The Phosphorus Index
A Phosphorus Assessment Tool
Technical Note
Subject: Engineering
Series Number: 1901
Reference: A Phosphorus Assessment Tool
Date: August, 1994
Phosphorus Concerns in the Environment
Eutrophication can be caused by the nutrient enrichment of
a water body. Nutrient movement in runoff and erosion from agricultural
nonpoint sources is a resource management concern. The movement
of phosphorus in runoff from agricultural land to surface water
can accelerate eutrophication. Undesirable aquatic plant growth
results from additions of phosphorus to the water. The net result
of the eutrophic condition and excess plant growth in water is
the depletion of oxygen in the water due to the heavy oxygen demand
by microorganisms as they decompose the organic material. Little
attention has been given to management strategies to minimize
the nonpoint movement of P in the landscape because of the easier
identification and control of point source inputs of P to surface
waters and a lack of direct human health risks associated with
eutrophication. Phosphorus is generally the limiting nutrient
in fresh water systems and any increase in P is usually results
in more aquatic vegetation. Society is concerned about maintaining
clean drinking water. This concern now includes a cost for removing
the color, taste, and odor associated with the high trophic condition
and vegetation growth in surface water due to excess nutrients.
Phosphorus
Movement Factors
The main factors influencing P movement can be separated into
the transport, phosphorus source, and phosphorus management factors.
Transport factors include the mechanism by which P moves within
the landscape. These are rainfall, irrigation, erosion and runoff.
Factors which influence the source and amount of P available to
be transported are soil P content and form of P applied. Phosphorus
management factors include the method of application, timing,
and placement in the landscape as influenced by the management
of application equipment and tillage.
Phosphorus
Movement in the Landscape
Phosphorus movement in runoff occurs as particulate P and dissolved
P. Particulate P is attached to mineral and organic sediment as
it moves with the runoff. Dissolved P is in the water solution.
In general, particulate P is the major portion (75-90%) of the
P transported in runoff from cultivated land. Dissolved P makes
up a larger portion of the total P in runoff from non- cultivated
lands such as pastures and fields with reduced tillage. In terms
of their impact on eutrophication of water bodies, particulate
P becomes less available to algae and plant uptake than dissolved
P because of the chemical form it has with the mineral (particularly
iron, aluminum, and calcium) and organic compounds. The availability
of particulate P to plants and algae is variable, ranging from
10 to 90% of the total P. yet can represent a long-term source
of P for algae and plant uptake from the water body. Dissolved
P is 100% bioavailable to plants. Added together, the bioavailable
portion of particulate P and the dissolved P represents the phosphorus
that promotes eutrophication of surface waters.
The method by which P in both particulate and dissolved form
moves within the landscape are simplified in the following description.
Eroding soil material is transported by runoff. During detachment
and movement of sediment in runoff, the finer clay-sized fraction
of the source material are preferentially eroded. The P content
and reactivity of the eroded material to P are usually greater
than the source soil from which it was eroded. The suspended sediment
in the runoff can rapidly adsorb the dissolved P in the runoff
water.
As runoff moves from the landscape and toward the water body
there is generally a progressive dilution of P through additions
of water and a reduction in the amount of sediment carried because
of sediment deposition. Phosphorus may become more bioavailable
by the sorption and Resorption processes and by the preferential
transport of claysized material as sediment moves over the landscape.
The movement of dissolved P begins with the Resorption, dissolution,
and extraction of P from the soil, plant, and organic material.
These processes occur when rain and runoff water interact with
the thin layer of surface soil (0.05 to 0.10 inches). Some water
infiltrates into the soil and percolates through the profile where
desorption of P will result in a low dissolved concentration in
subsurface and return flow. High dissolved P concentration can
be expected in the water percolating through organic, coarse-textured,
and oxygen depleted (reduced), water-logged soils. Soil pH also
affects the movement and availability of phosphorus.
The interaction between the particulate and dissolved P in
the runoff is very dynamic and the mechanism of transport is complex.
