soluble will minimize transport of TP (<0.45)
(Nelson et al., 1996). Weaver et al. (1988a, b) found that using coastal superphosphate as a
29
fertilizer resulted in a smaller concentration of P in the soil solution compared to ordinary and
lime superphosphate. Coastal superphosphate, is a granulated mixture of superphosphate, rock
phosphate, and elemental sulfur and is less water soluble compared to ordinary and lime
superphosphate. Similarly, to reduce P losses from animal litter, P solubility should be reduced
by adding amendments like slake lime or alum (Moore et al., 2000).
Phosphorus losses are also very dependent on the method and time of application
(Sharpley et al., 2000; Mikkelsen and Gilliam, 1995). Surface application increases losses while
incorporation of manure or fertilizer reduces P losses (Nelson et al., 1996). Surface application
of fertilizer increased TP (<0.45) concentration in runoff 100 times, compared to sites where the
fertilizer was incorporated 5 cm below the soil surface (Sharpley et al., 2000). Total P (<0.45)
losses are greater when the fertilizer or manure is applied after rather than before plowing on
very wet soils (Sharpley et al., 2000). Manure application before or during tillage will reduce
surface P accumulation and will increase its distribution in the root zone (Sharpley et al., 1994).
Phillips et al. (1981) recommends avoiding winter application of manure on areas that contribute
snowmelt runoff. They also found that non-winter manure applications had lower P
concentrations. In general, fertilizers and manure should be applied when plants reach maximum
nutrient uptake (Nelson et al., 1996). However, this is often not feasible because plants are too
large to get equipment in, therefore, fertilizer and manure should be injected.
The major portion of P losses in runoff generally results from one or two intense storms
(Haygarth et al., 1998a). This often happens during planting season, when intense spring rains
occur right after large amounts of P fertilizer and/or manure have been applied and the soil has a
minimum crop cover (Sharpley et al., 2000). In rainfall simulation experiments, delaying runoff
events from 1 h to 3 d after P application reduced P losses for fertilizers by 90 % because of the
increased time for P adsorption (Sharpley et al., 2000). Similarly, other researchers believe that
nutrient concentration in runoff is more dependent on the number of rainfalls since application,
than on the annual quantity of runoff or rainfall (Sauer et al., 2000).
Plant residue: Phosphorus from growing and decaying plant material can increase P
transport to lotic systems (Sharpley and Menzel, 1987; Schreiber, 1985; Schreiber and
McDowell, 1985; Sharpley, 1981). The amount of the leached P from growing or decaying
plants that will be transported depends on a number of factors. One of the major factors is
hydrology. Higher infiltration rates compared to runoff will increase P leachate recycling and
infiltrating in the soil while higher runoff rates will lead to more P leachate transported to aquatic
systems (Sharpley and Menzel, 1987).
For growing plants the age and species are very important, especially for TP (<0.45)
losses. Sharpley (1981) found for growing cotton (Gossypium hirsutum), sorghum (Sorghum
sudanense) and soybean (Glycine max.) that as the plants grew from 42 to 82 days plant leachate
contribution in overland flow increased from 20-60%. Seasonal variations in transported TP
(<0.45) in runoff can be accounted for, in part, by leaching of growing vegetation and the decay
of dead vegetation (Burwell et al., 1975; White and Williamson, 1973). Burwell et al. (1974)
and Gburek and Heald (1974) found that different types of vegetation lead to differences in P
losses. Alfalfa plots had much higher TP (<0.45) losses in overland flow (33 g P ha-1) than
forested (33 g P ha-1), oat (33 g P ha-1) and corn plots (33 g P ha-1) (Wendt and Corey, 1980).
The increased loses were attributed to more P leached from alfalfa. Finally, P leachate from soil
stressed plants may contribute up to 90% of the P transported in overland flow (Sharpley, 1981).
30
The most important factors for decaying plant material are the amount and type of plant
residue added and the application method. Addition of bean and barley residue in soil columns
as well as sucrose increased the amount of P movement (Hannapel et al., 1963a). The increase
of plant residue resulted in an increase in UP (unreactive P) in the soil solution. Mostaghimi et
al. (1988) found that no-till with 750 kg ha-1 of plant residue had lower TP (unf.) and TP (<0.45)
losses than no-till with 1500 kg ha-1 of plant residue. However, both no-till methods had lower
TP (unf.) and TP (<0.45) losses than conventional tillage. Surface applied residue may increase
the movement of TP (<0.45) in overland flow compared to residue incorporation (Sharpley and
Smith, 1989b). The percentages of P leached to TP in the residue (2.8-6%) was higher than the
percentages of N and C leached to the total N and C in the residue (<1.5%), for corn stover
(Schreiber, 1999). Finally, Havis and Alberts (1993) found that corn residue released more TP
(<0.45) than soybean residue, while soybean released more nitrate (NO3) and ammonia (NH4)
than corn residue.
2.2 Phosphorus Aquatic Processes
2.2.1 Aquatic processes in lotic systems
Aquatic processes include P form transformations in surface waters (Figure 9), P storage
and P additions from the streambanks and the streambed (Figure 1). As P is transported, it
changes forms between TP (<0.45) and TP (>0.45) by desorption and adsorption reactions
(Edwards et al., 2000). The instream processes responsible include aquatic plant uptake of TP
(<0.45), deposition of TP (>0.45) on the bed, and resuspension of TP (<0.45) from the
streambed, all of which change the stream water equilibrium between TP (<0.45) and TP (>0.45)
(Edwards et al, 2000; Sharpley and Syers, 1979a). These transformations are very dependent on
the time of year, amount and form of P entering from different sources, stream discharge, and
streambank and streambed properties (Edwards et al., 2000; Johnes and Hodgkinson, 1998).
These processes can be very important in P retention during its transport in the stream.
Several studies indicate significant assimilation of P in rivers, because of P adsorption by
sediments and uptake by aquatic plants (Hill, 1981). Sediments in streams can be either P
sources or sinks depending on their physical and chemical properties (Sharpley et al., 1996).
When sediments are sinks, they partially fix the TP (<0.45) but even then P may have to travel a
long distance in the stream before it becomes part of a sink. Ball and Hooper (1963) found that it
took 400-11,000 m of traveling before a P containing molecule was retained by the sediment.
The bed sediments retain P in the summer during low flow but when the sediment is resuspended
in autumn after the first intense rainfalls they act as a P source (House et al., 1998; Hill, 1981).
Stormflow from early major storms in autumn have higher P concentrations compared to those in
later winter storms due to resuspended P from the streambed (Sharpley and Syers, 1979a).
Plant and animal biomass, take up only small portions of the stream transported P (Pelton
et al., 1998) although this portion increases as water moves downstream. Interflow contributions
downstream become more significant and can dilute P concentrations in the stream water
(Sharpley and Tunney, 2000; Shirmohammadi et al., 1997) (Table 10). As drainage area
increases, attenuation of nutrient concentration can also be expected (Shirmohammadi et al.,
1997). All these processes lead to lower P concentration downstream that can be less by 50%
31
Algal/plants/
cyanobacteria
Precipitation
Fe, Al
compounds
Uptake
RP (>0.45)
suspended
Terrestrial and
Streambank Input
RP (<0.45)
RP (>0.45)
UP (unf.)
Adsorption
Desorption
Soluble
HxPO4-y
Adsorption
Desorption
In soil
pores
Die off
UP (unf.)
Resuspension
Deposition
RP
(>0.45)
Decomposition
UP (unf.)
Precipitation
Dissolution in
anaerobic conditions
Fe, Al
compounds
NOTE: HxPO4-y (form of orthophosphate) and Fe, Al
compounds are both included in RP (<0.45)
Figure 9. Phosphorus cycle in the aquatic systems (modified from USDA-NRCS, 1999). In
this cycle P moves from the water in the bed and from the bed to the water while also
transforming into different forms. Terrestrial and streambank sources provide the inputs.
Table 11. Phosphorus concentrations from headwater to downstream reaches. a
P Form
TP (<0.45)
AAP
TP (unf.)
a
Headwater
0.163
0.270
0.525
→ → → → → → → →
mg L-1
0.138
0.110
0.227
0.192
0.510
0.319
Downstream
0.084
0.140
0.258
Grubek et al. (2000).
compared to the upstream concentrations (Table 11) (Grubek et al., 2000). However, all these
instream processes are just a temporary delay for P transport (Edwards et al., 2000).
Resuspension of P stored in river sediments (House et al., 1998; Svensen et al., 1995;
Harms et al., 1978) and erosion from streambanks (Edwards and Withers, 1998; House et al.,
1998) have been recognized as major sources of P that can contribute up to 70 % of TP (unf.) in
32
the stream water (Sharpley and Syers, 1979a). Streambank erosion by itself has been reported to
contribute 56 % of the TP (<0.45) in the stream (Roseboom, 1987). However in other cases,
researchers found that sediment from streambanks can reduce TP (<0.45) concentrations because
they are P deficient (Kunishi et al., 1972; Taylor and Kunishi, 1971). Stabilizing streambanks,
reducing peak discharges and slowing water movement in the stream will reduce streambank
erosion and bed sediment resuspension (Nelson et al., 1996). Absence of instream macrophyte
beds and streambank vegetation will make the streams more susceptible to bed sediment
resuspension and streambank erosion (Smith, 1992).
2.2.2 Flow Regimes
Different flow regimes will have different P concentrations because various export
processes are active in P movement at different flow regimes. Dorioz et al. (1998) defined four
export regimes for the Foron watershed, in the Alps region of central Europe, based on unique
conditions of hydrology, TP (unf.) inputs and TP (unf.) storage in the river. The land uses in the
Foron watershed include natural forests (50%), agriculture (36%), urban uses (10%) and marshes
(4%). Dorioz et al. (1995) identified two regimes of the export (1 and 2) during baseflow
conditions and two others during (3 and 4) stormflow conditions. Total P (unf.) accumulation in
the streambed is significant in all regimes. These regimes are useful in understanding the major
P inputs under different hydrological conditions and point out the importance of understanding
the hydrology of the watershed to effectively manage for NPS P. Total P (unf.) inputs in this
example were agricultural and urban NPS, and point sources. Based on these four regimes
Dorioz et al. (1995) formulated an P export topology model. The P export topology can explain
TP (unf.) export patterns and TP (unf.) changes in streambed accumulations.
In regime 1, dry weather results in low stream discharges (<1.2 m3 s-1) and no active
runoff from the watershed. Total P (unf.) inputs are the natural background but consist
predominantly of point sources from wastewater plants and industrial facilities. During this
period, P inputs exceed P exports in the streambed and consequently there is an accumulation of
P in the streambed. Total P (unf.) accumulates in the riverbed through deposition, biological
uptake and adsorption mechanisms. Total P (unf.) is exported mainly as TP (<0.45) and AAP
forms.
In regime 2, weekly, average stream discharge continues to be low (<1.2 m3 s-1), but
small increases in weekly averages are induced by rainfall. These increases lead to TP (unf.)
exports exceeding P inputs in the streambed. The small increases in discharge change the
equilibrium of P in the flowing water and accumulated P that leads to resuspension and release of
stored TP (unf.). Additional inputs are point sources from wastewater plants and industrial
facilities, and urban overland flow. Agricultural NPS inputs and streambank erosion are
negligible. This regime is typical during summer time. Phosphorus associated with resuspended
sediment is highly exchangeable and highly algal available.
In regime 3, a rainfall episode causes a significant increase in discharge compared to
regime 1 and 2. This regime occurs after a regime 1, low flow period. Although there is
increased flow, soils are still dry and relatively permeable, so little runoff from agricultural fields
occurs. Point sources, from wastewater plants and industrial facilities, and urban overland flow
continue to be major inputs. Phosphorus exports exceed inputs and a portion of the stored P
from the river bottom is resuspended and is highly exchangeable and highly algal available.
33
Finally, in regime 4, frequent rainfall increases discharges substantially, and with the
high antecedent soil moisture in the watershed, a high percentage of the cultivated soil is crusted,
and high runoff from the cultivated areas occurs. Regime 4, typically occurs in autumn and
spring. Nonpoint source P from agricultural runoff is the predominant input entering the river
system. In this regime streambank erosion also contributes significantly to the P load.
Although evidence from adjacent watersheds found similar results to the Foron
watershed, additional studies should be conducted to determine if P export topology can be
applied to a broader range of watersheds. By understanding the hydrology of the watershed
through P export topology, the land manager has another tool to reduce P transport and
eventually eutrophication.
2.2.3 Critical phosphorus concentrations for surface waters
The sensitivity of surface waters to P, extends over several orders of magnitude making it
very difficult to define critical or threshold concentrations for eutrophication (Ferguson et al.,
1996). Although critical P concentrations have been recommended for all surface waters (Table
12), they are highly questionable. This is especially true for the critical P concentrations for
streams. The major reasons are because P concentrations are influenced by the changes in the
physical and chemical properties of the stream moving from the headwaters downstream (Table
13) and by the changes in P residence time and utilization in the channel.
Different nutrient levels lead to eutrophication from region to region. These differences
occur because of variations in geology and soil types. To accommodate these differences the
EPA has developed ecoregional nutrient criteria (USEPA, 2000c). In the United States, 14
different ecoregions (Figure 10) were developed based on geology, land-use, ecosystem type,
and nutrient condition. The EPA has developed nutrient criteria for P in lakes and reservoirs,
and stream and rivers for only eight of the ecoregions (Table 14).
Finally, Edwards et al. (2000) proposes that critical P concentration should be determined
by a new hierarchical classification system that combines the ecoregion concept with stream
order. He proposes that this classification integrate spatial and temporal affects of the slope,
land-use, flow velocity and water quality. The ecoregion concept (Figure 10) stratifies sites
based on similar eutrophic conditions and can be applied across a wide range of scales (e.g. from
the whole watershed, to the sub-watershed, to the reach, to the pool), while stream ordering of
independent stream segments is based on their position relative to other stream segments. The
most common stream ordering methods used are by Strahler (1957) and Shreve (1967). This
classification system proposed by Edwards et al. (2000) characterizes and groups streams or
stream reaches based on watershed attributes and can improve the prediction of the P
concentrations and loads that will cause eutrophication in streams.
