P Index Development and Implementation The Iowa Experience

1. P Index Developmentand ImplementationThe Iowa Experience Antonio Mallarino Iowa State University

2. Status of the P Index in Iowa • Iowa has a P index since late 2001. First developed for NRCS (590 guidelines). • Since last fall, its use is required by the State of Iowa for manure management plans for CAFOs. • Recently revised to use RUSLE2 instead of RUSLE and to add Mehlich-3 ICP soil test. • Validation research and education efforts for its use continue.

3. Origin of the Iowa P Index • In 1999, NRCS national policy suggested using one of three assessment tools to estimate risk of P loss: • soil-test P interpretations for crops • environmental soil-test P threshold • a P risk index. • The Iowa State Technical Committee established a subcommittee and later an expanded task force to provide advice as of what criterion Iowa would adopt.

4. Task Force Recommendations • Soil-test P classes for crops can’t be used: • not a good index of risk of P loss • penalizes animal production • A single, higher “environmental” soil P threshold sometimes is too low, too restrictive in others, and not field specific. • The P index integrates source and transport factors, is field specific, and suggests better soil conservation and P management practices to reduce P loss.

5. P Index Team Work • Use existing knowledge and scientific judgment. • Assess long-term risk of P loss. Not intended to predict P loss from a particular practice or event. • Arrange source factors within each main transport mechanism in a quantitative way. A key departure from early P indices. • It can’t be a complete P model, keep it as simple and practical as possible.

6. P Index Three Components Soil Erosion(Particulate P) Source Factors- soil P- application method, timing, and rate Water Runoff(Dissolved P) Tile Drainage(Dissolved P) Soil and water conservation practices

7. Major Concepts of the Index • Use NRCS tools to estimate impact of landscape, soils, and management on soil and water loss (RUSLE2, sediment traps, SDR, RCN). • At this time does not differentiate between P sources: more research needed, cropland, emphasizes long-term impacts, little or no manure to pastures. • Roughly estimates pounds of “effective” P delivered to the nearest stream.

8. Major Concepts of the Index • Uses soil-test P methods and sampling procedures commonly used for crops. • Considers P rate and application method since the last soil test. • The sum of three P loss estimates are used to establish five risk classes. • Can be applied to an entire field or field zones to recognize field variability and target field “hot spots” for P loss.

9. Soil Erosion Component P Bound to Sediment

10. Soil P Loss and Soil Erosion • Estimates P in sediment effectively transported to a stream: • Total soil P estimate (from soil-test P) • Soil loss (RUSLE2) estimate • Watershed sediment delivery “power” • Sediment traps, filter strips, sediment enrichment in fine particles and P • Distance to nearest stream • Availability of particulate P to algae

11. Surface Runoff Component P Dissolvedin Surface Runoff

12. Soil P Loss and Surface Runoff • Surface water flow estimate from NRCS runoff curve numbers. • Dissolved P concentration in runoff increases linearly with increasing soil-test P. Average equation for all Iowa soils. • Can use four soil P tests recommended by ISU for crops. • Sampling depth recommended for crop production (6 inches).

13. P Rate and Application Factors • Fertilizer or manure P application, since the last soil test. • P application increases soil-test P and, therefore, increases dissolved P loss with runoff. • Accounts for impacts of the P application rate, method of timing on dissolved P concentration in surface runoff.

14. When do P losses occur? • Recognizes increased risk of P loss from surface P application to frozen, snow-covered, or water-saturated soil. • When there is more runoff and higher probability of runoff events? • Late February to late June: • snow melt and high-rainfall periods • little or no crop canopy, tilled soil • water saturated soils, floods • reduced conditions (soluble Fe+2)

15. Subsurface Drainage Component P Dissolved in Subsurface Drainage

16. Soil P Loss and Subsurface Drainage • Subsurface water flow about 10% of annual precipitation. • Are tiles or sandy subsoil present? • assumes no P loss if answer is no. • Soil-test P drainage factor: • high risk for high soil P values • little loss risk if Bray-1 or M3 P is less than 100 ppm, Olsen < 60 ppm, and M3-ICP < 118 ppm

18. P Index Risk Ratings • Very Low (o-1) or Low (1-2): Excellent from for water quality or very little impairment. • Medium (2-5): Acceptable risk, but future practices should not increase P loss. • High (5-15): Obvious problem. New soil conservation and/or P management practices should be implemented. • Very High (>15): Extreme problem. New soil conservation and P management practices that may require no further P application should be implemented.

20. Implementation and Use • The index provides partial ratings for erosion, surface runoff and subsurface drainage components: • identify reasons of high loss risk • suggest new P management or soil conservation practices • The P index can and should be calculated for field zones: • how much P and what soil conservation practices to what field area?

21. Iowa P-Index Implementation Project 33 fields in six clusters

22. Index Partial Index Value component Average Range ------------------ % ------------------ Soil erosion 73 31 - 91 Surface runoff 24 7 - 58 Tile drainage 3 2 - 7 Index Component Contribution

23. 1.1 2.8 2.0 Zoning fields for P index calculation and management 1.2 5.1 IDNR-EPA & Iowa Soybean Association Supported Projects

24. Criteria to Delineate Zones • Yield, nutrient, topography differences: • Soil map units, including slope and erosion phases • Presence of terraces and tiles • Management practices, previous or current: tillage, crops, contour cropping, fertilization, soil-test P • Fits well with the concept of zone soil sampling for crop production.

25. P Index Use by Iowa NRCS • P index required for nutrient management plans if (and/or): • farmer enrolled in assistance programs • use of manure or organic sources • P impaired watershed • soil-test P is Very High (class for crops) • erosion exceeds tolerable soil loss (T) • Calculate for most erosive soil map unit of the field or field zone (conservation management units).

26. P Index Use for MMPs • CAFOs with greater than 500 animal units (about 3,000 in Iowa). • Use Index for 4-year manure management plans. Can update plan annually. • Can zone fields for P index calculation and manure management. • Use erosion rate of the most erosive soil map unit that is at least 10% of the field or zone or greater than 5 acres.

27. Soil Testing Requirements • Soil-test P and pH at least every 4 years. • Use certified labs - state program based on NAPT data. • Can any soil P test supported by ISU. Not not Bray-1 if pH >7.3. • Can use ISU recommended grid, zone, or soil type sampling methods, but should take at least one sample every 10 acres, unless P-based rates are used (20 acres).

28. Manure Application Rates • Very Low or Low (0-2) • N-based manure management • no consideration of P applied. • Medium (2-5) • N-based if practices will not increase the P risk to High • But can apply less than twice the P removed with crop harvest during the 4 years.

29. Manure Application Rates • High (5-15) • no manure application until practices reduce index to Medium • until 2008, P-based if P index rating is between 5 and 10. • Very High (>15) • no manure application

30. The P Index: A Useful Tool • Allows for reasonable agronomic and environmental manure P management. • Apply manure based on crop P nutrient needs or on N needs by watching P index ratings for the field or field zone. • Complementary practices: • Reduce manure P: low phytate grain, phytase enzyme, use reasonable P supplementation. • Incorporate the manure into the soil without increasing soil erosion. • Follow appropriate setbacks.