# The Phosphorus Index Wes Jarrell, Senior Scientist

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```					         The
Phosphorus Index
Wes Jarrell, Senior Scientist,
Discovery Farms Program
July 17, 2002
SUMMARY

Phosphorus that causes problems in water occurs
both in
particles (PP) and
dissolved in solution (SP) when it reaches surface
water.
The PI considers both particulate and dissolved P.
The numbers and relationships applied will become
more site-specific as experience is gained.
Total Risk Index for Phosphorus (PI):

PI = PP + SP + LP

PI = Total P index
PP = Particulate P
SP = Soluble P
LP = Leached P
PP: Depends on (1) erosion, (2) fraction of
eroded particles delivered to stream, and (3) P
concentration in the soil particles

Calculation:
Particulate P =
Rusle2 *
Sediment Delivery Ratio *
BufferEffectiveness*
Enrichment Ratio*
Soil particle P concentration (from Bray
P1)
Rusle2
Building on USLE and Rusle, Rusle2 will provide
- Erosion, T/acre/year

For soluble P it will provide
- Runoff volume, Liters/acre/year

Various studies around Wisconsin, including
Discovery Farms and Pioneer Farm, will be
testing and adjusting Rusle2 as needed
Sediment Delivery Ratio (SDR)

What fraction of the Rusle2 eroded sediment actually
reaches the waterbody in an “average” year?
Value between 1 (no removal) and 0 (100% removal)
Depends on
- slope, particle size, distance, roughness,
infiltration between the lower edge of field and the
waterbody.

- Uses an equation developed in late ’70’s and tested
since then
Enrichment ratio

[P] in delivered particles/[P] in bulk soil
As particles settle out, those still carried tend to be smaller
in diameter but have higher [P].

Values >1(usually 1 to 5)

Fractionating soils from around Wisconsin and looking at
changes in particle size/[P] relationships, and Bray P1
BufferEffectiveness
- Related to SDR

- Value between 1(no removal) and 0
(100% removal)

- Relates to how effective the field buffer is
at trapping particles.
Soil particle P concentration
(from Bray P1)

-Once we estimate the mass of particles entering
the water, we need to know the average
concentration of P in those particles

-Erosion (T/ A) * total [P] in particles, kg P/T

-But we usually don’t measure total [P]. So we
estimate it from Bray P1 for various soils from
across the state.
Pheasant Branch: TP vs total P, Elena Bennett
1600
y = 3.153x + 341.98
1400
R2 = 0.6721
1200
Total P, mg/kg

1000

800

600

400

200

0
0     50       100      150      200      250       300   350
Bray P1, mg/kg

Also:
Meyer, Lyne, Avila, Barak, UW Madison, Plano silt loam:
Total P = 2.5 (Bray P1) + 875
Soluble P: Depends on amount of runoff, P
concentration in the soil, and soluble P
concentrations in P-containing
amendments/fertilizers

Total of
SP from soil P
+ SP from unincorporated nutrients on unfrozen soil
+ SP from unincorporated nutrients on frozen soil
(SP release from plant tissue?)
a. For no fertilizer or incorporated
fertilizer:

Annual runoff volume (from Rusle2) *
Soil solution [P] (from Bray P1) *
“Extraction efficiency
“Extraction efficiency”
How efficiently does precipitation water
extract P from soil as it runs over the
surface?

Value = < 1
Bray P1 vs soil solution P

250
Bray P1, mg P/L

200        y = 30.1x + 17.4
2
R = 0.901
150

100
50

0
0.00       2.00         4.00    6.00      8.00
Soil solution [P], mg P/L
b. For surface-applied nutrients without
incorporation:

Soluble P in manure (lb/A) /
average days between runoff-generating
events
Soluble P in manure (lb/A)
Affected by diet, animal, and manure
handling
Effects of dietary [P] on [P]soluble in
feces (Satter, 2002)
125
Soluble P
105                                          52
41            in manure
31                    (Orthophosphate)
85     3           21
11
65
g/day

46     Insoluble P
36   38     40         42       44
45
25                                                  Milk
35   35     35         35       35    35     P
5
-15    -2

Mobilized 0.3      0.35   0.4       0.45      0.5   0.55
From Bone                    %Dietary P

72   84     96         108      120   133

g Dietary P / day
Average days between runoff-generating
events

The longer manure sits on soil surface, the
greater the likelihood that dissolved P
will be sorbed by the soil and protected
from runoff.

