I cannot find anything on how the test, itself, is conducted other than the few generic steps.

1. Get a soil sample (6" depth is recommended)
2. Mix with extractant solution (dependent on type of test)
3. Remove the solids
4. Test the extracted solution
5. Return solution to the sample

I know this is overly simplified as most people do not need to know the process. What kind of testing is done? How are the tests conducted? What machine(s), if any, are used?

Then, I started by looking into what solutions are used to extract the nutrient content for testing. I found a simple mixture (which leads me to believe that it is for limited testing), Mehlich-3, Mehlich-1 and a few others. Can you clarify, for me: which solution is used, and what they test for?