34
Table 12. Recommended critical or threshold P concentration for surface waters for
eutrophication. a
Concentration
µg L-1
10
10-30
100
50
50
10
1000
Comment
TP (<0.45) - critical concentration for lakes e, f
TP (<0.45) - critical concentration for natural waters a
TP (unf.) - critical concentration for streams d
TP (unf.) - critical concentration for lakes d
TP (<0.45) - concentration allowed to enter Florida Everglades c
TP (<0.45) - target concentration allowed to enter Florida
Everglades by the year 2000 c
Flow weighted annual TP (<0.45) - proposed allowable limit for
agricultural runoff d
a
Table adopted from Norfleet (1998).
c
Sharpley and Rekolainen (1997); USA vs. South Florida Water Management District (1994); dUSEPA (1986);
e
Vollenweider (1968); f Sawyer (1947).
b
Table 13. Changes in physical and chemical properties from headwaters to downstream
reaches and some biological implications. a
Physical aspects
A decrease in average slope,
frequency of extreme
hydrological events, river
flow velocity, and stream
length density, while both
stream order and depth tends
to increase.
a
Chemical aspects
An increase in pH, some
nutrient concentration,
total dissolved solids and
C availability
Biological aspects
An increase in species
diversity, productivity and
total biomass, length of
growing period, trophic
status, and a change from
benthic to planktonic
dominated algal
communities
Edwards et al. (2000).
Table 14. The EPA recommended critical TP (unf.) concentrations for some of the
ecoregions. Ecoregions are shown in Figure 10. a
Rivers and
Streams
Lakes and
Reservoirs
a
II
10.00
8.75
USEPA (2001).
TP (unf.) (µg L-1) concentrations for eight ecoregions
III
VI
VII
IX
XI
XII
21.8 76.25
33.00
36.56
10.00
40.00
8
37.5 14.75
8.00
20.00
8.00
10.00
0
XIV
31.25
17.50
35
Figure 10. The fourteen ecoregions in the United States (Omernik, 2000). The ecoregions
were differentiated based on geology, land-use, ecosystem type, and nutrient conditions.
36
3. LAND-USE PRACTICES,
AND BEST MANAGEMENT PRACTICES
3.1 Land-use practices
Land-use management is one the main factors influencing the concentration and amount
of NPS P transport to surface waters (Dorioz et al., 1998; Haygarth et al., 1998a; Smart et al.,
1985) (Appendix I). Land-use affects the percentage of exposed bare ground, the most important
factor for soil loss, and vegetation and ground cover, that are directly related to runoff (Hoffman
and Ries, 1991). Soil loss and runoff volumes are closely associated with NPS P loss (Hoffman
and Ries, 1991). Decreasing surface roughness, infiltration and evapotranspiration, as is the case
in most traditional land-use practices, increases overland flow, the major P pathway (Sharpley et
al., 2000) (Figure 11).
In the United States in 1997, 49, 163, and 152 million hectares were in pasture, range and
cultivated land, respectively (USDA-NRCS, 1997a). In this review, pasture and range are used
as synonymous terms. Even though pastures in the eastern U.S. are managed differently than the
rangeland of the west, most of the grazing literature comes from western rangelands and
inferences from that work can be applied to pastures. In general, traditional cropping practices
and livestock grazing systems have affected soil cover and stability and landscape hydrology,
and increased NPS P losses to surface waters (Edwards and Withers, 1998; Isermann, 1990).
Consequently, cultivated, pasture, and range lands are the major NPS P contributors (68%) to
surface waters (Figure 12). Although the effect of land-use on NPS P export is evident it is very
difficult to precisely estimate the P contribution from specific land-use practices to streams (Hill,
1981).
P Loss (g ha-1 yr-1)
Average TP (<0.45)
concentration (µg L-1)
TP (<0.45)
9
TP (>0.45)
12
14
18
25
37
71
0
50
100
150
200
250
Figure 11. Phosphorus concentrations (µg L-1), losses (g ha-1 yr-1) and forms (TP (<0.45)
and TP (>0.45)) in surface runoff under different land-uses (Pierzynski et al., 2000).
37
Cultivated land (40.2%)
Pasture and range (27.8%)
Forest (2.3%)
Other (1.3%)
Natural
Background (28.4%)
Figure 13. Nonpoint source P contributors in 1980. Cultivated, pasture and range land are
the major contributors (Council on Environmental Quality, 1989).
3.2 Pastures and Rangelands
Grazed pastures can be the most significant contributors of P to surface waters. Downing
et al. (2000) found in Iowa that tributaries with the greatest proportion of land in pastures,
compared to cultivated fields, had the highest losses of TP (unf.). Smart et al. (1985) found a
positive correlation between P losses and the percentage of pasture land in watersheds in the
Missouri Ozarks. This correlation was strongest when pastures occupied 40-100% of the
watershed. Finally, Thomas et al. (1992) found that pasture-dominated watersheds had higher P
losses than cropped and forested watersheds although the pasture watersheds were associated
with bedrock and soil that had high natural background soil P levels. However, in other studies
grazed watersheds had lower P export than cultivated (Correll et al., 1999; Nelson et al., 1996,
Jones et al., 1985), forested (Correll et al., 1999) and afforested watersheds (Smith, 1992).
Interestingly, Studdert et al. (1997) found that soil degradation may be reduced in conventional
cropping systems if pastures are included in the rotation. In such a rotation the pasture should be
maintained for at least three consecutive years. Finally, Shewmaker (1999) suggested that
herbivores (like cows and sheep) can be another vector to export P from the watershed and
reduce P transported to surface waters.
Grazed pastures have higher P losses than ungrazed pastures (Gilligham and Thorrold,
2000; Shirmohammadi et al., 1997; Thornley and Bos, 1985; Sharpley and Syers, 1979a)
indicating that livestock have a negative effect on water quality. An increase of 37% TP (unf.)
and 48 % TP (<0.45) losses was measured when grazing occurred (Schepers and Francis, 1982).
In some studies, there were no differences in P losses between no grazing and year-round grazing
(Owens et al., 1983 and 1989) because stocking rates were low. According to these studies,
year-round cattle grazing did not degrade stream water quality.
Livestock degrade water quality in various ways but primarily by their excretal returns
and trampling effects. Phosphorus is predominately excreted in the animal feces that have a
higher P content than soil (Haynes and Williams, 1993; Betteridge et al., 1986). According to
38
Barrow (1987), 60-99% of ingested nutrients are returned to the soil in feces and urine. The
deposition of feces increases P concentrations in the soil (During and Weeda, 1973; MacDiarmid
and Watkin, 1972). The increase in soil P is detected only in shallow surface soil layers and at
varying distances from the feces of up to five times the area of the feces patch. In a grazed
grassland, soil P in the 0-2 cm layer was 10-fold greater than P in the layer below 45 cm
(Haygarth et al., 1998b). In another study grazed areas had 100-200 mg kg-1 soil P (using
Mehlich-III), which was much higher than the 30 mg kg-1 soil P in adjacent wooded areas
(Gburek et al., 2000).
Typically, tile and overland flow P losses were lower from grazed plots compared to
cultivated plots with fertilizer (Sharpley and Syers, 1979b and 1976b) or ones with poultry litter
additions (Sauer et al., 1999). In these studies, pasture animal excretions were not significant P
sources relative to fertilizer and poultry litter. However, while fertilizer, manure, or animal litter
can be applied at specific times to avoid heavy rainfall and can be incorporated in the soil, but
this is not true with animal feces that are deposited daily on the surface of pastures. Therefore,
eroded animal feces on the surface of pasture soil can be more susceptible to overland flow and
can also be major sources of P to surface waters of agricultural watersheds (Sharpley and Syers,
1976a; Haygarth and Jarvis, 1997).
Another interesting factor in nutrient loss via runoff is the pattern of waste deposition that
can create a large degree of spatial and temporal variability in nutrient losses from grazed
pastures (Sauer et. al., 1999). Animals tend to deposit more excreta in loafing areas near shade,
water, and dietary supplement sources (Peterson and Gerrish, 1996) instead of uniform
deposition across the pasture. Additionally, animal feces deposition is on the soil surface, which
makes P transport easier and faster. This pattern of waste deposit leads to unequal nutrient
redistribution. Finally, direct deposits of animal waste in the stream cause elevated N and P
concentrations (Shirmohammadi et al., 1997).
Phosphorus losses from terrestrial pathways in pastures can be ranked highest to lowest
in the following order: overland flow, tiles, and interflow (Sharpley and Syers, 1979a). Grazing
increases P losses from overland flow (Sharpley and Syers, 1976a) and tiles (Sharpley and Syers,
1979b) compared to ungrazed sites. The addition of feces to the soil surface, soil disturbance,
and plant damage by livestock accentuated overland flow P losses (Nguyen et al., 1998; McColl
and Gibson, 1979; Sharpley and Syers, 1976a). However, overland flow varies with time and
depends on rainfall events of a certain magnitude (Johnes and Hodgkinson, 1998). Subsurface
drainage reduces TP (unf.) losses up to 50%, by reducing overland flow volumes (Sharpley and
Syers, 1979a; Sharpley and Syers, 1976a). Typically interflow P contributions are small.
The soil surface determines the routes of water flow and thus the pathway of nutrient
transport (Sauer et al., 2000). Trampling primarily around feeding and watering areas, exposes
bare soil that becomes a significant P source with increased TP (unf.) and TP (<0.45) losses
(Johnes and Hodgkinson, 1998; Shirmohammadi et al., 1997). In other cases, heavy animal
traffic decreases surface infiltration and damages soil structure which increases overland flow
(Russell et al., 2001) and TP (<0.45) and TP (>0.45) losses from grazed plots (Sharpley and
Syers, 1976a). Infiltration rates were lowest along cow paths and close to water tanks and
increased with distance from both (Radke and Berry, 1993). Many of these cow paths lead
directly to streams as cattle access the channel. Overgrazing can reduce vegetative cover and
increase bare soil, making it more susceptible to the erosive power of rainfall (Sauer et al., 2000).
Although animal feces, trampling and compaction are known to influence P losses in grazed
pastures, nobody has measured their actual impact (Haygarth and Jarvis, 1997). Larger scale,
39
long-term studies to assess the effects of the spatial and temporal variability of excretions and the
effects of animal traffic compaction and trampling are needed.
The P contributions from pastures depend on livestock stocking density, the length of the
grazing period, the time of the year grazing occurs and other grazing management practices
(discussed in more detail later in this section) (Ritter, 1988; Schepers and Francis, 1982;
Schepers et al., 1982). Increasing stocking rates increases P losses (Leinweber et al. 1997;
Isermann, 1990; Lambert et al., 1985). The amount of P lost is relative to the amount deposited.
More animals in the pasture lead to greater deposition of feces, more trampling and compaction,
and a greater chance of sediment and nutrient losses. Infiltration rates are higher for low
stocking rates compared to medium and high stocking rates (Radke and Berry, 1993). Heavy
grazing on a permanent pasture decreased infiltration by 80% compared to a lightly grazed
pasture (Heathwaite et al., 1990). High stocking density on a pasture also led to much higher
streambank erosion compared to an adjacent pasture with lower stocking density in central Iowa
(Zaimes, 1999). In contrast, Hawkins and Scholefield (1996), found that an increase in the
number of cattle and the number of grazing days did not influence P losses, contrary to what they
expected. Chichester et al. (1979) found P losses from pastures with winter-feeding and summer
grazing were greater than pastures grazed only in summer. Khaleel et al. (1978) reported that in
most cases water pollution from livestock is a result of overgrazing.
Lemunyon and Gilbert (1993) mention that TP (<0.45) is the major form of P lost from
grasslands, because runoff from pastures has a very low sediment load with very low adsorption
occurring (Sharpley et al., 1994). In contrast, Gilligham and Thorrold (2000) concluded that TP
(>0.45) is the dominant form. This would depend on how heavily the pastures are grazed.
Russell et al. (2001) found that increasing the height of the vegetation cover and decreasing
treading reduced soil losses. Overgrazing reduces vegetation cover, increases the percent of bare
soil and therefore increases soil and P losses. This is based on the fact that TP (unf.) yields have
been found to closely correlate with sediment (Sharpley and Syers, 1979a; Thorrold, 1999),
although in some cases they are unrelated (Gilligham and Thorrold, 2000). Gilligham and
Thorrold (2000) suggest that they are unrelated because as the sediment yield increases the P
enrichment and consequently the P yield decrease. Finally, from the chemical methods
classification standpoint, Heathwaite et al., (1990) found that most P was delivered as UP (unf.)
(unreactive P), and Owens et al. (1983) found UP (unf.) increased with grazing, but not RP
(reactive P) (unf.).
Many studies have found that streambank erosion is a major source of sediment in
streams (Zaimes, 1999; Lawler et al., 1999; Odgaard, 1984) but few tie that sediment loss to P
loss. Still many consider streambank erosion and bed sediment resuspension the main sources of
P losses in pastures. Streambank erosion and bed sediment resuspension contributed
approximately four-fold and two-fold greater amounts of TP (>0.45) and TP (unf.) respectively,
compared to overland flow (Sharpley and Syers, 1979a). In a study done in the Ngongotaha
River (New Zealand), intensive streambank erosion control reduced sediment by 85% but TP
only by 27% (Williamson et al., 1996). This suggests that overland flow in some cases could be
the major source for P losses because of higher P concentrations in the surface soil than in the
deeper profiles of the eroding banks. More work needs to be done to establish the relative roles
of overland flow and streambank erosion to soil P losses from pastures.
Fencing or herding the livestock animals from the stream water can reduce P losses
(Nelson et al., 1996). If animals have access to the streams, their direct deposits in the stream
will elevate P concentrations (Shirmohammadi et al., 1997). Additionally, free animal access to
40
the stream will enhance streambank erosion (Kauffman et al., 1983). If the stream is the only
source of water, it is recommended that armored crossing points should be built. Sheffield et al.
(1997) found that when cattle were given the choice, they drank water from a trough 92% of the
time, rather than from the stream. This resulted in a 77% reduction in streambank erosion and an
81% reduction of TP (unf.) that contradicts Williamson et al. (1996) results. Line et al. (2000)
found that livestock fencing and subsequent planting of tress along the riparian corridor reduce
TP (unf.) by 76%. In contrast, Owens et al. (1983) mentions that fencing is not always justified
because P losses are not always decreased. However, Owens et al. (1989) suggest that the
increase in sediment transport may warrant fencing livestock out. Reducing peak discharges can
also reduce streambank erosion. The best way to reduce peak discharges is by slowing terrestrial
water movement towards streams (Nelson et al., 1996) primarily overland flow. Approaches to
accomplish this are roughening the soil surface by maintaining sufficient soil vegetation,
increasing surface depression storage or infiltration rates and increased water use by plants. All
of these approaches can be accomplished by the establishment of riparian buffers (Schultz et al.,
2000; Isenhart et al., 1998)
Maximum stocking rates should be based on microclimate, soil vegetation, topography
and geology to reduce P losses to streams (Isermann, 1990). Grazing pressure should especially
be reduced adjacent to drainage waterways and stream courses (Ritter, 1988). Stocking rate and
length of grazing time should be managed to maintain sufficient vegetative cover (Nelson et al.,
1996) because it will minimize soil erosion (Russell et al., 2001) and overland flow. Finally, in
pasture areas in Australia prone to waterlogging, subsurface drainage should be considered
(Nelson et al., 1996). Reduced conditions cause dissolution of P associated with Al and Fe
oxides.
