Soil Phosphorus Tests in the Soil Phosphorus Tests in the North-Central RegionNorth-Central Region

Antonio MallarinoAntonio Mallarino

Iowa State UniversityIowa 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

Bray-1 Olsen

Bray-1 P (ppm)

0 10 20 30 40 50 60 70

Rel

ativ

e C

orn

Yie

ld (

%)

50

60

70

80

90

100

VL L H VHO

pH > 7.3

Olsen P (ppm)

0 5 10 15 20 25 30 3550

60

70

80

90

100

VL L H VHO

pH > 7.3

(low subsoil P)Classes

(low subsoil P)Classes

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 M3-K and the M3-P colorimetric in 1999.

M3- P is similar to Bray-1 P, except in some high-pH soils.

Ols

en P

(p

pm

)

Bray-1 P (ppm)0 20 40 60 80

0

20

40

60

80

Ols

en P

(p

pm

)

Mehlich-3 P (ppm)0 20 40 60 80

0

20

40

60

80

Meh

lich

-3 P

(p

pm

)

Bray-1 P (ppm)0 20 40 60 80

0

20

40

60

80

SOILS OF pH 7.3 OR LOWER

SOILS OF pH 7.4 OR HIGHER

Ols

en P

(p

pm

)

Bray-1 P (ppm)0 20 40 60 80

0

20

40

60

80

Ols

en P

(p

pm

)Mehlich-3 P (ppm)

0 20 40 60 800

20

40

60

80

Meh

lich

-3 P

(p

pm

)

Bray-1 P (ppm)0 20 40 60 80

0

20

40

60

80

Bray-1 Vs M3 and Soil pHM

ehlich-3

P (ppm

)

Bray-1 P (ppm)0 20 40 600 25 50 75 100

0

25

50

75

100pH<6.5

r = 0.96

0 25 50 75 100

pH 6.5-7.3 pH 7.4-8.2

r = 0.95

0 20 40 60

0

25

50

75

100M

ehlich-3

P (ppm

)

0 20 40 60

Mehlich-3

P (ppm

)

Olsen P (ppm)0 20 40 60

r = 0.91 r = 0.82r = 0.91

Bray-1 and M3 in Manured Soils

Bray P (ppm)

0 50 100 150 200 250 300

M3

P c

olo

rim

etri

c (p

pm

)

0

50

100

150

200

250

300

M3 = -1.8 + 1.10BPr 2 = 0.98

Swine manure

Poultry layers manure

Kansas: M3 vs Bray

y = 1.15 x Bray

r2 = 0.91

0

20

40

60

80

100

0 20 40 60 80 100Bray P (ppm)

M3

Co

l P (

pp

m)

Noncalcareous Soils

Calcareous Soils

D. Leikam

Wisconsin: M3 vs Bray

Bray P1, mg P kg-1

0 100 200 300 400

Me

hlic

h I

II,

mg

P k

g-1

0

100

200

300

400

Arlington

Lancaster

Fond du Lac

All sites (1998-2004)

M3 = -8 + 1.51Bray

r 2 = 0.97

L. Bundy

Iowa Field Calibrations: Bray and M3

Bray-1 M3 Colorimetric

Bray-1 P (ppm)

0 10 20 30 40 50 60 70 80

Rel

ativ

e C

orn

Yie

ld (

%)

50

60

70

80

90

100

VL L H VHO (low subsoil P)Classes

pH > 7.3

M3-colorimetric P (ppm)

0 10 20 30 40 50 60 70 80

VL L H VHO (low subsoil P)Classes

pH > 7.3

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 - MinnesotaS

oil-

Tes

t P

(p

pm

)

0

10

20

30

40

Soil P

Color ICP

Extra P to Extracts

Poly P

Color

ICP ATP

Color

ICP

ICP Measures Organic P - IowaR

eco

vere

d P

(p

pm

)

0

5

10

15

20

25

30

35

40 Ortho P ATP

Color ICP

Color

ICP

M3-ICP vs M3-Colorimetric or Bray

M3-COL (ppm)

0 10 20 30 40 50 60 70 80 90

M3-

ICP

(p

pm

)

0

10

20

30

40

50

60

70

80

90Y = 9.87 + 1.08Xr2 = 0.84P < 0.001

M3-COL (ppm)0 10 20 30 40 50 60 70 80 90

BR

AY

-1 P

(p

pm

)0

10

20

30

40

50

60

70

80

90Y = 1.21 + 0.84Xr2 = 0.97P < 0.001

pH 8.1(excluded)

pH 8.1

pH > 7.3

pH > 7.3

Kansas: M3 ICP vs Colorimetric

M3icp = 6.76 + 1.03M3colr2 = 0.98

0

20

40

60

80

100

0 20 40 60 80 100

M3-Col P (ppm)

M3-

ICP

P (

pp

m)

D. Leikam

Iowa: M3-ICP vs M3-Colorimetric

Relative Absolute

M3-COL (ppm)0 10 20 30 40 50 60 70 80

M3-

CO

L /

M3-

ICP

Rat

io

0.0

0.2

0.4

0.6

0.8

1.0

M3-COL (ppm)0 10 20 30 40 50 60 70 80

M3-

ICP

- M

3-C

OL

(p

pm

)0

5

10

15

20

25

30

Kansas: M3 Colorimetric/ICP Ratio

0.0

0.2

0.4

0.6

0.8

1.0

0 25 50 75 100

Mehlich 3-Col

M3-

Co

l :

M 3

-IC

P R

atio

D. Leikam

MN: M3-ICP vs M3-Colorimetric

ICP-Color Difference vs Mehlich P Colorimetric Value

-10

-5

0

5

10

15

20

0 20 40 60 80 100 120 140 160

Mehlich P Colorimetric (ppm)

M3

P IC

P-C

olo

rim

(p

pm

)

Iowa: M3 ICP & colorimetric and OM

Relative Absolute

But for this data set, high OM sites also had higher pH and lower P by any test.

