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The start of an answer to your question.

            1. Select the analytical lab based upon their ability to analyze reference samples from the National Proficiency Testing Program. If the lab doesn’t participate in this program or will not share their data for the last 3 years with you, keep looking. The nod goes to labs that physically weigh the soil aliquots used for analysis as opposed to using a “scoop”.

            2. Verify their performance by purchasing your own reference soils from the national program.  ~5% of your samples you submit to the analytical lab for analysis should come from those known reference samples. Those samples should be submitted blind, just like your real samples.

            3. Another 5% of the samples submitted for analysis should be replicated samples from your fields. Thes should also be submitted blind.

The above 3 steps will be the best money you will ever spend.

If you want to hire an ag consultant to do your sampling/analysis for you that doesn’t do this, don’t hire them. If they can’t tell you why each of these steps is important and how to use the data , don’t hire them either.