The Iowa Soil Testing Laboratory Certification Program requires laboratories that wish to be
certified in the State of Iowa to provide analyses on quarterly “check samples” as provided by the
North American Proficiency Testing (NAPT) Program. The laboratories are evaluated on the following tests:
Soil pH - 1:1 soil:water;
Buffer pH - SMP buffer (only required if pH < 7);
Phosphorus - Bray-1, Mehlich-3 (colorimetric or ICP), or Olsen;
Potassium - Ammonium Acetate or Mehlich 3;
Organic Matter - Walkley-Black, LOI, or dry combustion;
Zinc - DTPA.
To maintain certification, laboratories must maintain 80% proficiency every quarter. If a laboratory “fails” a
quarterly test, they will be removed from the list of certified laboratories. To regain certification, a laboratory
may ask for a different set of “check samples” and perform above 80% proficiency, or wait until they have had two consecutive tests above the 80% proficiency level.