Your chart has the numbers that I had been taught was how reliable soil test results are.

This chart shows the percent error of the zone average when compared to the individual samples (the chart is labeled incorrectly). In other words, the zone average over/underestimates the nutrient level by the listed percentage. Clearly we can see that for P & K, using the average for the zones results in a large amount of error. A person needs to keep in mind that over/underestimating the level of fertility directly affects the amount of fertilizer being applied by roughly the same percentage. So in effect, if I were to use the zone average as a basis for a fertilizer recommendation, I would be overapplying or underapplying P by approximately 40% and K by 20%. In my opinion, this is unacceptable.

This chart shows the percent error of the zone average when compared to the individual samples (the chart is labeled incorrectly). In other words, the zone average over/underestimates the nutrient level by the listed percentage. Clearly we can see that for P & K, using the average for the zones results in a large amount of error. A person needs to keep in mind that over/underestimating the level of fertility directly affects the amount of fertilizer being applied by roughly the same percentage. So in effect, if I were to use the zone average as a basis for a fertilizer recommendation, I would be over applying or under applying P by approximately 40% and K by 20%. In my opinion, this is unacceptable.

If I remember correctly the Journal of Production Agriculture had much the same end results, though I believe they approached it from a different angle. They were saying that pH is reasonable accurate, while P & K has roughly a 30% natural variability.

Add to the mix every lab has their own idea of what they should be finding even when using the same chemistry.

About 30 years ago a some what organic research outfit in Pennsylvania dug up a barrel full of dirt. They ground it and mixed it thoroughly and sent the samples off to ><100 different labs. They got back results that were all over the place. They published the results and their conclusions in the New Farm magazine.