Transcript Wisconsin P Index Approach to Soluble P Losses from Fields

Wisconsin P Index Approach to
Assessing Soluble P Losses from
Cropped Fields
L.W.Good
Department of Soil Science
University of Wisconsin Madison
Average Dissolved P (DP) Losses in
runoff are calculated for each crop
year for two P sources:
• DP released from soil surface
• DP released from fertilizer on surface
Example Field
Somewhat poorly drained silt loam
1% slope
Within 300 feet of stream
Soil test P = 35 ppm Bray1 P
Year 1: Corn grain, Fall chisel plowed, spring disking,
Maintenance fertilizer (DAP) incorporated by tillage spring.
Year 2: Soybean, Maintenance fertilizer (DAP) broadcast spring
Year 3: Winter wheat, Maintenance fertilizer (DAP), broadcast
fall
DP from Soil =
Surface Soil Test P (STP)x P Extraction Coefficient x
Runoff (RO) Volume x Units Conversion Factor
Surface STP = Plow layer STP x Stratification Factor
Maintenance Fertilizer P = Crop P Removal, so plow layer soil test P not expected to
change year-to-year, but surface soil test P will change with tillage.
Year 1:
Fall tilled corn
Year 2:
No till Soybean
Year 3:
No till W. Wheat
35 ppm
35 ppm
35 ppm
1.2
1.4
1.4
Est. Surface STP
42 ppm
49 ppm
49 ppm
Surf. STP further adj.
for fertilizer P
54 ppm
72 ppm
72 ppm
0.006
0.006
0.006
0.32 mg/L
0.43 mg/L
0.43 mg/L
1.5 in
3.8 in
4.1 in
0.2 lb/a/yr
0.3 lb/acre/yr
0.4 lb/a/yr
Plow layer STP
Soil P stratification
factor
Extraction
Coefficient
Est. Avg. RO DPsoil
Annual Runoff
An. loss RO DPsoil
DP from Fertilizer =
Fertilizer P on surface X Runoff/Precipitation x
O.034 exp (3.4 Runoff/Precip)
The ratio Runoff/Precipitation is for the season of application and can be thought of as
the potential for precipitation water to runoff, rather than infiltrate
Year 1:
Fall tilled corn
Year 2:
No till Soybean
Year 3:
No till W. Wheat
70 lb/acre,
Fall incorporated
55 lb/acre,
Spring broadcast
55 lb/acre,
Fall broadcast
Est. Soluble fertilizer
P on surface after
application and inc.
12 lb/acre
24 lb/acre
24 lb/acre
Runoff/Precipitation
for season of
application
0.02
0.06
0.05
0.01 lb/acre
0.06 lb/acre
0.05 lb/acre
P2O5 applied
Est. Fertilizer P
losses
Total Estimated Average Annual
Dissolved P Losses in Runoff for
Example Field
DP soil
year
DP
fertilizer
DP total
Year 1:
Fall tilled
corn
Year 2:
No till
Soybean
Year 3:
No till W.
Wheat
0.2 lb/a
0.3 lb/a
0.4 lb/a
0.01 lb/a
0.06 lb/a
0.05 lb/a
0.2 lb/a
0.4 lb/a
0.5 lb/a
WI P Index “Modeling” Approach
• Tested with field runoff data
• Shown to be providing a good estimate of field
runoff dissolved P losses when measured
runoff volume and precipitation are used in
the equation
• Current work on Wisconsin P Index focusing
on testing methods of assessing management
and soil affects on average runoff volume
Vadas -- APLE 2.4