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The Role of Proficiency Testing Programs InAssessing Soil Health

Robert O. Miller

Colorado State University

Fort Collins, CO

Background

Conduct Regional Research in: Soil

Sampling, Soil Fertility, Lab Analysis

and test calibration.

Affiliate Professor Colorado State University.

- Ph.D. Montana State University,

- Extension Soil Specialist UC Davis.

Coordinate the Agricultural Laboratory

Proficiency (ALP) Program; three other

programs, compost, bio-solids and

sports turf.

The ALP Program

ALP Program provides proficiency services to 85 laboratories for evaluating soil, plant, water and environmental analyses. The most experienced provider in North America.

Lab performance is evaluated for both accuracy and precision. ALP submits proficiency samples tri-annually, each analyzed in triplicate, 128 methods.

Soil Testing: A ChainSoil Testing: A Chain

Miller, 2013

Soil Sample

Soil Testing is based on three components, each required to make an accurate recommendation.

Test Method

CalibrationDatabase

A soil test, in general, is an index of fertility

and is not related directly to nutrient content

Miller, 2011

• Fertilizer: one pound fertilizer does not equal one pound increase in the soil test.

Soil Testing an Index of Fertility

• It is an index of probability of crop response, based on field nutrient research.

• Exception: soil nitrate, a quantification of NO3-N in the soils.

Why Proficiency Testing?

Miller, 2013

How good are your testing laboratory results?

Are your test results consistent?

Do two labs get the same test value?

Is a certified lab proficient?

Proficiency Programs

Agricultural Laboratory Proficiency Program

NAPT Laboratory Proficiency Program

Compost Proficiency Program (CAP)

Manure Analysis Proficiency Program (MAP)

Soil, Plants, Water and Environmental Soils

Soil, Plants and Water, Soil Science Society of America

Manure, Liquid and Solids

Compost, Inorganic and Biological

What is Proficiency Testing?

Miller, 2013

A coordinator provides test materials (i.e. soil, plants,

compost) to a testing laboratory for analysis based on

official or recognized methods.

PT programs evaluate laboratories a minimum of two

times per year, using multiple test materials.

Individual laboratory results are evaluated for bias

based on consensus population statistics, typically

median and 95% confidence limits. Specific programs

evaluate both lab bias and precision (CAP, MAP, ALP).

Proficiency Testing Methods

Miller, 2013

Test methods are based on scientifically proven and

excepted analytical procedures. For the Western Region

these are published in the WERA-103 Method Manual.

Soil Methods Plant Methods

pH N

EC Sat Paste P

P – Bicarb K

NO3-N Ca

K NH4oAc Mg

B Hot Water S

SOM Zn

http://www.spcouncil.com/


Miller, 2013

Soil saturated paste is the standard method for California

for assessing soil pH, soluble salts and sodicity.

Methods

Saturated Paste %

pH

EC dS/m

Ca meq/L

Mg meq/L

Na meq/L

SAR

Cl meq/L

HCO3 meq/L

B mg/L


Miller, 2013

Soil nitrate, phosphorus, potassium, micro nutrients,

SOM and CEC are assessed based on soil extractions.

Methods

NO3-N mg/kg

P Bicarb mg/kg

K NH4oAC mg/kg

Zn DTPA mg/kg

Cu DTPA mg/kg

SOM-LOI %

CEC cmol/kg

Standard Methods - CA


Miller, 2013

Plant analyses are based on quantitative analysis of

nutrients. Results reported on total basis.

Methods

N Dumas %

P %

K %

S %

Ca %

Mg %

Zn ppm

Cu ppm

B ppm

Standard Methods - CA

Most current soil methods have been developed

based on nutrient sufficiency and promoting growth

(i.e. EC, SAR). Although SOM is measure related to

soil productivity, it’s not a measure of biological

processes.

Solvita and similar test methods can be used to

assess biological potential. ALP is currently the only

PT program that evaluates the Solvita test on soils,

and CAP on compost materials.

Soil Analysis – Soil Health?

Proficiency Testing Basics

Miller, 2013

Proficiency materials are selected based on base

properties (pH, texture, NO3-N, P etc) and are:

pulverized, homogenized and packaged.

