P Index Development and Implementation The Iowa Experience Antonio Mallarino Iowa State University

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

Origin of the Iowa P Index l 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. l The Iowa State Technical Committee established a subcommittee and later an expanded task force to provide advice as of what criterion Iowa would adopt.

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

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

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

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

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

Soil Erosion Component P Bound to Sediment

Soil P Loss and Soil Erosion l 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

Surface Runoff Component P Dissolved in Surface Runoff

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

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

When do P losses occur? l Recognizes increased risk of P loss from surface P application to frozen, snow- covered, or water-saturated soil. l When there is more runoff and higher probability of runoff events? l 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 )

Subsurface Drainage Component P Dissolved in Subsurface Drainage

Soil P Loss and Subsurface Drainage l Subsurface water flow about 10% of annual precipitation. l Are tiles or sandy subsoil present? -assumes no P loss if answer is no. l 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

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

Implementation and Use l 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 l The P index can and should be calculated for field zones: -how much P and what soil conservation practices to what field area?

Iowa P-Index Implementation Project 33 fields in six clusters

Index Component Contribution Index Partial Index Value component Average Range % Soil erosion Surface runoff Tile drainage

Zoning fields for P index calculation and management IDNR-EPA & Iowa Soybean Association Supported Projects

Criteria to Delineate Zones l 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 l Fits well with the concept of zone soil sampling for crop production.

P Index Use by Iowa NRCS l 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) l Calculate for most erosive soil map unit of the field or field zone (conservation management units).

P Index Use for MMPs l CAFOs with greater than 500 animal units (about 3,000 in Iowa). l Use Index for 4-year manure management plans. Can update plan annually. l Can zone fields for P index calculation and manure management. l 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.

Soil Testing Requirements l Soil-test P and pH at least every 4 years. l Use certified labs - state program based on NAPT data. l Can any soil P test supported by ISU. Not not Bray-1 if pH >7.3. l 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).

Manure Application Rates l Very Low or Low (0-2) -N-based manure management -no consideration of P applied. l 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.

Manure Application Rates l 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. l Very High (>15) -no manure application

The P Index: A Useful Tool l Allows for reasonable agronomic and environmental manure P management. l Apply manure based on crop P nutrient needs or on N needs by watching P index ratings for the field or field zone. l 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.