Soil Phosphorus Tests in the
North-Central Region
Antonio Mallarino
Iowa State University
Basics of Soil Testing
 A relative measurement of nutrient
sufficiency for plants.
 Tests measure an amount of nutrient
that is proportional to the amount actual
available for plants.
 Many P tests are used and measure
different amounts of P. The amount
measured has no meaning by itself and
cannot by used without calibration.
Field Correlation/Calibration
 Gives a meaning to a soil test result.
 Establish critical concentration ranges
for different crops and soils.
 Establish probability of response for
different test results.
 Determine fertilization rates when test is
below a critical range: interpretation
classes or continuous formulas.
Soil Testing for P
 Bray-1 and Olsen have been extensively
used across the North-Central region.
 Bray-1 underestimates plant-available P
in many calcareous soils.
 Both tests have been used with a
colorimetric determination method,
intensity of a blue color.
 Colorimetric methods measure
orthophosphate P.
Common Extractive Solutions
 Bray-1: HCl + NH4F
 Olsen or bicarbonate: NaHCO3 (pH 8.5).
 Mehlich-3: CH3COOH + HNO3 +
NH4F + EDTA
NH4NO3 + EDTA
Iowa Field Calibrations: Bray & Olsen
Relative Corn Yield (%)
Bray-1
Olsen
100
100
90
90
80
80
pH > 7.3
70
70
60
60
VL L O
H
VH
pH > 7.3
Classes
(low subsoil P)
Classes
VL
(low subsoil P)
50
L
O
H
VH
50
0
10
20 30 40 50
Bray-1 P (ppm)
60
70
0
5
10 15 20 25
Olsen P (ppm)
30
35
The Mehlich-3 Extractant
 M3 extractant began to be used in the
NC Region for P, K, Ca, Mg, and some
micronutrients during the late 1990s.
 The NCR-13 committee included the M3
among procedures for P, K, Ca, and Mg.
 Iowa published field calibrations for M3K and the M3-P colorimetric in 1999.
 M3- P is similar to Bray-1 P, except in
some high-pH soils.
SOILS OF pH 7.3 OR LOWER
60
40
20
0
80
Olsen P (ppm)
80
Mehlich-3 P (ppm)
Olsen P (ppm)
80
60
40
20
60
40
20
0
0
20
40
60
80
0
0
Bray-1 P (ppm)
20
40
60
80
0
Bray-1 P (ppm)
20
40
60
80
Mehlich-3 P (ppm)
SOILS OF pH 7.4 OR HIGHER
60
40
20
0
80
Olsen P (ppm)
80
Mehlich-3 P (ppm)
Olsen P (ppm)
80
60
40
20
0
0
20
40
60
Bray-1 P (ppm)
80
60
40
20
0
0
20
40
60
Bray-1 P (ppm)
80
0
20
40
60
Mehlich-3 P (ppm)
80
Bray-1 Vs M3 and Soil pH
pH<6.5
pH 6.5-7.3
pH 7.4-8.2
Mehlich-3 P (ppm)
100
75
50
25
r = 0.96
r = 0.95
0
0
25
50
75
100 0
25
50
75
100 0
20
40
60
100
Mehlich-3 P (ppm)
Mehlich-3 P (ppm)
Bray-1 P (ppm)
75
50
25
r = 0.91
r = 0.91
r = 0.82
0
0
20
40
60
0
20
40
Olsen P (ppm)
60
0
20
40
60
Bray-1 and M3 in Manured Soils
300
M3 = -1.8 + 1.10BP
2
r = 0.98
M3 P colorimetric (ppm)
250
200
150
100
Swine manure
Poultry layers manure
50
0
0
50
100
150
Bray P (ppm)
200
250
300
Kansas: M3 vs Bray
100
y = 1.15 x Bray
r2 = 0.91
M3 Col P (ppm)
80
60
40
Noncalcareous Soils
Calcareous Soils
20
0
0
20
40
60
Bray P (ppm)
80
100
D. Leikam
Wisconsin: M3 vs Bray
400
All sites (1998-2004)
Mehlich III, mg P kg
-1
M3 = -8 + 1.51Bray
r 2 = 0.97
300
200
Arlington
Lancaster
100
Fond du Lac
0
0
100
200
Bray P1, mg P kg-1
300
400
L. Bundy
Iowa Field Calibrations: Bray and M3
Relative Corn Yield (%)
Bray-1
M3 Colorimetric
100
90
80
pH > 7.3
pH > 7.3
70
60
VL L O H
VH
Classes
VL L O H
(low subsoil P)
VH
Classes
(low subsoil P)
50
0
10 20 30 40 50 60 70 80
Bray-1 P (ppm)
0
10 20 30 40 50 60 70 80
M3-colorimetric P (ppm)
Now Comes the ICP!
