BACKGROUND AND BASIC CONCEPTS OF THE IOWA PHOSPHORUS INDEX

A Support Document to the NRCS Field Office Technical Note 25

Document prepared by:
Antonio P. Mallarino (Iowa State University), Barbara M. Stewart (Iowa NRCS),
James L. Baker, John A. Downing, and John Sawyer (Iowa State University)

Introduction

The phosphorus (P) index is a risk assessment tool that was developed to assess the potential for P delivery from fields to surface water resources. In April 1999, the United States Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS) issued national policy and general guidelines on nutrient management. Each state is required to revise NRCS state policy and guidelines by April 2001. These guidelines apply to nutrient management where nutrients are applied to the land, including organic by-products and animal manure. All NRCS staff will use these guidelines when providing technical assistance to producers. Third party vendors and other non-NRCS employees will use these guidelines when providing assistance through federal conservation programs for which NRCS has technical responsibility. The national policy and guidelines suggest the use of one of the following P risk assessment tools: soil-test P values, threshold limits, or a P risk index. The Iowa State Technical Committee recommended the use of the P index approach based on the advice of an interdisciplinary group of scientists, technical personnel, and representatives from various organizations. The index was developed by a core team of scientists and technical personnel from various institutions and agencies (a complete list of contributors at the end of the document). The P index is a tool that provides a relative rating as to the risk of P delivery from individual fields to surface water resources. This rating can be used to prioritize fields for nutrient and soil management practices. The purpose of this document is to provide a brief background and rationale for the first version of the Iowa P index.

The Need for an Environmental Phosphorus Assessment Tool

An increasing concentration of animal production in certain areas of the state is increasing the amount of manure being applied to the land. Often, the manure is applied at frequencies and rates that exceed the P required for optimizing crop yield or the amount of P removed in harvested crops. Animal manure can supply the nitrogen (N) and P needed by crops as well as other nutrients. Due to its relative N and P contents and potential N losses, however, continued use of rates that supply the N removed in harvested grain results in P accumulation in soils. Phosphorus accumulation in excess of crop needs may increase the potential for eutrophication of surface waters. Eutrophication occurs when nutrient levels in water are high and result in excessive algae growth, which during decomposition may reduce oxygen levels in water and often creates imbalances in water ecosystems and reduce the aesthetic value of lakes or streams. Contrary to the situation in oceans, P usually is the nutrient that limits (and controls) algae growth in freshwater bodies.

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Article published in the proceedings of the conference Agriculture and the Environment:

State and Federal Water Initiatives. p. 63-71. March 5-7, 2001. Ames, Iowa.

The potential problem of excessive P loss from agricultural fields is compounded because soils of many grain crop production areas already have soil-test P levels that are at or above levels that optimize crop production. The upper limit for amounts of manure that could be applied with minimal nutrient loss could be ultimately determined by the P level in the topsoil and the potential for soil erosion, water runoff, and/or P leaching through the soil profile that can reach surface water bodies. Thus, better estimates of the potential for P loss from agricultural soils, especially manured soils, are required.

The Index in Relation to Phosphorus Soil-Test Thresholds and Application Rates

The soil-test P level and the P application rate are the most frequently mentioned factors in relation to estimates of P delivery to surface waters and as the subject of possible regulation. However, consideration of only these two factors has serious limitations as tools to predict P delivery from soils. The amount of P delivered from a field depends only partly on the soil P concentration and the method or rate of P application. Additional major factors include soil P release characteristics of soils and transport mechanisms that control the amount of P that can move off a field and reach surface water resources. A specific soil-test P value or rate of P application may have a markedly different impact on P delivery from fields having different soil properties and landforms. Factors that influence the degree of soil erosion and volume of water runoff, the distance between fields and streams or lakes, and other factors that affect the transport of water or sediment from fields impact P delivery. Also, any P risk assessment tool based entirely or in part on soil-test P and P application rate should consider the depth from which soil samples are collected, the depth that is relevant to predict P transport, and the method of manure or fertilizer application. Thus, the P index approach is more comprehensive than relying only on a soil test P threshold value. Use of the P index provides a means of identifying fields that have high potential for P delivery to surface water resources through erosion, runoff, and/or subsurface drainage and, therefore, provide guidelines for improved soil conservation practices and nutrient management practices.

