When comparing results from one laboratory to the next it is important to determine the following:

Laboratory methods used by each laboratory for each analysis.
Units reported by each laboratory for each analysis.
Any conversion or calculation used by each laboratory.

If any of these variables are unknown, it will not be possible to complete a data comparison.

It is important to note that there is no perfect method for determining how much or little of an essential nutrient is available for plant uptake. Availability is determined by: soil moisture, temperature, soil pH, clay content, soil drainage, organic content, physical barriers, and many other factors. As any of these factors change, the availability will also change.

References used for Laboratory Methodologies.

Analytic procedures used in US EPA Report SW-846, Test Methods for Evaluating Solid Waste, 3rd Edition, November 1990, as revised.

Association of Official Analytical Chemists (AOAC) Official Methods of Analysis, 1990, 15th edition.

Methods of Soil Analysis, Parts I, II and III. Soil Science Society of America. 1996
North Central Regional (NCR) Publication No. 221 (Revised). Recommended Chemical Soil Test Procedures for the North Central Region Bulletin No. 499 (Revised) October 1988.

Standard Methods for the Examination of Water and Wastewater, 18th Edition, 1992.

The American Society for Testing and Materials (ASTM) Standard Test Methods, 1988.