 Some labs began using ICP (inductively
coupled plasma) for the M3 test.
 ICP measures uses a very hot flame that
breaks down all compounds.
 Great confusion! ICP measures more P
than the colorimetric method whatever
extractant is used.
 But problem with M3 because it is used
for several nutrients.
ICP Measures Organic P - Minnesota
Extra P to Extracts
Poly P
40
Soil-Test P (ppm)
ICP
ATP
ICP
30
Soil P
20
10
0
Color
ICP
Color
Color
ICP Measures Organic P - Iowa
40
Recovered P (ppm)
35
Ortho P
Color
ATP
ICP
ICP
30
25
20
15
10
5
0
Color
M3-ICP vs M3-Colorimetric or Bray
80
M3-ICP (ppm)
70
90
Y = 9.87 + 1.08X
r2 = 0.84
P < 0.001
80
60
50
pH > 7.3
40
30
20
pH 8.1
BRAY-1 P (ppm)
90
70
60
50
40
30
20
10
10
0
0
0 10 20 30 40 50 60 70 80 90
M3-COL (ppm)
Y = 1.21 + 0.84X
r2 = 0.97
P < 0.001
pH > 7.3
pH 8.1
(excluded)
0 10 20 30 40 50 60 70 80 90
M3-COL (ppm)
Kansas: M3 ICP vs Colorimetric
100
M3icp = 6.76 + 1.03M3col
r 2 = 0.98
M3-ICP P (ppm)
80
60
40
20
0
0
20
40
60
M3-Col P (ppm )
80
100
D. Leikam
Iowa: M3-ICP vs M3-Colorimetric
Relative
Absolute
30
M3-ICP - M3-COL (ppm)
M3-COL / M3-ICP Ratio
1.0
0.8
0.6
0.4
0.2
0.0
25
20
15
10
5
0
0 10 20 30 40 50 60 70 80
M3-COL (ppm)
0 10 20 30 40 50 60 70 80
M3-COL (ppm)
Kansas: M3 Colorimetric/ICP Ratio
1.0
M3-Col : M 3-ICP Ratio
0.8
0.6
0.4
0.2
0.0
0
25
50
Mehlich 3-Col
75
100
D. Leikam
MN: M3-ICP vs M3-Colorimetric
ICP-Color Difference vs M ehlich P
Colorimetric Value
M3 P IC P-C olorim (ppm)
20
15
10
5
0
0
20
40
60
80
100
-5
-10
Mehlich P Colorimetric (ppm)
120
140
160
Iowa: M3 ICP & colorimetric and OM
Relative
35
Y = 2.25 - 0.02X
r² = 0.07
P < 0.02
3.5
M3-ICP - M3-COL (ppm)
M3-ICP / M3-COL Ratio
4.0
Absolute
3.0
2.5
2.0
1.5
1.0
Y = 21.2 - 0.39X
r² = 0.16
P < 0.01
30
25
20
15
10
5
0
1
2
3
4
SOIL ORGANIC C (%)
5
1
2
3
4
SOIL ORGANIC C (%)
But for this data set, high OM sites also had higher pH
and lower P by any test.
5
MN: M3 ICP & colorimetric and OM
Colorimetric - ICP Difference
vs Soil Organic Matter
10
Mehlich P ICP-Colorim (ppm)
8
6
4
2
0
-2
0
2
4
6
-4
-6
-8
-10
Soil Organic Matter (% )
8
10
12
Iowa: M3 ICP & colorimetric and pH
Relative
Absolute
3.5
35
Y = 3.96 - 0.34X
r² = 0.14
P < 0.01
3.0
2.5
2.0
1.5
1.0
M3-ICP - M3-COL (ppm)
M3-ICP / M-3COL Ratio
4.0
30
Y = 58.3 - 6.7X
r² = 0.32
P < 0.001
25
20
15
10
5
0
5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5
5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5
SOIL pH
SOIL pH
But for this data set, high pH sites also had higher OM
and lower P by any test.
MN: M3 ICP & colorimetric and pH
M eh lich P IC P-Co lo rim etric D iffe re nc e
vs So il p H
M3 IC P - C o lo r D iffe ren c e (p p m )
20
15
10
5
0
5
5 .2
5 .4
5 .6
5 .8
6
6 .2
6 .4
6 .6
6 .8
7
-5
-1 0
So il p H
7 .2
7 .4
7 .6
7 .8
8
8 .2
8 .4
NCR-13 M3 ICP-Colorimetric Work
 ICP extra P seems to be derived from
soluble organic P compounds.
