Soil Grinding.

It is our goal to collect and test a sample that represents the actual fertility levels in the field we are sampling.  Today, my son is grinding samples for his master's project so I thought I would pass on how we assure we have a uniform sample to analyze.  Our normal sampling procedure is to pull 10 to 15 cores per sample area.  Usually we crumble the samples and take out about a cupful to send to the lab.  Sometimes when the soil is very wet or very high in clay or both, it does not crumble well.  After we dry the samples, we can either send the whole sample to the lab or if we want to save some space and weight, we can grind the samples ourselves.  The samples are ground and thoroughly mixed at the same time.  From the  sample we send to the lab, many of the tests use a 5 gram sample that is scooped out of what we send in.  When we send crumbled samples, the lab grinds them for us to assure thorough mixing and uniformity.  One thing you might want to look into is if your lab grinds the whole sample.  If not, then your tests may not tell you what you want to know because they do not have a representative 5 grams of the total sample. 
Grinder is on the right with samples waiting to be ground.

This what the inside of our grinder looks like.  Most are some variation of this.