SOIL ORGANIC C (%)

1 2 3 4 5

M3-

ICP

/ M

3-C

OL

Rat

io

1.0

1.5

2.0

2.5

3.0

3.5

4.0

SOIL ORGANIC C (%)

1 2 3 4 5M

3-IC

P -

M3-

CO

L (

pp

m)

0

5

10

15

20

25

30

35Y = 2.25 - 0.02Xr² = 0.07P < 0.02

Y = 21.2 - 0.39Xr² = 0.16P < 0.01

MN: M3 ICP & colorimetric and OM

Colorimetric - ICP Difference vs Soil Organic Matter

-10

-8

-6

-4

-2

0

2

4

6

8

10

0 2 4 6 8 10 12

Soil Organic Matter (%)

Me

hli

ch

P I

CP

-Co

lori

m (

pp

m)

Iowa: M3 ICP & colorimetric and pH

Relative Absolute

But for this data set, high pH sites also had higher OM and lower P by any test.

SOIL pH

5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5

M3-

ICP

/ M

-3C

OL

Rat

io

1.0

1.5

2.0

2.5

3.0

3.5

4.0

SOIL pH

5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5M

3-IC

P -

M3-

CO

L (

pp

m)

0

5

10

15

20

25

30

35Y = 58.3 - 6.7Xr² = 0.32P < 0.001

Y = 3.96 - 0.34Xr² = 0.14P < 0.01

MN: M3 ICP & colorimetric and pH

Mehlich P ICP-Co lorimetric D ifferencevs So il p H

-1 0

-5

0

5

1 0

1 5

2 0

5 5 .2 5 .4 5 .6 5 .8 6 6 .2 6 .4 6 .6 6 .8 7 7 .2 7 .4 7 .6 7 .8 8 8 .2 8 .4

Soil pH

M3

IC

P -

Co

lor

Dif

fere

nc

e (

pp

m)

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 high-pH 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

M3 Soil-Test P (ppm)

0 10 20 30 40 50 60 70 80 90

Rel

ativ

e C

orn

Yie

ld (

%)

50

60

70

80

90

100

110

M3 colorimetric

M3 ICP

Iowa M3 ICP Field Calibrations

M3 Soil-Test P (ppm)

0 10 20 30 40 50 60 70 80 90

Rel

ativ

e C

orn

Yie

ld (

%)

50

60

70

80

90

100

110

M3 colorimetric

M3 ICP

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

M3-colorimetric P (ppm)

0 10 20 30 40 50 60 70 80

Rel

ativ

e C

orn

Yie

ld (

%)

50

60

70

80

90

100

M3-ICP P (ppm)

0 10 20 30 40 50 60 70 80

VL L H VHOVL L H VHO (low subsoil P)Classes

(low subsoil P)Classes

pH > 7.3 pH > 7.3

Colorimetric ICP

Soil-Test CategorySoil 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----------------------- lb P2O5/acre ----------------------

Corn 100 75 55 0 0

Soybean 80 60 40 0 0

Soil-Test CategorySoil 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----------------------- lb P2O5/acre ----------------------

Corn 100 75 55 0 0

Soybean 80 60 40 0 0Maintenanceadjust basedon removal

Iowa Soil-Test P Interpretations

Four States Bray-1 P Classes

Class IA IL MN NE WI

- --------------------------- ppm ------------------------------

VL 0-8 na 0-5 0-5 0-7

L 9-15 na 6-11 6-15 8-12

O, M 16-20 22-32 12-15 16-24 13-18

H 21-30 32+ 16-20 25-30 19-28

NE, 8-inch sampling depth; others 6 to 7 inchesOptimum, Medium: IA, IL, WI apply maintenance

MN, NE starter or < maintenance

EnvironmentalEnvironmentalSoil Phosphorus TestsSoil Phosphorus Tests

Mehlich-3 P (ppm)0 20 40 60 80 100 120 140

Bray-1 P (ppm)0 20 40 60 80 100 120

To

tal D

isso

lved

P in

Ru

rno

ff (

mg

/L)

0.0

0.2

0.4

0.6

0.8

1.0

Olsen P (ppm)0 10 20 30 40 50 60 70 80 90

Iron-oxide P (ppm)0 10 20 30 40 50 60 70 80

0.0

0.2

0.4

0.6

0.8

1.0

FeO 0-6" vs FeO 0-2" H20 0-6" vs FeO 0-2"

Water extractable P (ppm)0 5 10 15 20 25 30 35

P saturation index (%)0 2 4 6 8 10 12 14 16

Y = 0.065 + 0.0047XR2 = 0.72

Y = 0.063 + 0.0043XR2 = 0.73

Y = 0.081 + 0.0073XR2 = 0.77

Y = 0.050 + 0.0076XR2 = 0.77

Y = 0.107 + 0.015XR2 = 0.75

Y = 0.046 + 0.038XR2 = 0.73

Dissolved P in Runoff

Bray-1 P (ppm)0 20 40 60 80 100 120

Dis

solv

ed P

(p

pm

)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

Olsen P (ppm)0 10 20 30 40 50 60

Iron-Oxide P (ppm)0 10 20 30 40 50 60 70 80 90

Dis

solv

ed P

(p

pm

)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

Soil P Saturation Index (%)0 5 10 15 20 25

Mehlich-3 P (ppm)0 20 40 60 80 100 120 140

Water Extractable P (ppm)0 5 10 15 20 25 30 35

P in Tile Drainage

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.