Rotational grazing and intensive management grazing are pasture systems gaining
acceptance because they can generate more monetary profit compared to continuous grazing
(USDA-NRCS, 1997d; Undersander et al., 1993). There are a number of definitions for
rotational grazing and intensive management grazing. Based on Barnhart et al. (1998) and the
USDA-NRCS (1997d), in rotational grazing the pasture is divided in more than one paddock
(typically 2-4) and livestock is moved usually on a calendar schedule. In intensive management
grazing there are more than two paddocks (typically 5-12) and livestock is moved according to
forage consumption (USDA-NRCS, 1997d; Barnhart et al., 1998). These systems not only
increase grazer’s profit but also can help reduce P losses to surface waters. Ritter (1988) and
Olness et al. (1980) found that continuous grazing results in higher P losses than rotational
grazing. Compared to continuous grazing, both these grazing practices decrease animal plant
selectivity that will lead to some parts of the pasture being overgrazed while other are
undergrazed (Barnhart et al., 1998). In overgrazed areas, bare, highly trampled, soil is very
common and that decreases infiltration. Both, large areas of bare soil and decreased infiltration
typically enhance P losses from overland flow. Plant and root growth that decrease soil erosion
and better distribution of animal feces throughout the pasture are also promoted by these systems
because animals are moved from paddock to paddock (USDA-NRCS, 1997d). Mathews et al.
(1994) found in continuous grazed pasture that P was accumulated near areas closest to water
sources, shade and supplemental feeders. However, they did not see this pattern for two
rotational grazed pastures. Finally, rotational and intensive management grazing increase
streambank stability compared to continuous grazing (Lyons et al., 2000; Clary, 1999). All these
advantages are more pronounced with intensive management grazing compared to rotational
grazing.
41
3.3 Cultivated fields
Cropland susceptibility to P losses has been documented since the late seventies (Alberts
et al. 1978; Olness et al., 1975), with losses sufficient to cause eutrophication (Catt et al., 1998).
Hill (1981) found a positive correlation between P export and increase of cropped area.
Intensive cropping systems rely on monocultures with high P fertilization and with little P
movement below the plow layer (Oldham, 1998) that has led to P accumulation in the surface
soil (Sawhney, 1978). Higher soil P can lead to higher P losses. Gburek et al. (2000) reported
that croplands had > 200 mg P kg-1 (using the Mehlich-III method) in the soil which was higher
than grazed pastures (100-200 mg kg-1) and woodlands (30 mg kg-1). In most cases, fertilized
cultivated fields had much higher P (using the hydroxide fusion of Smith and Bain, 1982) in the
soil compared to uncultivated fields (Maguire et al., 1998). When there were no differences in
soil P between cultivated and uncultivated soils, this was due to very low fertilizer inputs and
high initial natural background soil P levels. Additionally, larger aggregates were more
abundant in uncultivated soils compared to cultivated soils, which indicate a greater degree of
soil stability and more resistance to P losses by physical mechanisms.
Similar cropping systems can have different P losses that reflect differences in soil
properties (Catt et al., 1998). These differences in TP (unf.) losses under similar cropping
methods are the result of differences in aggregation of soil particles and loss of fine P enriched
particular matter in percolating water. Any factors that decrease aggregate stability, such as
decreases in organic matter content or cultivation practices that influence seedbeds, increase P
loss (Maguire et al., 1998).
Loss of cropland soil’s aggregate stability is very important because it increases soil
detachment that increases the risk of soil erosion and loss of applied P (Edwards and Withers,
1998). The absence of vegetative cover during part of the year also increases detachment of the
clay and silt fractions that are mainly associated with P losses (Maguire et al., 1998), and
increases overland flow after heavy rainfall (Catt et al., 1998). Additionally, root systems of
crop plants are shallow compared to forest or grassland plants that decrease infiltration. This
results in overland flow being the main pathway of P movement to surface water from cultivated
fields (Catt et al., 1998; Schwab et al., 1980) and TP (>0.45) is the dominant form of P transport
(Sharpley et al., 2000; Lemunyon and Gilbert, 1993). Culley and Bolton (1983) found that sheet
and rill erosion could contribute up to 65% of TP loads to streams while tiles contributed 25%
and streambank erosion 10%.
Different crops have different P removal rates (Table 15), periods they cover the soil and
plant physiology that leads to different P losses by overland flow (Figure 13) (Pierzynski and
Logan, 1993; Sharpley et al., 1993). However, higher P removal does not always mean lower P
losses. Plants with higher P removals, like vegetables and specialty crops, have higher P inputs
because they are very responsive to P additions even when soil P tests are high (Pierzynski and
Logan, 1993).
Cultivation techniques influence overland flow losses of P to surface waters (Figure 14)
(Catt et al., 1998; Pierzynski and Logan, 1993) because they influence surface residue cover and
soil disturbance. Reduced or no till increases the potential of P accumulation in the surface soil
(top 0-5 cm), (Sharpley et al., 1992; Amemiya, 1977). This results in higher TP (>0.45) and TP
(<0.45) concentrations in overland flow (Laflen and Tabatabai, 1984; Langdale et al., 1985)
because of the low P adsorption from the sediment load (Sharpley et al., 1994). Andraski et al.
(1985) found that algal available P (AAP) concentrations to TP (unf.) concentrations for chisel
42
Table 15. Phosphorus removal rates for different crops. Phosphorus removal (kg ha-1) is
estimated by the mean P concentration and the mean yield of the crop for the United
States. All values are expressed on a fresh weight basis. a
Crop or commodity
P removal
kg ha-1
Crop or commodity
P removal
kg ha-1
Specialty crops
Apple (Malus pumila Mill.)
Apricot (Prunus armeniaca L.)
5.9
5.9
Asparagus (Asparagus officinalis)
Bean, lima (Phaseolus limensis mac F.)
Bean, snap (Phaseolus vulgaris L.)
Beat, table (Beta vulgaris)
Blackcherry (Rubus ursinus Cham. & Schlect.)
Broccoli (Brassica olacea L. botrylis L.)
Cabbage (Brassica oleracea L. capitata L.)
Cantaloupe (Cucumis Melo cantalupensis)
Carrot (Daucus carota L.)
Cauliflower (Brassica oerlacea L. botrylis L.)
Celery (Apium graveolens)
Cucumber (Cucumis sativus L.)
Grape (Vitis vinifera L.)
Hops (Humulus lupulus L.)
Kale (Brassica olacea L. acephala D.C)
Lettuce (Lactuca sative L.)
Muskmelon (Cucumis melo L.)
Okra (Hibiscus esculentus L.)
Onion (Allium cepa L.)
19.6
7.7
11.6
3.9
3.9
11.6
11.6
18.7
11.6
13.3
72.3
3.9
11.6
17.8
7.7
11.6
5.9
3.9
15.6
Orange (Citrus sinensis)
Parsley (Petroselinum crispum Mill.
Manof.)
Parsnip (Pastinaca sativa L.)
Peach (Prunus persica L.)
Pear (Pyrus communis L.)
Pea (Pisum sativum)
Pepper (capsicum annum L.)
Potato (Solanum tuberosum l.)
Prune (Prunophora Focke)
Pumpkin (Curcubita pepo L.)
Raspberry (Rubus idealis L.)
Rutabaga (Brassica napus napobrassica)
Spinach (Spinacia oleracea)
Squash (Cucurbia ssp. L.)
Strawberry (Fragaria ssp. L.)
Sugar cane (sorhum saccharatum)
Sweet corn (Zea mays L.)
Sweet potato (Ipomoea batatas L. Lem.)
Tobacco (Nicotiana tabacum)
Tomato (Lycopersicon esculentum Mill.)
Turnip (Brassica rapa L. rapifera)
11.6
3.9
19.6
3.9
3.9
3.9
5.9
13.3
7.7
5.9
3.9
7.7
15.6
3.9
3.9
39.2
3.9
9.8
5.9
21.4
9.8
Row crops, small grains
Barley grain (Hordeum vulgare L.)
Barley straw (Hordeum vulgare L.)
Bean, dry (Phaseolus vulgaris L.)
Corn grain (Zea mays L.)
Corn stover (Zea mays L.)
Cotton, seed and lint (Gossypium hirsutum L.)
Cotton, stalks, leaves, burs (Gossypium hirsutum
L.)
Flax grain (Linum usitatissimum L.)
Flax straw (Linum usitatissimum L.)
5.9
2.0
9.8
20.5
14.3
7.8
3.9
Rice grain (Oryza sativa L.)
Rice straw (Oryza sativa L.)
Rye grain (Secale cerale L.)
Rye grain (Secale cerale L.)
Sorghum grain (Sorghum vulgare L.)
Sorghum stover (Sorghum vulgare L.)
Soybean grain (Glycine max Merrill.)
7.8
3.9
3.9
3.1
9.8
7.8
15.6
3.9
2.0
3.9
19.6
Oat grain (Avena sativa L.)
7.8
Oat straw (Avena sativa L.)
Peanut, nuts (Arachis hypogaea L.)
Peanut, vines(Arachis hypogaea L.)
5.9
5.9
11.6
Soybean straw (Glycine max Merrill. )
Sugar beet roots (Beta vulgaris
saccharifera L.)
Sugar beet tops (Beta vulgaris saccharifera
L.)
Sunflower (Helianthus annus L.)
Wheat grain (Triticum aestivum L.)
Wheat Straw (Triticum aestivum L.)
13.4
15.6
9.8
2.0
Forage Crops
Alfaalfa (Medicago sativa L.)
27.7
Bluegrass (Poa annua L.)
11.6
Coastal bermudagrass (Cynodon dactylon L.)
Corn silage (Zea mays L.)
56.2
31.2
Fescue, tall (Festuca arundicacea Schreb.)
Johnsongrass (Sorghum halepense L. Pers.)
Ladino clover (Trifolium rapens L.)
31.2
74.1
17.8
a
Pierzynski and Logan (1993).
Lespedeza (Lespedeza striata (Thunb.) H.
& A.)
Orchardgrass (Dactylis glomerata L.)
Red clover (Trifolium prantese L.)
Sudangrass (Soghum vulgare sudanense
(Piper) Hitchc.)
Sweet clover (Meliotis alba Desr.)
Timothy (Phleum pratense L.)
11.6
25.0
17.8
13.4
27.7
11.6
43
Tillage
P Added
kg kg
ha he-1
yr
yr-1
-1
-1
Erosion
TP(<0.45)
(<0.45)
RP
tons
yr-1
tonhe-1
he yr
yr
tons
ha
? g L-1
µg L-1
0.2
220
22
-1-1
-1-1
P loss (kg he-1)
SORGHUM
No till
0
RP (<0.45)
RP (>0.45)
Reduced
0
0.5
19
190
Conventional
24
17.5
13
130
No till
24
0.2
65
650
Reduced
0
1.0
180
18
Conventional
16
9.2
210
21
UP (unf.)
WHEAT
0
1
2
3
4
5
TP (unf.) (kg ha-1)
Figure 13. Phosphorus losses and its forms RP (<0.45) RP (>0.45), and UP (unf.) in
overland flow as a function of different crops and different tillage practices on several
Southern Plain watersheds, averaged over five years (modified from Sharpley et al., 1993).
plow, till-plant and no-till increased by 6, 8, and 28 %, respectively, compared to conventional
tillage. However, AAP losses for all three systems were less compared to conventional tillage.
Higher infiltration rates and surface roughness for no-till soils compared to chisel and moldboard
plowed soils (Radke and Berry, 1993) decreases surface erosion and volume of overland flow.
This leads to either a decrease of both TP (>0.45) and TP (<0.45) losses (Laflen and Tabatabai,
1984), or in other cases decreases only of TP (>0.45) losses while maintaining the same TP
(<0.45) losses (Langdale et al., 1985). In both cases, TP (unf.) losses decreased. Finally,
Mostaghimi et al. (1988), found that no-till decreased TP (unf.), TP (>0.45) and TP (<0.45)
losses by 97, 93 and 91 %, respectively, compared to conventional tillage.
Draining cultivated fields can also decrease overland flow, lower P enrichment and
consequently decrease P losses (Sharpley and Syers, 1979a). Common drainage systems are
open ditches, moles, and subsurface tiles. Drained fields typically have smaller P losses
compared to undrained systems (Catt et al., 1998; Hawkins and Scholefield, 1996; Sharpley and
Syers, 1979a).
Tiles are the most common drainage system in the Midwest. Their importance as P
pathway increases as the area of cultivated fields under tiles increases. Tiles in a watershed can
provide most of the baseflow in a stream. Xue et al. (1998) found that 86% of the flow of the
44
upper segment of the Embarras River in Illinois is from blind inlet tiles in land under maize and
soybean. Similarly, Isenhart and Crumpton (2000, unpublished data) estimated that over 90% of
the baseflow in Bear Creek in central Iowa is from tiles. Additionally, blind inlet tiles have been
found to transport significant amounts of TP (>0.45) (Bottcher et al., 1982). This is contrary to
common beliefs that expect tiles to primarily transfer TP (<0.45) (Xue et al., 1998).
Most early studies found P losses from tiles negligible (Baker et al., 1975) and tile
effluent with low P concentrations (Bolton et al., 1970). Tiles accounted for <1% of P inputs to
stream water (Phillips et al., 1982). In contrast, a study in Iowa under level terraces found that
10% of TP (unf.) was contributed from subsurface tile flow (Burwell et al., 1974). Culley and
Bolton (1983) found that tiles contributed even higher percentages of P losses. Tiles transferred
at least 25% of TP (unf.) and 50% of RP (<0.45) leaving the watershed. Recent studies by Sims
et al. (1998), Stamm et al. (1998), Heckrath et al. (1995), and Ulen (1995) have reinforced the
theory that appreciable losses of P can be through field tile drains. If many macropores are
present, preferential flow will increase P concentrations in tiles. During preferential flow, plant
root uptake and soil adsorption are ineffective because of the high hydraulic conductivity of the
flowing water (Sharpley et al., 2000).