Therefore, it is difficult to predict the transformation and ultimate
fate of P as it moves through the landscape.
The
Concept
The purpose of the Phosphorus Index is to provide field staffs,
watershed planners, and land users with a tool to assess the various
landforms and management practices for potential risk of phosphorus
movement to water bodies. The ranking of Phosphorus Index identifies
sites where the risk of phosphorus movement may be relatively
higher than that of other sites. When the parameters of the index
are analyzed, it will become apparent that an individual parameter
or parameters may be influencing the index disproportionately.
These identified parameters can be the basis for planning corrective
soil and water conservation practices and management techniques.
If successful in reducing the movement of phosphorus, the concern
of phosphorus enrichment will also be reduced.
For the first version, the Phosphorus Index will be a 8 x 5
matrix using a limited number of landform site and management
characteristics. The input to the matrix will be readily accessible
field data. This index will be used as a tool for understanding
the contribution that individual landform and management parameters
have toward risk of phosphorus movement and will provide a method
for developing management guidelines for phosphorus at the site
to lessen their impact on water quality.
A number of soil, hydrology, and land management site characteristics
will describe the landform. The Phosphorus Index (Table 1) uses
parameters that can have an influence on phosphorus availability,
retention, management, and movement. These include the erosion
rate in tons per acre per year, runoff, available phosphorus soil
test levels given in soil laboratory test units, phosphorus fertilizer
application rates in pounds available phosphate per acre, phosphorus
fertilizer application methods, organic phosphorus application
rates in pounds available phosphates per acre, and organic phosphorus
application methods. Field specific data for the eight site characteristics
selected for this version (Table 1) of the Phosphorus Index are
readily available at the field level. Some analytic testing of
the soil and organic material is required to determine the rating
levels. This soil and material analysis is considered essential
as a basis for the assessment.
The P Index is a simple 8 by 5 matrix that relates site characteristics
with a range of value categories. This first version (Table 1)
of the P Index has eight site characteristics. The eight characteristics
are:
- soil erosion
- irrigation erosion
- runoff class
- soil P test
- P fertilizer application rate
- P fertilizer application method
- organic P source application rate
- organic P source application method
The five value categories are:
- none
- low
- medium
- high
- very high
There are eight site characteristics used in the P Index to
assess a particular site. Each site characteristic is rated NONE,
LOW, MEDIUM, HIGH, or VERY HIGH by determining the range for each
category. Although ranges have been given in this version of the
index, it is imperative that each user establish their own range
of values for each characteristic. Each user group should consider
the cropping, soil, management conditions in the area designated
to use the Phosphorus Index. Consideration need to be made as
to base conditions for soil erosion, soil phosphorus test levels,
and crop requirements. From these local considerations the ranges
of the values category can be made. Any user group should include
NRCS, Extension Service, University and ARS researchers, as well
as state natural resource agencies and other groups interested
in water quality.
The definition of each of the eight site characteristics are:
Soil
Erosion
Soil erosion is defined as the loss of soil along the slope
or unsheltered distance caused by the processes of water and wind.
Soil erosion is estimated from erosion prediction models currently
used (USLE or RUSLE for water erosion and WEQ for wind erosion).
Erosion induced by irrigation is calculated by other convenient
methods. The value category is given in tons of soil loss per
acre per year (ton/acre/year). These soil loss prediction models
do not predict sediment transport and delivery to a water body.
The prediction models are used in this index to indicate a movement
of soil, thus potential for sediment and attached phosphorus movement
across the slope or unsheltered distance and toward a water body.
