It's not precise.
There's one clear methodology of how to conduct the soil test itself. However, scientific circles still dispute whether grid sampling or zone sampling
is the better way to take samples. That means that relying on a soil test's data when assessing fertility can be risky. It's expensive.
It costs about USD $20–50 to test just one soil sample. On average, we need to take one soil sample from each 0.3 ha of a field to create a true map of its chemical properties. If we have a 100 ha field, we need to take 300 soil samples. The whole test will cost us $6,000–15,000 (and that's without counting the cost of taking the samples themselves). In our experience, these investments in soil testing don't provide a return, even if we reach a higher yield.