Factors that affect P losses from tiles are soil P levels, crop cover, P fertilization rate, and
tile depth (Hawkins and Scholefield, 1996). Heckrath et al. (1995) found that if certain
concentration of Olsen-extractable P is exceeded in the soil plow layer, P losses through
subsurface runoff on clay loam soils were enhanced. Total P (unf.) losses in tiles were highest
under continuous corn, lowest under continuous bluegrass sod and intermediate for a rotation of
corn, oats and two years of alfalfa for both fertilized and unfertilized plots (Bolton et al., 1970).
Similarly Phillips et al. (1982) found higher concentrations of RP (<0.45) where tile flow in
catchments that were manured, plowed, disced and seeded in maize than catchments that were in
alfalfa, and alfalfa and barley. Total P (>0.45) loads were highest under continuous corn while
TP (<0.45) concentrations were highest in tile effluent from permanent bluegrass sod. Total P
(>0.45) increased with P fertilization rates and decreased with increasing soil plant cover over
tile lines (Culley et al., 1983). Sawhney (1978) found that soils under vegetable crops had higher
concentrations of P in tiles than under corn production. Finally, flooding may produce reducing
conditions, if the soil is saturated, resulting in greater P mobility and significantly increasing tile
drain effluent P concentration (Sawhney, 1978).
Streambank erosion can also contribute significantly to P losses although it is typically
less significant compared to overland flow in predominantly cultivated landscapes. Areas
without vegetation may have up to five times more detectable streambank erosion after flood
events compared to areas with riparian vegetation (Beeson and Doyle, 1995). In Iowa, as rowcrop agriculture becomes more intense, streams widen and deepen (Hamlett et al., 1983).
Zaimes (1999) found in central Iowa that sites with row-cropping to the edge of the stream had
three time higher streambank erosion rates compared to areas with a riparian forest. This is more
evident when farmers crop up to the edge of the streambank. The removal of the perennial
vegetation and the replacement with crops destabilizes streambanks. Crops cover the
streambanks only part of the year and have much shorter roots that provide less support to
streambank stability. Culley and Bolton (1983) found that streambank erosion contributed 10%
of watershed TP (unf.) export. Finally, P losses to groundwater from cultivated soils with deep
water tables are minimal. However, when the water table approaches the plow layer, P may
enter as groundwater (Sawhney, 1978).
45
Bowman and Halvorson (1997) introduced a new concept of assessing P content based on
cropping intensity. Intensive cropping systems allow P recycling through residue and plant litter
that will increase plant available P. Additionally, more residue and soil organic matter in the
long term should increase water infiltration, decrease overland flow, surface erosion and peak
discharges.
To reduce P losses in row-cropped areas, management should first aim to reduce the
rainfall erosive force by encouraging water storage in topsoil through cultivation practices that
enhance porosity and surface roughness (Haygarth et al., 1998a). The combination of minimal
tillage, maximizing crop cover, terracing and contouring has been effective (Catt et al., 1998;
Sharpley et al., 1993). Additionally, impoundments or small reservoirs and constructed wetlands
will reduce overland flow (Sharpley et al., 1993). Ponds and constructed wetlands decreased TP
(unf.) by 17 to 41%, with retention increasing with the surface area/watershed area ratio (UusiKämppä et al., 2000). Riparian buffers are very successful in reducing P losses and will be
discussed in more detail in a following section. Finally, decreasing peak flows will reduce
streambank erosion.
3.4 Reducing phosphorus inputs to surface waters
The reduction of NPS P requires control of both nutrient inputs and their subsequent
transport in runoff to prevent eutrophication and restore water quality (Withers and Jarvis, 1998).
Areas with high sources of P that can be easily transported should be primarily targeted
(Sharpley et al., 2000). Pionke et al. (1997) says that 10 % of the land area of an agricultural
watershed contributes more than 90 % of the P losses. These areas are called critical source
areas (Grubek et al., 2000). Identifying these areas is crucial because significant improvements
in water quality will occur only when these are managed properly (Nelson et al., 1996), and
because it is cheaper to treat the cause of eutrophication than the effects. In these critical source
areas, BMP’s should be implemented with the goal to reduce P export to streams.
This would be easier achieved by managing for P inputs than transport. However, it is
not feasible to eliminate terrestrial P inputs because these inputs are necessary for profitable
animal and crop production. Additionally, most agricultural soils have high P levels that can be
the major source of P transport. This leads to using best management practices (BMP’s) that
concentrate on P pathways. Overland flow is the major P transport pathway, and the most
stringent measures should be applied on high vulnerability sites to minimize overland flow
(Sharpley et al., 2000). This can be done by reducing the overland flow volume, and increasing
infiltration and sediment trapping. The ultimate goal should be to break the link between sources
and transport pathways (Grubek et al., 2000). Upland BMP’s such as reduced tillage, grassed
waterways, strip or contour cropping and terraces have been developed to accomplish to break
this link.
Most of the upland BMP’s are based on increasing vegetative cover. Vegetative cover
influences surface erosion and overland flow, and streambank erosion, the major pathways of P
transport. Vegetative cover is so important because it determines the ability of precipitation to
induce surface erosion and overland flow (Isermann, 1990) which influence the size of peak
discharges and the speed of water movement in streams that then affect streambank erosion
(Nelson et al., 1996) (Figure 14). Decreasing the effectiveness of these pathways to transport P
will reduce TP (unf.) losses to surface waters (Sharpley et al., 1992).
46
In most watersheds the majority of sediment and nutrient contribution to streams comes
from small areas near the stream course (Johnes and Heathwaite, 1997; Pionke et al., 1997;
Wilkin and Hebel, 1982). Gburek et al. (2000) found that high soil P levels within 60 m of the
stream were more important than the soil P of the rest watershed. Attenuation and uptake of
nutrients increases the further a P sources is from the stream. Consequently, upland BMP’s can
still lead to high P losses without BMP's in the riparian areas. Therefore, BMP’s that limit NPS
sediment and P movement should be primarily located along the stream edge. Additionally,
Peterjohn and Correll (1984) suggest that coupling natural systems and managed habitats such as
riparian buffers with agricultural practices within the watershed can reduce nutrient transport
significantly.
3.5 Riparian Buffers
Gilliam (1994) considers riparian areas “the most important factor influencing NPS
pollutants entering surface waters in many areas in U.S.A.” Riparian areas can be sinks or
sources of P (Cooper et al., 1995) because their sink capacity is finite. Their effectiveness as a P
remover is influenced by sediment burial of subsequent deposits, the seasonal nature of
maximum biological uptake and water fluctuations (Cooper and Gilliam, 1987).
Figure 14. Nontilled row-cropped fields have more overland flow and less total
evapotranspiration resulting in larger flow compared to riparian buffers that reduce
stormflow and increase baseflow due to higher infiltration and evapotranspiration.
Reducing overland flow and streambank erosion will decrease P losses (Schultz et al.,
2000).
47
Recent studies in the United States and other countries have demonstrated the
effectiveness of riparian buffers (Table 16). Most of these studies have been based on
established buffers adjacent to cultivated fields. There is a lot less information on newly
established buffers adjacent to cropfields or on grazed catchments (Lee et al., 2000, Nelson et al.,
1996; Schultz et al., 1995).
Buffer effectiveness is due to three mechanisms (Schultz et al., 2000) (Figure 15). The
first mechanism is reducing overland flow and streambank erosion. Buffers have vegetation that
covers the soil year round and increases surface roughness and infiltration thereby reducing
overland flow and streambank erosion. As a consequence, in forest and grasslands TP (<0.45) is
the dominant form of transported P, and not TP (>0.45) as in cultivated fields. The second
mechanism is plant assimilation and immobilization of P as long as they are actively
accumulating biomass. The third mechanism is the increased soil organic matter that leads to
larger microbial populations that are a nutrient sink and improve soil aggregation (infiltration).
Therefore, forests retain higher Bray-I P compared to pastures (Griffiths et al., 1997) and their
adsorption capacities can be up to 50% higher than agricultural A horizons (Beauchemin et al.,
1996). Wooded soils also have lower soil P compared to pastures and croplands because they
receive smaller inputs. Gburek et al. (2000) found that wooded soils had much lower soil P (30
mg kg-1 Mehlich-III), compared to pastures (100-200 mg kg-1 Mehlich-III) and cultivated lands
(>200 mg kg-1 Mehlich-III). It must be noted that all three mechanisms are more effective in
reducing TP (>0.45) than they are in reducing TP (<0.45) (Gilliam, 1994).
Figure 15. Phosphorus movement in riparian forest buffers. Sediment and TP (>0.45) are
filtered from overland flow and TP (<0.45) can be taken up by biota of the living filter
(Schultz et al., 2000).
48
Table 16. The effectiveness of different types of buffers with different widths in removing
NPS P from overland flow.
Country
Denmark
Type
b
Grass
Finland b
Norway b
Grass
Grass
Norway b
Grass
Sweden b
Grass
Sweden b
U.S.A. e
U.S.A. f
Grass
Grass
Grass
U.S.A. d
Forest
U.S.A. g
Forest
U.S.A. h
Forest
Finland a
Vegetative
(trees and
grasses)
Vegetative
(trees and
grasses)
Switchgrass
Norway a
U.S.A. c
Width
m
2
6
10
5
10
5
10
8
16
5
various
9.1
4.6
30
P form
Comments
Not stated
Reduction
%
65
97
38
88
96
51, 66
67, 82
66
95
-36
60
79
61
70-81
24-50
50
Not
stated
50
TP (unf.)
TP (>0.45)
80
74
3 kg TP (>0.45) removed per 1 ha of
riparian forest. TP losses divided evenly
through surface and subsurface
pathways.
10
TP (unf.)
27
5
10
TP (unf.)
75, 97
97, 96
Two short-term experiments
3
TP (unf.)
RP (<0.45)
TP (unf.)
RP (<0.45)
TP (unf.)
RP (<0.45)
TP (unf.)
RP (<0.45)
TP (unf.)
RP (<0.45)
TP (unf.)
RP (<0.45)
40
35
55
46
38
30
49
39
72
44
93
85
Switch grass more effective then coolseason grasses
TP (unf.)
RP (<0.45)
TP (unf.)
RP (<0.45)
46
28
81
35
1-h rainfall simulation at 6.9 cm/h
6
Cool-season
grass
3
6
U.S.A. a
Switchgrass
7.1
16.3
U.S.A. a
Multi-species
riparian
buffer
Switch grass
Multi-species
riparian
buffer
16.3
a
7.1
TP (unf.)
TP (unf.)
TP (unf.)
TP (unf.)
RP (<0.45)
TP (unf.)
TP (unf.)
TP (unf.)
TP (unf.)
Results from two short-term
experiments
Short-term experiments
Captures only 20% of RP (<0.45)
High inputs
Low inputs
2-h rainfall simulation at 2.5 cm/h
Lee et al. (2000); b Uusi-Kämppä et al. (2000); c Lee et al. (1999); d Palone and Todd (1997); e Daniels and Gilliam
(1996); f Dillaha et al., (1989); g Cooper and Gilliam (1987); h Peterjohn and Correll (1984).
49
Wider buffers are more effective at removing P (Uusi-Kämppä et al., 2000). Lee et al.
(1999) found that doubling the width of switchgrass (Panicum virgatum) buffers from 3 m to 6 m
increased removal of TP (unf.) by 15 % and RP (<0.45) by 11 %. For cool-season grass buffers
doubling the width (3 m to 6m) increased removal of TP (unf.) by 11 % and RP (<0.45) by 9 %.
Although the effectiveness of increasing the width is known, the major problem is convincing
the landowner to take his/her land out of crop-production. Most landowners prefer a minimum
acceptable width. This width is one that provides acceptable levels of all needed benefits at a
reasonable cost (Dosskey et al., 1997). The width will vary depending on the NPS pollutants
being addressed, the resource being protected, site characteristics (slope, soil), stream channel
characteristics (incised channels), watershed traits and adjacent land-use intensity (Palone and
Todd, 1997). The more intense the land-use, the greater the minimum effective buffer width.
Along meandering streams, the buffer widths tend to vary in order to provide straight edges
along the crop fields to facilitate cultivation and harvesting (Schultz et al., 2000). Finally, the
quality and continuity of the riparian buffer become the most important factor for their
effectiveness and their width beyond a certain minimum based on the topography and the
adjacent land uses (Pinay et al., 1990).
Three riparian buffer models have been developed to fit various landscape and land
management scenarios: a) filter strips, b) riparian forest buffer, and c) riparian management
practice.
3.5.1 Filter strips
Vegetation is primarily composed of cool-season grasses that remove P from runoff
(USDA-NRCS, 1997e). Although filter strips provide perennial vegetation, they do little to slow
overland flow (Dillaha et al., 1989). Cooper et al. (1995) mentions that grass buffers become
ineffective with time. If filter strips are planted to natural warm-season grasses they can more
effectively slow overland flow and trap P (Lee et al., 1999).
3.5.2 Riparian forest buffer
This buffer design was developed by the U.S. Department of Agriculture Forest Service
and the U.S. Department of Agriculture Natural Resource Conservation Service (NRCS) for the
row-crop region in the originally forested eastern USA (Welsch, 1991). Riparian forest buffers
consist of three zones (Figure 16). The first zone is 5 m wide and begins at the edge of the
stream. It contains unmanaged trees that provide streambank stability and woody debris for the
stream channel. The second zone is at least 18 m wide and consists of managed forest. This
zone provides maximum infiltration for overland flow and nutrient uptake and storage and
increases soil organic matter. The third zone is 6 m and consists of grass that filters sediment
from upland flow and allows infiltration of P to the root zone where nutrient uptake will take
place. This zone can be carefully grazed. To effectively reduce P losses from tiles, tile flow
should be brought to the surface and allowed to flow through the buffer rather than under it.
50
Figure 17. The riparian forest buffer. The first zone consists of unmanaged forest, which
protects the streambank and provides woody debris. In the second zone, the forest is
managed to maintain nutrient uptake through vigorous plant growth. The third zone has
grasses with controlled grazing allowed under certain conditions (Schultz et al., 2000).