Irrigation
Erosion
Potential P loss resulting from furrow irrigation induced erosion
is considered by inclusion of a rating system based on soil susceptibility
to particle detachment by hydraulic shear and flow rate of water
in the furrow. The susceptibility to detachment is given by a
relative ranking of soil erodibility classes under furrow irrigation
(table 2). These classes are an initial attempt at a relative
ranking based on inherent stable and static soil properties (i.e.,
texture and clay mineralogy). There are temporal variations in
the relative erodibility and actual amount of erosion with furrow
erosion. These changes in erodibility are a function in the soil
properties or a result of management. However, no attempt is made
to consider temporal soil properties nor management factors in
the rating. The introduced flow rate in the furrow (Q) is given
by the irrigation plan and recorded as gallons per minute (gal/min).
The furrow slope (S) of the site is given as a percentage (feet
per 100 feet). (See USDA-NRCS National Engineering Handbook 15,
chapter 5). The product of flow rate (Q) and slope (S) is used
to determine the value category.
Runoff
Class
The runoff class of the site can be determined from soil survey
data. Guidance in determining the runoff class is based on the
soil saturated hydraulic conductivity (KSat) and the
percent slope of the site. (See USDA-NRCS Soil Survey Manual,
Agricultural Handbook 18, 1993.) A more simplified table has been
developed using soil permeability classes (table 3). The result
of using the matrix relating soil permeability class and slope
provides the value categories: NEGLIGIBLE, VERY LOW, LOW, MEDIUM,
HIGH, and VERY HIGH.
Another method to determine runoff class is using the NRCS
curve number (CN) method. The major factors that determine curve
number are the hydrologic soil group. Landscape cover type, conservation
treatment, hydrologic condition, and antecedent runoff conditions.
The NRCS runoff curve number method is described in detail in
National Engineering Handbook number 4, 1985. A suggested guidance
matrix is given in table 4.
Soil
P Test
A soil sample from the site is necessary to assess the level
of "available P" in the surface layer of the soil. The
available P is the level customarily given in a sail test analysis
by the Cooperative Extension Service or commercial soil test laboratories.
The user of the P Index must determine the ranges of soil test
P in each value category (LOW, MEDIUM, HIGH, and EXCESSIVE). These
ranges of soil test P values will vary by soil test method and
region. The soil test level for "available P" does not
ascertain the total P in the surface soil. It does however, give
an indication of the amount of total P that may be present because
of the general relationship between the forms of P (organic, adsorbed,
and labile P) and the solution P available for crop uptake.
P
Fertilizer Application Rate
The P fertilizer application rate is the amount, in pounds
per acre (lb/acre), of phosphate fertilizer (P205)
that is applied to the soil. This phosphate fertilizer does not
include phosphorus from organic sources (recorded in Organic P
Sources).
P
Fertilizer Application Method
The manner in which P fertilizer is applied to the soil and
the amount of time that the fertilizer is exposed on the soil
surface until crop utilization effects potential P movement. Incorporation
implies that the fertilizer P is buried below the soil surface
at a minimum of two inches. The value categories of increasing
severity, LOW to VERY HIGH, depict the longer surface exposure
time between fertilizer application, incorporation, and crop utilization.
Organic
P Source Application Rate
The organic P application rate is the amount, in pounds per
acre (lb/acre), of potential phosphate (P2O5)
that is contained in the manure and applied to the soil. An analysis
of the organic material is necessary to determine the potential
phosphate content of the manure. The P content by analysis is
generally considered to be completely plant available. This organic
phosphate source does not include phosphorus from fertilizer sources
(recorded in P Fertilizer Application Rate).
Organic
P Source Application Method
The manner in which organic P material is applied to the soil
and the time that the organic material is exposed on the soil
surface until crop utilization can determine potential P movement.
Incorporation implies that the organic P material is buried below
the soil surface at a minimum of two inches. The value categories
of increasing severity, LOW to VERY HIGH, depict the longer surface
exposure time between organic P material application, incorporation,
and crop utilization.