Main Characteristics of Phosphorus Indices

All versions of P indices include a number of site characteristics related to the source, transport, and management of P. Early versions of the P index included factors or site characteristics such as soil erosion, soil runoff class, soil-test P, P fertilizer application rate, P fertilizer application method, organic P application rate, and organic P application method. Each site characteristic was assigned a relative potential P loss rating with a corresponding numerical value. For example, None (0), Low (1), Medium (2), High (4), or Very High (8). Also, each characteristic was given a weighting factor reflecting its relative importance in contributing to P runoff (for example, 0.5 to 1.5). The P index was calculated by multiplying each potential P loss rating by its corresponding weighting factor and summing the results. The P index for an individual field placed it into a category (for example, Low, Medium, High, or Very High) with associated interpretations and recommendations for nutrient management. Later versions of P indices developed for specific regions included other factors and changed how potential P loss ratings are calculated to obtain a P index. Additional factors included (or substituted for some of the factors mentioned above) were distance to water body; tillage, vegetation, or grazing management; site hydrology (for example, slope gradient and length, flooding frequency, drainage class, subsurface drainage, etc.); and estimates of the degree of soil P saturation.

In most early P index versions, the factors were additive. This means that all factors were considered equivalent (with adjustments for variable weighting of individual factors) and there was no accounting for interaction among terms. A modification introduced in indices proposed since the late 1990s uses a multiplicative approach. The various factors are arranged into two distinct groups: P transport factors (for example, soil erosion, runoff class, and distance) and P source factors (soil test P, P rate, and application method of both fertilizer and organic P source). The P transport factors receive rating values of less than one, and are multiplied together to yield an overall P transport potential with a value between zero and one. The P source potential value (the sum of individual source factor values, as in earlier versions) is then multiplied by the P transport potential value. Thus, the P transport potential value serves as a scaling coefficient that reduces the full P source potential by an amount proportional to the P that can potentially move off the field. This means, for example, that a field with a very high soil-test P level (a high P source potential) but with a low P transport potential would not likely receive a high P index rating because there is a low probability that P would be transported off the field.

Rationale for the Iowa Phosphorus Index

The most recent conceptual change concerning P index development focuses on relating more clearly the index output with the fundamental processes controlling P delivery from fields. Most early indices, although recognizing these basic processes, did not integrate the different processes into a quantitative set of components directly related to estimates of P loads. The major advantage of the earlier approach was that it produced a simpler index and did not require (although may have included) assumptions concerning functional relationships between source and transport factors and estimates of amounts of P delivered from fields. This approach results in two limitations. One, the index would not achieve its full potential to integrate an understanding and description of the basic process with the mechanics of calculating the risk rating. Two, and perhaps most important, lack of consideration of estimates of P delivery complicates the comparison (or normalization) of different indices developed for various regions. Indices that integrate estimates of P delivery from fields with a risk index can be reasonably normalized across regions. The Iowa P index is based on this concept of P index development.

Current knowledge about the processes that contribute to P delivery to surface waters underlies the main characteristics and concepts of the Iowa P index. The index is based on current research data, survey results, and scientific judgement when the data is not yet available. Ongoing research designed to validate the various components of the index will produce results useful to modify the index as needed. The pace of the research suggests that the index may need to be revised every two years. Some of the most important fundamental concepts of the Iowa P index can be summarized in the following points.