3.5.3 Multi-species buffer and Riparian management systems
A similar model was independently developed for the agricultural region of the
Midwestern United States (Schultz et al., 2000; 1995). The riparian forest buffer is not as
effective in the Midwest, because streams in these landscapes are of low relief and there is
extensive use of field tile drains. This has resulted in concerns by landowners that woody debris
in streams can slow flow and restrict tile drainage. The multi-species riparian buffer (MRB) was
developed to address these concerns and function in these landscapes (Figure 17). The basic
design of MRB zone is 21 m wide from the stream edge, but can vary from 10-45 m. The first
zone (14 m wide) consists of managed trees and shrubs that provide streambank stability and a
long-term P sink. The second zone (7 m wide) consists of grasses that intercept and dissipate the
energy from overland flow, trap the sediment and P and increase infiltration and microbial
activity. Warm- season grasses are preferred to cool-season, because they have taller and stiffer
stems, and deeper roots. Additionally, sediments can more easily bury cool-season grasses than
warm-season grasses. Lee et al. (2000) in central Iowa compared the effectiveness of P removal
from switchgrass buffers (7.1 m width) to switchgrass- woody plant buffers (16.3 m width) for
different rainfall intensities. When rainfall intensity was 69 mm h-1 and applied for 1 h the
switchgrass-woody plant buffer increased removal of TP (unf.) by 35% and TP (<0.45) by 7%
compared to the switchgrass buffer. When the rainfall intensity was 25 mm h-1 and applied for 2
h the switchgrass-woody plant increased TP (unf.) by 21% and TP (<0.45) by 41% compared to
51
Figure 17. The multi-species riparian buffer model. The first zone, is located along
consists of a managed tress and shrubs. They provide bank stability, wildlife habitat, are a
nutrient and sediment sink, and modify the aquatic environment. The second zone, consist
of native grass and forbs that intercept overland flow, increase infiltration and intercept
NPS pollutants (Schultz et al., 2000).
the switchgrass buffer. The authors attributed the increase in P removal to the high infiltration
capacity provided by the deep-rooted plant that allowed clay and soluble nutrient trapping,
although the fact that the woody-switchgrass buffer was wider than the switchgrass buffer was
also influential. In a study on the same riparian forest buffer in central Iowa the riparian forest
buffer reduced streambank erosion up to 70% compared to reaches with row-crops and pastures
on the same creek (Zaimes, 1999). The MRB is part of the Riparian Management System
(RiMS) (Figure 18) that provides additional BMP’s to address potential NPS pollution and
channel morphological problems. The RiMS package provides streambank bioengineering to
stabilize streambanks, constructed wetlands for intercepting field tile drainage water, in-stream
structures such as boulder weirs for controlling channel morphology, and rotational grazing
practices that reduce the impact of grazing on the riparian zone and the stream channel. Any one
or more of these practices can be used with the MRB to address potential P pollution problems.
52
Figure 19. The Riparian Management System (RiMS) consists of five practices: i) multispecies riparian buffer with woody plants and warm-season grasses that intercept NPS
from adjacent land practices, ii) streambank bioengineering that provide bank stability, iii)
constructed-restored wetlands that intercept and filter NPS from subsurface tiles, iv) coolseason or warm-season grasses can replace MRB that may be used for rotational grazing
with stream fenced out and v) instream structures like boulder weirs (Schultz et al., 2000).
53
4. OTHER FACTORS THAT INFLUENCE PHOSPHORUS
TRANSPORT
No one factor can be singled out to reduce P losses to surface waters. A
combination of factors that varies even between neighboring sites can lead to excess P in
surface water. Catchment shape can influence the P form and the amount transported
because it affects the average soil/water contact time (Stevens et al., 1999). As soil/water
contact time increases TP (<0.45) decreases because of the increase in P adsorption.
Watershed slope gradients (Keup, 1968) and stream drainage density (Kirchner, 1975)
also have been correlated with stream P levels. As both these factors increase terrestrial
P losses and stream P levels both increase. Hill (1981) mentions that although land-use is
important, additional landscape variables should be analyzed to better understand the
relationship between P export for different types of watersheds.
Edward and Withers (1998) said that to establish the movement of P to surface
waters it is necessary to know: a) the primary mechanisms of P storage to determine if a
landscape patch serves as a P sink or source; b) spatial organization of each patch to
understand its position within the transport pathway from land to surface waters; and c)
the residence time of P within each component of the ecosystem in order to understand
the kinetics and transport of P.
4.1 Different scale categories
Depending on the objectives of the research, different plot scales should be used.
Johnes and Hodgkinson (1998) divide scale levels into four categories: 1) plot scale (0.550 m2) studies to estimate P export under individual crop management systems along
different pathways; 2) field scale (1-10 km2) studies to investigate P transport from
upslope to downslope along different pathways; 3) small catchment (1-10 km2) studies to
investigate the spatial location and extent of P export zones; and 4) watershed scale (> 10
km2) studies that use models based on the results from the three previous scales that will
allow a sound understanding of P transport in the watershed. Some researchers found
that P transport results differed depending on the scale used and that there are limitations
to extrapolating to the watershed level (Truman et al., 1993; Sharpley and Syers, 1979a).
4.2 Watershed scale studies
Many studies have quantified losses from specific sources at the plot, field and
small catchment scale under varying management practices and for different pathways.
In contrast, not many studies have considered the losses from various sources and
quantified them at the watershed level (Sharpley and Tunney, 2000). To reduce P from
NPS, watershed scale studies would be ideal. Unfortunately, studies at this scale are also
very costly (Sharpley et al., 1994). Instead, plot, field and small catchment scale studies
should be adopted and the results extrapolated to provide realistic and reliable data at the
watershed scale. However, to extrapolate the results it is necessary to understand how
specific soil characteristics, hydrological conditions, different crop covers, and
54
management practices influence P transport in different climatological and geological
conditions (Johnes and Hodgkinson, 1998). An understanding of these processes will
identify the pathways that P follows at the watershed scale, which is essential for
modeling.
55
5. PHOSPHORUS-INDEX SYSTEM
The P-index system was developed by the USDA-NRCS to assess the various
topographic, land-use, and hydrologic factors that influence P transport to water bodies,
and to determine whether the risk of P loss is high or low (Lemunyon and Gilbert, 1993).
It is based on a field-oriented matrix system. Sites are ranked by integrating soil P
availability, fertilizer and animal manure, feces, and litter transport phenomena. The Pindex system can be used at site specific locations to provide general conservation
recommendations, or at the watershed scale to target critical P source areas. The first Pindex system was developed by Lemunyon and Gilbert (1993) (Appendix II). The major
advantage of the P-index system is that it allows delineation of the critical source areas
that control P losses where the appropriate BMP’s should be implemented. This allows
evaluation of the trade-offs between protecting the stream and allowing agricultural
activities to continue (Gburek et al., 2000). The original P-Index has been modified to
the watershed scale (Grubek et al., 2000) and currently the original P-Index is being
modifying by most States. The reason for the modification at the State level is to meet
each States hydrologic characteristics. The watershed-modified P-Index and the Iowa Pindex will be discussed in the following two sections.
5.1 Watershed-modified Phosphorus-Index
The original assumption of the P-index (Lemunyon and Gilbert, 1993) was that
processes that indicate P vulnerability at the field scale were similar to those for the
watershed scale. The assumption was incorrect and a watershed-modified P-index was
developed (Grubek et al., 2000). There are two basic differences between these two Pindex systems. In the watershed-modified P-index, the P source and transport
characteristics are evaluated separately and the hydrological return period is incorporated
in the transport characteristics.
The source characteristics in the watershed-modified P-index system are soil P
test, P fertilizer application rate (kg P ha-1) and application method and organic P (animal
manure and litter) application rate (kg P ha-1) and application method (Table 17). The
transport characteristics are soil erosion, runoff class, and return period/contributing
distance (Table 17). These characteristics were selected because they influence P
availability, uptake, retention, movement, and management at the watershed scale.
Higher P in the soil often increases the TP (<0.45) concentration in the runoff
water. Similarly, increased P fertilizer or P rates from animal manure, feces, and litter
and crop residue can increase P losses. In the application of fertilizer or animal manure,
feces, and litter and crop residue, the main consideration is how long P remains on the
soil surface. The longer it is exposed the higher the chances for losses via overland flow
and soil erosion. Therefore application methods become very important with those that
incorporate either the inorganic or organic P generally reducing the potential for P
transport.
For this index, water and wind processes that lead to soil losses and TP (>0.45)
transport along a slope refer to soil erosion. The runoff class is based on the saturated
56
Table 17. The transport and source characteristics with their respective weight
factors, and the five P loss-rating levels of the watershed-modified P-index system.
The transport characteristics include soil erosion, soil runoff class and return
period/distance. The source characteristics include soil P test, P fertilizer rate, P
fertilizer application method, organic P application rate, and organic P application
method. a
TRANSPORT
CHARACTERISTIC
(Weight Factor)
Soil Erosion
(1.5)
Soil Runoff Class
(0.5)
Return
period/distance
(1.0)
SOURCE
CHARACTERISTICS
(Weighing Factor)
Soil Test P
(1.0)
P Fertilizer Rate
(kg P ha-1)
(0.75)
P Fertilizer
Application Method
(0.5)
Organic P Source
Application Rate
(kg P ha-1)
(1.0)
Organic P Source
Application Method
(1.0)
a
None
(0.6)
N/A
PHOSPHORUS LOSS RATING (VALUE)
Low
Medium
High
(0.7)
(0.8)
(0.9)
< 10 Mg ha-1 10-20 Mg ha-1
20-30 Mg ha-1
Very High
(1)
> 30 Mg ha-1
N/A
Very Low
or Low
Medium
High
Very High
None
(0.2)
> 10 yr
> 170 m
Low
(0.4)
6-10 yr
130-170 m
Medium
(0.6)
3-5 yr
80-130 m
High
(0.8)
1-2 yr
30-80 m
Very High
(1)
< 1 yr
< 30 m
None
(0)
N/A
PHOSPHORUS LOSS RATING (VALUE)
Low
Medium
High
(1)
(2)
(4)
Low
Medium
High
N/A
< 15
15-45
56-75
> 75
N/A
Placed with
planter
deeper than
5 cm
Incorporate
immediately
before crop
Surface applied
> 3 months
before crop
N/A
< 15
15-45
Incorporate
> 3 months
before crop or
surface applied
> 3 months
before crop
56-75
N/A
Injected
deeper than
5 cm
Incorporate
immediately
before crop
Incorporate
> 3 months
before crop or
surface applied >
3 moths
before crop
Surface applied
to pasture
> 3 months
before crop
Very High
(5)
Excessive
> 75
Gburek et al. (2000).
hydraulic conductivity of the soil and the percentage of slope of the site. These data are
obtained from the soil survey. The hydrologic return period quantifies the probability
that overland flow will impact the stream. It is described by two characteristics, return
period and contributing distances. The return period represents the probability of rainfall
57
or flood events of a given magnitude. Typically, it is expressed in years. The
contributing distances refer to the distance of the field from the stream. It is a function of
hydrology and watershed geometry and is estimated with Geographic Information
Systems (GIS) or aerial photos. As the distance increases the risk of P pollution
decreases. All characteristics have five levels with assigned numerical values, and a
weighting factor (Table 17). The numerical values that correspond to the appropriate
levels for each characteristic are multiplied by the weighting factor to get the
characteristic’s rating. The rating system can be changed to meet localized conditions.
To get the site vulnerability value, the product from the rating of the soil erosion, the
runoff class, and the return period/distance, is added to the sum of the rating of the source
parameters. The formula for P index site vulnerability is:
P Index = { [ (Erosion rating x weight factor) x (Runoff rating x weight factor) x
(Return period rating x weight factor) ] x Σ (Source characteristic rating
x weight factor) }
The general conservation recommendations for each site are based on the total
vulnerability chart (Table 18). Additional characteristics that have been considered are
total soil P instead of soil P test, clay mineralogy, cation exchange capacity, particle size
distribution, soil erodibilty and subsurface P losses. These characteristics are not used
because their contributions are not fully understood and they are difficult to obtain.
5.2 Iowa Phosphorus-Index
The Iowa P-index was developed to assess the potential of P transport from
individual fields to surface water based on local management practices and soil and field
Table 18. The site vulnerability chart of the watershed-modified P-index system
that indicates the potential of a site to deliver P to surface waters. a
Phosphorusindex for site
<5
Generalized interpretations of P-index for site
LOW potential for P movement from the site. If farming practices are maintained as
the current level there is a low probability of an adverse impact to surface waters
from P losses at the site
MEDIUM potential for P movement from the site. The chance for an adverse
5-9
impact to surface water exists. Some remedial actions should be taken to lessen the
probability of P loss.
HIGH potential for P movement from the site and for an adverse impact to surface
9-22
water to occur unless remedial action is taken. Soil and water conservation as well
as P management practices are necessary to reduce the risk of P movement and water
quality degradation.
VERY HIGH potential for P movement from the site and for an adverse impact to
> 22
surface water exists. Remedial action is required to reduce the risk of P loss. All
necessary soil and water conservation practices, plus a P management plan must be
put in place to avoid the potential for water quality degradation.
a
adapted from Gburek et al. (2000) and Pierzynski et al. (2000).
58
characteristics (USDA-NRCS, 2001). The Iowa P-index consists of three components:
i)erosion ii) runoff and iii) subsurface drainage. The erosion component describes the
potentially mobile P that may be delivered to surface water as TP (>0.45), the runoff
component describes the potentially mobile P that may be delivered to surface water as
TP (<0.45), and the subsurface drainage component describes the potentially mobile P
that may be delivered to surface water from subsurface drainage. The major difference
between the Iowa P-Index systems and the original P-Index is that it considers subsurface
flow. This was necessary because of the vast area of land in Iowa that has been tiled. All
three components have the same weight and the Iowa P-Index is estimated by the
following equation:
P Index = Erosion component + Runoff component + Subsurface Drainage
component
The sum of the three P-Index components provides the site vulnerability rating.
The rating ranges from 0 - >15 (no units) and is divided in five hazard classes: i) very
low, ii) low, iii) medium, iv) high and v) very high (Table 19). To estimate each
individual component the following formulas are used:
Erosion component = Gross erosion x (Sediment trapping factor or Sediment
delivery
ratio) x Buffer factor x Enrichment factor x Soil P test
erosion factor
Runoff component = Runoff factor x Precipitation factor x (Soil P test runoff factor
+ P application factor)
Subsurface drainage component = Precipitation factor x Flow factor X Soil P test
Factor
In the erosion component, gross erosion is estimated from the Revised Universal
Soil Loss Equation (RUSLE). The sediment trap factor considers how certain
conservation practices trap sediment while sediment delivery ratio considers the amount
of sediment delivered form a field based on the Iowa landform region it is in and the
fields distance to nearest perennial or ephemeral streams. The buffer factor accounts for
the removal of sediment when vegetative buffers are present. The increase of finer
particles that are transported in the eroded sediment that leads to higher P concentration is
represented by the enrichment factor. The enrichment factor is highly influenced by land
practices. Finally, the soil P test erosion factor refers to the amount of TP (>0.45) in the
delivered soil sediment that reaches the surface waters.