PT programs typically submit 3-5 test materials each PT

cycle, to evaluate lab performance across a range of

analyte values.

Participating lab results are evaluated each PT cycle.

Specific programs (MAP) provide annual summary.

Miller, 2012

Bias (accuracy) and precision is best depicted by the target bulls eye.

Analysis Bias and Precision

Bias evaluates soil test consistency between labs, important to the industry, whereas precision defines the uncertainty of the soil test within a laboratory.

http://www.amrl.net/AmrlSitefinity/Newsletter/images/Spring2012/

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Assessing Lab Bias

Miller, 2013

For each sample, each method:

population median and variance based

on the median deviations are

determined.

Laboratories exceeding 95%

confidence limits are flagged for bias,

Median ± 95% CL.

Note: PT Program performance is

based on single blind results whereby

the lab is testing known proficiency

sample – unknown value.

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Histogram plot of P-Bicarb

Miller, 2013

0

5

10

15

20

25

30

35

40

45

50

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34

P (mg/kg)

Number: 120

Median: 14.8 mg/kg

95% CL: 4 mg/kg

Skew: 2.5

ALP Soil SRS-1214

Soil ID Median 95% CL Lab A Lab B

2004-111 7.90 ± 0.27 7.61 7.98

2004-112 5.92 ± 0.43 5.93 6.08

2004-113 5.38 ± 0.35 5.47 5.43

2004-114 5.76 ± 0.20 5.88 5.73

2004-115 7.40 ± 0.28 7.44 7.42

Soil pH (1:1) PT Result Comparison

Miller, 2013Source NAPT 2004, 76 labs

Plant ID N 95% CL P 95% CL

SRB-1201 3.78 ± 0.19 0.369 ± 0.061

SRB-1202 1.09 ± 0.17 0.403 ± 0.078

SRB-1203 1.87 ± 0.18 0.292 ± 0.055

SRB-1204 4.33 ± 0.27 0.387 ± 0.058

SRB-1205 2.62 ± 0.18 0.288 ± 0.064

SRB-1206 2.29 ± 0.16 0.227 ± 0.038

Plant N and P Result Comparison

Miller, 2013Source: ALP 2012, 35 labs

Miller, 2013

Analysis Median

N % 3.10

P % 0.302

K % 2.40

S % 0.24

Ca % 0.78

Mg % 0.24

30 laboratories providing results. N Results based on Dumas NWithin lab uncertainty based on 95% confidence level, 3 replications.

ALP SRB-1106, Tissue Leaf

Within - Lab Stdev

0.040

0.008

0.036

0.008

0.022

0.011

Plant Analysis within Lab Performance

The lab client has

the most interest

knowing the within

lab uncertainty.

Consesus results based on within lab

standard deviation of 35 participating labs.

Lab Certifications

Soils: ALP - State Programs, NE, MN, WI, IL, IA, MO

Compost: CAP national: United States Compost Council

Manure: MAP National Minnesota Dept of Agriculture

STA Seal of Testing Assurance, 2 labs in CA

MAP Certification, National

National and State – Program Dependent

SSSA NAPT -Performance Assessment Program

Miller, 2013

What Constitutes Certified?

Program Dependent

Minnesota Dept of Ag soils program, certified labs must provide 80% of results within 95% Confidence Limits of median each soil test.

NAPT-PAP program soils, labs must provide 80% of results within 90% CL of PAP lab pool (n=24 labs). Soil test limited. Not certification.

Illinois Soil Testing Association, certified labs must

provide 80% of results within ± 0.20 pH units and

95% CL of soil test for P and K.

Miller, 2013

Why is Certification Important?

With the Central Coast Regional Water Quality

Control Board’s Agricultural Order R3-2012-0011

Adopted March 15, 2012, specific requirements

of the order require the use of certified labs.