 IA, MN, MO: No relationship with soil
organic matter or manure history.
 Not clear results for pH. Sometimes ICP Colorimetric difference is smaller for highpH soils, but often have less P.
 Many public and private labs: P from ICP
correlates well with colorimetric P but
there is no reliable, constant factor.
Iowa M3 ICP Field Calibrations
Relative Corn Yield (%)
110
100
90
M3 colorimetric
80
M3 ICP
70
60
50
0
10
20
30
40
50
60
M3 Soil-Test P (ppm)
70
80
90
Iowa M3 ICP Field Calibrations
Relative Corn Yield (%)
110
100
90
M3 colorimetric
80
M3 ICP
70
60
50
0
10
20
30
40
50
60
M3 Soil-Test P (ppm)
70
80
90
Critical Soil P Concentrations
Model
Bray-1
M3-Col
M3-ICP
------------------- ppm P ------------------
Cate-Nelson
13
16
20
Lin-Plateau
20
21
32
Quad-Plateau
26
28
43
Iowa M3 ICP & Color Interpretations
Relative Corn Yield (%)
Colorimetric
ICP
100
90
80
pH > 7.3
pH > 7.3
70
60
VL L O H
VH
Classes
(low subsoil P)
VL
L
O
H
Classes
VH (low subsoil P)
50
0
10 20 30 40 50 60 70 80 0
M3-colorimetric P (ppm)
10 20 30 40 50 60 70 80
M3-ICP P (ppm)
Iowa Soil-Test P Interpretations
Soil-Test Category
Soil Test Method
Very low Low Optimum High Very high
-------------------------- ppm P -------------------------Olsen
0-5
6-10
11-14
15-20
21+
Bray-1 & M3 colorim.
0-8
9-15
16-20
21-30
31+
M3-ICP
0-15
16-25
26-35
36-45
46+
Crop
P Fertilizer Recommendation
Corn
Soybean
----------------------- lb P 2O5/acre ---------------------100
75
55
0
0
80
60
40
0
0
Maintenance
adjust based
on removal
Four States Bray-1 P Classes
Class
IA
IL
MN
NE
WI
- --------------------------- ppm ------------------------------
VL
L
O, M
H
0-8
9-15
16-20
21-30
na
na
22-32
32+
0-5
6-11
12-15
16-20
0-5
6-15
16-24
25-30
0-7
8-12
13-18
19-28
NE, 8-inch sampling depth; others 6 to 7 inches
Optimum, Medium: IA, IL, WI apply maintenance
MN, NE starter or < maintenance
Environmental
Soil Phosphorus Tests
Dissolved P in Runoff
1.0
0.8
Total Dissolved P in Rurnoff (mg/L)
Y = 0.063 + 0.0043X
R2 = 0.73
Y = 0.065 + 0.0047X
R2 = 0.72
Y = 0.081 + 0.0073X
R2 = 0.77
0.6
0.4
0.2
0.0
0
1.0
0.8
20 40 60 80 100 120 0 20 40 60 80 100 120 140 0 10 20 30 40 50 60 70 80 90
Olsen P (ppm)
Bray-1 P (ppm)
Mehlich-3 P (ppm)
Y = 0.050 + 0.0076X
R2 = 0.77
Y = 0.107 + 0.015X
R2 = 0.75
Y = 0.046 + 0.038X
R2 = 0.73
0.6
0.4
FeO 0-6"
H20
0-6" vs
vs FeO
FeO 0-2"
0-2"
0.2
0.0
0 10 20 30 40 50 60 70 80
Iron-oxide P (ppm)
0
5 10 15 20 25 30 35 0
Water extractable P (ppm)
2 4 6 8 10 12 14 16
P saturation index (%)
P in Tile Drainage
Dissolved P (ppm)
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
0
20
40
60
80
100
120 0
20
Bray-1 P (ppm)
40
60
80
100 120 140 0
10
Mehlich-3 P (ppm)
20
30
40
50
60
Olsen P (ppm)
Dissolved P (ppm)
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
0
10 20 30 40 50 60 70 80 90 0
Iron-Oxide P (ppm)
5
10
15
20
25
30
Water Extractable P (ppm)
35 0
5
10
15
20
Soil P Saturation Index (%)
25
Soil P Testing in the NC Region
 No problem with Bray and Olsen tests
in the NC Region. Known high pH issue.
 M3-COL is similar to Bray in acid or
neutral soils, better in IA high-pH soils.
Some different results in WI.
 M3-COL and M3-ICP are different soil
tests!! Getting M3-COL or Bray from
M3-ICP is a very risky business.
 Base interpretations on calibrations!
 Used in P indices for NMPs or MMPs.