In the runoff component, the runoff factor is based on the Runoff Curve Number
(RCN) (adapted for this index). The RCN converts the amount of precipitation water that
runs off the field. The precipitation factor is based on the annual precipitation for each
county. The soil P test runoff factor provides the TP (<0.45) concentration in overland
flow and P application factor considers the affects of P application.
59
In the subsurface drainage component, the precipitation factor is 10 % of the
runoff precipitation factor. Presence or absence of subsurface/substrata flow determines
the flow factor while the soil P test factor is based on soil P test values.
Table 19. The site vulnerability rating (P hazard class) for the Iowa P-index. a
P hazard
class
Very Low
Rating
Interpretations
0-1
A field in which movement of P offsite will be Very Low. If soil
conservation and P management practices are maintained at current
levels, impacts on surface water resources from P losses from the
field will be small.
A field in which movement of P offsite will be Low. Although P
delivery to surface water bodies is greater than from a field with a
very low rating, current soil conservation and P management
practices likely do not pose a threat to water quality.
A field in which movement of P offsite will be Medium. Impacts on
surface water resources will be higher than for a field with a low
rating, although P delivery potential still is not likely to produce water
quality impairment. However, careful consideration should be given
to soil conservation and P management practices that could reduce the
risk of significant p delivery.
A field in which movement of P offsite will be High. Impacts of
surface water resources will be high. Remedial action is required to P
movement to surface water bodies. New soil and water conservation
and/or P management practices are necessary to reduce potential for
offsite P movement and probable water quality degradation.
A field in which movement of P offsite will be Very High. Impacts
on surface water resources are extreme. Remedial action is required
to reduce P movement. All necessary soil and water conservation
practices plus a P management plan, which may require discontinuing
of P application, must be put in place to reduce potential of water
quality degradation.
Low
1-2
Medium
2-6
High
6-15
Very High
>15
a
USDA-NRCS (2001)
60
6. Conclusion
In many landscapes excessive inputs of terrestrial P to surface waters threatens the biotic
integrity of the aquatic ecosystems. Eliminating all terrestrial P inputs in cropping systems is not
a realistic solution because they are essential for profitable crop and animal production (Hedley
and Sharpley, 1998). The ultimate answer to solving the problem of NPS P pollution in
watersheds is to establish a balance between terrestrial P inputs and outputs (Gburek et al.,
2000). The basic concept of achieving balance should be based on maximizing P use efficacy in
agricultural systems, but also reducing the negative impacts of P on surface waters (Frossard et
al., 2000).
Reducing P inputs to levels that can be efficiently used in the agricultural systems would
be part of the solution. However, more importantly, to accomplish this balance it is necessary to
have a good understanding of the pathways NPS P follows to reach surface waters (terrestrial
processes) and the way P behaves (aquatic processes) once it reaches those waters. In most
cases, controlling the processes and pathways that transport NPS P to surface waters is the most
realistic solution (Gburek et al., 2000). Current land-use practices like cultivation and grazing
have altered the processes and pathways taken by P to surface waters in natural settings (forests,
prairies) and have generally increased the amounts of NPS P reaching surface waters.
Best management practices like conservation tillage, terracing, intensive management
grazing, and the establishment of riparian buffers may reduce overland flow (the major P
terrestrial pathway) and streambank erosion (the major P aquatic input) that current land-use
practices have enhanced. To implement the most effective BMP’s it is necessary to recognize
the potential impact of agricultural NPS P on surface waters, understand that different landscapes
and varying hydrologic conditions cause temporal and spatial variation in P losses and that
similar land-use practices under different management systems have different risks of NPS P
losses (Sharpley et al., 2000).
Although in each watershed P inputs, pathways and processes are similar, each watershed
is unique and requires a different management plan to achieve P balance between inputs and
outputs. Most BMP’s are suited for many watersheds but should be modified to each specific
watershed. The modifications should be based on watershed characteristics such as degree and
lengths of slopes, land-use practices (row-cropfields, pastures, forests), precipitation amounts,
soil types, and land drainage among other factors. The watershed-modified P-index is a tool that
can assess the risks of NPS P losses.
However, solving agricultural NPS P pollution will be extremely difficult because of the
problems associated with the measurement and regulatation of NPS P. Nonpoint source P has
temporal variations due to weather and temporal and spatial variations associated with the large
areas of agricultural lands that may provide the P lost to surface waters (Carpenter et al., 1998).
This makes it difficult to trace the source of P and the pathways it follows. In many cases
solving NPS P pollution may take a long time. Even if all P inputs were eliminated, the
eutrophication problem due to P would not be eliminated immediately. Many soils, have soil P
levels that are so high that even when P fertilizer or manure are not applied, P losses still remain
high (Addiscott et al., 2000). In surface water, even after terrestrial P loads have been reduced
eutrophication levels may not decrease substantially because of the steady source of P in the
sediments in the bed of the stream or lake that have accumulated P and continue to release it
(Sharpley et al., 1994).
61
Future research must concentrate on developing a better understanding of P transport
from agricultural sources through NPS pathways. These studies should be conducted at larger
scales and over wider ranges of regions that are diverse and under different combinations of
BMP’s.
Appendix I
Case Studies:
Phosphorus Losses Under Different
Land-use Practices
62
7. References
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phosphorus levels. J. Environ. Qual. 24:132-138.
Addiscott, T.M., D. Brockie. J.A. Catt, D.G. Christian, G.L. Harris, K.R. Howse, N.A. Mirza, and T.J.
Pepper. 2000. Phosphate losses through field drains in a heavy cultivated. J. Environ. Qual. 29: 522-532.
Ahuja, L.R. 1986. Characterization and modeling of chemical transfer to runoff. Adv. Soil Sci. 4:149-188.
Alberts, E.E and R.G. Spomer. 1985. Dissolved nitrogen and phosphorus in runoff from watersheds in
conservation and conventional tillage. J. Soil Water Conserv. 40:153-157.
Alberts, E.E., G.E. Schuman, and R.E. Burwell 1978. Seasonal runoff losses of nitrogen and phosphorus
from Missouri Valley watersheds. J. Environ. Qual., 7:203-207.
Amemiya, A. 1977. Conservation tillage in the western Corn Belt. J. Soil Water Conserv. 32:29-36.
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role of phosphorus in agriculture. ASA-CSSA-SSSA, Madison, WI.
Andraski, B.J., D.H. Mueller, and T.C. Daniel. 1985. Phosphorus losses in runoff as affected by tillage.
Soil Sci. Soc. Am. J. 49:1523-1527.
Baker, J.L., K.L. Campbell, H.P. Johnson, and J. J. Hanway. 1975. Nitrate, phosphorus, and sulfate in
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Baker, J.C. and J.P. Zublena. 1995. Livestock manure nutrient assessment in North Carolina, N.C.
Cooperative Extension Service, Raleigh, NC.
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Appendix I
Case Studies:
Phosphorus Losses Under Different
Land-use Practices
Appendix I -Table 1. Phosphorus (P) losses and concentrations under different land-use management’s, through different
pathways and various P forms.
Location
California, U.S.A.
Ontario, Canada
New York, U.S.A.
Vermont, U.S.A
Pathway
tiles
tiles
NO FERTILIZER
Rotation:
Corn
Oats + alfalfa
Alfalfa 1st yr.
Alfalfa 2nd yr.
Corn-continuous
Bluegrass sod
continuous
FERTILIZER
Rotation:
Corn
Oats + alfalfa
Alfalfa 1st yr.
Alfalfa 2nd yr.
Corn-continuous
Bluegrass sod
continuous
100% pine and
In the stream
hardwoods
50% pasture and 50%
crops and hay
Tiles
Cropland
Alfalfa-cropland
P form
TP (unf.)
P concentration
mg L-1
0.079
(0.05-0.23)
P export
kg ha-1 yr-1
Johnston et al.
1965
Bolton et al.
1970
TP (<0.45)
TP (unf.)
References
0.13
0.13
0.13
0.08
0.26
0.01
0.2
0.2
0.18
0.17
0.17
0.17
0.24
0.13
0.15
0.22
0.29
0.12
0.22
0.19
0.21
0.27
0.19
0.19
0.015
Comments
Fertilizer
application small
but consistent
increase in p losses.
79
Ohio, U.S.A.
Land-use
management
cropland
Taylor et al.
1971
0.022
RP (<0.45)
Tiles
RP (<0.45)
Overland flow
0.004-0.01
<0.02
0.8
0.008-0.046
Zwerman et al.
1972
Benoit 1973
Neither crop
management nor
fertilizer affected
RP (<0.45)
concentrations.
Corn-cropland
Hay-pasture
Iowa, U.S.A.
Ontario, Canada
Corn
Overland flow TP (unf.)
TP (<0.45)
Tiles
TP (unf.)
TP (<0.45)
Cultivated mucklands Tiles
TP (unf.)
with vegetables
TP (<0.45)
Uncultivated
TP (unf.)
mucklands
TP (<0.45)
Corn
Tiles
Florida, U.S.A.
Citrus production
Surface tillage
Deep tillage
Deep tillage plus
liming
Corn
Corn
Alfalfa
Undrained grazed
pasture plots
1
Tiles
Ohio, U.S.A.
New Zealand
2
3
TP (unf.)
TP (<0.45)
TP (<0.45)
0.72
Hanway and
Laflen 1974
0.015
1.56
0.19
0.34
Nichols and
MacCrimmon
1974
0.031
0.007-0.182
0-0.038
0.0003
Baker et al.
1975
Calvert 1975
0.53
0.17
0.09
Tiles
TP (<0.45)
0.07 (0.0-0.88)
0.11 (0.0-0.77)
0.22 (0.03-1.8)
0.3
0.3
0.1 (lb/ac/yr)
Logan and
Schwab 1976
Sharpley and
Syers 1976
Overland flow
TP (unf.)
TP (<0.45)
RP (<0.45)
TP (unf.)
TP (<0.45)
RP (<0.45)
TP (unf.)
TP (<0.45)
RP (<0.45)
1.53
0.90
0.53
4.90
2.00
1.45
1.59
0.92
0.47
Cultivated marshes
higher P losses
because of
increased
fertilization,
oxidation of
organic matter and
mineralization of
organic P.
Deep tillage and
deep tillage plus
liming reduced P
losses compared to
surface tillage.
80
Iowa, U.S.A.
0.7
0.9
High 3.0
2.08
0.06
0.02
0.01
4 Fertilizer added
Undrained ungrazed
pasture plots
5
6 Fertilizer added
Drained ungrazed
pasture plots
7
8 Fertilizer added
Contour corn
P applied kg ha-1yr-1
40
63 farms
New Zealand
Pasture, sheep (15
animal ha-1 )
P applied kg ha-1yr-1
0
75
TP (unf.)
TP (<0.45)
RP (<0.45)
TP (unf.)
TP (<0.45)
RP (<0.45)
0.82
0.46
0.31
1.64
1.89
3.64
TP (unf.)
TP (<0.45)
RP (<0.45)
TP (unf.)
TP (<0.45)
RP (<0.45)
0.23
0.12
0.10
0.74
0.45
0.42
Overland flow
Burwell et al.
1977
DP
PP
DP
PP
TP
67
Europe
4.38
2.36
1.81
Unspecified
0.19
0.71
0.25
1.27
TP (unf.)
Overland flow TP (<0.45)
TP (>0.45)
Overland flow TP (<0.45)
0.11
0.40
0.13
0.68
0.1-31
0.32
0.01
0.06
0.03
0.01
0..20
0.05
Frissel 1977
McColl et al.
1977
81
Minnesota, U.S.A.
TP (unf.)
TP (<0.45)
RP (<0.45)
High temporal and
spatial variation
typical of soil
erosion and of P
losses.
New Zealand
New York, U.S.A.
Canada
Sheep and cattle
Unspecified
grazing sheep (11
animals ha-1)
Cultivated mucklands Tiles
i)organic material
ii) calcareous mineral
material
Wheat-summer
fallow
Applied P kg ha-1yr-1
0
TP (>0.45)
TP (unf.)
RP (<0.45)
0..30
1.6
0.9-30.7
5.4-7.8
0.2-1.8
Overland flow
Duxbury and
Peverly 1978
Some sheet and rill
and some bank
erosion.
Sources of P
mineralization of
organic matter (~50
kg ha-1 yr-1) and
fertilization (~40
kg ha-1 yr-1).
Vegetables
Corn
Orchard
Hay
Tile
New Zealand
Sheep grazing
Unspecified
TP (unf.)
Ontario, Canada
Intensively cropped
Organic soils
Mineral soils
Pasture
Tiles
RP (<0.45)
0.30
1.80
3.70
7.40
0.052-0.124
0.018-0.077
0.012-0.036
0.014-0.052
Sawhney 1978
0.11
1.14-18.2
0.015-0.072
Overland flow TP (unf.) TP
(<0.45) TP
(>0.45) RP
(<0.45)
Interflow
TP (unf.) TP
(<0.45) TP
(>0.45) RP
(<0.45)
Tiles
TP (unf.) TP
(<0.45) TP
0.18
1.25
1.07
2.59
1.6-36.8
0.03-0.24
1.00
0.47
0.53
0.37
0.60
0.09
0.51
0.06
0.43
0.25
McColl and
Gibson 1979
Miller 1979
Sharpley and
Syers 1979
Flooding may
produce reducing
conditions resulting
in greater P
mobility.
82
Connecticut,
U.S.A.
54
Bargh 1978
Nicholaichuk
and Read 1978
TP (<0.45)
TP (>0.45)
TP (<0.45)
TP (>0.45)
TP (<0.45)
New Zealand
0.14
Pasture fertilized
P applied
50 kg ha-1yr-1
(>0.45) RP
(<0.45)
Stream flow
TP (unf.) TP
(<0.45) TP
(>0.45) RP
(<0.45)
Overland flow TP (unf.) TP
(<0.45) TP
(>0.45) RP
(<0.45)
0.18
0.20
2.74
0.92
1.82
0.71
4.36
2.19
2.16
2.06
TP (unf.) TP
(<0.45) TP
(>0.45) RP
(<0.45)
TP (unf.) TP
(<0.45) TP
(>0.45) RP
(<0.45)
TP (unf.)
43.27
8.36
34.93
5.78
61.53
18.46
43.03
15.86
Subwatershed
Bank erosion
and resuspension of
sediments
Stream flow
Ohio, U.S.A.
Pasture no mineral
fertilizer
rotationally grazed
continuously grazed
Rotation: corn, oat,
fallow, alfalfa and
soybean
Rainfall inputs
Indiana, U.S.A.
