Determining the Phosphorus Index for a Field


Agdex#:	531/743
Publication Date:	10/05
Order#:	05-067
Last Reviewed:	10/05
History:	Replaces OMAFRA Factsheet Determining the Phosphorus Index for a Field, Order No. 03-109
Written by:	Don Hilborn - Engineer/OMAFRA; Robert Stone - Engineer/OMAFRA

PDF Version - 163 KB

Introduction
Background
Calculation Procedure for the P Index
Effect of Best Management Practices on P Index
Interpretation of Phosphorus Index for Ontario
Summary
P Index Calculation Sheet

Introduction

The Phosphorus (P) Index has been introduced into Ontario as a means to:

rank the relative risk of surface water contamination resulting from phosphorus application on crop land,
select management strategies that can be used to reduce this risk,
determine the distance that phosphorus applications must be set back from surface water,
set restrictions on rates of phosphorus applied to a field.

This Factsheet details the information and steps required to determine and utilize the P Index.

Background

Phosphorus is an important plant nutrient for crop production, and is provided either from the soil or from soil amendments, such as commercial fertilizers or animal manures. Phosphorus application for crops is generally determined by a soil test-based program.

Environmental concerns with phosphorus centre around eutrophication - an increase in the fertility status of natural waters that causes accelerated growth of algae or water-plants. Reducing phosphorus entry to surface water limits the rate of aquatic plant growth. This practice decreases the chances of fish kills caused by oxygen depletion when plants die and decay.

Phosphorus binds tightly to soil particles, so one key component in water contamination is soil erosion (influenced by soil type, slope, and water runoff potential). Thus, even though phosphorus levels in the soil may be quite high, good management strategies for phosphorus will allow crops to be produced with minimum risk to surface water contamination.

The P Index considers many factors, such as the conditions of a field (phosphorus levels in the soil, soil erosion and soil runoff risk), the quantity of nutrients to be applied along with their methods of application, and the distance to the nearest surface water.

The P Index assigns a number - 0, 1, 2, 4, 8 or 16 - to each of the conditions which can affect phosphorus losses, where 0 is the lowest P loss potential and 16 is the highest P loss potential. This is completed according to the probability of P loss from the site. Furthermore, each site characteristic is assigned a weighting factor that indicates the seriousness of the P loss potential of that individual site characteristic. All of the weighted conditions are added together to obtain the P Index.

Current agricultural nutrient management standards indicate that a Phosphorus Index must be determined if the P soil test for a particular field is above 30 ppm. A Phosphorus Index can still be calculated if the P soil test is below 30 ppm, but the data gathered would be primarily for the farmer's management information.

Ontario's Phosphorus Index was adapted by the University of Guelph from the 1993 U.S. index of Lemunyon & Gilbert, and modified to suit local conditions. The current coefficients reflect professional judgment, but have not yet been calibrated or validated against actual water contamination data.

The Phosphorus Index is referred to in the OMAFRA Publication 818, Nutrient Management Workbook and the NMAN Computer Software, and is a recommended tool for developing manure setbacks from surface water within nutrient management plans.

The steps below lead one through the calculation of the P Index for a field.

Calculation Procedure for the P Index

To complete an index for field or section of a field, use the P Index Calculation Sheet (found at the end of this Factsheet) as a template. A completed sample is on the back.

Find the rating for each site characteristic (using the number shown in the brackets).
Multiply each of the ratings obtained by their respective weightings.
Sum all the weighted rating values obtained in (B) above to get the P Index.
Use Table 9 to determine the affect on nutrient application.

Soil Erosion (Weighting = 2.0)

The Universal Soil Loss Equation (USLE) is used to estimate soil erosion. For details on how to calculate the soil loss, refer to OMAFRA Factsheet, Universal Soil Loss Equation (USLE), Order No. 00-001. Once the estimated soil erosion is determined, obtain the corresponding rating factor from Table 1.

Soil erosion and the corresponding rating factor may be reduced by adjusting tillage/cropping practices to increase the amount of surface residue or by decreasing the slope length.

**Table 1.** Soil Erosion
Soil Erosion (tonnes/ha/year)	Soil Erosion (tons/ac/year)	Rating
<12	<5	Very Low (1)
12 - 25	5 - 11	Low (2)
26 - 37	12 - 17	Moderate (4)
>37	>17	High (8)

Water Runoff Class (Weighting = 1.0)

The water runoff class estimates the potential for surface runoff from a field based on soil texture and field slope. Increased runoff results in the potential for higher P losses.

