One can obtain well-characterized soil samples from a number of agencies such
as NIST (National Institute of Science and Technology), the National Research
Council of Canada, and other agencies. Generally these soils are
well-characterized chemically and serve as reference standards for soil
research. I would assume that the USDA would also have reference soils.
Hope this is helpful.
Southwest Texas State University
San Marcos, Tx 78666
Email: RM12 at swt.edu