Variety agronomic
crops
Unspecified
Olness et al.
1980
0.20
0.76
TP (unf.)
2.2 a- 1.9 b
0.8 a
Surface drains
Shallow pipes
(0.5m)
Deep pipes
(1.0m)
Rainwater
Tiles
Schwab et al.
1980
1.2 a - 1.6 b
4.6 b
RP (<0.45)
UP (<0.45)
TP (>0.45)
0.04
0.03
0.21
0.035
0.026
0.16
Bottcher et al.
1981
83
Oklahoma, U.S.A.
a
Mean for 1969.
Mean for 19721977.
43% of TP (unf.) in
tiles was sediment
bound.
76% of TP (unf.) in
surface drains was
sediment bound.
High rainfall
inputs.
TP (>0.45) can be
the major source of
P in tiles.
b
New York, U.S.A.
Ontario, Canada
Ontario, Canada
Nebraska, U.S.A.
Cropland (corn,
soybean and wheat)
Manure applied
kg ha-1
0
35000
200000
Continuous silage
corn
fertilized plots
manure plots
Continuous corn
Watershed 1
Watershed 2
Pasture
Ungrazed
Tiles
RP (<0.45)
Tiles
Tiles
Overland flow
Tiles
RP (<0.45)
Phillips et al.
1981
0.01
0.02-0.17
0.12-1.95
0.01-0.07
0.01-0.12
Runoff
TP (unf.) TP
(<0.45) TP
(unf.) TP
(0.45)
Row crops 62% and
cereal crops 27% of
watershed
Tiles
No Fertilizer
Corn
Bluegrass sod
Rotation corn
Oats
Alfalfa 1st yr.
Alfalfa 2nd yr.
Corn
Bluegrass sod
Rotation corn
Oats
Alfalfa 1st yr.
Tiles
TP (unf.)
RP (<0.45)
TP (<0.45)
TP (>0.45)
TP (<0.45)
concentration in
tiles where highest
immediately after
application of
manure.
1.28
0.83
2.14
1.38
0.188
(0.015-3.30)
0.077
(0.001-0.450)
0.10
0.15
0.09
0.10
0.10
0.11
0.29
0.09
0.26
0.19
0.22
Phillips et al.
1982
Schepers and
Francis 1982
Culley and
Bolton 1983
Culley et al.
1983
Wildlife activities
and leaching of
nutrient from plant
material can also
degrade water
quality.
25% of TP (unf.)
and 50% of RP
(<0.45) came from
tile drainage.
34% TP (unf.) load
in tiles TP (>0.45).
84
Ontario, Canada
Hergert et al.
1981
0.011
0.014
0.218
Grazed
Ontario, Canada
TP (<0.45)
Ohio, U.S.A
Mississippi, U.S.A. No till corn for grain
P applied kg ha-1yr-1
0
30
No till corn for silage
P applied kg ha-1yr-1
0
30
Conventional corn
P applied kg ha-1yr-1
0.17
TP (<0.45)
TP (>0.45)
TP (<0.45)
TP (>0.45)
Overland flow TP (unf.)
Unspecified
0.36
3.29
1.02
1.10
0.97
1.08
0.42
0.21
0.21
0.30
0.26
0.36
0.36
0.16
0.47
0.18
0.1 (<0.1-0.2)
0.1 (<0.1-0.2)
<0.1 (<0.1-0.1)
TP (unf.)
0.1
0.1
0.1
0.40
Overland flow
TP (<0.45)
TP (>0.45)
TP (<0.45)
TP (>0.45)
0.23
0.46
0.57
0.51
0.98
1.96
1.61
1.43
TP (<0.45)
TP (>0.45)
TP (<0.45)
TP (>0.45)
0.23
0.43
0.39
0.49
0.63
1.16
0.71
0.89
Owens et al.
1983
van Roon et al.
1983
McDowell and
McGregor 1984
85
New Zealand
Alfalfa 2nd yr.
Fertilizer 30 kg ha-1
Corn
Bluegrass sod
Rotation corn
Oats
Alfalfa 1st yr.
Alfalfa 2nd yr.
Corn
Bluegrass sod
Rotation corn
Oats
Alfalfa 1st yr.
Alfalfa 2nd yr.
Depth 0.6m
Depth 1.0m
Depth 0.6m
Depth 1.0m
Ungrazed pasture
Grazed pasture
Forest
Grazed pasture
15
30
Florida, U.S.A.
Potato crops
Sheep grazing
Cattle rotational
grazing
Missouri, U.S.A.
Different River and
creeks
1. Pasture 74%,
forest 21%, urban 1%
2. Pasture 70%,
forest 24%, urban 1%
3. Pasture 66%,
forest 24%, urban 9%
4. Pasture 62%,
forest 37%, urban 1%
5. Pasture 55%,
forest 40%, urban
<1%
6. Pasture 49%,
forest 46%, urban 3%
7. Pasture 40%,
forest 60%,
0.07
3.57
0.11
9.71
0.17
0.26
0.27
13.48
0.18
15.63
0.05
0.29
0.43
0.10
1.10
0.30
TP (unf.)
0.70
1.50
Campbell et al.
1985
Lambert et al.
1985
Smart et al.
1985
TP (unf.)
TP (<0.45)
TP (unf.)
TP (<0.45)
TP (unf.)
TP (<0.45)
TP (unf.)
TP (<0.45)
TP (unf.)
TP (<0.45)
0.065
0.049
0.029
0.018
0.054
0.031
0.039
0.027
0.039
0.025
TP (unf.)
TP (<0.45)
TP (unf.)
TP (<0.45)
0.031
0.019
0.019
0.013
c
Overland flow
measurements were
taken in the fields
that were tile
drained.
RP (<0.45)losses
60% less with
subsurface
irrigation and tile
drains than water
furrow irrigation
and surface ditches
Sheep grazing 85%
TP (>0.45). Cattle
rotational grazing
TP (>0.45) 91%.
P losses related to
land-uses rather
than bedrock
geology or soil
association.
86
New Zealand
1. Tile
2. Overland
flow c
3. Total 1 and
2
4. Water
furrow
TP (<0.45)
TP (>0.45)
TP (<0.45)
TP (>0.45)
RP (<0.45)
urban <1%
8. Pasture 34%,
forest 60% urban 0%
9. Pasture 34%,
forest 64% urban
1.4%
10. Pasture 34%,
forest 66% urban 1%
11. Pasture 128%,
forest 67%, urban 4%
12. Pasture 14%,
forest 86%, urban
<1%
13. Only urban
14. Only pasture
15. Only forest
Louisiana, U.S.A.
New Zealand
Georgia, U.S.A.
Average ten livestock In the stream
watersheds
Average ten nonlivestock watersheds
Corn
Undrained
Drained
Native pedocarp/
mixed hardwoods
Continuously grazed,
30 kg mineral P
fertilizer/ year
Cropland % of the
watershed or the
Unspecified
In the stream
0.014
0.008
0.027
0.018
TP (unf.)
TP (<0.45)
TP (unf.)
TP (<0.45)
TP (unf.)
TP (<0.45)
0.025
0.010
0.058
0.047
0.016
0.012
TP (unf.)
TP (<0.45)
TP (unf.)
TP (<0.45)
TP (unf.)
TP (<0.45)
TP (unf.)
0.106
0.079
0.046
0.031
0.020
0.014
2 (0.8-4)
Thornley and
Bos 1985
87
Ontario, Canada
TP (unf.)
TP (<0.45)
TP (unf.)
TP (<0.45)
<1
TP (unf.)
7.8
5.0
TP (unf.)
0.12
TP (unf.)
1.67
Bengston et al.
1988
Cooper and
Thomsen 1988
Lowrance and
Leonard 1988
Undrained soils all
losses via overland
flow. In drained
soils most losses
from overland flow
and only 6% from
tiles.
subwatershed
1. 40.4
2. 33.9
3. 30.8
4. 31.8
5. 34.6
6. 29.0
7. 26.3
8. 31.7
9. 32.2
11. 32.6
U.S.A
Great Plains
Region U.S.A.
Coastal Delaware,
U.S.A.
Seepage from
stacked manure
Feedlot runoff
Feedlot runoff
Agricultural
Watershed
Minnesota, U.S.A.
Forest
Upland native prairie
1. Apl. dairy manure
2. Apl. dairy manure
Precipitation d
Overland flow
Overland flow TP (unf.)
0.19
0.014
0.17
0.013
0.18
0.014
0.17
0.014
0.22
0.023
0.18
0.014
0.15
0.015
0.29
0.034
0.16
0.014
0.16
0.014
0.27
0.33
0.02-0.04
190-280
0.71
0.05
0.79
0.06
0.70
0.06
0.81
0.07
0.87
0.10
0.70
0.06
0.99
0.10
0.94
0.12
0.86
0.07
0.82
0.07
0.93
0.12
0.05-0.10
290-360
47-300
10-260
Precipitation d TP (unf.)
Overland flow
Precipitation d TP (unf.)
Overland flow
0.011-0.042
0.04-1.20
88
10. 30.4
TP (unf.)
TP (<0.45)
TP (unf.)
TP (<0.45)
TP (unf.)
TP (<0.45)
TP (unf.)
TP (<0.45)
TP (unf.)
TP (<0.45)
TP (unf.)
TP (<0.45)
TP (unf.)
TP (<0.45)
TP (unf.)
TP (<0.45)
TP (unf.)
TP (<0.45)
TP (unf.)
TP (<0.45)
TP (unf.)
TP (<0.45)
TP (unf.)
Ritter 1988
d
normalized to
precipitation of 760
mm yr-1.
Ritter 1988
1.45-1.58
0.39-0.46
Ritter 1988
0.10
0.08
0.13
Ritter 1988
1.8-49.9
0.5-0.6
d
Normalized to
precipitation of 760
mm yr-1.
d
Normalized to
precipitation of 760
mm yr-1.
New York, U.S.A.
Ohio, U.S.A.
Oklahoma, U.S.A.
South Carolina,
U.S.A.
Texas, U.S.A.
Virginia, U.S.A
Vermont, U.S.A.
Ohio, U.S.A.
Florida, U.S.A.
Overland flow TP (unf.)
8.5-39.5
Overland flow TP (unf.)
0.011-0.020
0.020-0.023
0.56-0.83
1.21-1.32
7.5-8.9
1. Apl. dairy manure
2. Apl. dairy manure
3. seepage from
stacked manure
Forest
Unimproved pasture
Overland flow TP (unf.)
Overland flow TP (unf.)
Ditches
Sugarcane
Overland flow TP (unf.)
Overland flow TP (unf.)
Overland flow TP (unf.)
Ritter 1988
0.011-0.020
0.06
0.89
3.29
8.2-13.5
0.04
0.07
Ritter 1988
Ritter 1988
Ritter 1988
Overland flow TP (unf.)
Overland flow TP (unf.)
Ritter 1988
Ritter 1988
0.12-0.19
0.10-0.60
0.28
0.54
0.19-0.31
0.41-0.65
7-255
0.21
0.88
Ritter 1988
0.08
0.4-0.7
Ritter 1988
< 0.1
0.1
0.1
0.1
Owens et al.
1989
0.28
0.17
0.25
0.20
0.56
0.72
0.43
0.88
0.51
1.38
Izuno et al. 1991
Parent rock has a
greater effect of
stream water
content then landuse. Phosphate
content in these
stream lower than
18 years ago.
51-156
Radish
Cabbage
TP (unf.)
TP (<0.45)
TP (unf.)
TP (<0.45)
TP (unf.)
Year-round cattle
grazing would not
degrade the stream
water quality.
89
Wisconsin, U.S.A
Dairy barnyard
runoff
Forest
Farmland
Grass, rotat. grazing
Grass, cont. grazing
Applied. dairy
manure
Grass, rotat. grazing
Grass, cont. grazing
Piedmont
Silvicultural
Agricultural
Coastal Plain
Poorly-drained
Well-Drained
Dairy barnyard
runoff
Oklahoma and
Texas, U.S.A.
Native grass
No till, wheat
Reduced-till, wheatsorghum-fallow
rotation
Conventional till
wheat
Peanut-sorghum
rotation
TP (<0.45)
TP (unf.)
TP (<0.45)
TP (unf.)
TP (<0.45)
TP (unf.)
TP (<0.45)
TP (unf.)
TP (unf.)
TP (<0.45)
TP (>0.45)
AAP
AAP (>0.45)
TP (unf.)
TP (<0.45)
TP (>0.45)
AAP
AAP (>0.45)
TP (unf.)
TP (<0.45)
TP (>0.45)
AAP
AAP (>0.45)
TP (unf.)
TP (<0.45)
TP (>0.45)
AAP
AAP (>0.45)
TP (unf.)
TP (<0.45)
TP (>0.45)
AAP
AAP (>0.45)
TP (unf.)
TP (<0.45)
TP (>0.45)
AAP
0.31
0.43
0.28
1.03
0.51
0.16
0.08
0.07
0.72
0.59
0.34
3.82
0.97
0.72
0.200
0.122
0.078
0.176
0.054
2.310
1.178
1.132
1.456
0.278
0.398
0.087
0.311
0.203
0.116
0.529
0.046
0.483
0.142
0.096
6.852
0.391
6.461
0.974
0.583
6.292
0.237
6.055
1.796
Sharpley et al.
1992
90
No-till, wheatsorghum-fallow
rotation
Drained
fallow
Flooded
fallow
Main farm
canals
Precipitation
Runoff
New Zealand
Pasture
Riparian pine 1
Riparian pine 2
Kentucky, U.S.A.
AAP (>0.45)
TP (unf.)
TP (<0.45)
TP (unf.)
TP (<0.45)
TP (unf.)
TP (<0.45)
RP (<0.45)
In the stream
Streams
1. Pasture, 2%
tobacco
2. Pasture, 54%
wooded
3. Wooded 75%, corn
and pasture
remainder
4. Corn, soybean and
hay
1
2
5. Pasture, corn,
tobacco
6. Corn, soybean, hay
and 5% wooded
7. 20% wooded,
remainder in pasture
tobacco and corn
Corn, conventional
Overland flow
till
Microplots (60 sq.-ft)
TP (<0.45)
AAP
AAP (>0.45)
Mesoplots (6500 sq.TP (<0.45)
ft)
AAP
1.559
0.99
0.53
1.42
0.37
1.43
0.55
0.28 e - 0.17 f
Smith 1992
Thomas et al.
1992
Completely
pastured
catchments Lower
P losses then
afforested
watersheds.
e
Mean in 1972.
f
Mean in 1990.