The
Procedures for Making an Assessment
The site characteristics have been assigned a weighting based
on the reasoning that particular site characteristics may be more
prominent than others in allowing potential phosphorus movement
from the site. There is scientific basis for concluding that these
relative differences exist; however, the absolute weighting factors
given are based currently on professional judgment. The site characteristic
weighting factors are:
- soil erosion (1.5)
- irrigation erosion (1.5)
- runoff class (0.5)
- soil P test (1.0)
- P fertilizer application rate (0.75)
- P fertilizer application method (0.5)
- organic P source application rate (1.0)
- organic P source application method (1.0)
The value categories are rated using a log base of 2. The greater
ratings, the proportionally higher are the values. The higher
the value, the higher potential for significant problems related
to phosphorus movement.
The value ratings are:
- none = 0
- low = 1
- medium = 2
- high = 4
- very high = 8
To make an assessment using the P Index, select a rating value
for each site characteristic using the categories NONE, LOW, MEDIUM,
HIGH, or VERY HIGH. Multiply the site characteristic weight factor
by the rating value to get the weighted value for that characteristic.
Proceed to rate and factor each characteristic of the index. Sum
the weighted values for all eight characteristics, and compare
the total with site vulnerability chart.
A description of vulnerability is given to appraise the assessment
of the P Index.
Interpretations
of Site Vulnerability Rating. for the P Index
LOW - This site has a LOW potential for P movement from
the site. If farming practices are maintained at current level,
the probability of an adverse impact to surface water resources
from P losses from this site would be low.
MEDIUM - This site has a MEDIUM potential for P movement
from the site. The probability for an adverse impact to surface
water resources is greater than that from a LOW vulnerability
rated site. Some remedial action should be taken to lessen the
probability of P movement.
HIGH - This site has a HIGH potential for P movement
from the site. There is a high probability for an adverse impact
to surface water resources unless remedial action is taken. Soil
and water conservation as well as phosphorus management practices
are necessary to reduce the risk of P movement and probable water
quality degradation.
VERY HIGH - This site has a VERY HIGH potential for
P movement from the site. The probability for an adverse impact
to surface water resources is very high. Remedial action is required
to reduce the risk of P movement. All necessary soil and water
conservation practices plus a phosphorus management plan must
be put in place to reduce the potential of water quality degradation.
An Example in using the Phosphorus Index (Table 1):
soil erosion (weight = 1.5) is 7.5 ton/ac/yr (=MEDIUM, value = 2) |
1.5 x 2 = 3.0 |
irrigation erosion (weight = 1.5) is not applicable (=NONE, value = 0) |
1.5 x 0 = 0 |
runoff class (weight = 0.5) is LOW (value = 1) |
0.5 x 1 = 0.5 |
soil P test (weight = 1.0) is 82 lb P (=HIGH, value = 4) |
1.0 x 4 = 4.0 |
P fertilizer application rate (weight = 0.75) is 25 lb/ac (=LOW, value = 1) |
0.75 x 1 = 0.75 |
P fertilizer application method (weight = 0.5) is placed with planter (=LOW, value = 1) |
0.5 x 1 = 0.5 |
organic P source application rate (weight = 1.0) is 95 lb/ac (=VERY HIGH, value = 8) |
1.0 x 8 = 8.0 |
organic P source application method (weight = 1.0) is surface applied a month before notill planting (=HIGH, value =4) |
1.0 x 4 = 4.0 |
Sum Total of all weighted values = 20.75
Site Vulnerability is HIGH
HIGH - This site has a HIGH potential for P movement
from the site. There is a high probability for an adverse impact
to surface water resources unless remedial action is taken. Soil
and water conservation as well as phosphorus management practices
are necessary to reduce the risk of P movement and probable water
quality degradation.
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Table 1.
Phosphorus Index for assessing the vulnerability
of a land unit. Summation of the weighted rating value is used
to determine the site vulnerability.
