The Minnesota Phosphorus Index: Assessing risk of phosphorus loss from cropland

aerial photograph of farmfields and a lake
Statewide, agriculture is one of several sources of phosphorus to lakes and streams.

Phosphorus is essential for crop production, but can be lost to runoff from soil, plant residue and fertilizer and manure that’s applied to the soil surface.

The challenge is keeping phosphorus on the land and out of the water. Farmland can be a significant source of the phosphorus that gets into lakes and streams, where it causes excess algae growth.

Here, we explain when and how to assess the risk of phosphorus loss from cropland using the Minnesota Phosphorus Index.

Excess phosphorus

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Assessment tool overview

Factors that influence P runoff from uplands into surface water include erosion rates, manure application rates, soil test P levels and distance between the field and the surface water.

Because direct measurements of P loss are rarely practical, we use models or indicators to estimate current or future P losses under various management scenarios (Table 1). Use these major indicators to roughly identify sites likely to lose phosphorus.

Table 1: Phosphorus loss assessment tools

NRCS 590 standards and MPCA 7020 rules* Rapid P Index Minnesota P Index
Type Indicators and practices Indicators Mechanistic model of three pathways
Uses To control major P loss risk factors related to manure application To reduce the number of sites where the Minnesota P Index will be applied Farm nutrient management planning, and watershed planning to control P loss
Inputs Distance to water, soil test P, erosion, filter strip, manure method and timing Distance to water, soil test P, erosion, P fertilizer and manure rate and method Distance to water, soil test P, erosion, P fertilizer and manure rate and method, tillage type and direction, county and crop rotation
Output Manure application rate restrictions Indicates whether field is above or below risk threshold Relative rating of risk of P loss
*These are not primarily P loss assessment tools, but comprehensive feedlot and nutrient management guidelines.
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Minnesota Phosphorus Index: Using the model and understanding results

The Minnesota Phosphorus Index (Minnesota P Index) is a user-friendly model for estimating P loss risk.

How to download and use the Minnesota P Index

The Minnesota P Index is a management decision-making tool. Inputs are easily available and it generates reliable results for making farm-level decisions or for watershed planning.

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How to assess P loss risk

The method of P loss risk assessment differ for a farm manager planning practices that don’t threaten water quality, and a watershed planner identifying and addressing sources of P to a lake or stream.

Farm manure and nutrient management

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Watershed risk assessments

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High-risk sites

Generally, a site with a Minnesota P Index value over four has a high risk of P loss.

You can set a different threshold depending on your objective and the sensitivity of the receiving water. A threshold of two or three may be appropriate if a lake or stream is highly sensitive to P or if watershed planners aim to eliminate nearly all P inputs.

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Education

You can use the Minnesota P Index to help explain the movement of phosphorus across the landscape and illustrate the effect of various management practices.

Here are some examples of educational uses:

  • Run scenarios in the Minnesota P Index program with an individual or group of land managers. Allow them to choose the management inputs. Show how the changes do or do not affect the P loss risk estimate.

  • Use the Reports function in the program to create graphs comparing the effect of alternative management practices for your local soils and cropping systems. Illustrate the effect of how multiple P loss risk factors interact.

  • Use diagrams of the Minnesota P Index model to explain sources and transport of phosphorus.

Critical P sources

Estimates of erosion and P loss risk represent the whole field, yet only a single runoff path in the field is used to estimate erosion.

This is appropriate because sediment and phosphorus are not evenly lost from all parts of a field, but come from a few critical source areas called the “most limiting areas of significant extent,” according to the Minnesota NRCS.

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The rapid phosphorus index

The Rapid Phosphorus Index (RPI) is a set of indicators and thresholds based on the Minnesota P Index.

It’s less powerful but quicker to use than the Minnesota P Index. While the Minnesota P Index is easy to learn and use, it requires a modest amount of time to collect input data.

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Ann Lewandowski, Research and Outreach Coordinator, John Moncrief, Professor Emeritus, and Matt Drewitz, Minnesota Department of Agriculture

Acknowledgements

The development of the Minnesota Phosphorus Index was a cooperative effort of the University of Minnesota, Minnesota Department of Agriculture (MDA), Minnesota Pollution Control Agency (MPCA) and the Natural Resource Conservation Service (NRCS), with funding from the Minnesota Environmental Quality Board, University of Minnesota Extension Service and the U.S. Environmental Protection Agency 319 grant sponsored by the MDA.

Contributors:

  • University of Minnesota: John Moncrief, Paul Bloom, Neil Hansen, David Mulla, Ann Lewandowski, Ed Dorsey (who wrote the Windows-based P Index program), Carl Rosen, John Lamb, Jeff Strock and Gyles Randall.

  • MPCA: Dave Wall, Jim Klang and Chris Zadak.

  • NRCS: Jeff St.Ores, Pete Cooper and Robin Martinek.

  • MDA: Matt Drewitz and Mark Dittrich.

Reviewed in 2018

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