0.08 e - <0.01 f
0.01 e - <0.01 f
0.04 e - 0.03 f
1.79 f
0.10 e - <0.01 f
91
Georgia, U.S.A.
In the stream
0.03 e - <0.01 f
0.006 e - <0.01 f
0.13-0.18 g
0.28-0.29 g
0.26 g
0.22-0.29 g
0.35-0.49 g
Truman et al.
1993
g
Units are
kg ha-1 per event.
TP (<0.45) losses
decrease with days
since fertilization.
Microplots
experienced higher
P loses than
mesoplots.
Florida, U.S.A.
Sugarcane
Slow drainage
Ditches
TP (unf.)
TP (<0.45)
TP (unf.)
TP (<0.45)
Fast drainage
Ontario, Canada
England
Finland
RP (<0.45)
Tiles
Overland flow
Ridge till
Tiles
Overland flow
Conventional till
Tiles
Overland flow
Cropland (continuous Tiles
TP (unf.)
wheat and rotation of
fallow, potato and
three successive
wheat)
Fertilization
No
High
Grassland used for
dairy production
1981-82
1990-91
Tiles
Barley
Grass ley
Barley
Overland flow TP (unf.)
Grass ley
0.24
0.29
0.34
0.55
0.54
1.02
0.01-0.12
Coale et al. 1994
0.03-0.24 h
0.38
0.18
0.53
0.44
0.87
0.65
Gaynor and
Findlay 1995
Heckrath et al.
1995
0.03-0.23
0.55-2.75
RP (<0.45)
Smith et al.
1995
0.20
0.29
Tiles
TP (unf.)
RP (<0.45)
TP (unf.)
RP (<0.45))
0.42
0.33
0.79
0.11
1.27
0.17
Turtola and
Jaakkola 1995
h
Units are
kg ha-1 per event.
TP (<0.45) and TP
(unf.) differences in
concentration or
export were not
significant between
the sugarcane and
uncropped fallow
fields.
Extensive cracking
enhances
preferential flow.
Installation of new
tiles increased P
concentration.
Disturbance of soil
increases
preferential flow.
Fertilized plots 7886% of TP (unf.)
was RP (<0.45).
Increase in soil test
P (Olsen P) lead to
increase of P
concentration in
tiles
In tiles 63-82% of
TP (unf.) as TP
(>0.45) for barley
and 38-55% for
grass ley. In grass
ley heavy
92
Northern Ireland
Corn
Zero till
0.13-0.38
0.04-0.27
0.08-0.22
0.04-0.14
h
Finland
Barley
Tiles
Timothy-fescue-red
clover pasture
Quebec, Canada
England
England
Coastal Plain,
Maryland and
Delaware, U.S.A.
1. Permanent pasture
(91%) grazed by
sheep
2. Permanent pasture
(93%) grazed by
sheep and cattle
Pasture
Native hardwoods/
pine
57.8% crops
0.04
0.2
0.03
0.2
TP (Unf.)
Overland flow RP (<0.45)
Field drains
Tiles
Unspecified
TP (unf.)
TP (<0.45)
RP (<0.45)
TP (unf.)
TP (<0.45)
RP (<0.45)
Overland flow
No fertilizer
RP (<0.45)
TP (unf.)
Fertilizer
RP (<0.45)
TP (unf.)
Unspecified
TP (unf.)
RP (<0.45)
UP (unf.)
TP (unf.)
RP (<0.45)
Turtola and
Paajanen 1995
0.01-1.17
0.11-0.22
0.05-0.10
0.00-0.27
0.35
0.048
0.016
0.40
0.305
0.209
0.041
0.112
0.252
0.458
1.04
0.14
Beauchemin et
al. 1996
Hawkins and
Scholefield
1996
Nelson et al.
1996
1.11
0.84
Haygarth and
Jarvis 1997
0.092
4.641 l
0.044
0.009
0.035
0.59
0.35
Jordan et al.
1997a
RP (<0.45) losses
unaffected by
grazing strategies
but increased with
P fertilizer rate.
Concentrations in
both streams
exceed desirable
limits.
93
Australia
Forest and hay fields
on dairy farms
Permanent grazed
grassland
TP (<0.45)
TP (>0.45)
TP (<0.45)
TP (>0.45)
fertilization
increased. RP
(<0.45)
concentrations
Improved drainage
practices (new tiles,
wood chip backfill
above drains)
decreased TP
(<0.45)
concentrations in
tiles but increased
PP (>0.45)
concentrations.
Piedmont,
Maryland, U.S.A.
Native hardwoods
Unspecified
62.4% crops
Delaware, U.S.A.
Piedmont,
Maryland, U.S.A.
Switzerland
England
Illinois, U.S.A.
Grazed grassland
Corn-soybean
rotation
Ditches
Summer
Winter
Spring
Unspecified
0.24
0.37
0.09
0.28
0.47
0.16
0.31
0.15
0.05
0.06
TP (unf.)
RP (<0.45)
TP (unf.)
RP (<0.45)
RP (<0.45)
0.11
0.05
2.50
1.53
4.80 (high)
0.6-1.9
TP (unf.)
RP (<0.45)
Overland flow TP (unf.)
(undrained
RP (<0.45)
soil)
Overland flow TP (unf.)
(undrained
RP (<0.45)
soil)
Overland flow TP (unf.)
and interflow
(undrained
soil)
Overland flow
and interflow
(drained soil)
Mole and tile
River
TP (unf.)
Tiles
0.26-0.60
0.03-0.07
1.50
0.13-0.85
0.37-0.91
0.047-0.24
3.07
1.0-1.2
0.63-5.98
0.06-0.94
0.03-1.22
0.002-0.184
Tiles
Drainflow
Jordan et al.
1997b
Sallade and
Sims 1997
Shirmohammadi Average of three
et al. 1997
years.
0.232
Stamm et al.
1997.
Catt et al. 1998
Haygarth et al.
1998
0.152
0.132
0.19
0.14
0.45
0.42
Highest losses
shortly after
manure application.
Xue at al. 1998
94
England
Unspecified
(major agronomic
crops in the region
corn, soybean, wheat,
barley and sorghum)
Watershed 1A, no
dairy operation
Watershed 1B, dairy
cows (3.25 cows/ha)
Manure grassland
with intensive swine
production
Cultivated arable
land
UP (unf.)
TP (unf.)
RP (<0.45)
UP (unf.)
TP (unf.)
RP (<0.45)
UP (unf.)
TP (<0.45)
Draining reduced P
losses by 30 %.
Rhode river,
Maryland, U.S.A.
Old growth
Unspecified
deciduous hardwoods
64% crops
rotationally grazed,
no mineral fertilizer
Denmark
Australia
Forest (eucalyptus)
With ground cover
range of grasses and
clover
Watershed 1.
Sub-watershed A
Sub-watershed B
Watershed 2.
Sub-watershed A
Sub-watershed B
Oats
Treatments
Standard tilth
Finer tilth
1. When fertilizer
was added after
plowing
Average
Rotational
crops
Simulated
rainfalls
Natural
rainfall
Overland flow
and interflow
0.43
0.19
0.24
2.54
1.40
1.14
0.40
0.20
0.20
Correll et al.
1999
Laubel et al.
1999
TP (<0.45)
TP (>0.45)
TP (<0.45)
TP (>0.45)
TP (unf.)
0.04-0.103
0.18-0.88
570-1750 i
710-5920 i
Stevens et al.
1999
0.13
0.31
i
Units are
kg ha-1 mm-1.
Data from both
natural rainfall
events and
simulations.
Watershed 1. No
domestic animal
grazing. Grazed by
kangaroos.
Watershed 2.
Domestic animal
grazing.
0.37
1.02
Addiscott et al.
2000
Moles
TP (<0.45)
RP (<0.45)
UP (<0.45)
TP (<0.45)
RP (<0.45)
UP (<0.45)
0.087
0.028
0.059
0.057
0.037
0.023
TP (<0.45)
0.22
1.00
RP (<0.45) was
determined by a
different method
each year.
Fertilization after
plowing increased
losses.
P losses increase
with increased
mole spacing
95
England
Tiles
TP (unf.)
RP (<0.45)
UP (unf.)
TP (unf.)
RP (<0.45)
UP (unf.)
TP (unf.)
RP (<0.45)
UP (unf.)
Application of
phosphate (kg ha-1)
33
16.5
Mole spacing (m)
2
4
2. When fertilizer
was added before
plowing
Average
16.5
Illinois, U.S.A.
Iowa, U.S.A.
Mole spacing (m)
2
4
Not given
River Flux
Percent of land-use in In the stream
subbasins
1. Corn 51, farmstead
0, forest 0, grass 0,
0.028
TP (<0.45)
UP (<0.45)
RP (<0.45)
TP (<0.45)
UP (<0.45)
RP (<0.45)
0.35
0.032
0.32
0.17
0.024
0.15
UP (<0.45)
UP (<0.45)
0.17
0.30
TP (<0.45)
RP (<0.45)
0.08
0.02
TP (<0.45)
UP (<0.45)
RP (<0.45)
TP (<0.45)
UP (<0.45)
RP (<0.45)
0.097
0.024
0.073
0.076
0.018
0.059
UP (<0.45)
UP (<0.45)
TP (unf.)
0.060
0.075
0.7-1.1
0.12
0.40
0.10
96
Application of
phosphate (kg ha-1)
33
RP (<0.45)
David and
Gentry 2000
TP (unf.)
Downing et al.
2000
1.4
Of TP (unf.) 38%
was TP (<0.45) and
62 % was TP
(>0.45).
2.6
1.2
1.7
0.8
2.2
97
hay 0, pasture 10,
soybean 36
2. Corn 47, farmstead
2, forest 0, grass 2,
hay 3, pasture 14,
soybean 30
3.Corn 41, farmstead
1, forest 1, grass 3,
hay 4, pasture 25,
soybean 23
4. Corn 40, farmstead
2, forest 1, grass 2,
hay 4, pasture 24,
soybean 26
5. Corn 51, farmstead
0, forest 0, grass 2,
hay 1, pasture 15,
soybean 29
6.Corn 41, farmstead
2, forest 1, grass 2,
hay 4, pasture 23,
soybean 25
7. Corn 43, farmstead
2, forest 0, grass 2,
hay 7, pasture 19,
soybean 25
8. Corn 35, farmstead
2, forest 5, grass 1,
hay 1, pasture 37,
soybean 17
9. Corn 38, farmstead
0, forest 1, grass 2,
hay 1, pasture 27,
soybean 30
10. Corn 38,
farmstead 2, forest 1,
grass 2, hay 1,
0.7
25.2
0.2
1.0
20.7
6.7
0.8
0.4
98
pasture 14, soybean
30
11. Corn 29,
farmstead 3, forest 9,
grass 0, hay 12,
pasture 39, soybean 5
12. Corn 32,
farmstead 2, forest 4,
grass 1, hay 5,
pasture 38, soybean
15
13. Corn 41,
farmstead 2, forest 1,
grass 2, hay 4,
pasture 23, soybean
25
14. Outlet. Corn 35,
farmstead 1, forest 3,
grass 2, hay 4,
pasture 24, soybean
22
99
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Appendix II
Original Phosphorus-Index
104
Appendix II - Table 1. The site characteristics, their weight factors, and the five P
loss levels of the original P index system (Pierzynski et al., 2000; Sims et al., 1996;
Lemunyon and Gilbert, 1993).
TRANSPORT
CHARACTERISTIC
(Weight Factor)
Soil Erosion
(1.5)
Irrigation Erosion
(1.5)
None
(0.6)
N/A
N/A
PHOSPHORUS LOSS RATING (VALUE)
Low
Medium
High
(0.7)
(0.8)
(0.9)
< 10 Mg ha-1 10-20 Mg ha-1
20-30 Mg ha-1
Very High
(1)
> 30 Mg ha-1
Infrequent
irrigation on
well-drained
soils
Very Low
or Low
Moderate
irrigation on
soils with
slope <5%
Medium
Frequent
irrigation on
soils with slope
2-5%
High
Frequent
irrigation on
soils with slope
>5%
Very High
Soil Runoff Class
(0.5)
N/A
Soil Test P
(1.0)
P Fertilizer Rate
(kg P/ha)
(0.75)
P Fertilizer
Application Method
(0.5)
N/A
Low
Medium
High
Excessive
N/A
< 15
15-45
56-75
> 75
Placed with
planner
deeper than
5 cm
Incorporate
immediately
before crop
< 15
15-45
Injected
deeper than
5 cm
Incorporate
immediately
before crop
Organic P Source
Application Rate
(kg P/ha)
(1.0)
Organic P Source
Application Method
(1.0)
N/A
Incorporate
> 3 moths before
crop or surface
applied
> 3 moths before
crop
Surface applied
> 3 moths
before crop
N/A
N/A
56-75
> 75
Incorporate
> 3 moths before
crop or surface
applied > 3
moths
before crop
Surface applied
to pasture
> 3 moths
before crop
105
Appendix II - Table 2. The site vulnerability chart for the original P index system
that indicates the potential of a site to deliver P to surface waters ((Pierzynski et al.,
2000; Sims et al., 1996; Lemunyon and Gilbert, 1993).
Phosphorus Index
for Site
Generalized Interpretations of Phosphorus Index for Site
<8
LOW potential for P movement from the site. If farming practices are maintained as
the current level there is a low probability of an adverse impact to surface waters
from P losses at the site
MEDIUM potential for P movement from the site. The chance for an adverse
impact to surface water exists. Some remedial actions should be taken to lessen the
probability of P loss.
HIGH potential for P movement from the site and for an adverse impact to surface
water to occur unless remedial action is taken. Soil and water conservation as well
as P management practices are necessary to reduce the risk of P movement and water
quality degradation.
VERY HIGH potential for P movement from the site and for an adverse impact to
surface water exists. Remedial action is required to reduce the risk of P loss. All
necessary soil and water conservation practices, plus a P management plan must be
put in place to avoid the potential for water quality degradation.
8-14
15-32
> 32
106
Appendix II – References
Lemunyon, J. L., and R. G. Gilbert. 1993. The concept and need for a phosphorus assessment tool. J. Prod.
Agric. 6(4): 483-486.
Pierzynski G.M., J.T. Sims, and G.F. Vance. 2000. Soils and environmental quality. Second edition. CRC
Press, Boca Raton, FL.
Sims, J.T. 1996. The phosphorus index: a phosphorus management strategy for Delaware’s Agricultural
soils, Fact Sheet ST-08, College of Agricultural Sciences and Cooperative Extension, University of
Delaware, Newark, DE.