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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/
Proficiency Testing Methods
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
Proficiency Testing Methods
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
Proficiency Testing Methods
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 (%)
Miller 2013, within lab uncertainty,
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
Miller 2013, within lab uncertainty,
data from ALP Program
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
Miller 2013, within lab uncertainty,
data from ALP Program
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.
ALP SRB-1106, Tissue Leaf
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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ALP SRB-1106, Tissue Leaf
Intra - Lab
Stdev
1.96
0.53
15
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