Lesson 34: Phosphorus Index

Environmental Stewardship Assessment:

The goal of this assessment tool is to help you confidentially evaluate the risk associated with high soil P.

Caution: Many states have a state-specific P Index. Although the principles of most P Index tools are similar, individual factors or weighting of those factors varies among states. If available, review your own state’s P Index.

Name of Operation:
Date:

1. Soil Erosion: The annual soil erosion loss value can be found in a conservation plan. Multiply that value by 2 for the P Index Risk Value. If soil erosion is unknown, the following values may be used:

Very low: 0 tons/acre/yr Medium: 3-4 tons/acre/yr High: 5-6 tons/acre/yr
Low: 1-2 tons/acre/yr   Very High: 7+ tons/acre/yr

Risk Value =  tons/acre/yr

2. Runoff Class: The runoff class is a combination of the slope and the soil permeability class for a field.

Instructions:

  1. From a conservation plan or county soil survey map, determine the field’s slope.
  2. Using a county soil map, determine the soil name/map unit for the field. Refer to the Soil Survey table summarizing Physical or Chemical Properties or Engineering Properties to determine the permeability (inches/hr) of the upper 12 to 18 inches of soil profile for the soil name/map unit identified for the field.
  3. Using the table below,
    1. Along the top of the table, locate the appropriate Soil Permeability column based on the permeability value found in the Soil Survey.
    2. Locate the appropriate Slope row along the left side of the table.
    3. The value where the appropriate Soil Permeability column and Slope row intersect is the Runoff Class Risk Value.

  Soil Permeability Class1
Very rapid (6-20 in/hr) Moderately rapid and rapid (2-6 in/hr) Moderately slow and moderate (0.20-2 in/hr) Slow (0.06-0.2 in/hr) Very slow (< 0.06 in/hr)
Slope, %  
Concave
< 1
1-5
5-10
10-20
> 20
Corresponding magnitude of risk
 
1Permeability class of the least permeable layer within the upper 12-18 inches of the soil profile. Permeability classes for specific soils can be obtained from a published soil survey or from local USDA-SCS field offices.

3. Subsurface Drainage

Instructions:

  1. In the left column, identify the depth to seasonal high water table (or presence of drainage system) that matches your field conditions.
  2. From the soil drainage classes, identify a description that best describes your field conditions.
  3. The soil drainage risk value for the Phosphorus Index is found where the identified row and column intersect.

Depth to Seasonal High Water Table, ft Soil Drainage Class
Very poorly drained Poorly Drained Somewhat Poorly Drained Moderately Well Drained Well Drained Somewhat Excessively Drained Excessively Drained
0-1  
1-3
3-6
> 6  
Artificial subsurface drainage (any depth)  
Corresponding magnitude of risk
 

4. Leaching Potential
Clay, sandy clay, and silty clay Corresponding magnitude of risk
 
Clay loam, silty clay loam, and silty clay
Loam, silt loam, and silt
Sandy loam
Sand and loamy sand

5. Connection to Surface Water: Through field observation, you need to determine if the field is connected, partially connected, or not connected to a water body.
Field is far from water body, and field runoff does not enter surface water. Corresponding magnitude of risk
 
Field is separated from surface water, and runoff reaches surface water only during large, intense storms.
OR
Field adjoins surface water with > 30 ft of permanent vegetated buffer.
Field is separated from surface water, and runoff reaches surface water during snow melt or low-intensity storms.
OR
Field adjoins surface water with 10- to 30-ft permanent vegetated buffer.
Field adjoins surface water with < 10 ft of vegetated buffer between field and surface water.

 
Sum of Transport Factors/35:

 

6. Soil Test P: The soil test level is taken from a soil test report as either PPM (multiply by 0.25 for Risk Value) or lbs P2O5 /acre (multiply by 0.06 for Risk Value).

7. Loss Rating for P Application Method and Timing

Placed with planter or injected > 5 cm deep Corresponding magnitude of risk
 
Incorporated < 1 wk after application
Incorporated > 1 wk or not incorporated following application in late spring to early autumn
Incorporated > 1 wk or not incorporated following application in late autumn to early spring
Surface applied on frozen or snow-covered soil

8. Fertilizer P Risk: the product of

Fertilizer Application Rate (lbs P2O5 /acre)
Corresponding magnitude of risk

9. Manure P Risk: the product of

Manure Application Rate (lbs P2O5 /acre)
Corresponding magnitude of risk

 
Sum of Management Factors:

 


P Index:
Magnitude of risk:

 

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