Phosphorus Index for Nutrient Management

D.K. Bhumbla
WVU Extension Service
Soils and Water Extension Specialist

Purpose:

The Phosphorus Index (P Index) is a tool that can identify farm fields that are a potential source of phosphorus (P) pollution of surface waters. Using the P Index can help a farmer identify fields and management practices that have the greatest potential to pollute bodies of water with phosphorus. The P index can help land users assess management strategies to minimize P loss from agricultural areas.

Concept:

On agricultural land when annual application of phosphorus exceeds its removal by crops, then phosphorus will accumulate in soils. Phosphorus accumulation in soils leads to high soil test values for phosphorus. A soil testing high for P can be a source of phosphorus pollution. Movement of phosphorus from crop fields into bodies of water may lead to excessive growth of algae (algal bloom). Algal bloom adds easily decomposable organic matter to water. Decomposition of organic materials requires oxygen. Thus, algal bloom caused by phosphorus movement to a body of water leads to reduction of oxygen in water. This lack of oxygen can kill aquatic animals such as fish. However, most P initially added to land through fertilizer or manure reacts with soil components, converting to an insoluble form or attaching to soil particles. Thus, most P loss in agriculture is associated with loss of soil particles.

A large number of factors determine phosphorus loss from a field. These include a soil test value for phosphorus; source, method, rate, and timing of P application; susceptibility of a given soil to erosion; and management practices. The P index quantitatively determines the relative risk of P movement from a given field by considering most of the factors that govern P losses. The P Index for a field can be calculated by using the following worksheet.

Worksheet for Calculating P Index for a Field

P Index rating value from soil test
If soil test value is 0 to 50 lbs P/acre, enter 1 in box A.
If soil test value is 51 to 80 lbs P/acre, enter 2 in box A.
If soil test value is 81 to 200 lbs P/acre, enter 4 in box A.
If soil test value is more than 200 lbs P/acre, enter 8 in box A.
Box A	___________________________________

P Index rating value from manure application
If applying no manure, enter 0 in box B.
If applying 1 to 30 lbs P₂O₅/acre from manure, enter 1 in box B.
If applying 31 to 60 lbs P₂O₅/acre from manure, enter 2 in box B.
If applying 61 to 90 lbs P₂O₅/acre from manure, enter 4 in box B.
If applying more than 90 lbs P₂O₅/acre from manure, enter 8 in box B.
Box B	___________________________
P Index rating value from fertilizer application
If applying no fertilizer, enter 0 in box C.
If applying 1 to 30 lbs P/acre from fertilizer, enter 1 in box C.
If applying 31 to 90 lbs P/acre from fertilizer, enter 2 in box C.
If applying 91 to 150 lbs P/acre from fertilizer, enter 4 in box C.
If applying more than 150 lbs P/acre from fertilizer, enter 8 in box C.
Box C	______________________________
P Index rating value from manure/fertilizer application method
If applying no manure/fertilizer, enter 0 in box D.
If placing manure/fertilizer deeper than 2 inches, enter 0.5 in box D.
If incorporating manure/fertilizer immediately before crop, enter 1 in box D.
If incorporating manure/fertilizer more than 3 months before crop or are surface applying manure/fertilizer less than 3 months before crop, enter 2 in box D.
If surface applying manure/fertilizer (no incorporation of manure/fertilizer into soil) more than 3 months before crop or are surface applying manure to a pasture land, enter 4 in box D.
Box D	_________________________________

P Index rating value from soil erosion
If soil loss from this field is less than 5 tons/acre/year, enter 1.5 in box E.
If soil loss from this field is 5 to 10 tons/acre/year, enter 3 in box E.
If soil loss from this field is 10 to 15 tons/acre/year, enter 6 in box E.
If soil loss from this field is more than 15 tons/acre/year, enter 12 in box E.
Box E	___________________________

P Index rating value from surface runoff
If surface runoff from this field is less than 0.1 cm enter 0 in box F
If surface runoff from this field is 0.1 to 1.0 cm enter 0.5 in box F
If surface runoff from this field is 1.0 to 5.0 cm enter 1.0 in box F
If surface runoff from this field is 5.0 to 10.0 cm enter 2 in box F
If surface runoff from this field is more than 10 cm enter 4 in box F
Box F	_________________________

Total P Index rating value for the site

Add the P Index value points from boxes A, B,C, D, E and F and enter the total in box G.

The value in box G represents Total P Index value for the site.

Box G

_________________________________

Site vulnerability to P-loss as a function of Total P Index rating values

Site vulnerability rating	Total P Index rating value
Low	<8
Medium	8 to 14
High	15 to 32
Very High	>32

Low to medium site vulnerability ratings indicate that current management practices are adequate for protection of surface waters from phosphorus pollution. High and very high site vulnerability ratings indicate a need for improved management practices.

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