1. The index uses a multiplicative approach to combine source and transport factors in estimating P delivered to water resources. The source factors are combined in a multiplicative manner within three major components based on the major transport mechanisms: an Erosion Component (sediment loss), a Runoff Component (water loss), and a Subsurface Drainage Component (water loss through tiles and/or coarse subsoil/substrata). Each component provides a rough (or proportional) estimate of amounts of P delivered from fields through each transport mechanism that would be available for aquatic ecosystems (lb P/acre/year). The outputs from the three components are summed to get an overall approximate estimate of the total P delivered. The resulting number (one per field or one per each conservation management unit within a field) is placed into one of five risk classes that range from very low to very high. These classes are based on current knowledge concerning the impact of P loads on eutrophication of water resources.

2. The index makes use of common tools used by NRCS field staff to estimate the impact of landscape forms, soil types, and management practices on soil and water loss from fields. Some of these tools have been modified as needed to better estimate these losses for the most representative area of individual agricultural fields. Thus, the index uses existing databases for soil classification, landscape forms, and major soil physical properties; the Revised Universal Soil Loss Equation (RUSLE) to estimate sediment loss through sheet, rill, and ephemeral gully erosion; sediment delivery ratio (SDR) or sediment trap efficiency of conservation practices (terraces and ponds, for example) to estimate sediment delivery off fields; runoff curve numbers (RCN) to estimate water runoff; and county historical precipitation data to estimate precipitation. This approach utilizes background information available through producers and/or NRCS field offices. In contrast to some other indices, the Iowa P index does not require that producers collect complicated field measurements such as slope gradient and length. It requires only an estimate of the distance from the center of the field to the nearest perennial or intermittent stream or lake, and information on tillage and vegetation cover management (grain harvest, silage, grazing, hay, etc.) normally used for other purposes.

3. The index considers losses of P dissolved in water and P bound to sediment (or particulate P). The dissolved P is readily available to aquatic organisms, whereas a large proportion of the particulate P will be released to the water over a variable period of time depending on various factors. Aquatic research demonstrates that a large proportion of the particulate P can be made available through chemical, biological, and hydrological processes. Thus, the index weighs particulate P losses heavily when the erosion risk is high and considers potentially high loss of dissolved P during short periods relative to the long-term potential annual loss. Current knowledge suggests that particulate P has more short-term influence in the amount of dissolved P in water bodies (mainly in lakes) than previously thought. This influence is difficult to predict because it depends on many factors such as water body chemistry, depth, input and output patterns, and usage (recreation, motorized boats, etc.). Iowa scientists believe early indices have under-emphasized the long-term impact of particulate P losses on lake ecosystems.

4. The index does not differentiate between commonly used P sources, and gives similar weight to fertilizer, manure, and other organic sources. Scientists recognize that differences in water solubility of the P in some organic sources may have a large influence on the short-term impact of P application on P loss through runoff or subsurface drainage. For example, it is possible that dissolved P delivered through runoff or leaching processes immediately after applying solid manures (especially when it is mixed with bedding) and compost is lower than for other manures (such as liquid swine manure or poultry manure) because solid manures often have a lower proportion of water soluble P. However, this may not necessarily be the case when longer term losses (one or more years) and total P delivered with eroded sediment are considered.

5. Only soil P tests and sampling procedures currently recommended for crop production are needed for the index. Iowa State University (ISU) supports the Bray-1, Mehlich-3, and Olsen P tests. The soil-test P value is used to estimate dissolved P concentration in water runoff and subsurface drainage and to estimate total soil P concentration in sediment. The basic concept underlying this decision is that available research does not clearly support the need for a change to other soil testing procedures (extractant, sample depth, and sample strategies). The routine soil P tests used for crop production do not measure total soil P or water soluble P, but extract an amount of P proportional to the amount of P available for a crop. Interpretations of the tests supported by ISU for crop production are described in the Extension Publication Pm-1688. Ongoing research suggests that recently proposed testing procedures aimed solely at environmental evaluations provide similar results to current agronomic testing procedures and are less practical or more expensive for producers. Many years of research have produced field calibrations that were used for developing interpretation ranges for agronomic soil tests and fertilization recommendations. The concept of soil test calibration also applies when the main objective of soil testing is to estimate amounts of soil P that could potentially reach surface water supplies. Data from ongoing research in Iowa and other states have been utilized.