SOIL EROSION (1.5) |
NOT APPLICABLE |
<5 TONS/AC |
5-10 TONS/AC |
10-15 TONS/AC |
>15 TONS/AC |
IRRIGATION EROSION (1.5) |
NOT APPLICABLE |
TAILWATER RECOVERY or QS<6 for very erodible soils or QS<10
for other soils |
QS>10 for erosion resistant soils |
QS>10 for erodible soils |
QS>6 for very erodible soils |
RUNOFF CLASS (0.5) |
NEGLIGIBLE |
VERY LOW or LOW |
MEDIUM |
HIGH |
VERY HIGH |
SOIL P-TEST (1.0) |
NOT APPLICABLE |
LOW |
MEDIUM |
HIGH |
EXCESSIVE |
P-FERTILIZER APPLICATION RATE (0.75) |
NONE APPLIED |
1-30 P2O5 LBS/AC |
31-90 P2O5 LBS/AC |
91-150 P2O5 LBS/AC |
> 150 P2O5 LBS/AC |
P-FERTILIZER APPLICATION METHOD (0.5) |
NONE APPLIED |
PLACED WITH PLANTER DEEPER THAN 2 INCHES |
INCORPORATED IMMEDIATELY BEFORE CROP |
INCORPORATED > 3 MONTHS BEFORE CROP or SURFACE APPLIED < 3 MONTHS BEFORE CROP |
SURFACE APPLIED > 3 MONTHS BEFORE CROP |
ORGANIC P SOURCE APPLICATION RATE (1.0) |
NONE APPLIED |
1-30 P2O5 LBS/AC |
31-60 P2O5 LBS/AC |
61-90 P2O5 LBS/AC |
> 90 P2O5 LBS/AC |
ORGANIC P SOURCE APPLICATION METHOD (1.0) |
NONE |
INJECTED DEEPER THAN 2 INCHES |
INCORPORATED IMMEDIATELY BEFORE CROP |
INCORPORATED > 3 MONTHS BEFORE CROP or SURFACE APPLIED < 3 MONTHS BEFORE CROP |
SURFACE APPLIED TO PASTURE, or > 3 MONTHS BEFORE CROP |
< 8 |
LOW |
8 -14 |
MEDIUM |
15 - 32 |
HIGH |
> 32 |
VERY HIGH |
Table 2. Furrow IRRIGATION EROSION site characteristics
I. QS value
Q = flow rate of water introduced into the furrow (in gallons
per minute, GPM).
S = furrow slope (in feet per 100 feet, percent).
Example: For a 5 gpm flow rate and a 2% furrow grade:
QS = 5 gpm * 2% grade =
10
II. Relative ranking of soil erodibility under furrow irrigation
Use local criteria to determine the relative erodibility of
the soil in question. If no local criteria are established, use
the following for guidance:
A. Very Erodible Soils
Soils in which the surface layer texture is silt, or silt loam
with < 15% nonmontmorillonitic clay, or fine and very fine
sandy loam with < 15% nonmontmorillonitic clay, or loamy fine
sand, or loamy very fine sand. Contact a soil scientist for clay
content and mineralogy.
B. Erosion-resistant Soils
Soils that have the following characteristics in the upper
5 cm of the surface layer:
silty clay, clay, or sandy clay texture, weak or massive
structure, and mixed or montmorillonitic clay mineralogy.
other soils that have medium or coarse blocky structure or
coarse granular structure (i.e. natural aggregates > 10 mm)
and very firm or firmer rupture resistance class in the moist
state (i.e. requires at least strong force between thumb and forefinger
to cause failure of a moist soil aggregate).
See the Soil Survey Manual (1993), chapter 3 for description
of soil structural aggregates (peds), and table 3-14 for soil
rupture-resistance classes.
C. Erodible Soils
Soils that have a surface layer not fitting any of the above
criteria.
Table 3.
