  |
|
|
Wyoming Home
About Us
News
Programs
Technical Resources
Partnerships
Contact Us
|
|
|
|
|
|
Section 12 - Nutrient Basis for Manure Application RatesThe Phosphorus IndexBeginning in the mid-1980s, scientists researching soluble movement of phosphorus in surface runoff began documenting a relationship between soil phosphorus levels and phosphorus movement in runoff solution. There is now a substantial body of research addressing the relationship between soil phosphorus levels, phosphorus transport and phosphorus impact on aquatic ecosystems.The Phosphorus Index (PI) is a site-specific, qualitative, vulnerability assessment tool. The PI is to be used to assess the potential movement of phosphorus from Agricultural Waste application sites, to identify those sites which should be considered for more careful management of phosphorus, and to aid in the determination of manure application rates. Factors other than soil test phosphorus levels impact potential off-site losses of phosphorus. Factors such as soil erosion, soil test phosphorus levels, and rate, timing and method of application differentially impact the potential of phosphorus loss.
Procedure for Using the Phosphorus IndexThe site characteristics which impact potential phosphorus loss have been placed in categories and assigned a weight factor based on their relative impact on potential phosphorus movement from the site. Instructions are provided for each category, however, only categories that apply to the site receiving manure applications are to be used. Producers are encouraged to contact the NRCS field office for assistance in completing the Phosphorus Index.Phosphorus Loss Categories and Weighting Factors
Risk Ratings for Each Category Are As Follows:
Category InstructionsIndividual sections from Table 4, Phosphorus Index Site Vulnerability, are shown for each category to assist in determining the weighted risk factor for the category.Soil Erosion
Soil erosion is the detachment and transport of soil from the site. This category is expressed as the actual soil loss for a given soil. Soil erosion should be predicted using the Revised Universal Soil Loss Equation (RUSLE) found in Section I of the NRCS Field Office Technical Guide. Erosion estimates consider climate, soil characteristics, slope length and percentage, crop rotation, tillage system, and other practices such as contouring or cross-slope farming. (NOTE: If users of the PI are not familiar with the use of the RUSLE technology, they are STRONGLY encouraged to contact the nearest NRCS field office, or contact the State Conservation Agronomist.)
Irrigation EfficiencyFlow and application rate, as well as land slope all impact the potential off-site movement of phosphorus via irrigation erosion. However, irrigation efficiency (absolute efficiency, NOT potential efficiency) directly or indirectly impacts this potential off-site movement, and can be quantified with some ease. Efficiency values should be determined using the Farm Irrigation Rating System (FIRS).(NOTE: Evaluate only the irrigation type most representative of the site.)
Runoff Class
The runoff class of a site is based on the least permeable soil layer in the top three feet. Permeability classes for specific soils can be found in the soil map unit description in the county soil survey. Soil survey information is available through the local NRCS office. Slope and soil permeability class must be determined to calculate the runoff class. See Table 2, Runoff Class, to determine Runoff Class. Table 2. Runoff Class
Soil Phosphorus Test Level
The Olsen (sodium bicarbonate) soil test is normally utilized on soils with a pH greater than 7.0 and contain calcium carbonate (CaCO3). Since most Wyoming soils fall into this category, soil phosphorus levels should normally be determined using this method. In addition, the University of Wyoming Guide to Fertilizer Recommendations bases phosphorus application recommendations on the Olsen extractant method. Bray P1 soil tests are typically used on soils with pH < 7.0. (Note: Use either the Olsen or Bray method; do NOT use both tests.) Commercial Phosphorus Fertilizer Application Method
Method of phosphorus fertilizer placement, time to incorporation, and stage of crop all affect potential phosphorus losses. The categories are intended to quantify the potential effects on off-site losses of surface exposure time, incorporation and crop uptake to commercially applied phosphorus fertilizer. Commercial Phosphorus Fertilizer Application Rate
Commercial phosphorus fertilizer application rate is the amount in pounds
per acre (lbs/ac) of phosphate fertilizer (P2O5)
that is applied. This does not include phosphorus from organic sources.
Manure/Organic Phosphorus Application Method
Method of manure/organic placement, time to incorporation, and stage
of crop all affect potential phosphorus losses. The categories are
intended to quantify the potential effects on off-site losses of surface
exposure time, incorporation and crop uptake to land-applied manure/organic
fertilizer.
Manure/Organic Phosphorus Fertilizer Application Rate
Manure/organic phosphorus fertilizer application rate is the amount in pounds per acre (lbs/ac) of phosphate fertilizer (P2O5) contained in manure and/or organic fertilizer that are applied. This does not include phosphorus from commercial sources. Refer to Agronomy Tech Note 14 or site-specific manure tests for information on the nutrient content of agricultural wastes. Distance to Surface Water
This category is an estimate of distance between the application site, and surface water. Use zero for the distance if manure or fertilizer is applied directly to a concentrated flow area that delivers runoff water to an intermittent or perennial stream, or lake or other water body. Soil Phosphorus Sorption Capacity
The soil parameters pH and surface texture affect a soil's phosphorus sorption ability. See Table 3, Determining Soil Sorption Capacity, to determine the Soil Phosphorus Sorption Capacity. Table 3. Determining Soil Sorption Capacity
Completing the Risk AssessmentEach category factor in Table 4 is multiplied by the weighting factor to get a weighted value. All applicable categories are rated, and the risk assessment rating is the sum of these products. Once a rating is determined, record the rating in Table 5, and use Table 6 for application guidelines.Table 4. Phosphorus Index Site Vulnerability
Table 5. Site/Field Vulnerability to Phosphorus
Loss
|
Total of Weighted Values |
Site Vulnerability Rating |
Site/Field Numbers |
|---|---|---|
|
< 11 |
Low |
|
|
12 - 21 |
Medium |
|
|
22 - 35 |
High |
|
|
> 35 |
Very High |
Medium - This site has some potential for off-site movement of phosphorus. There is a greater potential of impact to surface water than the LOW category. Phosphorus management practices (applying manure at lower rates, installing filter strips, injection/incorporation changes) should be considered as an alternative.
High - This site is likely to have some off-site movement of phosphorus. Phosphorus management practices will be documented in the alternatives section of the conservation plan and manure application rates will be phosphorus-based.
Very High - This site is very likely to have some off-site movement of phosphorus. Phosphorus management practices will be documented in the alternatives section of the conservation plan and no additional manure applications will be made on the site until such time as remedial action is taken to reduce the site's risk assessment rating.
Conservation Practices to minimize potential phosphorus off-site movement vary by site. Site categories having the highest loss potential are the most critical, and should be addressed first. Keep in mind that the PI is only intended as a risk assessment tool and can be used to identify parameters that MIGHT contribute to off-site movement of phosphorus.
Phosphorus Index Rating |
Manure or Organic Application |
|---|---|
|
Low |
Nitrogen-based |
|
Medium |
Nitrogen-based |
|
High |
Phosphorus-based up to crop removal- |
|
Very High |
No Application |
|
|