Components and Factors of the Iowa Phosphorus Index

1. Erosion Component.

The output of this index component is an approximate (proportional) estimate of the amount of P delivered with sediment (lb P/acre/year) that is likely to become available to aquatic ecosystems. It considers sheet and rill soil erosion; ephemeral and classic gully erosion; SDR or sediment trap efficiency of conservation practices to estimate sediment delivery off a field; total P in the topsoil; the impact of landforms and management on the P enrichment of eroded sediment; the impact of vegetative buffers on sediment losses from a field; and the proportion of tightly bonded particulate P that may become available for aquatic organisms.

Gross erosion is estimated using the NRCS Field Office Technical Guide to calculate the tons/acre/year of soil loss through sheet and rill erosion with RUSLE (Section I, Erosion Prediction) plus ephemeral and classical gully erosion (Section I, C-3, Gully Erosion). The SDR is derived from a modification of existing procedures in use by NRCS to estimate sediment delivery for watersheds based on area (Field Office Technical Guide, Section I, Erosion and Sediment Delivery). The modification allows the use of the basic concept to estimate sediment delivery for individual fields by transforming area to linear distance from the center of the field to the nearest perennial or intermittent stream down the slope by means of Equation 1.

Equation 1: Distance = 0.7 x Area0.6.

Measurement units are feet for distance and square miles for area. A support chart included with the index summarizes output SDR values for four major Iowa landform regions. The output values from the chart are unitless, and range from 0.03 to 1.0 to account for situations when very little sediment reaches the stream (0.03) to situations when all the sediment likely reaches the stream (1.0). Another support chart provides coefficients for sediment trap efficiency of specific conservation practices such as level terraces, ponds, etc. These practices can reduce the sediment delivery to a field edge by 80 to 100%.

The buffer factor refers to a vegetative buffer that meets NRCS standards for filter strips. A support table included with the index provides values for three classes arranged by buffer widths that range from 0 to 75 feet or wider. The output values from the chart are unitless. Values range from 0.5 to 1.0 to account for situations when the buffer is most effective in retaining sediment (about 50%) to situations in which the buffer does not exist or is insufficient and all the sediment leaves the field (1.0).

The enrichment factor accounts for the increase in the proportion of fine soil particles contained in eroded sediment, which tends to have a higher concentration of P when certain land treatments are present. Five classes with varying enrichment coefficients according to tillage system, grain or forage crops, and presence or absence of a buffer strip at least 20 feet in width are shown in a support table included with the index. The output values from the chart are unitless. Values range from 1.1 for situations in which little enrichment is expected (without a buffer and tillage used) to 1.3 for situations in which enrichment is the highest (with a buffer and no-till management or forages).

The total P factor is based on an estimate of the total P concentration of the surface 6-inch layer of soil that may become available to aquatic organisms. Total soil P is calculated from the average amount of total P in low P testing Iowa soils and the increase in total P due to application of fertilizers or manure estimated from a recent measurement of soil-test P. The average value used for total topsoil P (6-inch depth) is 500 ppm. This value is increased according to the most recent soil-test P measurement using Equation 2. Estimates represent the current total P to a depth of 6 inches when soil-test P (STP) is measured with the Bray-1 or Mehlich-3 tests. The 3.0 coefficient reflects the assumption (based on available research data) that in the long-term a 1 ppm increase in soil-test P corresponds to an increase in total soil P of approximately 3 ppm. When the Olsen test is used, the soil-test value should be divided by 0.6 to account for the lower P extraction with this test.

Equation 2: Total P = 500 + (3.0 x STP).

In addition, total P is multiplied by a 0.7 coefficient to account for the assumption (supported by aquatic research) that on average only 70% of the particulate P effectively delivered to a lake will be biologically available within a long but reasonable time period for algae growth. A support table included with the index provides factor output values for various Bray-1, Mehlich-3, and Olsen soil-test P values.

