The Virginia Tech Soil Testing Laboratory participates in the North American Proficiency Testing (NAPT) program, http://www.naptprogram.org/, which assists agricultural labs in their performance through inter-laboratory sample exchanges and statistical evaluations of the analytical data.
Recommendation Guidebook (PDF)
About ICP analysis
The Soil Testing Laboratory uses an Inductively Coupled Plasma Atomic Emission Spectrometer for analyzing the nutrient content of soil extracts. A description of ICP analysis is here