E.g., John Norman’s PALMS model
4“
Rainfall 2000   Runoff Events 2000
N-S Chisel Plow

Runoff Events 2000
Arlington, Wisconsin
Contour Ridge Till
c. For snow-melt events with nutrients
spread on frozen soil:
Soluble P in manure (lb/acre)*(Slope
percentage)2/200
-This is related to the Potential Energy on the
slope that can move water down.
-The higher this energy, the more likely the
manure will reach water.
- Proposed modification: slope <6: 0.4 *Soluble
P in manure; >6: above relationship
**NOT YET INCORPORATED INTO MODEL**

d. For loss from plant material
SP = Soluble P in residue (kg P/acre)*
Runoff volume at susceptible time (L/acre)*
d. For loss from plant material - tests

Runoff plots at Pioneer Farm will provide insight into
dissolved P loss from alfalfa, possibly other crop
plants.
**NOT YET INCORPORATED INTO MODEL**
e. LP = P lost through leaching

LP =
P concentration in soil solution (from Bray P1)/
(depth to tile * retention coefficient * recharge volume)
New PI interface
New PI interface
Preliminary Interpretation:
Actions to be taken based on PI values

0 – 2 Low risk: Low probability of being a problem
2 – 6 Intermediate risk: Should not allow it to
increase
6 - 10 High risk: Lower to 6 over course of one
rotation or four years
>10 Very high risk: Lower to 6 over course of two
rotations or eight years.
Decrease P in
Improved management practices to decrease PI if values are too high
feed
General comment:                                    Decrease fertilizer/           Apply to other fields
Increase Infiltration!                              manure rates
Export from farm
Plant high-P-uptake
Lower Bray P1
crops

Increased residue

Decrease erosion             Reduced tillage/ no till
Particulate P
too high                                              Cover crop

Contour tillage
Grassed waterways                 Increase width,
Trap eroded                Riparian buffers                  effectiveness
particles
Settling ponds/levies/basins

Lower Bray PI (see above for particulate P)

Low-disturbance
manure incorporation
Soluble P                                              Eliminate mineral
too high                                               P supplements
Decrease soluble P
in manure                    Chemically sorb or
precipitate P

Storage
Change winter
sites
Continued Validation of P index

Ultimate: correlate land management with water
quality
Interpretation: Compare calculated PI(s) with actual
measured P load entering a water body
Methods for validation
- Small plots, natural and simulated rainfall
- Field-scale runoff: Pioneer Farm, Discovery Farms
(including forested sites, overwintering sites)
- Stream flow: Discovery Farms, other DNR-related
sites
The Phosphorus Risk Index –
Predicting future values

Case 1 - degradation
Phosphorus
Risk                                    Case 2 - balance
Index

Case 3 - restoration

1   2     3      4     5     6    7
Years
Bragger farm, Buffalo County
North flume
The Phosphorus Risk Index –
Field trials, Pioneer Farm –
Defines “Successful PI” – if it works!
7
6                                    Flume 4
Phosphorus    5               Flume 1
Risk
4
Index
Calculation   3                         Flume 3
Flume 2
2
1

1    2     3    4    5     6     7
P load, lb/acre/year, measured at flumes
From data run by Todd Andraski and Larry Bundy
- small simulated runoff plots

Relationship between Particulate P Index and PP load
in runoff at Arlington, Lancaster, and Madison.
2000

No Manure
r2 = 0.71
Spring Manure
Particulate P load (g ha )

1500
-1

r2 = 0.72

1000

500

0
0           1              2             3             4
Particulate P Index
From data run by Todd Andraski and Larry Bundy
- small simulated runoff plots

Relationship between Soluble + Particulate P Index
and TP load in runoff at Fond du Lac.

2000
Total P load (g ha )

1500
-1

1000

No Manure
r2 = 0.46
500                                            Spring Manure
r2 = 0.65

0
0           1          2             3           4       5
Soluble + Particulate P Index
From data run by Todd Andraski and Larry Bundy
- small simulated runoff plots
Relationship between Soluble P Index and DP load in runoff.
1200

1000         1. Runoff / Rainfall = actual value
No Manure
2. Soil/Runoff Interaction:             r2 = 0.98
800             Mod. permeability = 0.2
DP load (g ha )

Spring Manure
-1

Slow permeability = 0.6
r2 = 0.84
600

400

200

0
0                  1                2           3             4
Soluble P Index
Integrated farm analysis for nutrient management

Common User Interface

SNAP            P Balancer            Wisconsin          Rusle2
2004            Erb,                  PI
NPM                   Jarrell,          Yoder,
Pearson,                              Discovery         NRCS
Discovery                             Farms
Farms,                                Management
and Erb,                              recommendation
loop
NPM

WI Comprehensive Nutrient Management Plan
Components:
Land management plan, Nutrient management
plan, Record keeping, Manure management,
Feed management

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