Table 2 rates the probability of runoff occurring in the vicinity of surface water. These numbers are fixed; they cannot be altered by changing cropping or tillage practices.

**Table 2.** Water Runoff Class
Hydrologic Soil Group (Drainage Class)	Maximum Field Slope within 500 ft (150 m) of Top of Bank of Surface Water
Hydrologic Soil Group (Drainage Class)	< 3%	3 - < 6%	6 - < 9%	9 - 12%
A (Rapid)	Very Low (1)	Very Low (1)	Low (2)	High (8)
B (Moderate)	Very Low (1)	Low (2)	Mod (4)	High (8)
C (Slow)	Low (2)	Mod (4)	High (8)	Very High (16)
D (Very Slow)	Mod (4)	High (8)	High (8)	Very High (16)

Hydrologic Soil Groups corresponding to the various soil series names found in Ontario are listed in OMAFRA Publication 29, Drainage Guide for Ontario. Group A is often associated with sand, Group B with loam, Group C with clay loam and Group D with clay soil textures.

Phosphorus Soil Test (Weighting = 2.0)

Phosphorus levels will build up in the soils if phosphorus is applied above crop removal. It is estimated that 35 lbs. of unused phosphate is required to increase the soil test level by 1 ppm (although the exact number will vary with soil texture). Good nutrient management practices should be used to minimize build-up in soils with higher phosphorus soil test levels.

Use Table 3 to determine the rating factor for the field's soil test level. If a soil test is not available, then assume a very high rating.

**Table 3.** Phosphorus Soil Test
P Soil Test (ppm)	Rating
< 15	Very Low (1)
15 - 30	Low (2)
31 - 60	Moderate (4)
61 - 100	High (8)
> 100	Very High(16)

Commercial Fertilizer Application Rate (Weighting = 0.5)

All commercial fertilizer is to be considered in calculating this rating factor. Most starter fertilizer rates result in a Very Low to Low category. The use of any starter fertilizer should be carefully considered when there already are higher levels of phosphorus in the soil.

Use Table 4 to determine the rating factor for the fertilizer application rate. If no fertilizer is used, assume a rating of zero.

**Table 4.** Fertilizer Application Rate
Fertilizer Application Rate (lb P₂O₅)/ac	Fertilizer Application Rate (kg P₂O₅)/ha	Rating
< 22	< 25	Very Low (1)
22 - 45	25 - 50	Low (2)
46 - 67	51 - 75	Moderate (4)
> 67	> 75	High (8)

Commercial Fertilizer Application Method (Weighting = 1.5)

The sooner a fertilizer is incorporated after application, the less chance there is for runoff to occur during rainfall.

Use Table 5 to determine the rating factor for the fertilizer application method. If no commercial fertilizer is applied then assume a rating of zero.

**Table 5.** Fertilizer Application Method
Fertilizer Application Method	Rating
Placed with Planter	Very Low (1)
Incorporated < 2 weeks	Low (2)
Incorporated > 2 weeks	Moderate (4)
Not Incorporated	High (8)

Manure/Biosolid (Organic P) Application Rate (Weighting = 0.5)

Most manure application rates will be in the Low to Moderate category. A high application rate of manure combined with a high concentration of nutrients will typically cause a rating factor of High. Conversely, a low application rate of manure with high levels of bedding will typically be in the Very Low category. The phosphate application rate is based on long-term soil build-up value (80% availability). This is twice the amount available to this year's crop. Use OMAFRA Publication 818, Nutrient Management Workbook or the NMAN Computer Software, to determine the actual nutrient values.

Use Table 6 to determine the rating factor for the manure/biosolid (Organic P) application rate. If no manure is used then assume a rating of zero.

**Table 6.** Manure/Biosolid Application Rate
Manure/Biosolid Application Rate (lb P₂O₅)/ac	Manure/Biosolid Application Rate (kg P₂O₅)/ha	Rating
< 11	< 12	Very Low (1)
11 - 32	12 - 36	Low (2)
33 - 54	37 - 60	Moderate (4)
> 54	> 60	High (8)

Manure/Biosolid (Organic P) Application Method (Weighting = 1.5)

The longer the period between application and incorporation, the greater the opportunity for runoff caused by rainfall.

Use Table 7 to determine the rating factor for the manure/biosolid (Organic P) application method. If no manure is used then assume a rating of zero.