2. Runoff Component.

This component estimates the amount of dissolved P delivered with water runoff (lb P/acre/year). The estimate of dissolved P includes dissolved orthophosphate P (often referred to as dissolved reactive P) and other dissolved P fractions. Runoff P is estimated by use of NRCS runoff curve numbers, historic annual county precipitation data, an equation that estimates the impact of soil-test P on the concentration of dissolved P, an equation that estimates the impact of recent P application on soil-test P, and the impact of the timing and method of P application on the concentration of dissolved P in runoff.

The runoff curve number (RCN) term in the component expresses runoff volume as a fraction of the average annual precipitation for the county. Users select fraction values from support charts included with the index that include RCN and county precipitation data. The RCN numbers were developed by NRCS with consideration of precipitation intensity, landforms, vegetative cover, and other factors (NRCS Engineering Field Manual, Chapter 2). The calculations for the P index include a 0.5 coefficient to reduce the runoff volume fraction because approximately 50% of the rainfall does not produce significant runoff. The average annual precipitation for each county is divided by the constant 4.415 to convert inches of rain to millions of lb of water/acre.

The soil-test P runoff factor estimates the total dissolved P concentration in water runoff using Equation 3, which describes a linear relationship between P concentration in runoff and soil-test P. The soil-test P value in the equation (STP) corresponds to samples collected to a depth of 6 inches when the Bray-1 and Mehlich-3 tests are used. When the Olsen test is used, the soil-test value is divided by 0.6 to account for the lower P extraction. The 0.05 coefficient is the intercept of the equation and represents the concentration of dissolved P in runoff at very low soil-test P levels. The 0.005 coefficient is the slope of the relationship (the average increase in dissolved P per unit of soil-test P). Output factor values are shown in a support table included with the index for various Bray-1, Mehlich-3, and Olsen P values.

Equation 3: Dissolved P = 0.05 + (STP x 0.005).

The rate, method, and time of P application factor estimates the additional impact of recent P applications on soil-test P since the last soil sampling and before growing a crop. The P2O5 term of Equation 4 represents the P application rate (fertilizer, manure, or other organic sources) and the 4.58 coefficient transforms lb of P2O5 to ppm of P (elemental) assuming that a 6-inch slice of topsoil one acre in size weighs two million pounds. The 0.5 coefficient transforms this value into effective soil-test P increase by assuming that 50% of applied P within 100 days after the application is measured by the Bray-1 or Mehlich-3 soil tests.

Equation 4: Additional Dissolved P = ((P2O5/4.58) x 0.5) x Met&Time x 0.005.

Values for the method and time of P application term (Met&Time) are unitless and modify the impact of P applications on dissolved P with runoff. The values for four classes that consider methods and time of application are provided in a support table included with the index. Values range from 1.5 when P is surface-applied to snow covered/frozen ground, water saturated soil, or flood plains (full impact) to 0.4 when the P is injected into the soil or incorporated within 24 hours of the application. The 0.005 coefficient represents the slope of the relationship between soil-test P and the concentration of dissolved P in the runoff. The assumed soil bulk density value is within average observed values for the major agricultural soils in Iowa. The 50% recovery value of applied P by the Bray-1 or Mehlich-3 soil tests was derived from Iowa research that included various soil series and commonly used P fertilizers.

3. Subsurface Drainage Component.

This component estimates the amount of total dissolved P delivered to surface water resources through subsurface drainage, mainly through tile in poorly drained soils (lb P/acre/year). It uses existing databases (soil classification, landscape forms, and major soil physical properties), an estimate of water flow, historic county precipitation data, and an estimate of the impact of soil-test P on the concentration of dissolved P in the water. The precipitation data is the same used for the runoff component.