The surface RUNOFF CLASS site characteristic
determined from the relationship of the soil permeability class
and field slope. Adapted from Soil Survey Manual (1993) Table
3-10.
|
Very Rapid |
Moderately Rapid and Rapid |
Moderately Slow and Moderate |
Slow |
Very Slow |
Concave** |
N |
N |
N |
N |
N |
< 1 |
N |
N |
N |
L |
M |
1 - 5 |
N |
LV |
L |
M |
H |
5 - 10 |
LV |
L |
M |
H |
HV |
10 - 20 |
LV |
L |
M |
H |
HV |
> 20 |
L |
M |
H |
HV |
HV |
* Permeability class of the least permeable layer
within the upper 39 inches (one meter) of the soil profile. Permeability
classes for specific soils can be obtained from a published soil
survey or from local USDA-NRCS filed offices.
Soil Permeability Classes in inches per hour (in/hr):
very slow (<0.06 in/hr), slow (0.06 - 0.20 in/hr), moderately
slow (0.20 - 0.60 in/hr), moderate (0.60 -2.00 in/hr), moderately
rapid (2.00 - 6.00 in/hr), rapid (6.00 - 20.00 in/hr), very rapid
(>20.00 in/hr).
** Area from which no or very little water escapes
by overland flow
*** RUNOFF CLASS: N = negligible, LV = very low,
L = low, M = medium, H = high, HV = very high.
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Table 4.
The surface RUNOFF CLASS
site characteristic determined from the relationship of the NRCS
curve number and field slope. Refer to National Engineering Handbook
number 4, 1985.
|
< 50 |
50 - 60 |
60 - 70 |
70 -80 |
> 80 |
< 1 |
N |
N |
N |
N |
M |
1 -2 |
N |
N |
LV |
L |
M |
2 - 4 |
N |
N |
L |
M |
H |
4 - 8 |
N |
LV |
M |
H |
HV |
8 - 16 |
LV |
L |
M |
HV |
HV |
> 16 |
LV |
L |
H |
HV |
HV |
* RUNOFF CLASS: N = negligible, LV = very low, L
= low, M = medium, H = high, HV = very high.
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Precautions in the Use of the Phosphorus Index
The Phosphorus Index is an assessment tool to be used by planners
and landusers to assess the risk that exists for phosphorus leaving
the landform site and travelling toward a water body. It also
can be used to identify the critical parameters of soil, topography,
and management that most influence the movement. Using these parameters,
the index then can help select in the selection of management
alternatives that would significantly address the potential impact
and reduce the risk. The index is intended to be part of the planning
process that takes place between the landuser and resource planner.
It can be used to communicate the concept, process, and results
that can be expected if various alternatives are used in the management
of the natural resources at the site. THE
PHOSPHORUS INDEX IS NOT INTENDED TO BE AN EVALUATION SCALE FOR
DETERMINING WHETHER LANDUSERS ARE ABIDING WITHIN WATER QUALITY
OR NUTRIENT MANAGEMENT STANDARDS THAT HAVE BEEN ESTABLISHED BY
LOCAL, STATE, OR FEDERAL AGENCIES. Any attempt
to use this index as a regulatory scale would be grossly beyond
the intent of the assessment tool and the concept and philosophy
of the working group that developed it. As discussed in this technical
note the Phosphorus Index is proposed to be adapted to local conditions
by a process of regional adaptations of the site characteristic
parameters. This local development process must involve those
local and state agencies and resource groups that are concerned
with the management of phosphorus. After the index is adapted
to a locality, it must be tested by the development group to assure
that the assessments are giving valid and reasonable results for
that region. Field testing of the index is one of the most appropriate
methods for assessing the value of the index.
Alternative
Approaches for Use of the Phosphorus Index
Using the same site characteristics, but a more complex equation,
with the assumptions listed below, another approach has been developed
to show potential of phosphorus movement. The Phosphorus Index
can easily be modified by altering the basic equation, by developing
a factoring method.
The basic assumptions used in this modification of the Phosphorus
Index is given below:
- If soil erosion and runoff were both nearly zero, then the
phosphorus movement vulnerability should be low.
- If application of P has been nearly zero and the soil test
P levels are low, then the phosphorus movement vulnerability
should be low.
