Evaluating Runoff Phosphorus Risk on Agricultural Lands:

Pleasant Valley Experience

Photo: M Godfrey

Laura Ward Good

Wisconsin P Index

Photo: M Godfrey

Estimates average annual

P lb/acre/year delivered to surface water from field

Field

Annual P delivery to

stream (P Index)

Annual “field-edge” runoff

losses estimated for each crop

year: Sediment-bound P

Dissolved P from soil

Dissolved P from manure and

fertilizer

Feld to stream delivery ratio

Stream

Runoff

Erosion

WI P Index Basics Developed for Nutrient Management Planning

“Conservative” planning assumptions:

• Uses RUSLE2 erosion and runoff calculated for Dominant Critical Area for whole field

• Assumes continuous channelized flow similar to a grassed waterway

Does not account for P in gully erosion

For equations: wpindex.soils.wisc.edu

Good, L.W., P.Vadas, J.C. Panuska, C.A. Bonilla, W.E. Jokela, 2012. Testing the Wisconsin Phosphorus Index with Year-Round Field-Scale Runoff Monitoring. Journal of Environmental Quality 41:1730-1740.

X

=

RUSLE2

P Index calculator

WI P Index (P b/a/yr):

• Rotation Average

• Annual

Dissolved

Particulate

Erosion (sed t/a/yr)

P Trade (lb/fld/yr)

Uses predominant soil

for erosion and runoff

Nutrient application

rate calculator

Soil Test P and Organic Matter

Field Slope

Field Slope Length

Tillage

Rotation crops and yields

Manure Applications

P Fertilizer Applications

Downfield Slope to Surface Water

Soil Type

Distance to Surface

Water

Phosphorus Index

SnapPlus Inputs and Outputs Inventory

Calculations Results by field

Baseline P Index Distribution

Project watershed rotational P Index by acres

All > 6 down to 6, reduce loads by 1/4 All >3 down to 3, reduce loads by 1/2

Flat field (1% slope) in continuous corn silage with excessively high soil test P (200 ppm)

Example High P Loss Field

No one-size-fits-all solutions

Reductions went below runoff standards

First focus: Fields with P Index above 6 Second focus: Fields with P Index between 3 and 6

Reality: Farmers applied practices across many fields, not just high P Index fields

Participating Farms Reduced Runoff P Loss and Erosion

Acres

P Index

reduction

(lb/yr)

Erosion

reduction

(ton/yr)

No-till/reduced

till/residue

management

1840 3300 2000

Pasture systems

(stream crossings,

fencing, seeding)

315 1100 100

Estimated average annual runoff P and erosion reductions in Pleasant Valley from cost-shared managements: Implementation period (2010-2013) compared to baseline (2006-2009)

Estimated average annual runoff P losses for

participating farms, baseline (2006-2009) and 2013

Participating farms cut runoff P losses in half

Watershed tracking finding:

Runoff P losses increased on other farms

Estimated P delivery from all agricultural acres in watershed was reduced 12%

Largest increases were from grassland conversion to cropland

Summary

WI P Index inventory and tracking allowed project to: • Identify croplands and pastures with higher

P runoff contributions to stream • Identify alternative managements • Estimate runoff phosphorus reductions with

changing practices