P Index DevelopmentP Index Developmentand Implementationand Implementation

The Iowa ExperienceThe Iowa Experience

Antonio MallarinoAntonio Mallarino

Iowa State UniversityIowa State University

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.

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.

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.

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.

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

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.

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.

Soil Erosion Component

P Bound to SedimentP Bound to Sediment

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

Surface Runoff Component

P DissolvedP Dissolvedin Surface Runoffin Surface Runoff

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).

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.

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)

Subsurface Drainage Component

P Dissolved in Subsurface DrainageP Dissolved in Subsurface Drainage

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

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.

SOIL TEST-P (ppm)0 20 50 100 150 200 250 300

P I

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20IMPACT OF SOIL-TEST P AND SOIL EROSION

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A. Mallarino, ISU

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?

Iowa P-Index Implementation Project

33 fields in six clusters

Index Component Contribution

Index Partial Index Value component Average Range

------------------ % ------------------

Soil erosion 73 31 - 91

Surface runoff 24 7 - 58

Tile drainage 3 2 - 7

Zoning fields forP index calculationand management

IDNR-EPA & Iowa Soybean Association Supported Projects

2.8

1.1

2.0

1.25.1

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.

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).

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.

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).

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.

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

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.