Using these two assumptions and also assuming that the other
key parameters are already identified, then the basic equation
needs to be modified to produce the expected answer. There are
many possible alternates to the basin equation, this is only an
example to show one possibility.
One possible formula would be:
Phosphorus Index rating =
((SE)X(C1) + ((RC) x (C2))) x (((Porg.applic rate) + (Pfert.
rate) x (AM) x (C3) + (Psoil test) x (C4))
Where:
SE = Soil Erosion
C1 = Coefficient of weighting erosion, that must be increased
if irrigation water is used.
RC = Runoff Class (this should reflect the amount or rate
of runoff.
C2 = Coefficient of weighting runoff.
Porg. applic. rate = Organic P application rate.
Pfert. rate = Commercial fertilizer P application rate
AM = Application method.
C3 = Coefficient of weighting the application method.
Psoil test = Phosphorus level in the soil.
C4 = Coefficient of weighting the phosphorus test level.
The values for these parameters would be the same as used in
the basic equation. The four coefficients would be dimensionless,
and locally adjusted to calibrate the equation.
A test for this alternative algorithm, or any developed for
a specific region, would be to perform field validation. After
customizing the algorithm it should be tested using data not used
in the development. A site known to be effected by P movement
problems should obtain a higher Phosphorus Index rating number
than a site that is known not to have any P related problems.
Some explanation and adjustment of the algorithm may be needed
in order to produce a logical rating with the modified algorithms.
Future Development of the Phosphorus Index
The short term objectives in development of a phosphorus assessment
tool, like the Phosphorus Index, are:
- To develop a procedure for field staffs involved in resource
management to assess the relative potential that exist for phosphorus
leaving the landform site and traveling toward a water body.
- To develop a method of using the assessment procedure with
in the Phosphorus Index in identifying the critical parameters
that most strongly influence the index.
- And then, to select management practices that when applied
to the landscape would decrease the site's vulnerability to phosphorus
loss.
A long term objective in the development of a phosphorus assessment
tool, like the Phosphorus Index, is the use with existing watershed
designations and/or water quality models that provide information
on sensitivity of water bodies to inputs of phosphorus. The water
quality and limnology models need to be sensitive to the form
of phosphorus (either particulate or soluble) entering the water
body and the specific impact these forms of the nutrient have
in the short and long term on water quality. There is a need to
link the movement of phosphorus in the landscape to the delivery
of phosphorus to the water body. An additional site characteristic
that addresses potential for transported P to reach water bodies
from the edge of the field would be desirable.
Phosphorus Index Core Team (PICT) Development
Assessing the potential for soil P to move in the landscape
and play a significant role in eutrophication of surface waters
is a prerequisite for an effective, prioritized nutrient management
and water quality program. This phosphorus assessment tool, the
Phosphorus Index, as described in this technical note has been
developed by a national group of scientist with the USDA, universities,
extension, private agencies, and industry. This group was titled
the Phosphorus Index Core Team (PICT). The objective of the group
was to develop a P indexing procedure that would identify soil,
landforms, and management practices with varying degrees of potential
risk to have unfavorable impacts on water bodies because of the
potential for P movement. This group recognized the need for a
simple, field-based index using readily available information
that could be used to assess site conditions and potential vulnerability.
As this index evolves more detailed research and field information
can be added to advance the science and methodology of P assessment.
The work group has expanded and diversified its effort (now known
as the Southern Regional Extension and Research Information Exchange
Group) and continues to develop the science of P behavior and
management in the soil.
Use of the Phosphorus Index in the Natural Resources Conservation Service
The Phosphorus Index is a planning tool that can be used in
resource management plans, for water and soil quality, nutrient
management, and ecosystem based planning assistance in watersheds.
It is intended to be used by the planner to communicate to the
landuser the relative potential for phosphorus movement in the
landscape. The NRCS does not condone or promote the use of the
index for placing any restrictions on land use or other regulatory
purposes that could be construed by manipulating the parameters
of the index.
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