Miller, 2013

Water sample analysis requires a certified

laboratories a list of which can be found at the

California Dept of Public Health (CDPH) website:

www.cdph.ca.gov/certlic/labs/Documents/ELAPLablist.xls

Tim Hartz, UC Davis

Central Coast RegionalWater Quality Control Board

Agricultural Regulatory Program

Tim Hartz, UC Davis

CCRQWCB Nutrient Management

Nutrient Management is component of Tier I and

Tier II with Nitrogen Balance Ratio targets (total nitrogen applied : crop need).

Nitrogen management is the focus, along with phosphorus and soluble salts.

Miller, 2013

Lab Testing and Uncertainty

Miller, 2013

With regard to nutrient analysis and

management, with each soil test value there exist

a lever of uncertainty.

What is uncertainty?

Soil AnalysisSoil Analysis

Miller, 2013

pH : Measurement of acidity or alkalinity

Lab A 7

Lab B 7.1

Lab C 7.08

Which Lab is more accurate?

Issue of significant digits

± 0.5

± 0.05

± 0.05

Default interpretation is ½ interval of the reported value

Uncertainty and ManagementUncertainty and Management

Miller, 2013

With agricultural management we mustalways assess the measurement uncertainty in the decision.

Field 1 Field 2

Example soil EC dS/m

Example, there is no difference in soil EC for the two fields

2.5 ± 1.0 3.5 ± 0.3

Measurement UncertaintyMeasurement Uncertainty

Miller, 2013

Every measurement has associatedwith it, an uncertainty limit

This is true for any measurement

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pH, EC, P, K, yield value, speedometer, sprayer output, temperature, etc.

± 0.5 volts

Uncertainty CalculationUncertainty Calculation

Equation

Miller, 2013

where:

t(n-1) is a Student-t distribution with

(n-1) degrees of freedom, x is the

sample mean and s the unbiased

estimate of the true standard

deviation. Note often known as

the t-test.

x ± t s / o n

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DF 90 % CL 95% CL 99% CL

2 2.92 4.30 9.93

3 2.35 3.18 5.84

4 2.13 2.78 4.60

5 2.02 2.57 4.03

Values of Student’s t

pH and SalinitypH and Salinity

Method Uncertainty

pH ± 0.2 across the pH range

99% of agronomists and consultants samples

are analyzed in a single laboratory.

EC dS/m Uncertainty

0.5 ± 0.05

1.8 ± 0.09

3.7 ± 0.29

EC (saturated paste)

Miller 2013, within lab uncertainty,

Data from ALP Program

Saturated Paste MoistureSaturated Paste Moisture

Soil saturated paste moisture percentage (%)


data from ALP Program

20 30 40 50 60

± 2.4%

Lab Uncertainty

± 3.3% ± 3.8%

Uncertainty changes with analysis range

Soil NitrateSoil Nitrate



Data based on within lab stdev, average of 30 Labs, uncertainty 95% confidence limits.

Soil ID Median(ppm)

Uncert(ppm)

SRS-0804 4.8 ± 1.2

SRS-0907 13.6 ± 2.7

SRS-1101 27.4 ± 3.2

SRS-1115 54.1 ± 4.6

SRS-1210 84.5 ± 6.8

Absolute uncertainty

increases with NO3-N

content.

Uncertainty of the soil

analysis is method specific

with Cd reduction the least

and ISE the greatest.

Method Performance

Soil Analysis / Sample

K (ppm)

SRS-1107

SRS-1111

SRS-1112

SRS-1113

P Bicarb (ppm)

SRS-1107

SRS-1111

SRS-1112

SRS-1113

Stdev

1 Summary statistics based on ALP 2011 data base.2 Uncertainty based on α 0.05 and 3 replications.

6.2

3.6

5.2

14.4

0.9

1.1

3.7

1.0

±±±± 15.2

±±±± 8.8

±±±± 12.8

±±±± 35.4

±±±± 2.3

±±±± 2.7

±±±± 9.4

±±±± 2.6

Intra-Lab Precision

Mean 1

58

83

160

479

10.6

21.0

80.9

15.4

Uncertainty 2

Miller, 2013

Soil Test P and KSoil Test P and K



Phosphorus Bicarbonate (ppm)

10 20 30 40 50

± 3.2 ± 4.2 ± 6.2

Potassium (ppm)

50 100 150 200 250 300

± 10 ± 16 ± 20

± 15%

± 12%

1 Data based on within lab stdev, 45 Labs, uncertainty 95% level.

Method relative uncertainty was

calculated (uncertainty/median x 100)

and indicates similar extraction

methods are not equivalent.

