The rating system which indicates if an extracted level of a nutrient is high, medium or low will differ from lab to lab. Even if two labs extract the same amount of a nutrient using the same extractant, one lab may call that amount medium while anoth er lab will indicate that it is a high level for that nutrient. The fertilizer recommendations may also vary for each lab. The recommendations are ideally derived from Experiment Station data specific for a particular region's climate and soils. Some labs will use data from adjacent states if they don't have data from their own Experiment Station for a particular crop.
Some private labs use data from universities.
So, if a farmer gets results back from two separate labs, they may differ because the sample sent to the labs were not mixed properly, the labs may be using different extractants, they may be using a different rating system for the nutrient levels, or they may be following different guidelines for their recommendations. Ultimately, with only two sets of results for comparison, it is extremely difficult to say which is best to follow. Many farmers will choose the lab results that show what they would like to see even though it may not be the most correct for their situation. It is best to stick with one laboratory and develop a long term relationship with their services as many have done with Clemson's Lab. If you have any questions about a sample, the labs will be happy to re-test the sample.