Illinois Soil Testing Association
ISTA's Lab Proficiency Testing Program
The Bulls-Eye of Consistent Quality and Consistent Results
ISTA knows that farmers and others in need of soil analysis require reliability,
consistency and accuracy. ISTA membership is proof positive that a soil testing
laboratory strives to meet these expectations.
The Illinois Soil Testing Association Lab Accreditation Program
The Illinois Soil Testing Association Lab Accreditation Program (ISTA-LAP) has been
developed to assure soil test results provided by participating labs serving the citizens of
Illinois are within industry statistical norms of the test method. Based on this premise,
soil test results from ISTA-LAP approved labs shall be accepted under the U.S.
Department of Agriculture NRCS (Natural Resource Conservation Services) code 590 of
nutrient management, revised January 2012. The ISTA-LAP program is a voluntary
program for laboratories seeking accreditation for soil testing in the state of Illinois. At
the beginning of each year, the ALP coordinator will require a letter of acceptance from
the participating laboratories allowing the release of proficiency data to the ISTA-LAP
The ISTA-LAP proficiency assessor performs statistical analysis of the data as specified
by the ISTA-LAP program. Laboratories providing soil test results within performance
limits, will be placed on the ISTA list of approved labs and posted on the associations
web site. Laboratories that fail to meet performance limits of the ISTA-LAP program, will
be offered a retest, and program for reinstatement.
Laboratories that wish to participate in the ISTA-LAP program will be required to:
(1) A member of ISTA
(2) Enrolled in the ALP Program for the calendar year
(3) Provide soil analyses by methods recognized by the ISTA, each ALP cycle.
(Recommended Chemical Soil Test Procedures for the North Central Region. (Revised)
1998. Missouri Agricultural Experiment Station SB 1001.)
The required soil test analyses methods are:
- pH H2O (1:1) and/or pH (1:1) 0.01 M CaCl2
- Phosphorus: Bray-1 P /or Mehlich 3 P SPEC or Mehlich 3 P ICP
- Potassium: NH4oAc Extactable /or Mehlich 3 K
Additional soil tests analyses are encouraged (i.e. buffer pH, exchangable cations, micro
nutrients and soil organic matter) but are not required.
ISTA-LAP criteria for accreditation
Assessment of laboratory performance will be based on the laboratory analysis of five
ALP soils (ALP test Code 801) each proficiency cycle by the proficiency assessor. Soil
test performance limits criteria are listed in Table 1 by analysis method. Individual
laboratory soil test results exceeding the listed performance limits of the statistical
median, for each test method will be flagged for failure. ISTA accreditation shall be
based on a 80% level of performance for each soil test method, across the five soils of
each proficiency cycle. A maximum of only one soil test result could be flagged, to meet
accreditation requirements of the ISTA-LAP program each
Laboratories failing to meet accreditation requirements shall be offered a retest soil
sample set for the soil test method in question, and with a successful retest, their
accreditation reinstated. Re-test soil sets will be provided within 15 days by the ALP
Program Technical Director at a cost of $100. Laboratories which choose not to retest or
provide retest results within 30 days will be removed from the ISTA accreditation list.
Laboratories failing the retest soil sample set, shall have an independent consultant
review the lab’s method operation and performance at their costs, until it is determined
by the ISTA-LAP assessor, they be provided a 2nd retest set at an additional cost of $75.
Laboratories failing on two retest sets will be removed from the accreditation list for
1 year. Once annually a yearend report will be provided to each ISTA-LAP laboratory
listing a summary of the laboratory performance.
Table 1. ISTA-LAP Soil test performance limits criteria.
Soil Test Method Soil Test Ranges and Performance Limits*
pH (1:1) H2O or 0.01 M CaCl2 ± 0.20 or ± 95% CL, whichever is greater
Phosphorus - all methods (lbs/ac) > ± 95% CL or ± 4 lbs/ac, whichever is greater
Potassium - all methods (lbs/ac) > ± 95% CL or ± 20 lbs/ac whichever is greater
* Performance limits are based on a 95% confidence Limits (CL) of the consensus ISTA
median soil test value, or as specified.
Annually the ISTA-LAP soil test method performance and accreditation criteria shall be
reviewed by a the ISTA-LAP committee, comprised of a member of the ISTA board, the
program assessor, a representative of the University of Illinois and a representative of
the ALP Program. Modifications of the accreditation criteria or addition of soil test
methods shall require a majority vote of the ISTA organization.