The flow factor is determined by presence or absence of subsurface flow. If tile or coarse-textured subsoils/substrata are known to be present the flow factor takes a value of one (unitless). Otherwise it has a value of zero and, therefore, it is assumed that no P is delivered to surface water resources via this transport mechanism. It is assumed that 10% of precipitation will flow through tile or coarse-textured subsoils/substrata, so a constant (average) value of 0.1 is included in the equation. If it is unknown whether tile or coarse-textured subsoil/substrata is present, tile is assumed to be present if the field is cropped to grain crops, the predominant soil series of the field has 5% slope or less, 40% clay or less, and the soil is in the poor or very poorly internal drainage class. A list of the soil map units that have these characteristics is provided in a support table included with the index. Another support table lists the soil map units with coarse-textured subsoil and/or substratum.

The soil-test P subsurface drainage factor consists of two classes. Available research data from Iowa does not support use of a continuous relationship between soil P and dissolved P movement through soil profiles at this time. The factor has a value of 0.1 if the Bray-1 or Mehlich-3 soil-test value (or Olsen P divided by 0.6) is 100 ppm or less in the top 6 inches of soil, which represents an average dissolved P concentration in the subsurface flow water of 0.1 ppm. The factor value is 0.2 if soil-test P is higher than 100 ppm.

Phosphorus Delivery Risk Interpretation Classes

Very Low (0-1): Soil conservation and P management practices result in small impacts on surface water resources.

Low (1-2): The P delivery to water resources is greater than from a site with a very low rating, but current practices keep water quality impairment low.

Medium (2-5): The P delivery may produce some water quality impairment. Consideration should be given to future soil conservation and/or P management practices that do not increase the risk of larger P delivery.

High (5-15): The P delivery produces large water quality impairment. Remedial action is required. New soil and water conservation and/or P management practices are necessary to reduce offsite P movement.

Very High (higher than 15): Impacts on surface water resources are extreme. Remedial action is urgently required. Soil and water conservation practices plus a P management plan, which may require discontinuing P applications, must be implemented.

Summary and Precautions in the Use of the P Index

The Iowa P index considers P management and soil conservation practices that influence P delivery from fields to surface water resources. The index does not include a built-in soil-test P or P application limit. Instead, it provides information as to likely causes of high P delivery to surface water resources, and provides information useful to choose from alternative P management and soil conservation practices that would reduce the risk of offsite P delivery.

The P index is an assessment tool intended to be used by planners and land users to assess the risk that exists for P moving off a field to a water body. It can also be used to identify the critical parameters of soil, topography, and management that most influence that movement. Using these parameters, the index can help in the selection of P management and soil conservation practices that would significantly reduce P loss and the risk of water impairment. The index is intended to be part of the planning process that takes place between the land user and resource planner. It can be used to communicate the main concepts and processes involved, and results that can be expected if various alternatives are used in the management of the natural resources at the site.

The P index is not intended to be an evaluation scale for determining whether land users are abiding within water quality standards that have been or may be established by local, state, or federal agencies. Any attempt to use this index as a regulatory scale would be beyond the intent of the assessment tool and the concept and philosophy of the working group that developed it.

The P index has been developed for local conditions on the basis of available Iowa research, information from other states that could apply to Iowa conditions, and from scientific judgement when research data were incomplete. This version of the index will be tested and modified periodically as new research data becomes available.

Members of the Iowa Phosphorus Index Team

The team that developed the index was coordinated by Gerald A. Miller, Associate Dean, College of Agriculture, Iowa State University. Team members are listed below following an alphabetical order. Other scientists from Iowa and other states who are not listed were consulted for specific aspects of the index.

Iowa State University:

James L. Baker (Department of Agricultural and Biosystems Engineering)

John A. Downing (Department of Animal Ecology)

Thomas E. Fenton, Antonio P. Mallarino, John E. Sawyer, and Regis D. Voss (Department of Agronomy)

Iowa USDA-NRCS: Mark Jensen, Douglas Johnson, and Barbara M. Stewart

USDA-ARS National Soil Tilth Laboratory: John L. Kovar and Thomas J. Sauer