**Table 7.** Manure/Biosolid Application Rate
Manure/Biosolid Application Method	Rating
Injected in Season	Very Low (1)
Incorp. - < 5 days	Low (2)
Not Incorp. - Pretilled	Moderate (4)
Not Incorp. - Crop Residue	Moderate (4)
Not Incorp. - Standing Crop	Moderate (4)
Not Incorp. - Bare Soil	High (8)

Effect of Best Management Practices on P Index

Best Management Practices (BMP) can be used to lower an index number. Table 8 demonstrates the ways in which the farming practices used on a field can be adjusted to lower the P Index.

**Table 8.** Effect of Best Management Practices on P Index
Site Characteristic	Management Practices that will Lower P Index	Example of BMP
Soil Erosion	Any practice to reduce soil erosion.	Reduce slope length; increase surface residue; plant cover crops.
Water Runoff Class	None	-
Phosphorus Soil Test	The management of fertilizer and manure application methods/rates will control the rate at which the phosphorus level in the soil changes.	The phosphorus level of a field can be lowered on a long-term basis by reducing or eliminating application rates of manure/fertilizer and/or using crops with higher P removal capabilities.
Commercial Fertilizer Application Rate	Applying less fertilizer to a field will lower the level of phosphorus accordingly.	A reduction in the commercial fertilizer application rate from 60 lbs P₂O₅/acre to 30 lbs P₂O₅/acre will reduce the P Index by 1 point.
Commercial Fertilizer Application Method	The use of an application method that incorporates the fertilizer quickly and efficiently will result in a lower rating factor.	By changing the application method from Non-Incorporated to Placed with Planter, the P Index is reduced by 10.5 points.
Manure /Biosolid Application Rate	Applying less manure to a field will lower the level of phosphorus accordingly.	A reduction in the manure/biosolid application rate from 60 lbs P₂O₅/acre to 30 lbs P₂O₅/acre will reduce the P Index by 3 points.
Manure /Biosolid Application Method	The use of an application method that incorporates the manure quickly and efficiently will result in a lower rating factor.	Changing the application method from Non-Incorporated on Bare Soil to Injected will cause the P Index to be reduced by 10.5 points.

Interpretation of Phosphorus Index for Ontario

The P Index can impact a nutrient management plan in 2 separate ways:

Sets minimum separation distances for nutrient application close to surface water.
Determines maximum phosphorus application rates in vicinity of surface water.

The following table recommends redeeming actions according to the P Index value.

**Table 9.** Phosphorus Application Rates and Setback Distances for P Index Ranges
Phosphorus Index for Site	Generalized Interpretation of Phosphorus Index for Site	Minimum Setback¹ from Surface Water if P₂O₅ is applied up to crop removal ² [ft (m)]	Minimum Setback from Surface Water if P₂O₅ is applied over crop removal [ft (m)]
< 15	VERY LOW potential for P movement from the site. If farming practices are maintained at the current level there is a small chance that P losses from this site will have an adverse impact on surface waters.	10 (3)	100 (30)
15 - 29	LOW potential for P movement from the site. The chance for an adverse impact to surface water exists. Some remedial action should be taken to lessen the potential for P loss if application is close to surface water.	10 (3)	100 (30)
30 - 50	MODERATE potential for P movement from the site and for an adverse impact on surface waters to occur unless remedial action is taken. In areas close to surface water, soil and water conservation along with P management practices are needed in order to reduce the risk of P movement and water quality degradation.	10 (3)	200 (60)
> 50	HIGH potential for P movement from site and for an adverse impact on surface waters. Remedial action is required to reduce the risk of P movement. All necessary soil and water conservation practices plus a P management plan must be put in place to avoid the potential for water quality degradation.	100 (30)	Do not apply over crop removal

¹With manure application, it is recommended that the minimum separation distance be met in order to address direct surface runoff concerns. See Section Q, Table 15: Minimum Separation Distance (with established buffer zone) in OMAFRA Publication 818, Nutrient Management Workbook, for more details.
²The maximum allowable application rate is recommended to be the lowest rate calculated in the Table in Section S - Maximum Rates, OMAFRA Publication 818, Nutrient Management Workbook.

Summary

The phosphorus index is used as a means to address the risk of surface water environmental concerns resulting from phosphorus. Using this index you can determine if phosphorus can be applied above crop removal and/or you can set the minimum setback from surface water. In many cases, best management practices can be used to lower the index number.

See O. Reg 267/03, Section 42 and 45 for setbacks for application of non-agricultural source materials adjacent to surface water. See O. Reg 267/03, Section 14 regarding application rates and setbacks when more than one application rate or setback applies.