P Olsen had the lowest relative

uncertainty for K.

Micronutrient (B, Zn, Mn, Cu)

uncertainty was generally > 15% for

DTPA methods.

Method PerformanceIntra-Laboratory Summary - Soils

pH (1:1) H2O ±±±± 2.4 %

EC (dS/m) ±±±± 8.5 %

P Olsen (ppm) ±±±± 15%

K (ppm) ±±±± 11 %

Zn DTPA (ppm) ±±±± 18 %

Cu DTPA (ppm) ±±±± 21 %

SOM-LOI (%)±±±± 12 %

AnalysisRelative

Uncertainty

1 Based on 45 ALP samples 2009-2012, soil P values < 100 ppm, pH < 7.5, removed.

Miller, 2013

Method PerformanceIntra-Laboratory Summary - Soils

1 Based on 50 ALP samples 2009-2012, soil P values < 100 ppm, pH < 7.5, removed.

Relative Uncertainty

Soil Analysis Method (Ranked low to high)

1 – 5 %pH, Buffer pH, Saturated Paste %,

Bray-K,

5 – 10 % NO3N-CTA, TOC EC sp

10 – 15%X-Ca, X-K, NO3N-Cd,

SOM-LOI, X-Mg, Bray-P

15 – 25 %CEC, TKN, Clay %, EC, P-Bicarb,

DTPA-Zn, DTPA-Cu

25 – 35 %Hot W-B, Cl, Extr. SO4-S,

SAR, NH4-N

Miller, 2013

What is your uncertainty?

Olsen P = 16 ppm

Lab Uncertainty ± 416 ± 4 ppm

What Management Value is Grid Sampling?

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12 15 18

14 13 17

19 14 20

Mapping P UncertaintyMapping P Uncertainty

Miller, 2013

Analysis Median

N % 3.10

P % 0.302

K % 2.40

S % 0.24

Ca % 0.78

Mg % 0.24

30 laboratories providing results. N Results based on Dumas NWithin lab uncertainty based on 95% confidence level, 3 replications.


Intra - Lab

Stdev

0.040

0.008

0.036

0.008

0.022

0.011

Plant Analysis Performance - MacrosPlant Analysis Performance - Macros

Intra - Lab

Uncertainty

± 0.119

± 0.024

± 0.107

± 0.023

± 0.065

± 0.033

± 3.8%

± 8.3%

± 8.0%

± 4.6%

± 9.5%

± 14%

Miller, 2013

Analysis Median

Zn (ppm) 40.9

Cu (ppm) 11.6

Fe (ppm) 194

Mn (ppm) 61.1

B (ppm) 15.0

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Intra - Lab

Stdev

1.96

0.53

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1.5

1.0

Plant Analysis Performance - MicrosPlant Analysis Performance - Micros

Intra - Lab

Uncertainty

± 5.8

± 1.6

± 44

± 4.4

± 2.9

30 laboratories providing results. Within lab uncertainty based on 95% confidence level, 3 replications.

ConclusionsConclusions

Miller, 2013

Proficiency programs verify lab quality, which is continuing process.

Soil and plant methods vary in precision.

All three aspects associated with lab testing have an associated uncertainty: the sample, the test method and the recommendation.

Verifying Lab Quality

Miller, 2013

1. Verify the Laboratory is enrolled in a Proficiency Testing Program, ALP, CAP, NAPT…..

2. Duplicate soils, measure lab precision… This requires skill in preparation. Submit on a routine basis.

3. Purchase known soil reference standards (ALP) to quantify lab bias…..double blind evaluation.

Soil Testing: A ChainSoil Testing: A Chain

Miller, 2013

Soil Sample

Remember the lab is responsible for the analysis… the sample is your responsibility.

Test Method

CalibrationDatabase

Thank You for Your Time and Attention