After three years or if the crop changes, the soil should be retested and a new recommendation determined. The actual laboratory data from the analysis of your soil sample are included at the bottom of the report along with the results of any optional tests performed. These data generally are not necessary once the interpretation and recommendation have been determined. This information is useful if you compare analytical results from different labs. Direct comparisons can be made only between labs using exactly the same procedures.
Many different methods are in use around the country, each with strong and weak points.
Research on how well the tests work under local conditions determines which test will be used in a given area. The above tests have been determined to work best for Pennsylvania conditions. Bray 1 extractable phosphorus and neutral 1 N ammonium acetate extractable potassium, calcium, and magnesium tests used at Penn State before 1991 are very similar to the Mehlich 3 extractable amounts of these nutrients. It is easy to send a soil sample to a lab anywhere in the country, and that lab may do an excellent job in its area. Always use a soil testing laboratory that bases its interpretations and recommendations on local conditions.