The Phosphorus Index
A Phosphorus Assessment Tool

Technical Note

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 (K_Sat) 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.

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.

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:

1. If soil erosion and runoff were both nearly zero, then the phosphorus movement vulnerability should be low.
2. 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:

1. 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.
2. 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.
3. 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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